ANRV384-PY47-18
ARI
ANNUAL
REVIEWS
2 July 2009
19:16
Further
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
Click here for quick links to
Annual Reviews content online,
including:
• Other articles in this volume
• Top cited articles
• Top downloaded articles
• Our comprehensive search
Ustilago maydis as a Pathogen
Thomas Brefort, Gunther Doehlemann,
Artemio Mendoza-Mendoza, Stefanie Reissmann,
Armin Djamei, and Regine Kahmann
Max Planck Institute for Terrestrial Microbiology, Department of Organismic Interactions,
D-35043 Marburg, Germany; email: [email protected]
Annu. Rev. Phytopathol. 2009. 47:423–45
Key Words
The Annual Review of Phytopathology is online at
phyto.annualreviews.org
corn smut, dimorphism, signal transduction, host response, fungal
effectors
This article’s doi:
10.1146/annurev-phyto-080508-081923
c 2009 by Annual Reviews.
Copyright All rights reserved
0066-4286/09/0908/0423$20.00
Abstract
The Ustilago maydis–maize pathosystem has emerged as the current
model for plant pathogenic basidiomycetes and as one of the few models
for a true biotrophic interaction that persists throughout fungal development inside the host plant. This is based on the highly advanced genetic system for both the pathogen and its host, the ability to propagate
U. maydis in axenic culture, and its unique capacity to induce prominent
disease symptoms (tumors) on all aerial parts of maize within less than
a week. The corn smut pathogen, though economically not threatening, will continue to serve as a model for related obligate biotrophic
fungi such as the rusts, but also for closely related smut species that
induce symptoms only in the flower organs of their hosts. In this review we describe the most prominent features of the U. maydis–maize
pathosystem as well as genes and pathways most relevant to disease.
We highlight recent developments that place this system at the forefront of understanding the function of secreted effectors in eukaryotic
pathogens and describe the expected spin-offs for closely related species
exploiting comparative genomics approaches.
423
ANRV384-PY47-18
ARI
2 July 2009
19:16
INTRODUCTION
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
Hemibasidiomycete smut fungi comprise more
than 1500 species, and among them are plant
pathogens of considerable economic importance. Most smuts have a narrow host range
and are specialized on members of the Poaceae
(grasses) that include valuable crop species such
as maize, sorghum, sugar cane, wheat, and barley. A recent investigation involving five smut
species indicates that contemporary smut taxa
diverged millions of years ago, i.e., before the
time of domestication and modern agriculture,
presumably through coevolution within natural populations of the ancestors of our current
crop plants (86).
In all smut fungi, pathogenic development
is coupled to sexual development. Haploid cells
of opposite mating type have to fuse and generate a dikaryotic form that is then able to
invade the host plant either through natural
openings or directly. Most smut fungi infect
plants during the seedling stage and then continue growth within meristematic tissue during
plant development. Usually, this phase is systemic, and symptom development is restricted
to the female and/or male inflorescences of
the host. In the flowers, massive fungal proliferation and spore formation occurs. As a
consequence, seeds are replaced by smut sori
consisting of masses of black teliospores (74).
Teliospores represent the resting stage that can
survive harsh environments for long periods.
Under appropriate conditions teliospores germinate, undergo meiosis, and produce haploid
progeny. Ustilago maydis, in contrast to the other
smut fungi, produces prominent symptoms on
all aerial parts of its host plant, maize. In practice, maize seedlings can be infected at the
three-leaf stage, and about one week later the
symptoms, in this case tumor formation, can be
scored.
U. maydis represents a fully developed
genetic system that is amenable to highly
efficient reverse genetics and cell biological approaches (11, 58, 98). The first steps
toward understanding the disease were undertaken by the characterization of the mating
424
Brefort et al.
type loci (1), which enabled the design of
solopathogenic haploid strains that cause
disease without the need for a mating partner.
Such strains allowed unbiased approaches
to analyze pathogenicity (9, 92). In 2006 a
new era began with the publication of the
20.5 Mb U. maydis genome sequence (59).
The genome is highly compact and codes for
approximately 6900 protein encoding genes.
The majority of these genes lack introns, and
the genome is largely devoid of repetitive
DNA (59). More than 99% of the sequence is
represented in 24 large scaffolds that correspond to the 23 chromosomes. Access to the
manually annotated genome sequence (http://
www.broad.mit.edu/annotation/genome/
ustilago maydis/Home.html and http://
mips.gsf.de/genre/proj/ustilago/) allowed
detection of a large set of novel genes that have
crucial roles during pathogenesis. Many of
these novel genes code for secreted proteins,
so-called effectors (59). Effectors have so far
mostly been studied in prokaryotic pathogens,
where they have been shown to be translocated
to plant cells and interfere with a variety
of defense responses (43). Discovery of a
large repertoire of effectors in oomycete plant
pathogens has led to the definition of two effector classes: those that function in the apoplast
and those that are taken up by plant cells via a
conserved motif (RXLR) (57, 103). Effectors
secreted by fungi including U. maydis lack
such a common motif (34). In the U. maydis
genome, effector genes were detected by
serendipity through their organization in
clusters that are upregulated during plant
colonization (59).
In this review, we will focus on aspects that
relate to pathogenic development of U. maydis.
This includes morphological transitions, signaling networks, nutrition in the host environment, plant defense responses, and secreted effectors. A section on comparative genomics and
what we can expect from such approaches is included, as well as an outlook on questions that
might be addressed in the near future. For all
other aspects of U. maydis biology the reader is
referred to a number of recent comprehensive
ANRV384-PY47-18
ARI
2 July 2009
19:16
reviews (1, 56, 64, 87, 97, 98) and to a special
issue of Fungal Genetics and Biology (46) that
highlights several distinct features of the
genome.
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
THE DISEASE CYCLE
The haploid yeast-like form of U. maydis can be
propagated on artificial media. However, this
form is unable to cause disease. The infectious
form is generated when two compatible haploid
strains fuse and generate a dikaryotic filament.
This process is controlled by a tetrapolar mating system that is specified by the biallelic a
locus and the multiallelic b locus (1, 56). Strains
that differ in a and b can fuse and form dikaryotic hyphae. On agar plates containing activated
charcoal, such dikaryotic filaments extend into
the air and profusely cover the colonies, resulting in a white, fuzzy appearance (2). The a locus encodes a pheromone and receptor system
that allows haploid cells of the opposite a mating type to sense each other and to fuse. The
fate of the resulting dikaryon depends on the b
locus, which codes for a pair of homeodomain
transcription factors, termed bE and bW. bE
and bW proteins dimerize when derived from
different alleles, and the heterodimeric bE/bW
complex triggers filamentation as well as sexual and pathogenic development (1). The filamentous dikaryon of U. maydis establishes a
biotrophic interaction that persists throughout
the disease cycle (Figure 1).
The dikaryotic hyphae show tip-directed
growth, and the cytoplasm accumulates in the
tip cell compartment, whereas older parts of the
hyphae become vacuolated and are sealed off
by regularly spaced septa. Often, these empty
sections collapse. On the plant surface, hyphae stop polar growth in response to an as
yet unidentified signal, and their tips swell to
form poorly differentiated, nonmelanized appressoria (Figure 1a). These develop infection
hyphae (Figure 1b) that directly penetrate into
the plant tissue, presumably aided by lytic enzymes. The plasma membrane of the plant cell
invaginates and tightly surrounds the intracellular hyphae. This creates a special biotrophic
interface, an interaction zone in which fungal
secretion leads to an accumulation of deposits
that make up a vesicular matrix along the contact area (7; Figure 1c).
U. maydis does not form specialized feeding structures (haustoria) suggesting that signal exchange and nutrient uptake have to occur via the biotrophic interface. During early
in planta development, the fungus grows intracellular, and cell-to-cell passages into deeper
layers of the infected tissue are observed. The
formation of empty sections ceases, and mitotic divisions take place, concomitant with
the development of clamp-like structures that
allow the correct sorting of nuclei (91) and
branching. After these early stages the fungus
is found preferentially in and near the vasculature (Figure 1d ). Five to six days after infection
massive fungal proliferation, presumably of the
diploid form, is observed in tumor tissue, and
this occurs intracellularly and extracellularly in
apoplastic cavities where large fungal aggregates form (27; Figure 1e). In these aggregates
hyphae are embedded in a mucilaginous matrix that could have a protective role. Proliferation is followed by sporogenesis where hyphal
sections fragment, round up, and differentiate
into heavily melanized diploid teliospores (3,
94, 95). The spores are released when the tumors dry up and rupture. Under favorable conditions these spores germinate, the diploid nucleus undergoes meiosis, and budding off from
a promycelium, haploid cells are again produced. So far it has not been possible to reconstitute this cycle in vitro, indicating that many
of its steps require stage-specific signals from
the host.
Mating and Pathogenic Development
Require cAMP and MAPK Signaling
During mating the lipopeptide pheromone is
perceived, and the signal is transmitted by
two conserved signaling cascades: a cAMPdependent protein kinase A (PKA) pathway and
a mitogen-activated protein kinase (MAPK)
module that have been comprehensively reviewed recently (40, 64, 87). Components of
www.annualreviews.org • Ustilago maydis as a Pathogen
425
ANRV384-PY47-18
ARI
2 July 2009
19:16
addition, prf1 transcription is controlled by the
two MAPKs Kpp2 and Crk1 (41, 85) through a
complex interplay of at least four downstream
transcription factors that bind to discrete elements in the prf1 promoter: Prf1 itself, Rop1,
Hap2, and an as yet unidentified factor downstream of Crk1 acting via an upstream activation
sequence (15, 41, 48, 78, 105). Except for the
recently identified dual-specificity phosphatase
Rok1 that downregulates mating (23), all
←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
Figure 1
Plant responses
U. maydis effectors
a
Rep1
Pep1
Protease inhibitors
Receptor kinases
5 μm
b
Pep1
JA signaling
Cluster 5A
No SA signaling
24 h
PR genes*
12 h
PR genes
JA signaling
Mig2_5-6
Cell death suppression
5 μm
c
Rsp1/Hum3
Cell death suppression
Mig1_1
Antioxidants
48 h
PR genes*
Mig2_1–6
Photosynthesis
5 μm
d
Photosynthesis
Mig1_1
Shikimate pathway
Mig2_1–6
Hexoses
4d
Antioxidants
Cluster 19A
Auxin and GA
25 μm
Redirection of carbon flow
e
Photosynthesis
Cluster 19A
Shikimate pathway
Cluster 2A
Auxin and GA
Cell division and enlargement
426
Brefort et al.
50 μm
8d
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
these pathways with crucial roles during mating
and their epistatic relationships are depicted in
Figure 2a. After pheromone-induced activation both signaling pathways converge on the
key transcription factor Prf1, which in turn induces transcription of a large set of genes including the a and b mating-type genes (112) and
is thus essential for mating. To induce expression of the a genes, mfa and pra, Prf1 needs to be
phosphorylated by the PKA Adr1, whereas induction of the b genes requires Prf1 phosphorylation by Adr1 and the MAPK Kpp2 (55, 85). In
Stage specificity of plant responses and activity of
secreted U. maydis effectors. Prominent plant
responses to U. maydis infection (left), micrographs
of typical infection structures (middle), and secreted
effectors that are required at distinct stages of
infection between 12 hpi and 8 dpi are shown.
Green arrows: Induced plant genes or processes.
Red arrows: Repressed plant genes or processes.
Asterisks: Expression reduced compared to the 12 h
time point. U. maydis effectors that are required for
virulence are shown in red letters; effectors that
restrict virulence are shown in green. Effectors
without known function for pathogenicity are shown
in black. For microscopy, maize plants were infected
with solopathogenic haploid strains of U. maydis that
expressed fluorescently tagged fusion proteins in (c).
(a) 12-h postinfection, filaments of a solopathogenic
U. maydis strain are forming appressoria on the
maize epidermis. Blue: Fungal hypha stained with
calcofluor white. (b) 24-h postinfection, confocal
projection of an appressorium of the same
solopathogenic strain as in (a); the penetrating
hypha has entered the maize epidermis cell (arrow).
Blue: Fungal hypha stained with calcofluor white.
(c) Confocal projection of a hypha that is growing
intracellularly in a maize epidermis cell, 48-h
postinfection. Blue: Plant-cell-wall
autofluorescence. Red: Cytoplasmic red fluorescent
protein (RFP) expressed by U. maydis. Green:
Mig2-6:GFP fusion protein, which is secreted from
the hypha to the biotrophic interface. (d ) Confocal
projection of U. maydis hyphae colonizing maize
tissue in and around a vascular bundle, 4-days
postinfection. Red: Plant-cell-wall structures stained
with propidium iodide. Green: Hyphae of U. maydis
stained with WGA-AF488. (e) Confocal projection
of U. maydis hyphae in maize leaf tumor tissue,
8-days postinfection. Fungal hyphae form large
aggregates between the enlarged maize tumor cells.
Green: U. maydis hyphae stained with WGA-AF488.
Blue: Plant-cell-wall autofluorescence.
ANRV384-PY47-18
a
ARI
2 July 2009
19:16
b
Mating
a1b1
Plant invasion
a2b2
Cutin monomers/
?
a3
Gpa3
Gγ
cAMP
ATP
Ras2
Pra1
a3
Gpa3
Gγ
cAMP
U
Ubc2
Adr1
Ras1
?
ATP
U
Ubc2
Adr1
Kpp4/Ubc4
Ubc1
Kpp4
Ubc1
Fuz7/Ubc5 P
cAMP
Adr1
Crk1
Ubc1
P
Fuz7
cAMP
Adr1
Kpp2/Ubc3 P
Crk1
Ubc1
P
P
P
P
Kpp6 P
Rok1
Rok1
P
?
P
P
Sql2
?
Kpp2
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
Uac1
Ras1
Bpp1
Uac1
Ras2
?
Pra1
Rop1
P
Hap2
P
Prf1
P
P
a mating type genes:
mfa1, pra1
prf1
P
b mating type genes:
bE1, bW1
P
?
Hgl1
P
Hap2
P
Prf1
P
prf1
b mating type genes
a mating type genes:
mfa1, pra1
bE1
bW2
Figure 2
cAMP and MAPK signaling during mating and pathogenic development. Components of the cAMP ( green) and MAPK (red ) signaling
pathways and their interactions are indicated ( green or red arrows, respectively). Phosphorylation is symbolized by small circles labeled
P in the color of the respective upstream kinase. Transcriptional activation is symbolized by black arrows. Postulated interactions and
unknown components are indicated by question marks. (a) Components required for mating in a haploid a1 b1 cell. (b) Components
required for pathogenic development in the dikaryon. Components that are specifically required during pathogenesis are highlighted in
bright yellow. Components or domains with reduced or no function during pathogenic development are drawn smaller in size and
fainter in color or are not shown, respectively. The seven transmembrane receptor Pra for perception of the Mfa pheromone and
postulated receptors for environmental cues and plant-derived stimuli are depicted in the schematic membrane. Characterized
components of the cAMP pathway are the α and β subunits of a heterotrimeric G-protein, Gpa3 and Bpp1, respectively; the small
G-protein Ras1; the adenylate cyclase Uac1; the inhibitory and catalytic subunits of PKA Ubc1 and Adr1, respectively; and the putative
regulatory protein Hgl1. Hg1 is predicted to become inactive upon phosphorylation by Adr1, and this is indicated by brackets. Gpa3,
Bpp1, and Ras1 all activate the adenylate cyclase Uac1. Components of the MAPK pathway are the small G-protein Ras2; the putative
CDC25-like guanyl nucleotide exchange factor for Ras2, Sql2; and the Ste50p-like adaptor protein Ubc2. The N terminus of Ubc2 is
essential for mating but dispensable for pathogenic development, whereas the C terminus is essential for pathogenic development but
plays no role in mating. The core MAPK module consists of the hierarchical kinases Kpp4 (Ubc4, MAPKKK), Fuz7 (Ubc5, MAPKK),
and Kpp2 (Ubc3, MAPK) and the two alternative MAPKs, Crk1 and Kpp6. Downstream the transcriptional regulators Rop1 and
Hap2, a postulated Crk1-induced factor, and the central regulator Prf1 (blue) bind to four distinct elements in the prf1 promoter
(indicated by different shapes and colors). Phosphorylated Prf1 induces expression of the a and b mating type genes. After cell fusion
the bE/bW heterodimer is formed that is the key regulator of pathogenic development. (1, 12, 15, 23, 29, 40, 41, 44, 47, 48, 55, 63, 64,
66, 68, 76, 78, 79, 83, 84, 85, 105, 112).
www.annualreviews.org • Ustilago maydis as a Pathogen
427
ARI
2 July 2009
19:16
components shown in Figure 2a are required
for efficient cell fusion in axenic culture.
The analysis of mutants in solopathogenic
strains has revealed that most components required for pheromone signaling play additional
roles during pathogenesis, indicating that the
core constituents of the cAMP and MAPK
pathways transmit signals from the plant environment in addition to pheromone. However, some additional components are required
only for pathogenesis, whereas others are dispensable (Figure 2a,b). Interestingly, the resulting signaling outputs during pathogenic development are fundamentally different from
the responses during mating. We attribute the
prime differences in signaling output between
haploid strains (Figure 2a) and dikaryotic (or
solopathogenic) strains to the absence and presence of an active bE/bW heterodimer, respectively (Figure 2) (see following section for details). Support for this scenario is provided by
the putative adaptor protein Ubc2, where it
has been demonstrated that the N-terminal
domain is required for mating, whereas the
basidiomycete-specific C terminus is essential
for pathogenic development but plays no role
in mating (Figure 2; 63, 76). This could indicate that the distinct Ubc2 domains mediate
the assembly of specific MAPK-signaling complexes that differ during mating and pathogenic
development. On the plant surface the transcription factor Rop1 and the β subunit bpp1
of the heterotrimeric G-protein are dispensable, although both are needed for cell fusion in
axenic culture (15, 84). It has been speculated
that the role of Rop1 is taken over by a yet unknown regulator supposedly involved in sensing
plant surface cues and acting via the upstreamactivating sequence (UAS) in the prf1 promoter
(15, 48). Bpp1 might be replaced by an unconventional Gβ subunit (Figure 2b). Signaling
components that are required for pathogenesis
but not for mating are the Kpp2-related MAPK
Kpp6, Sql2, a Cdc25-like guanyl nucleotide exchange factor predicted to control activity of
the small G-protein Ras2, and the putative regulatory protein Hgl1 (12, 29, 68, 83). The alternative MAPK Kpp6 shares partially redundant
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
428
Brefort et al.
functions with Kpp2 during mating but has a
highly specific function in appressoria, where it
is required for penetration of the plant cuticle
(12). The phosphorylation status of Kpp2 and
Kpp6 is regulated by the dual specificity phosphatase Rok1, and as a result rok1 mutants are
hypervirulent (23). The specific need for Sql2
during pathogenesis could indicate an involvement in transmission of plant-derived signals
that feed into the MAPK module.
The function of the PKA target Hgl1, which
is predicted to be inactivated upon phosphorylation by Adr1, becomes apparent late during the infection cycle, where it is essential for
progression from hyphal fragments to mature
teliospores, possibly because of low PKA activity during this stage (29). The need for tightly
controlled PKA activity also becomes apparent
in ubc1 mutants lacking the inhibitory subunit
of the PKA or gpa3QL mutants in which cAMP
signaling is constitutive. Both mutants are able
to colonize plants, but ubc1 mutants fail to induce tumors, whereas gpa3QL mutants induce
tumors that do not contain teliospores but tend
to develop shoot-like structures (44, 66). Regulated PKA activity thus emerges as a critical determinant for the production and/or function
of U. maydis–emitted signals leading to tumor
induction and affecting tumor morphology.
The bE/bW Master Regulator
for Pathogenic Development
After fusion of compatible haploid cells, an active heterodimeric bE/bW homeodomain transcription factor is generated that recognizes a
sequence containing a TGA-N9 -TGA motif
(56). The formation of this heterodimer is central for pathogenic development and cannot be
bypassed by other means.
A U. maydis strain that expresses an inducible combination of bE1/bW2 genes was instrumental to obtain a comprehensive view of
the genes that are regulated by the bE/bW
complex and because it made it possible to obtain a time-resolved view (13). Application of
such b-inducible strains in RNA fingerprinting and Affymetrix array analyses defined a set
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
ARI
2 July 2009
19:16
of 347 b-regulated genes, of which 212 were
upregulated, and 135 were repressed after induction of the bE/bW heterodimer (12, 13,
56). Functional classification revealed that cellular processes such as cell wall remodeling,
lipid metabolism, cell cycle control, mitosis, and
DNA replication were differentially regulated
by the active bE/bW heterodimer (56). Given
that promoters of the majority of these genes
lack putative bE/bW binding sites, the bE/bW
heterodimer must induce a transcriptional cascade. Surprisingly, only a small fraction of the
bE/bW-regulated genes proved to be required
for pathogenic development (13).
A bE/bW-regulated protein with an important role during plant colonization is the zinc
finger transcription factor Biz1 (37). biz1 mutant cells are unaffected in mating and filamentation, but appressoria formation in biz1
mutants is about 10-fold reduced compared to
wild type, and the few hyphae that invade the
plant are found only in the epidermal layer (37).
biz1 is expressed throughout biotrophic development, and this has been interpreted to indicate that the protein could have additional functions during these stages (37).
Another bE/bW-regulated protein is the
MAPK Kpp6 (see previous section). Using a
Kpp6 version that cannot be phosphorylated
by the upstream MAPK, it was shown that respective mutants are able to form appressoria, but these fail to penetrate (12). This could
suggest that these mutants are defective in the
production of cell-wall-degrading or cell-wallloosening enzymes.
One of the potential direct bE/bW targets
is clp1 (91). The clp1 gene product is related to
Clp1 from Coprinus cinereus, where it is required
for clamp cell formation (53), structures that
serve to guarantee correct nuclear distribution
in the mitotically dividing dikaryotic filament.
In a haploid strain of U. maydis, clp1 overexpression has no effect, but when clp1 is expressed
in strains with an active bE/bW complex, filamentation is strongly attenuated. clp1 is essential for host colonization; i.e., clp1 mutants form
appressoria, and these penetrate, but growth is
arrested prior to the first mitotic division. The
defect could be tracked to the inability of clp1
mutant strains to form clamp-like structures,
resulting in disturbed nuclear distribution (91).
Based on the finding that bE and bW expression
is unaffected by clp1 overexpression but a number of genes that are upregulated by the bE/bW
heterodimer are repressed, Scherer et al. (91)
hypothesized that Clp1 could affect the regulatory activity of the bE/bW heterodimer on the
protein level. Because the bE and bW proteins
are needed throughout fungal development inside the plant, this inhibition must be of a transient nature and has been speculated to operate
at the level of nuclear localization of Clp1 (91).
An additional direct target of the bE/bW
heterodimer is lga2, a gene in the a2 locus. lga2
is upregulated through the pheromone cascade,
and additional strong induction is conferred
by the bE/bW heterodimer. The Lga2 protein
localizes to mitochondria, and overexpression
of lga2 (as is expected to occur in the dikaryon)
interferes with cell growth, induces mitochondrial fragmentation, and lowers mitochondrial
respiratory activity (10). lga2 interferes with mitochondrial fusion (10), and recent studies have
demonstrated that Lga2, together with Rga2,
another mitochondrial protein encoded by the
a2 locus, directs uniparental mitochondrial
inheritance and represses recombination of mitochondrial DNA during sexual development
of U. maydis (36). The other genes in the a locus,
i.e., the pheromone and receptor genes, on the
other hand, are downregulated by the bE/bW
heterodimer (105). This effect is likely to be
indirect, as no potential binding sites for the
bE/bW heterodimer could be detected in the
respective promoters. Downregulation of the a
mating type genes provides an explanation for
the observed attenuation of mating in strains
expressing an active bE/bW heterodimer (67).
These studies illustrate that the processes
regulated through the bE/bW heterodimer
are diverse and concern developmental steps
on the leaf surface and after penetration. It is
also becoming evident that the cascade that
is triggered through bE/bW is not a mere
transcriptional cascade but involves feedback
loops operating on the posttranscriptional
www.annualreviews.org • Ustilago maydis as a Pathogen
429
ANRV384-PY47-18
ARI
2 July 2009
19:16
level. A comprehensive characterization of
this cascade was once thought to identify the
complete pathogenicity program of U. maydis.
However, it is now becoming increasingly clear
that additional levels of regulation are switched
on once the fungus has come into contact
with the plant, and these have stimulatory or
inhibitory effects on the processes regulated by
the bE/bW master regulator for pathogenicity.
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
Morphological Transitions Are
Intimately Coupled to Cell
Cycle Control
During its life cycle, U. maydis undergoes a
number of discrete morphological transitions
whose timing and execution are decisive for
pathogenic development. The most dramatic
of these transitions is the switch from budding
to polarized hyphal growth that is initiated after
perception of pheromones and continues after
cell fusion through much of the disease cycle.
Polarized growth of hyphae requires both
microtubules and F-actin. As a consequence,
pharmacological disruption of actin cables or
microtubules leads to defects in conjugation
hyphae, cell fusion, and development of dikaryotic hyphae. This is supported by the finding
that Myo5, a class V myosin predicted to
transport membranous vesicles along F-actin
filaments, is required for hyphal growth as well
as pathogenicity. Microtubules, on the other
hand, are specifically required for long-distance
transport in all U. maydis hyphae that extend
beyond 50–60 μm, and this is reflected by the
need of kinesin-1 and kinesin-3 motors (98).
To set up a new morphogenetic program
like the switch from budding to filamentous
growth requires the cell cycle to pause. The
finding that overexpression of the G1 cyclin
cln1 blocks sexual development but that its
absence enables cells to express mating type
genes has led to the proposition that an active
cell cycle represses mating, whereas the induction of mating arrests the cell cycle; i.e., these
choices become incompatible (17). Under
poor nutrient conditions (as are expected to
occur on the leaf surface) both mitotic cyclin
430
Brefort et al.
genes of U. maydis are downregulated, and
this was shown to stimulate prf expression and
facilitates entry into the mating program (17).
Upon pheromone perception, the haploid cells
arrest budding growth in the G2 phase (39) and
start formation of conjugation tubes (96). Conjugation tubes are filled with cytoplasm, fuse at
their tips, and establish the dikaryotic hyphae.
These hyphae can be up to 20 times the length
of haploid cells and develop the characteristics
of hyphae seen on the plant surface, that is, they
show tip growth, the cytoplasm accumulates in
the tip cell compartment, and the older parts of
these hyphae appear highly vacuolated and are
sealed off by regularly spaced septae (38, 93).
After cell fusion the cell cycle arrest is maintained, at least partially through the action of
Biz1, one of the bE/bW-induced transcription
factors. Biz1 is required for appressorium formation and proliferation inside the host plant
(37). Biz1 represses the expression of the cyclin
clb1, resulting in G2-arrest (37). Based on these
observations, it was suggested that the Biz1induced cell cycle arrest is important though
probably not sufficient for appressorium formation (37). The fact that biz1 is highly expressed
during the entire infection process might indicate that U. maydis cells growing within the
plant tissue continuously need to go through
cell cycle arrest stages. We consider it an attractive possibility that this may turn out to be particularly important during intracellular growth
when cells traverse from one cell to the next.
This process is very similar to the penetration
step of the epidermis (25) and may need a cell
cycle–arrested hyphae to reform appressorialike structures.
However, the open question is how the cell
cycle arrest is (intermittently) released during
biotrophic development. It is evident that the
hyphae penetrating the epidermis are still cell
cycle arrested, as they show the typical empty
sections also seen in the dikaryotic filaments
growing on the leaf surface (94). We consider
it feasible that Clp1, the protein needed for
clamp formation, assumes this role: clp1 mutants are unable to release the bE/bW-triggered
cell cycle arrest, and the forced expression of
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
ARI
2 July 2009
19:16
clp1 in strains where filamentation is induced
by an active bE/bW heterodimer suppresses filamentation, that is, the cell cycle block (91).
Currently, it is not clear how this presumed
cycling could be regulated, but we would like
to speculate that the timing may be set by the
appressorium-like structures that allow hyphae
to spread from one cell to the next. Later, when
the hyphae grow in the apoplast, branch, form
lobed structures and proliferate massively, fragment, and produce rounded spores, there must
be additional regulators that control these morphogenetic events. Their identity is currently
unknown, and without availability of a system
where these events can be triggered in axenic
culture, it will be difficult to identify the genes
involved. The tight link between cell cycle and
pathogenic differentiation makes it mandatory
to study in detail the mechanisms that regulate
the cell cycle during the biotrophic phase.
Sensing the Plant Surface Involves
Chemical and Physical Cues
The plant cuticle is composed of two layers, an inner layer consisting of intracuticular waxes associated with a polyester matrix of
cutin and a continuous surface layer of epicuticular wax without cutin (32, 65). Cutin consists of a network of interesterified hydroxyl
and epoxy derivatives of C16 and C18 fatty acids
(89). Plant surfaces thus provide a reservoir of
chemical and physical cues, of which epicuticular waxes and cutin monomers, as well as topography, hardness, and hydrophobicity, have
been shown to trigger morphogenetic events in
phytopathogenic fungi (101). On the leaf surface, compatible haploid strains of U. maydis
mate and then switch to filamentous growth,
whereas solopathogenic haploid strains switch
to filamentous growth without prior mating.
Following hyphal tip swelling, nonmelanized appressoria are formed that directly
penetrate the epidermis, most likely aided by
cell wall–degrading enzymes (92, 94). Recently,
a marker gene that is specifically expressed in
the tip cell of those hyphae that have formed
an appressorium has been identified, and after
fusion to green fluorescent protein (GFP),
this allowed simple scoring of appressorium
formation (79). With the help of this marker,
it was demonstrated that hydrophobicity is an
essential signal for appressoria development
(79). Hydroxy-fatty acids, like cutin monomers
from maize leaves, enhance appressorium
development on hydrophobic surfaces (79).
These morphogenetic events require an active
bE/bW transcription factor, suggesting that
the program that is induced by these cues
from the plant surface relies on the program
that is triggered by bE/bW. Hydroxy-fatty
acids induce short and nonseptated filaments
resembling conjugation tubes, whereas a
hydrophobic surface induces long and septated
filaments that look like the filaments observed
on the plant surface (79, 94). In haploid
cells hydroxy-fatty acids induce pheromone
gene transcription but not conjugation tube
formation (79). It is currently unknown how
the hydroxy-fatty acid signal is perceived
and transmitted to induce the pheromone
genes. Also, in solopathogenic haploid strains
where the pheromone genes are repressed by
the bE/bW heterodimer, the hydroxy-fatty
acid signal elevates pheromone gene expression. The following increase in autocrine
pheromone stimulation is responsible for the
observed morphological response (79).
The effect of hydroxy-fatty acids on U. maydis development depends on the phosphorylation of Kpp2. The genetic activation of kpp2
bypasses the need of hydroxy-fatty acids for appressorium formation but is not able to bypass
the need for the hydrophobic stimulus (79).
The hydroxy-fatty acid signal is also dispensable when rok1, which negatively modulates
the phosphorylation status of Kpp2, is deleted
(Figure 2b; 23). Kpp2 activity is also essential
for hydrophobicity-induced filamentation, but
so far it is unknown how hydrophobicity is perceived. Pth11, a nine-transmembrane protein
in Magnaporthe grisea has been suggested as a
putative surface-sensing protein, and pth11 mutants are impaired in their response to both hydrophobicity and cutin monomers (22). However, U. maydis lacks an obvious ortholog to
www.annualreviews.org • Ustilago maydis as a Pathogen
431
ARI
2 July 2009
19:16
this protein. Because in U. maydis hydroxy-fatty
acids and hydrophobicity are both needed for
the efficient induction of appressoria, a certain
threshold activation of the underlying MAPK
signaling pathway might be needed that would
require combination of both signals (79). On
the leaf surface, U. maydis differentiation could
then proceed in two stages. After cell fusion, filamentation could be initially induced through
the hydrophobic surface. The filament might
then secrete cutin degrading enzymes that liberate cutin monomers. This would amplify the
filamentation response and at the same time
induce appressoria. Based on the finding that
lipids induce a filamentous phenotype that resembles the infectious cell type, lipids are also
proposed to act as one of the signals that promotes and maintains filamentous growth of
U. maydis in the host environment (62).
Identification of cutin monomers and hydrophobicity as in vitro cues for filamentation and appressoria development in U. maydis now opens the possibility to analyze how
these signals are perceived and how they trigger
differentiation and function of nonmelanized
appressoria.
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
U. maydis Infection Induces Distinct
Plant Responses and Suppresses
Induction of Programmed Cell Death
As a biotrophic pathogen, U. maydis depends on
survival of colonized host cells. Nevertheless,
one does observe early symptoms that become
macroscopically visible 12–24 h after inoculation and include chlorosis and small necrotic
spots at the sites of infection. In some cases
these symptoms are likely to result from unsuccessful penetration events, but in addition,
the plant appears to recognize U. maydis hyphae during intracellular development in the
epidermal layer and during cell-to-cell movement. Usually, the colonized plant cells stay
alive, whereas plant cells containing older hyphae that lack cytoplasm undergo cell death (26,
27). At later stages, U. maydis–induced tumors
are formed by enlargement and proliferation
of plant cells. Large fungal aggregates form in
432
Brefort et al.
tumors, and this occurs without the elicitation
of programmed cell death in the surrounding
plant tissue (27). Induction of tumor growth
is also accompanied by accumulation of anthocyanins, resulting in a red pigmentation of infected tissue. The transcriptional responses of
maize plants were initially studied by differential display and subsequently at distinct stages
after infection with a solopathogenic U. maydis strain by using the Affymetrix microarray
system (4, 26). This latter approach shows an
initial recognition of the fungus on the leaf surface that elicits strong, rather unspecific plant
defense responses (Figure 1a; 26).
The early transient upregulation of plant
defense genes suggests that the plant recognizes
U. maydis via conserved pathogen-associated
molecular patterns (PAMPs). Recently, it has
been shown that specific PAMP receptors are
transcriptionally upregulated after elicitation
(113, 114). Similarly, during the early phase
of U. maydis infection, upregulation of two
putative membrane-bound leucine-rich repeat
(LRR) receptor-like kinases is observed.
Moreover, a somatic embryogenesis receptor
(SER)-like kinase is induced that belongs to
a group including BAK1, which has recently
been shown to act as a positive regulator
of infection-induced cell death signaling
(18, 61). These observations suggest that
PAMP-induced signaling is activated prior
to and during the penetration phase of U.
maydis. With establishment of the biotrophic
interaction 24 h after infection, these initial
responses are attenuated (Figure 1b; 26). In
parallel, induction of genes coding for cell
death suppressors such as Bax-Inhibitor-1 (31)
and the repression of caspases is observed
(26). This is likely to reflect the suppression of
programmed cell death by U. maydis once the
biotrophic interface has been established.
Reactive oxygen species (ROS) that are
formed in response to fungal pathogens are
important regulators of cell death (109).
Necrotrophic pathogens such as Botrytis cinerea
massively induce the production of H2 O2 and
superoxide radicals, which leads to runaway cell
death in the infected tissue (45). It is obvious
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
ARI
2 July 2009
19:16
that massive accumulations of ROS must be
prevented in a biotrophic interaction. For
U. maydis it has been shown that the fungus
actively needs to detoxify H2 O2 to be fully virulent, and this occurs through a system related
to the Saccharomyces cerevisiae Yap1p regulator
(80). U. maydis yap1 mutants show higher sensitivity to H2 O2 than wild-type cells and display
reduced proliferation in the infected tissue (80).
Around intracellularly growing yap1 mutant hyphae, H2 O2 accumulates, which is never seen
around infecting wild-type hyphae. Because
U. maydis does not code for NADPH oxidase
genes, pharmacological inhibition of NADPH
oxidase could be used to specifically target plant
enzymes in infected tissue. Such conditions
largely restore virulence of yap1 deletion strains
(80). This illustrates that virulence of U. maydis
depends on its ability to detoxify ROS.
A number of plant proteins such as glutathione S-transferases (GSTs) are associated with scavenging of oxygen radicals after
pathogen attack (75). Just 12 h after infection
with U. maydis, seven genes coding for maize
GSTs are transcriptionally induced. Moreover,
Tau-class GSTs, which have been shown to suppress Bax-mediated cell death induction (60),
are strongly induced 24 h postinfection when
biotrophy is established (26). The total content of glutathione is also increased throughout U. maydis infection. This likely reflects the
requirement for a higher antioxidative capacity in colonized tissue (26), which may aid in
prevention of cell death.
Progression of U. maydis infection is accompanied by distinct transcriptional changes
of plant hormone signaling genes. Whereas
necrotrophic pathogens induce salicylic acid
(SA)-dependent cell death responses including
expression of defense-related genes such
as PR1 (106), biotrophic pathogens induce
primarily JA and ethylene responses during
compatible interactions (42). These responses
are associated with an induction of tryptophan
biosynthesis, the accumulation of secondary
metabolites, and the induction of plant genes
encoding defensins (14, 42, 108). In line
with this, PR1 expression was undetectable
during the early biotrophic phase of U. maydis
infection, whereas activation of typical JAresponsive defense genes such as defensins and
Bowman-Birk-like proteinase inhibitors was
observed (Figure 1c,d; 26).
At later stages, when tumors are formed,
both auxin synthesis and auxin-responsive
genes are induced (Figure 1d,e; 26). This
suggests a role of plant-derived auxin in cell
enlargement during tumor formation. Because
auxin signaling was shown to be antagonistic
to SA signaling in Arabidopsis thaliana (107),
auxin could contribute to the suppression of SA
signaling and thereby promote fungal growth
and host susceptibility to U. maydis. The observation that U. maydis–induced plant tumors
contain up to 20-fold higher auxin levels than
noninfected tissue was made almost 50 years
ago (102). U. maydis can produce auxin, and
in the meantime the underlying biosynthetic
pathways for auxin have been identified (90).
The starting point of auxin biosynthesis is
tryptophan, which can be deaminated to
indole-3-pyruvate (IP) by the enzymes tam1
and tam2. This is followed by the decarboxylation to indole-3-acetaldehyde, which is then
converted to indole-acetic acid (IAA) by two
indole-3-acetaldehyde dehydrogenases Iad1
and Iad2. U. maydis strains in which tam1,
tam2, iad1, and iad2 are simultaneously deleted
are not compromised in tumor formation,
but these tumors contain significantly lower
levels of free IAA compared to tumors induced
by wild-type strains (90). This rules out
involvement of U. maydis–produced auxin in
tumor formation. However, the possibility
remains that elevated auxin biosynthesis, as was
observed in U. maydis–infected maize plants
(26), stimulates tumor development through
host cell enlargement.
Together, these observations suggest that
U. maydis is initially recognized by the plant innate immune system and induces a nonspecific,
PAMP-triggered response. Once U. maydis has
entered the host tissue, a biotrophic interface
is established in which these defense responses
and, in particular, programmed cell death are
suppressed.
www.annualreviews.org • Ustilago maydis as a Pathogen
433
ANRV384-PY47-18
ARI
2 July 2009
19:16
Resistance to U. maydis Infection
Is a Polygenic Trait
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
In the U. maydis–maize system, gene-for-gene
interactions have not been identified, although
they provide durable resistance in other smut
pathosystems. For example, in the 14 races of
Ustilago hordei, six avirulence genes have been
identified that have corresponding resistance
genes in barley cultivars (99). In current work
these avirulence genes have been mapped, and
their molecular characterization is under way
(70). U. maydis resistance in maize, on the other
hand, is a polygenic trait, and specific loci may
contribute to resistance in either the ear or the
tassel (8). In the most recent study using two
recombinant inbred populations, three quantitative trait loci (QTL) mapping to similar regions in both populations have been identified
that contribute to the frequency and severity
of U. maydis infection over the entire plant (8).
Depending on the population, several QTLs
contribute to the frequency or severity of infection in only the tassel, the stalk, or the ear
tissue (8). Currently, it is not clear whether
the tissue-specific resistance provided by these
QTLs is direct or operates through secondary
traits such as husk cover, surface properties, or
flowering time that could limit access of the fungus to specific plant organs. Interestingly, several of the QTL regions defined in the study
by Baumgarten et al. (8) contain genes with
a known role in pathogen resistance such as
nucleotide-binding site (NBS)-LRR resistance
gene homologs, a pathogenesis-related protein,
a chitinase, a basal antifungal protein, and a
wound-inducible protein. Future studies will
show whether these genes contribute to the activity of a given QTL. It will also be of interest to elucidate whether U. maydis mutants that
lack specific effectors (see below) show a differential response on the respective inbred lines,
as this could indicate that certain QTL gene
products might be targeted by the respective
effectors. Another interesting area of investigation would be to assess whether the QTLs
mapped for U. maydis resistance also provide
resistance to Sporisorium reilianum, the cause
434
Brefort et al.
of head smut in corn, a disease with the more
typical smut symptoms occurring only in the
inflorescences.
Secreted Effectors Counteract
Plant Defenses
To
establish
compatibility,
biotrophic
pathogens need to overcome the basal, PAMPtriggered plant defense mechanisms (54).
To counteract defense responses, microbial
pathogens secrete a variety of proteins, socalled effectors, that interfere with plant
defenses and thereby trigger susceptibility (54).
Effector proteins show a high structural and
functional variety and are often completely
novel proteins that lack conserved functional
domains. To establish and maintain biotrophy,
effectors have to interfere with various processes in the host tissue such as primary (and
secondary) metabolic pathways, cell-death
regulation, hormone signaling and/or cell
growth (43). To cope with plant defense
responses, fungal pathogens have developed
several strategies: (a) They can modify their cell
wall to evade host recognition, (b) they can sequester breakdown products of fungal cell walls
that trigger host defenses, (c) they can counteract the activity of antimicrobial enzymes
produced by the plant via apoplastic effectors,
or (d ) they can interfere with plant defense
responses via intracellular effectors that are
translocated to plant cells (88). For two rust
fungi and Colletotrichum graminicola, it has been
demonstrated that invading hyphae modify
their chitin to chitosan, and this is discussed as a
mechanism that could protect invading hyphae
from hydrolysis by host chitinases or to avoid
the generation of PAMPs that could trigger
defense responses (33). The U. maydis genome
contains six putative chitin deacetylase genes,
but whether these genes have the expected
function has not been investigated so far, nor
has it been shown whether they contribute to
escape from host recognition. Recently, two
proteins with a LysM domain have been identified in the U. maydis genome (20). Both could
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
ARI
2 July 2009
19:16
potentially be secreted. The LysM domain is a
carbohydrate-binding module that is proposed
to sequester chitin oligosaccharides that could
elicit host immune responses and/or protect
fungal hyphae against chitinases (20). Based
on the finding that the Cladosporium fulvum
LysM effector Ecp6 is a virulence factor (20),
it will be rewarding to determine whether the
U. maydis LysM proteins also have a virulence
function.
U. maydis encodes 386 putatively secreted
proteins that are not predicted to have an enzymatic function, and of these, 272 code for either
U. maydis–specific proteins or conserved proteins lacking defined InterPro domains. These
secreted proteins have recently been grouped
into repetitive proteins with and without internal Kex2 protease cleavage sites, cysteine-rich
proteins, proteins with a presumed regulatory
function, and a large group of proteins without recognizable features (81). Only a small
fraction of the corresponding genes has been
characterized molecularly (Figure 1). These include the repellent gene rep1 that encodes 11
secreted repellent peptides generated by Kex2dependent cleavage of a precursor protein. In
addition, two hydrophobin genes have been
characterized. hum2 encodes a typical class I hydrophobin, whereas hum3 encodes a repeated
domain likely to be processed by Kex2 fused
to a hydrophobin domain (100). In contrast to
other fungi, the repellents, rather than the hydrophobins, are required for hyphal hydrophobicity, aerial hyphae formation, and attachment
to hydrophobic surfaces. However, neither the
deletion of rep1 nor the simultaneous deletion
of hum2 or hum3 affects pathogenic development (100, 110). This makes it unlikely that
these molecules, which attach tightly to the fungal cell wall, have a protective function during
pathogenesis. Interestingly, a third secreted and
Kex2-processed repetitive protein, Rsp1, has a
role during pathogenesis, but this becomes apparent only when hum3 is deleted simultaneously (82). The fact that the hum2/hum3 double
deletion strains are fully virulent strongly suggests that the processed peptides from Rsp1 and
the repetitive domain of Hum3 have redundant
functions. The rsp1/hum3 double mutants are
unaffected in mating and penetration but arrest
growth during early intracellular development
without signs of enhanced plant defense (82).
This could indicate that the respective peptides
have a signaling function. Alternatively, they
might facilitate nutrient uptake by changing the
interface between fungus and host membrane.
Another family of secreted effectors that
were originally identified because of their enormous transcriptional upregulation in infected
tissue is encoded by the mig1 and mig 2 gene
clusters (Figure 1). These clusters consist of
two and six genes, respectively, that each code
for small related proteins with a characteristic spacing of Cys residues reminiscent of fungal Avr gene products from Cladosporium fulvum (5, 6, 21, 35). However, under the tested
conditions neither the mig1 nor the mig2 genes
are required for pathogenic development. This
could suggest additional redundancy with other
cysteine-rich effectors. Alternatively the mig
gene products might indeed confer avirulence
on some hosts which carry the cognate resistance gene(s). In order to identify such host varieties, one might screen different accessions of
teosinte, the closest wild relative to maize (24),
which is the only additional host for U. maydis
(19).
U. maydis was the first eukaryotic pathogen
in which it was discovered that novel effectors
with relevant functions for pathogenic development are situated in gene clusters that are
transcriptionally upregulated in tumor tissue
(59). Twelve such gene clusters were identified,
which all code for novel secreted proteins, and
in several instances these belong to small gene
families. Four of the cluster deletion mutants
are significantly attenuated in virulence and
show defects at different stages of pathogenic
development. Remarkably, deletion of one cluster results in increased virulence (59). The existence of secreted effectors negatively affecting
virulence indicates that U. maydis does not use
its full virulence potential, presumably because
this might result in host death prior to completion of the life cycle and spore formation.
One cluster encoding 24 secreted effectors is
www.annualreviews.org • Ustilago maydis as a Pathogen
435
ARI
2 July 2009
19:16
of particular interest with respect to tumor development, as mutants still proliferate in the infected tissue but fail to induce tumors (59). Currently, it is not clear which or how many effectors from a given cluster are responsible for the
observed phenotypes or with which processes
they interfere. Nevertheless, because individual cluster mutants are affected at discrete steps
of biotrophic development (Figure 1), the responsible effectors must have distinct cellular
targets. To elucidate their functions, the subcellular localization of the respective proteins
needs to be determined, in particular whether
they act in the apoplast and biotrophic interface or after being translocated into plant cells.
Furthermore, we expect that identification of
host proteins interacting with secreted effectors will provide the crucial leads to effector
function.
In addition to these gene clusters, a novel
secreted effector has recently been shown to be
essential for establishment of the biotrophic interaction of U. maydis with maize plants. This
protein, termed Pep1, is dispensable for saprophytic growth of U. maydis, but deletion mutants are completely blocked in biotrophic development (25). Confocal microscopy showed
that pep1 mutants are able to penetrate the plant
cell wall, but subsequent invasion of the host
cell stops immediately after invagination of the
plant plasma membrane. At the same time, the
mutant induces strong plant defense responses
including programmed cell death of attacked
epidermis cells. Fluorescently tagged versions
and immunolocalization show that Pep1 protein is secreted from intracellular hyphae into
the biotrophic interface, and particularly strong
accumulations are seen when hyphae invade
other cells. The molecular function of Pep1 is
presently unclear. Because a Pep1 ortholog of
the barley-covered smut fungus U. hordei is able
to complement U. maydis pep1 deletion mutants
and because U. hordei pep1 deletion strains arrest at a similar stage as corresponding U. maydis
mutants, it has been proposed that Pep1 proteins are essential compatibility factors in smut
fungi (25).
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
436
Brefort et al.
Extracellular Enzymes and Nutrition
in the Host Environment
Genome
comparisons
between
the
necrotrophic fungi Magnaporthe grisea and
Fusarium graminearum with the biotroph
U. maydis revealed a relatively small number
(33 versus 138 and 103) of hydrolytic secreted
enzymes (59, 81). This is in line with the
biotrophic lifestyle of U. maydis, in which
extensive tissue damage that could result in the
production of cell-wall degradation products
that may trigger host defense responses (28)
is avoided. The genes for cell-wall-degrading
enzymes follow distinct expression profiles at
early and late stages of tumor development
(27), but so far no systematic analysis of
mutants has been performed. For the set of
three genes coding for pectinolytic enzymes,
deletion mutants exist, but neither single nor
triple mutants show defects in pathogenic
development (27). Based on these studies, it
has been speculated that U. maydis uses its set
of plant-cell-wall-degrading enzymes largely
for softening the plant-cell-wall structure as a
prerequisite for in planta growth rather than
for feeding on carbohydrates derived from the
digestion of plant-cell-wall material (27). For
none of the other secreted enzyme families,
that is, the 23 proteases or the 11 lipases
(81), has an attempt been made to generate
mutants, as the fear has been that owing to
functional redundancy no phenotype would
be observed. With the establishment of the
FLP-FRT recombination system for U. maydis
(Y. Khrunyk and R. Kahmann, unpublished)
such experiments may be feasible in the near
future.
As part of the establishment of the
biotrophic lifestyle, U. maydis must enter a competition with his host for carbohydrates and
trace elements. So far only the aspect of iron acquisition by U. maydis has been studied in detail.
The essential trace element iron is taken
up by two high-affinity iron uptake systems
in U. maydis. One system is permease based;
the other relies on the secreted hydroxamate siderophores ferrichrome and ferrichrome
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
ARI
2 July 2009
19:16
A (16, 30). Both siderophores are cyclic peptides produced by nonribosomal peptide synthases. The genes necessary for the synthesis of siderophores and for the permease-based
high-affinity iron uptake are organized in three
clusters that are under negative control by
the iron-responsive GATA transcription factor Urbs1 (30, 69). Under low-iron conditions the biosynthesis genes are induced and
siderophores are secreted. The siderophore system is not required for virulence (77), but
strains with deletions in the permease-based
high-affinity iron uptake system show a drastic virulence reduction (30). Recent data suggest that the siderophore system is essential
for iron storage in U. maydis spores and is upregulated transcriptionally during spore formation (B. Winterberg, R. Kahmann, and J.
Schirawski, personal communication).
Transcript profiling of infected and uninfected maize leaves at different stages of development provided significant new insights
into the influence of U. maydis infection on
metabolic regulation and carbon fluxes. During development, maize leaves undergo a transition from a sink to a source tissue. When
leaves emerge from the stem and the blade unrolls, it becomes exposed to light, which induces
massive transcriptional changes in the developing leaf tissue. Major aspects of these developmental changes are the activation of the
photosynthetic apparatus, RNA synthesis, and
sucrose as well as starch synthesis (26). On the
metabolic level, a significant decrease of free
hexoses can be observed during leaf development, which is indicative of photosynthetic active source tissue (50). These transitions during
normal leaf development are massively changed
in U. maydis–infected tissue. Transcriptional activation of genes involved in photosynthesis is
almost completely blocked, and the switch from
C3 to C4 metabolism that is observed during development of noninfected maize leaves does not
occur (26, 50). At the same time, genes involved
in sucrose degradation, glycolysis, and the tricarboxylic acid cycle are strongly induced in infected tissue, whereas sucrose synthesis genes
are downregulated. In line with this, the content
of free hexoses remains at the high level characteristic for immature sink leaves. These observations indicate that sucrose is imported from
uninfected tissue to the developing U. maydis
tumors. Tumors thus represent an artificial sink
organ supporting fungal growth by providing
plant-derived hexoses (26).
For acquisition of these carbon sources,
U. maydis is equipped with 19 potential hexose transporters. Of these, one general hexose
transporter and one novel transporter for sucrose were recently shown to be required for
full virulence (R. Wahl, K. Wippel, J. Kämper
and N. Sauer, personal communication). This
is an exciting discovery, as it links the physiological status of the infected tissue to the
carbon sources preferred by U. maydis during
biotrophic growth.
Comparative Genomics: From
Sequence Information
to Biological Insights?
Comparative genomics of the few sequenced
basidiomycetes, U. maydis, Laccaria bicolor, Cryptococcus neoformans, Phanerochaete chrysosporium,
and Malazezzia globosa, reveal dramatic differences in genome size. Whereas the ectomycorrhizal symbiont L. bicolor (65 Mb; 20,614
predicted genes) to date carries the largest sequenced fungal genome, the skin-inhabiting
fungus M. globosa contains only 4285 predicted
genes in an assembled genome sequence of
about 9 Mb, which is among the smallest for
sequenced free-living fungi (72, 111). U. maydis (20.5 Mb; 6785 predicted genes) and the
opportunistic human pathogen C. neoformans
(20 Mb; 6574 predicted genes) have similar size
genomes and gene numbers, whereas the white
rot fungus P. chrysosporium has an intermediate genome size of 30 Mb (11,777 predicted
protein-encoding genes) (52, 59, 71, 73).
In addition to these significant differences
in size, the sequenced genomes show tremendous variation in structure, that is, with respect
to the abundance of introns and repetitive transposon-derived sequences. Furthermore, comparison of these genomes reveals a
www.annualreviews.org • Ustilago maydis as a Pathogen
437
ARI
2 July 2009
19:16
remarkable absence of synteny. Even M. globosa and U. maydis, where 82% of the predicted
M. globosa proteins show significant sequence
conservation to U. maydis proteins (52% average amino acid identity), show strikingly little
synteny. In addition, the clusters of secreted effector genes, which are crucial determinants for
the biotrophic lifestyle in U. maydis, do not exist
in the M. globosa genome (111). Gene inventories in the sequenced basidiomycetes strongly
reflect their specialized environmental niches.
The Malassezia species that live in close association with human and animal skin lack a functional fatty acid synthase gene and satisfy their
need for fatty acids from external sources by
an expanded set of secreted lipases, phospholipases, and acid sphingomyelinases (111). The
saprophytic, wood-degrading basidiomycete
P. chrysosporium, on the other hand, encodes
large number of secreted oxidases, peroxidases,
and hydrolytic enzymes that cooperate in lignin
degradation (73). The genome sequence of the
ectomycorrhizal fungus L. bicolor, a biotrophic
symbiont of trees, shows striking expansions in
genes, which are predicted to play roles in signal transduction (72). This might reflect an intimate cross talk between symbiont and host as
well as the need to coordinate the complex differentiation processes during the development
of multicellular fruiting bodies. Interestingly,
the L. bicolor gene inventory revealed some similarities to the biotrophic pathogen U. maydis,
that is, a restricted number of enzymes for the
degradation of plant-cell-wall polysaccharides
(59, 72) and an array of small secreted effectortype proteins with unknown functions that are
specifically expressed in symbiotic tissue (72).
Analogous to the role of the U. maydis gene
clusters encoding secreted effectors (see previous section; 59), this set of mycorrhizal proteins might play decisive roles in the establishment and subsequent beneficial colonization of
the host. However, most (82%) of these 2931
predicted secreted proteins from L. bicolor are
exclusively found in this fungus (72).
It is tempting to speculate that the speciesspecific effector sets may manipulate leaf and
flower tissue for parasitic colonization by
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
438
Brefort et al.
U. maydis and facilitate the ectomycorrhizal
symbiosis with roots by L. bicolor, respectively.
These comparisons illustrate that with respect
to pathogenicity, relatively little can be gained
from a comparison of fungal genomes that
are all classified as basidiomycetes but inhabit
very different ecological niches. A much more
promising approach that has been pioneered
in oomycete pathogens is genome comparisons of closely related species: Many of the
oomycete genomes are syntenic (104), but effector gene families, despite their common evolutionary origin, are highly diverse and are
evolving rapidly, presumably owing to coevolutionary pressure from the host species (103).
We have recently sequenced the genome of the
head smut fungus S. reilianum, a close relative
of U. maydis, which also infects maize but elicits distinct disease symptoms only in the flower
tissue. The S. reilianum genome shows a very
high level of synteny to the U. maydis genome.
Interestingly, many of the effector genes identified in the U. maydis genome are also found in
S. reilianum, but they display low sequence conservation and often differ in copy number. In
addition, a number of species-specific genes encoding effectors were found ( J. Schirawski and
R. Kahmann, unpublished). This serves as an
important guide for experimental approaches
(i.e., for prioritizing which genes to delete),
will permit study of the relevance of speciesspecific constriction and expansions of effector
gene families, and might help map functionally
relevant protein domains.
CONCLUSIONS
The studies of U. maydis as a pathogen have
achieved a new level of understanding that
goes far beyond what has started it all, that
is, the characterization of the mating-type
loci and how they control morphogenesis and
pathogenic development. We are now in a situation where the processes that are controlled
by the central regulator for pathogenicity, the
bE/bW complex, are studied on the genomewide level and where plant inputs into these
processes can be analyzed. The same holds for
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
ARI
2 July 2009
19:16
the fungal signaling pathways that were initially
studied because of their role during mating, and
it is now clear that they assume new roles during virulence. The connection between morphological transitions and the cell cycle has
been established, and from this insights into the
elaborate control of cell cycle progression during fungal in planta development are emerging. The cues that trigger U. maydis differentiation on the leaf surface up to the formation
of appressoria have been uncovered, and this
should pave the way for elucidating how these
signals are perceived. Access to the genome
sequence has provided unprecedented insights
into how U. maydis, with the help of novel secreted effector proteins, adapts to growth and
development in the plant environment and establishes a biotrophic relationship with its host
plant. One of the so far unique features of U.
maydis is that mutants in single effector genes
or effector gene clusters have rather dramatic
effects on biotrophic development. Such a low
degree of genetic redundancy is in stark contrast
to oomycetes, where it seems likely that large
numbers of effector genes with similar functions exist (103). Given the low level of genetic
redundancy in the effector repertoire of U. maydis and given the ease with which gene replacement mutants can be generated, a systematic
effector gene knockout strategy promises insights into all aspects and steps of biotrophic
development that are modulated by effectors.
The genome-wide analysis of plant responses to
infection by U. maydis has uncovered several key
processes that are changed, that is, defense responses, cell death suppression, phytohormone
signaling, and photosynthesis. The challenge
ahead will be to connect these processes with
specific effectors and to elucidate which of these
processes are necessary for the establishment
of a compatible relationship between U. maydis
and maize plants.
SUMMARY POINTS
1. U. maydis continues to serve as an instructive model for biotrophic fungal plant pathogens.
2. Studies of the U. maydis mating program, morphological transitions, cell cycle, regulatory
cascades, signal transduction pathways, and nutrition during the biotrophic phase are
leading to an integrated view of the fungal processes that need to be adjusted for growth
and differentiation in the plant environment.
3. Plant responses to infection by U. maydis are providing leads to those processes in the
plant that need to be reprogrammed for compatibility.
4. U. maydis uses a multilayered system to deal with plant responses that relies on the
detoxification of ROS as well as on secreted effectors.
5. The growing repertoire of secreted U. maydis effectors with crucial roles during different
stages of pathogenic development provides insights into the different levels of fungal
communication with the host plant.
FUTURE ISSUES
1. The established framework for the signaling pathways that are relevant for disease should
allow determination of how the outputs of these pathways are modulated by plant signals.
2. Based on the success of mutant complementation in U. maydis by genes from more
distantly related biotrophic pathogens such as rust fungi (51), it should be possible to determine whether effectors from such obligate biotrophs can complement the phenotype
of U. maydis effector mutants.
www.annualreviews.org • Ustilago maydis as a Pathogen
439
ANRV384-PY47-18
ARI
2 July 2009
19:16
3. With respect to the effectors of U. maydis, one of the big challenges will be to determine
where they function, that is, whether they reside in the apoplast or are taken up by plant
cells, which processes they control in the host plant, and which of these processes are
relevant for disease progression.
4. It is presently an open question whether it will be possible to study effectors from smut
fungi that parasitize on grasses in A. thaliana, where a wealth of mutants and tools would
be available to speed up functional analyses. Therefore, the accessibility of hosts such
as maize for reverse genetics needs major improvement, and efficient transient gene
silencing systems need to be established.
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
5. The manually annotated genome sequence of U. maydis is expected to serve as a scaffold
for the assembly of the genomes of closely related pathogens that can now be costeffectively sequenced by 454-technology. By extending comparative approaches to closely
related smuts that parasitize on different hosts, we expect to obtain insights not only into
the repertoire of critical determinants for disease and their evolution but also into host
range.
DISCLOSURE STATEMENT
The authors are not aware of any affiliations, memberships, funding, or financial holdings that
might be perceived as affecting the objectivity of this review.
ACKNOWLEDGMENTS
We apologize to those whose original work could not be cited in this review because of space
limitations. We thank the groups of Jörg Kämper and Jan Schirawski for sharing unpublished
data and Gertrud Mannhaupt for bioinformatics support. Our work has been supported through
the DFG Collaborative Research Center 593, the DFG Research Group FOR 666, DFG grant
DJ64/1-1 to A. D., a Humboldt fellowship to A. M.-M., and funds from the Max Planck Society.
LITERATURE CITED
1. Banuett F. 2007. History of the mating types in Ustilago maydis. See Ref. 49, pp. 351–75
2. Banuett F, Herskowitz I. 1989. Different a alleles of Ustilago maydis are necessary for maintenance of
filamentous growth but not for meiosis. Proc. Natl. Acad. Sci. USA 86:5878–82
3. Banuett F, Herskowitz I. 1996. Discrete developmental stages during teliospore formation in the corn
smut fungus. Ustilago maydis. Development 122:2965–76
4. Basse CW. 2005. Dissecting defense-related and developmental transcriptional responses of maize during
Ustilago maydis infection and subsequent tumor formation. Plant Physiol. 138:1774–84
5. Basse CW, Kolb S, Kahmann R. 2002. A maize-specifically expressed gene cluster in Ustilago maydis.
Mol. Microbiol. 43:75–93
6. Basse CW, Stumpferl S, Kahmann R. 2000. Characterization of a Ustilago maydis gene specifically induced
during the biotrophic phase: evidence for negative as well as positive regulation. Mol. Cell. Biol. 20:329–39
7. Bauer R, Oberwinkler F, Vánky K. 1997. Ultrastructural markers and systematics in smut fungi and
allied taxa. Can. J. Bot. 75:1273–314
8. Baumgarten AM, Suresh J, May G, Phillips RL. 2007. Mapping QTLs contributing to Ustilago maydis
resistance in specific plant tissues of maize. Theor. Appl. Genet. 114:1229–38
440
Brefort et al.
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
ARI
2 July 2009
19:16
9. Bölker M, Böhnert HU, Braun KH, Görl J, Kahmann R. 1995. Tagging pathogenicity genes in Ustilago
maydis by restriction enzyme-mediated integration (REMI). Mol. Gen. Genet. 248:547–52
10. Bortfeld M, Auffarth K, Kahmann R, Basse CW. 2004. The Ustilago maydis a2 mating-type locus genes
lga2 and rga2 compromise pathogenicity in the absence of the mitochondrial p32 family protein Mrb1.
Plant Cell 16:2233–48
11. Brachmann A, König J, Julius C, Feldbrügge M. 2004. A reverse genetic approach for generating gene
replacement mutants in Ustilago maydis. Mol. Genet. Genomics 272:216–26
12. Brachmann A, Schirawski J, Müller P, Kahmann R. 2003. An unusual MAP kinase is required for efficient
penetration of the plant surface by Ustilago maydis. EMBO J. 22:2199–210
13. Brachmann A, Weinzierl G, Kämper J, Kahmann R. 2001. Identification of genes in the bW/bE regulatory cascade in Ustilago maydis. Mol. Microbiol. 42:1047–63
14. Brader G, Tas E, Palva ET. 2001. Jasmonate-dependent induction of indole glucosinolates in Arabidopsis
by culture filtrates of the nonspecific pathogen Erwinia carotovora. Plant Physiol. 126:849–60
15. Brefort T, Müller P, Kahmann R. 2005. The high-mobility-group domain transcription factor Rop1 is
a direct regulator of prf1 in Ustilago maydis. Eukaryot. Cell 4:379–91
16. Budde AD, Leong SA. 1989. Characterization of siderophores from Ustilago maydis. Mycopathologia
108:125–33
17. Castillo-Lluva S, Pérez-Martin J. 2005. The induction of the mating program in the phytopathogen
Ustilago maydis is controlled by a G1 cyclin. Plant Cell 17:3544–60
18. Chinchilla D, Zipfel C, Robatzek S, Kemmerling B, Nürnberger T, et al. 2007. A flagellin-induced
complex of the receptor FLS2 and BAK1 initiates plant defence. Nature 448:497–500
19. Christensen J. 1963. Corn smut caused by Ustilago maydis. Monograph No. 2, Am. Phytopathol. Soc., St.
Paul
20. De Jonge R, Thomma BPHJ. 2009. Fungal LysM effectors: extinguishers of host immunity? Trends
Microbiol. 17:151–157
21. De Wit PJ. 2007. How plants recognize pathogens and defend themselves. Cell Mol. Life Sci. 64:2726–32
22. DeZwaan TM, Carroll AM, Valent B, Sweigard JA. 1999. Magnaporthe grisea Pth11p is a novel plasma
membrane protein that mediates appressorium differentiation in response to inductive substrate cues.
Plant Cell 11:2013–30
23. Di Stasio M, Brefort T, Mendoza-Mendoza A, Münch K, Kahmann R. 2009. The dual specificity
phosphatase Rok1 regulates mating and pathogenicity in Ustilago maydis. Mol. Microbiol. In press
24. Doebley J. 2004. The genetics of maize evolution. Annu. Rev. Genet. 38:37–59
25. Doehlemann G, Van Der Linde K, Assmann D, Schwammbach D, Hof A, et al. 2009. Pep1, a secreted
effector protein of Ustilago maydis is required for successful invasion of plant cells. PLoS Pathogens 5(2):
e1000290
26. Doehlemann G, Wahl R, Horst RJ, Voll LM, Usadel B, et al. 2008. Reprogramming a maize plant:
transcriptional and metabolic changes induced by the fungal biotroph Ustilago maydis. Plant J. 56:181–95
27. Doehlemann G, Wahl R, Vranes M, de Vries RP, Kämper J, Kahmann R. 2008. Establishment of
compatibility in the Ustilago maydis/maize pathosystem. J. Plant Physiol. 165:29–40
28. D’Ovidio R, Mattei B, Roberti S, Bellincampi D. 2004. Polygalacturonases, polygalacturonase-inhibiting
proteins and pectic oligomers in plant-pathogen interactions. Biochim. Biophys. Acta 1696:237–44
29. Dürrenberger F, Laidlaw RD, Kronstad JW. 2001. The hgl1 gene is required for dimorphism and
teliospore formation in the fungal pathogen Ustilago maydis. Mol. Microbiol. 41:337–48
30. Eichhorn H, Lessing F, Winterberg B, Schirawski J, Kämper J, et al. 2006. A ferroxidation/permeation
iron uptake system is required for virulence in Ustilago maydis. Plant Cell 18:3332–45
31. Eichmann R, Schultheiss H, Kogel KH, Hückelhoven R. 2004. The barley apoptosis suppressor homologue BAX inhibitor-1 compromises nonhost penetration resistance of barley to the inappropriate
pathogen Blumeria graminis f. sp. tritici. Mol. Plant-Microbe Interact. 17:484–90
32. Eigenbrode SD, Espelie KE. 1995. Effects of plant epicuticular lipids on insect herbivores. Annu. Rev.
Entomol. 40:171–94
33. El Gueddari NE, Rauchhaus U, Moerschbacher BM, Deising HB. 2002. Developmentally regulated
conversion of surface-exposed chitin to chitosan in cell walls of plant pathogenic fungi. New Phytol.
156:103–12
www.annualreviews.org • Ustilago maydis as a Pathogen
441
ARI
2 July 2009
19:16
34. Ellis JG, Dodds PN, Lawrence GJ. 2007. The role of secreted proteins in diseases of plants caused by
rust, powdery mildew and smut fungi. Curr. Opin. Microbiol. 10:326–31
35. Farfsing JW, Auffarth K, Basse CW. 2005. Identification of cis-active elements in Ustilago maydis mig2
promoters conferring high-level activity during pathogenic growth in maize. Mol. Plant-Microbe Interact.
18:75–87
36. Fedler M, Luh K-S, Stelter K, Nieto-Jacobo F, Basse CW. 2009. The a2 mating-type locus genes lga2 and
rga2 direct uniparental mtDNA inheritance and constrain mitochondrial DNA recombination during
sexual development of Ustilago maydis. Genetics 181:847–60
37. Flor-Parra I, Vranes M, Kämper J, Perez-Martin J. 2006. Biz1, a zinc finger protein required for plant
invasion by Ustilago maydis, regulates the levels of a mitotic cyclin. Plant Cell 18:2369–87
38. Fuchs U, Manns I, Steinberg G. 2005. Microtubules are dispensable for the initial pathogenic development but required for long-distance hyphal growth in the corn smut fungus Ustilago maydis. Mol. Biol.
Cell 16:2746–58
39. Garcia-Muse T, Steinberg G, Perez-Martin J. 2003. Pheromone-induced G2 arrest in the phytopathogenic fungus Ustilago maydis. Eukaryot. Cell 2:494–500
40. Garcia-Pedrajas MD, Nadal M, Bölker M, Gold SE, Perlin MH. 2008. Sending mixed signals: redundancy vs. uniqueness of signaling components in the plant pathogen, Ustilago maydis. Fungal Genet. Biol.
45(Suppl 1):S22–30
41. Garrido E, Voss U, Müller P, Castillo-Lluva S, Kahmann R, Perez-Martin J. 2004. The induction of
sexual development and virulence in the smut fungus Ustilago maydis depends on Crk1, a novel MAPK
protein. Genes Dev. 18:3117–30
42. Glazebrook J. 2005. Contrasting mechanisms of defense against biotrophic and necrotrophic pathogens.
Annu. Rev. Phytopathol. 43:205–27
43. Göhre V, Robatzek S. 2008. Breaking the barriers: microbial effector molecules subvert plant immunity.
Annu. Rev. Phytopathol. 46:189–215
44. Gold SE, Brogdon SM, Mayorga ME, Kronstad JW. 1997. The Ustilago maydis regulatory subunit of a
cAMP-dependent protein kinase is required for gall formation in maize. Plant Cell 9:1585–94
45. Govrin EM, Rachmilevitch S, Tiwari BS, Solomon M, Levine A. 2006. An elicitor from Botrytis cinerea
induces the hypersensitive response in Arabidopsis thaliana and other plants and promotes the gray mold
disease. Phytopathology 96:299–307
46. Gow N, ed. 2008. Fungal Genetics and Biology. Vol. 45. St. Louis, MO: Academic Press. 94 pp.
47. Hartmann HA, Kahmann R, Bölker M. 1996. The pheromone response factor coordinates filamentous
growth and pathogenicity in Ustilago maydis. EMBO J. 15:1632–41
48. Hartmann HA, Krüger J, Lottspeich F, Kahmann R. 1999. Environmental signals controlling sexual
development of the corn smut fungus Ustilago maydis through the transcriptional regulator Prf1. Plant
Cell 11:1293–306
49. Heitman J, Kronstad JW, Taylor JW, Casselton LA, eds. 2007. Sex in Fungi: Molecular Determination and
Evolutionary Implications. Washington, DC: ASM Press
50. Horst RJ, Engelsdorf T, Sonnewald U, Voll LM. 2008. Infection of maize leaves with Ustilago maydis
prevents establishment of C4 photosynthesis. J. Plant Physiol. 165:19–28
51. Hu G, Kamp A, Linning R, Naik S, Bakkeren G. 2007. Complementation of Ustilago maydis MAPK
mutants by a wheat leaf rust, Puccinia triticina homolog: potential for functional analyses of rust genes.
Mol. Plant-Microbe Interact. 20:637–47
52. Idnurm A, Bahn YS, Nielsen K, Lin X, Fraser JA, Heitman J. 2005. Deciphering the model pathogenic
fungus Cryptococcus neoformans. Nat. Rev. Microbiol. 3:753–64
53. Inada K, Morimoto Y, Arima T, Murata Y, Kamada T. 2001. The clp1 gene of the mushroom Coprinus
cinereus is essential for A-regulated sexual development. Genetics 157:133–40
54. Jones JD, Dangl JL. 2006. The plant immune system. Nature 444:323–29
55. Kaffarnik F, Müller P, Leibundgut M, Kahmann R, Feldbrügge M. 2003. PKA and MAPK phosphorylation of Prf1 allows promoter discrimination in Ustilago maydis. EMBO J. 22:5817–26
56. Kahmann R, Schirawski J. 2007. Mating in the smut fungi: from a to b to the downstream cascades. See
Ref. 49, pp. 377–87
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
442
Brefort et al.
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
ARI
2 July 2009
19:16
57. Kamoun S. 2006. A catalogue of the effector secretome of plant pathogenic oomycetes. Annu. Rev.
Phytopathol. 44:41–60
58. Kämper J. 2004. A PCR-based system for highly efficient generation of gene replacement mutants in
Ustilago maydis. Mol. Genet. Genomics 271:103–10
59. Kämper J, Kahmann R, Bölker M, Ma LJ, Brefort T, et al. 2006. Insights from the genome of the
biotrophic fungal plant pathogen Ustilago maydis. Nature 444:97–101
60. Kampranis SC, Damianova R, Atallah M, Toby G, Kondi G, et al. 2000. A novel plant glutathione
S-transferase/peroxidase suppresses Bax lethality in yeast. J. Biol. Chem. 275:29207–16
61. Kemmerling B, Schwedt A, Rodriguez P, Mazzotta S, Frank M, et al. 2007. The BRI1-associated kinase
1, BAK1, has a brassinolide-independent role in plant cell-death control. Curr. Biol. 17:1116–22
62. Klose J, de Sa MM, Kronstad JW. 2004. Lipid-induced filamentous growth in Ustilago maydis. Mol.
Microbiol. 52:823–35
63. Klosterman SJ, Martinez-Espinoza AD, Andrews DL, Seay JR, Gold SE. 2008. Ubc2, an ortholog of
the yeast Ste50p adaptor, possesses a basidiomycete-specific carboxy terminal extension essential for
pathogenicity independent of pheromone response. Mol. Plant-Microbe. Interact. 21:110–21
64. Klosterman SJ, Perlin MH, Garcia-Pedrajas M, Covert SF, Gold SE. 2007. Genetics of morphogenesis
and pathogenic development of Ustilago maydis. Adv. Genet. 57:1–47
65. Kolattukudy PE. 2001. Cutin from Plants. Weinheim, Germany: Wiley-VCH, 35 pp.
66. Krüger J, Loubradou G, Wanner G, Regenfelder E, Feldbrügge M, Kahmann R. 2000. Activation of the
cAMP pathway in Ustilago maydis reduces fungal proliferation and teliospore formation in plant tumors.
Mol. Plant-Microbe Interact. 13:1034–40
67. Laity C, Giasson L, Campbell R, Kronstad J. 1995. Heterozygosity at the b mating-type locus attenuates
fusion in Ustilago maydis. Curr. Genet. 27:451–59
68. Lee N, Kronstad JW. 2002. ras2 controls morphogenesis, pheromone response and pathogenicity in the
fungal pathogen Ustilago maydis. Eukaryot. Cell 1:954–66
69. Leong SA, Winkelmann G. 1998. Molecular biology of iron transport in fungi. Met. Ions Biol. Syst.
35:147–86
70. Linning R, Lin D, Lee N, Abdennadher M, Gaudet D, et al. 2004. Marker-based cloning of the region
containing the UhAvr1 avirulence gene from the basidiomycete barley pathogen Ustilago hordei. Genetics
166:99–111
71. Loftus BJ, Fung E, Roncaglia P, Rowley D, Amedeo P, et al. 2005. The genome of the basidiomycetous
yeast and human pathogen Cryptococcus neoformans. Science 307:1321–24
72. Martin F, Aerts A, Ahren D, Brun A, Danchin EG, et al. 2008. The genome of Laccaria bicolor provides
insights into mycorrhizal symbiosis. Nature 452:88–92
73. Martinez D, Larrondo LF, Putnam N, Gelpke MD, Huang K, et al. 2004. Genome sequence of the
lignocellulose degrading fungus Phanerochaete chrysosporium strain RP78. Nat. Biotechnol. 22:695–700
74. Martinez-Espinoza AD, Garcia-Pedrajas MD, Gold SE. 2002. The Ustilaginales as plant pests and model
systems. Fungal Genet. Biol. 35:1–20
75. Mauch F, Dudler R. 1993. Differential induction of distinct glutathione-S-transferases of wheat by
xenobiotics and by pathogen attack. Plant Physiol. 102:1193–201
76. Mayorga ME, Gold SE. 2001. The ubc2 gene of Ustilago maydis encodes a putative novel adaptor protein
required for filamentous growth, pheromone response and virulence. Mol. Microbiol. 41:1365–79
77. Mei B, Budde AD, Leong SA. 1993. sid1, a gene initiating siderophore biosynthesis in Ustilago maydis:
molecular characterization, regulation by iron, and role in phytopathogenicity. Proc. Natl. Acad. Sci. USA
90:903–7
78. Mendoza-Mendoza A, Eskova A, Weise C, Czajkowski R, Kahmann R. 2009. Hap2 regulates the
pheromone response transcription factor prf1 in Ustilago maydis. Mol. Microbiol. 71(4):895–911
79. Mendoza-Mendoza ABP, Djamei A, Weise C, Linne U, Marahiel M, et al. 2009. Physical-chemical
plant-derived signals induce differentiation in Ustilago maydis. Mol. Microbiol. 71:895–911
80. Molina L, Kahmann R. 2007. An Ustilago maydis gene involved in H2 O2 detoxification is required for
virulence. Plant Cell 19:2293–309
81. Müller O, Kahmann R, Aguilar G, Trejo-Aguilar B, Wu A, de Vries RP. 2008. The secretome of the
maize pathogen Ustilago maydis. Fungal Genet. Biol. 45(Suppl 1):S63–70
www.annualreviews.org • Ustilago maydis as a Pathogen
443
ARI
2 July 2009
19:16
82. Müller O, Schreier PH, Uhrig JF. 2008. Identification and characterization of secreted and pathogenesisrelated proteins in Ustilago maydis. Mol. Genet. Genomics 279:27–39
83. Müller P, Katzenberger JD, Loubradou G, Kahmann R. 2003. Guanyl exchange factor Sql2 and Ras2
regulate filamentous growth in Ustilago maydis. Eukaryot. Cell 2:609–17
84. Müller P, Leibbrandt A, Teunissen H, Cubasch S, Aichinger C, Kahmann R. 2004. The Gbeta-subunitencoding gene bpp1 controls cyclic-AMP signaling in Ustilago maydis. Eukaryot. Cell 3:806–14
85. Müller P, Weinzierl G, Brachmann A, Feldbrügge M, Kahmann R. 2003. Mating and pathogenic development of the smut fungus Ustilago maydis are regulated by one mitogen-activated protein kinase.
Eukaryot. Cell. 2:1187–99
86. Munkacsi AB, Stoxen S, May G. 2007. Domestication of maize, sorghum, and sugarcane did not drive
the divergence of their smut pathogens. Evolution 61:388–403
87. Nadal M, Garcı́a-Pedrajas MD, Gold SE. 2008. Dimorphism in fungal plant pathogens. FEMS Microbiol.
Lett. 284:127–34
88. O’Connell RJ, Panstruga R. 2006. Tête à tête inside a plant cell: establishing compatibility between
plants and biotrophic fungi and oomycetes. New Phytol. 171:699–718
89. Purdy RE, Kolattukudy PE. 1975. Hydrolysis of plant cuticle by plant pathogens. Purification, amino
acid composition, and molecular weight of two isozymes of cutinase and a non-specific esterase from
Fusarium solani f. pisi. Biochemistry 14:2824–31
90. Reineke G, Heinze B, Schirawski J, Buettner H, Kahmann R, Basse CW. 2008. Indole-3-acetic acid (IAA)
biosynthesis in the smut fungus Ustilago maydis and its relevance for increased IAA levels in infected tissue
and host tumour formation. Mol. Plant Pathol. 9:339–55
91. Scherer M, Heimel K, Starke V, Kämper J. 2006. The Clp1 protein is required for clamp formation and
pathogenic development of Ustilago maydis. Plant Cell 18:2388–401
92. Schirawski J, Böhnert HU, Steinberg G, Snetselaar K, Adamikowa L, Kahmann R. 2005. Endoplasmic
reticulum glucosidase II is required for pathogenicity of Ustilago maydis. Plant Cell 17:3532–43
93. Snetselaar KM, Bölker M, Kahmann R. 1996. Ustilago maydis mating hyphae orient their growth toward
pheromone sources. Fungal Genet. Biol. 20:299–312
94. Snetselaar KM, Mims CW. 1993. Infection of maize stigmas by Ustilago maydis: light and electron
microscopy. Phytopathology 83:843–50
95. Snetselaar KM, Mims CW. 1994. Light and electron microscopy of Ustilago maydis hyphae in maize.
Mycol. Res. 98:347–55
96. Spellig T, Bölker M, Lottspeich F, Frank RW, Kahmann R. 1994. Pheromones trigger filamentous
growth in Ustilago maydis. EMBO J. 13:1620–27
97. Steinberg G. 2007. On the move: endosomes in fungal growth and pathogenicity. Nat. Rev. Microbiol.
5:309–16
98. Steinberg G, Perez-Martin J. 2008. Ustilago maydis, a new fungal model system for cell biology. Trends
Cell Biol. 18:61–67
99. Tapke VF. 1945. New physiological races of Ustilago hordei. Phytopathology 35:970–76
100. Teertstra WR, Deelstra HJ, Vranes M, Bohlmann R, Kahmann R, et al. 2006. Repellents have functionally replaced hydrophobins in mediating attachment to a hydrophobic surface and in formation of
hydrophobic aerial hyphae in Ustilago maydis. Microbiology 152:3607–12
101. Tucker SL, Talbot NJ. 2001. Surface attachment and pre-penetration stage development by plant
pathogenic fungi. Annu. Rev. Phytopathol. 39:385–417
102. Turian G, Hamilton RH. 1960. Chemical detection of 3-indolylacetic acid in Ustilago zeae tumors.
Biochim. Biophys. Acta 41:148–50
103. Tyler BM. 2009. Entering and breaking: virulence effector proteins of oomycete plant pathogens. Cell
Microbiol. 11:13–20
104. Tyler BM, Tripathy S, Zhang X, Dehal P, Jiang RH, et al. 2006. Phytophthora genome sequences uncover
evolutionary origins and mechanisms of pathogenesis. Science 313:1261–66
105. Urban M, Kahmann R, Bölker M. 1996. Identification of the pheromone response element in Ustilago
maydis. Mol. Gen. Genet. 251:31–37
106. van Loon LC, Rep M, Pieterse CMJ. 2006. Significance of inducible defense-related proteins in infected
plants. Annu. Rev. Phytopathol. 44:135–62
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
444
Brefort et al.
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
ANRV384-PY47-18
ARI
2 July 2009
19:16
107. Wang D, Pajerowska-Mukhtar K, Culler AH, Dong X. 2007. Salicylic acid inhibits pathogen growth in
plants through repression of the auxin signaling pathway. Curr. Biol. 17:1784–90
108. Wasternack C. 2007. Jasmonates: an update on biosynthesis, signal transduction and action in plant stress
response, growth and development. Ann. Bot. (Lond.) 100:681–97
109. Wojtaszek P. 1997. Oxidative burst: an early plant response to pathogen infection. Biochem. J. 322:681–92
110. Wösten HA, Bohlmann R, Eckerskorn C, Lottspeich F, Bölker M, Kahmann R. 1996. A novel class of
small amphipathic peptides affect aerial hyphal growth and surface hydrophobicity in Ustilago maydis.
EMBO J. 15:4274–81
111. Xu J, Saunders CW, Hu P, Grant RA, Boekhout T, et al. 2007. Dandruff-associated Malassezia genomes
reveal convergent and divergent virulence traits shared with plant and human fungal pathogens. Proc.
Natl. Acad. Sci. USA 104:18730–35
112. Zarnack K, Eichhorn H, Kahmann R, Feldbrügge M. 2008. Pheromone-regulated target genes respond
differentially to MAPK phosphorylation of transcription factor Prf1. Mol. Microbiol. 69:1041–53
113. Zipfel C, Kunze G, Chinchilla D, Caniard A, Jones JD, et al. 2006. Perception of the bacterial PAMP
EF-Tu by the receptor EFR restricts Agrobacterium-mediated transformation. Cell 125:749–60
114. Zipfel C, Robatzek S, Navarro L, Oakeley EJ, Jones JD, et al. 2004. Bacterial disease resistance in
Arabidopsis through flagellin perception. Nature 428:764–67
RELATED RESOURCES
Broad Institute Ustilago maydis database: http://www.broad.mit.edu/annotation/genome/
ustilago maydis/Home.html
MUMDB: MIPS Ustilago maydis database: http://mips.gsf.de/genre/proj/ustilago/
Special Ustilago maydis issue: Fungal Genetics & Biology 2008. 45 (Suppl. 1):S1–S94
www.annualreviews.org • Ustilago maydis as a Pathogen
445
AR384-FM
ARI
14 July 2009
23:48
Annual Review of
Phytopathology
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
Contents
Volume 47, 2009
Look Before You Leap: Memoirs of a “Cell Biological” Plant
Pathologist
Michèle C. Heath p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 1
Plant Disease Diagnostic Capabilities and Networks
Sally A. Miller, Fen D. Beed, and Carrie Lapaire Harmon p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p15
Diversity, Pathogenicity, and Management of Verticillium Species
Steven J. Klosterman, Zahi K. Atallah, Gary E. Vallad, and Krishna V. Subbarao p p p p p p p39
Bacterial/Fungal Interactions: From Pathogens to Mutualistic
Endosymbionts
Donald Y. Kobayashi and Jo Anne Crouch p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p63
Community Ecology of Fungal Pathogens Causing Wheat Head Blight
Xiangming Xu and Paul Nicholson p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p83
The Biology of Viroid-Host Interactions
Biao Ding p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 105
Recent Evolution of Bacterial Pathogens: The Gall-Forming
Pantoea agglomerans Case
Isaac Barash and Shulamit Manulis-Sasson p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 133
Fatty Acid–Derived Signals in Plant Defense
Aardra Kachroo and Pradeep Kachroo p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 153
Salicylic Acid, a Multifaceted Hormone to Combat Disease
A. Corina Vlot, D’Maris Amick Dempsey, and Daniel F. Klessig p p p p p p p p p p p p p p p p p p p p p p p p 177
RNAi and Functional Genomics in Plant Parasitic Nematodes
M.N. Rosso, J.T. Jones, and P. Abad p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 207
Fungal Effector Proteins
Ioannis Stergiopoulos and Pierre J.G.M. de Wit p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 233
Durability of Resistance in Tomato and Pepper to Xanthomonads
Causing Bacterial Spot
Robert E. Stall, Jeffrey B. Jones, and Gerald V. Minsavage p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 265
v
AR384-FM
ARI
14 July 2009
23:48
Seed Pathology Progress in Academia and Industry
Gary P. Munkvold p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 285
Migratory Plant Endoparasitic Nematodes: A Group Rich in Contrasts
and Divergence
Maurice Moens and Roland N. Perry p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 313
The Genomes of Root-Knot Nematodes
David McK. Bird, Valerie M. Williamson, Pierre Abad, James McCarter,
Etienne G.J. Danchin, Philippe Castagnone-Sereno, and Charles H. Opperman p p p p p 333
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
Viruses of Plant Pathogenic Fungi
Said A. Ghabrial and Nobuhiro Suzuki p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 353
Hordeivirus Replication, Movement, and Pathogenesis
Andrew O. Jackson, Hyoun-Sub Lim, Jennifer Bragg, Uma Ganesan,
and Mi Yeon Lee p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 385
Ustilago maydis as a Pathogen
Thomas Brefort, Gunther Doehlemann, Artemio Mendoza-Mendoza,
Stefanie Reissmann, Armin Djamei, and Regine Kahmann p p p p p p p p p p p p p p p p p p p p p p p p p p p p 423
Errata
An online log of corrections to Annual Review of Phytopathology articles may be found at
http://phyto.annualreviews.org/
vi
Contents
AR384-FM
ARI
14 July 2009
23:48
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
Related Articles
From the Annual Review of Analytical Chemistry, Volume 2 (2009)
Nanoparticle PEBBLE Sensors in Live Cells and In Vivo
Yong-Eun Koo Lee, Ron Smith, and Raoul Kopelman
Micro- and Nanocantilever Devices and Systems for Biomolecule Detection
Kyo Seon Hwang, Sang-Myung Lee, Sang Kyung Kim, Jeong Hoon Lee,
and Tae Song Kim
Applications of Aptamers as Sensors
Eun Jeong Cho, Joo-Woon Lee, and Andrew D. Ellington
From the Annual Review of Biochemistry, Volume 78 (2009)
The Structural and Functional Diversity of Metabolite-Binding Riboswitches
Adam Roth and Ronald R. Breaker
Sphingosine 1-Phosphate Receptor Signaling
Hugh Rosen, Pedro J. Gonzalez-Cabrera, M. Germana Sanna, and Steven Brown
From the Annual Review of Biomedical Engineering, Volume 11 (2009)
Fluorescent Probes for Live-Cell RNA Detection
Gang Bao, Won Jong Rhee, and Andrew Tsourkas
From the Annual Review of Biophysics, Volume 38 (2009)
Comparative Enzymology and Structural Biology of RNA Self-Cleavage
Martha J. Fedor
From the Annual Review of Cell and Developmental Biology, Volume 24 (2008)
Auxin Receptors and Plant Development: A New Signaling Paradigm
Keithanne Mockaitis and Mark Estelle
Systems Approaches to Identifying Gene Regulatory Networks in Plants
Terri A. Long, Siobhan M. Brady, and Philip N. Benfey
vii
AR384-FM
ARI
14 July 2009
23:48
From the Annual Review of Ecology, Evolution, and Systematics, Volume 39 (2008)
Sanctions, Cooperation, and the Stability of Plant-Rhizosphere Mutualisms
E. Toby Kiers and R. Ford Denison
The Performance of the Endangered Species Act
Mark W. Schwartz
Pandora’s Box Contained Bait: The Global Problem of Introduced Earthworms
Paul F. Hendrix, Mac A. Callaham, Jr., John Drake, Ching-Yu Huang,
Sam W. James, Bruce A. Snyder, and Weixin Zhang
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
From the Annual Review of Entomology, Volume 54 (2009)
Adaptation and Invasiveness of Western Corn Rootworm: Intensifying Research
on a Worsening Pest
Michael E. Gray, Thomas W. Sappington, Nicholas J. Miller, Joachim Moeser,
and Martin O. Bohn
Impacts of Plant Symbiotic Fungi on Insect Herbivores: Mutualism in a
Multitrophic Context
Sue E. Hartley and Alan C. Gange
Cellular and Molecular Aspects of Rhabdovirus Interactions with Insect
and Plant Hosts
El-Desouky Ammar, Chi-Wei Tsai, Anna E. Whitfield, Margaret G. Redinbaugh,
and Saskia A. Hogenhout
From the Annual Review of Genetics, Volume 42 (2008)
How Saccharomyces Responds to Nutrients
Shadia Zaman, Soyeon Im Lippman, Xin Zhao, and James R. Broach
The Organization of the Bacterial Genome
Eduardo P.C. Rocha
Genomic Insights into Marine Microalgae
Micaela S. Parker, Thomas Mock, and E. Virginia Armbrust
From the Annual Review of Genomics and Human Genetics, Volume 9 (2008)
Phylogenetic Inference Using Whole Genomes
Bruce Rannala and Ziheng Yang
From the Annual Review of Microbiology, Volume 62 (2008)
Evolution of Intracellular Pathogens
Arturo Casadevall
Chlamydiae as Symbionts in Eukaryotes
Matthias Horn
viii
Related Articles
AR384-FM
ARI
14 July 2009
23:48
From the Annual Review of Pharmacology and Toxicology, Volume 49 (2009)
Lipid Mediators in Health and Disease: Enzymes and Receptors as Therapeutic
Targets for the Regulation of Immunity and Inflammation
Takao Shimizu
From the Annual Review of Plant Biology, Volume 60 (2009)
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
DNA Transfer from Organelles to the Nucleus: The Idiosyncratic Genetics
of Endosymbiosis
Tatjana Kleine, Uwe G. Maier, and Dario Leister
Jasmonate Passes Muster: A Receptor and Targets for the Defense Hormone
John Browse
A Renaissance of Elicitors Perception of Microbe-Associated Molecular Patterns
and Danger Signals by Pattern-Recognition Recepts
Thomas Boller and Georg Felix
Biosynthesis of Plant Isoprenoids: Perspectives for Microbial Engineering
James Kirby and Jay D. Keasling
Roles of Plant Small RNAs in Biotic Stress Responses
Virginia Ruiz-Ferrer and Olivier Voinnet
Related Articles
ix
AR384-FM
ARI
14 July 2009
23:48
Annual Reviews is a nonprofit scientific publisher established to promote the advancement of the
sciences. Beginning in 1932 with the Annual Review of Biochemistry, the Company has pursued as its
principal function the publication of high-quality, reasonably priced Annual Review volumes. The volumes
are organized by Editors and Editorial Committees who invite qualified authors to contribute critical
articles reviewing significant developments within each major discipline. The Editor-in-Chief invites those
interested in serving as future Editorial Committee members to communicate directly with him. Annual
Reviews is administered by a Board of Directors, whose members serve without compensation.
Annu. Rev. Phytopathol. 2009.47:423-445. Downloaded from arjournals.annualreviews.org
by Washington State University on 02/23/10. For personal use only.
2009 Board of Directors, Annual Reviews
Richard N. Zare, Chairman of Annual Reviews, Marguerite Blake Wilbur Professor of Chemistry,
Stanford University
John I. Brauman, J.G. Jackson–C.J. Wood Professor of Chemistry, Stanford University
Peter F. Carpenter, Founder, Mission and Values Institute, Atherton, California
Karen S. Cook, Chair of Department of Sociology and Ray Lyman Wilbur Professor of Sociology,
Stanford University
Sandra M. Faber, Professor of Astronomy and Astronomer at Lick Observatory, University of California
at Santa Cruz
Susan T. Fiske, Professor of Psychology, Princeton University
Eugene Garfield, Publisher, The Scientist
Samuel Gubins, President and Editor-in-Chief, Annual Reviews
Steven E. Hyman, Provost, Harvard University
Sharon R. Long, Professor of Biological Sciences, Stanford University
J. Boyce Nute, Palo Alto, California
Michael E. Peskin, Professor of Theoretical Physics, Stanford Linear Accelerator Center
Harriet A. Zuckerman, Vice President, The Andrew W. Mellon Foundation
Management of Annual Reviews
Samuel Gubins, President and Editor-in-Chief
Richard L. Burke, Director for Production
Paul J. Calvi Jr., Director of Information Technology
Steven J. Castro, Chief Financial Officer and Director of Marketing & Sales
Jeanne M. Kunz, Human Resources Manager and Secretary to the Board
Annual Reviews of
Analytical Chemistry
Anthropology
Astronomy and Astrophysics
Biochemistry
Biomedical Engineering
Biophysics
Cell and Developmental Biology
Clinical Psychology
Earth and Planetary Sciences
Ecology, Evolution, and
Systematics
Economics
Entomology
Environment and Resources
Financial Economics
Fluid Mechanics
Genetics
Genomics and Human
Genetics
Immunology
Law and Social Science
Marine Science
Materials Research
Medicine
Microbiology
Neuroscience
Nuclear and Particle
Science
Nutrition
Pathology: Mechanisms of
Disease
Pharmacology and Toxicology
Physical Chemistry
Physiology
Phytopathology
Plant Biology
Political Science
Psychology
Public Health
Resource Economics
Sociology
SPECIAL PUBLICATIONS
Excitement and Fascination of
Science, Vols. 1, 2, 3, and 4