Journal of Experimental Botany, Vol. 52, Roots Special Issue,
pp. 435±443, March 2001
Nitrogen nutrition and the role of root±shoot nitrogen
signalling particularly in symbiotic systems
Richard Parsons1 and Robert J. Sunley
University of Dundee, Dundee DD1 4HN, Scotland, UK
Received 9 May 2000; Accepted 24 August 2000
Abstract
To obtain and concentrate reduced N from the
environment, plants have evolved a diverse array of
adaptations to utilize soil, biotic and atmospheric N.
In symbiotic N2-fixing systems the potential for oversupply exists and regulation of activity to match
demand is crucial. N status in plants is likely to be
most strongly sensed in the shoot and signals
translocated to the roots may involve phloem transported amino compounds or very low concentrations
of specific signal molecules. The mechanism for
sensing N status in plant cells is not understood at
the molecular level although it may be expected to be
similar in all plants. Mechanisms for the regulation of
symbiotic N2 fixation may be very different in the
different symbiotic types. Rhizobia, Frankia and
cyanobacteria are all symbiotic with different species
of plants and the provision of O2, carbohydrate or
other nutrients may control symbiotic activity to
varying extents in the different symbioses.
Key words: N status, regulation, Gunnera, Nostoc, Lotus,
actinorhizal.
Introduction
Novel strategies to obtain nitrogen from the environment
have evolved in the plant kingdom many times. These
include an array of wonderful trapping mechanisms to
catch organisms from terrestrial, air, water, and soil
environments, symbiotic associations with nitrogen-®xing
micro-organisms, mycorrhizal associations, and parasitic
plants interactions. Most plants utilize NO 3 , NHz
4 , urea,
and amino acids as N substrates and responses to these
compounds vary among species. Some have evolved
1
strategies that favour one speci®c substrate or a combination of substrates. Most crop species grow optimally
with a mixture of ammonium and nitrate, the latter
generally being the most abundant N form in freely
drained aerobic soil environments (Crawford and Glass,
1998). NHz
4 becomes an increasingly important substrate
on ammonia- fertilized soils or on poorly drained, acidic
soils where nitri®cation by micro-organisms is limited
(Rice and Pancholy, 1972).
Whatever the mode of nitrogen acquisition be it by
physiological adaptationuassociation or direct uptake from
the rhizosphere, some form of regulation is generally
required to match N uptake and assimilation to the N
demands of plant growth and storage. Imbalances in
uptake and assimilation for N compounds do occur with
NO 3 frequently accumulating in leaves and recent work
has identi®ed a role for water in nitrate homeostasis
(Cardenas-Navarro et al., 1999). Progress so far in understanding regulatory control is limited, particularly in the
®eld of symbiotic nitrogen-®xing systems. The following
is a review of the mechanisms for nitrogen uptake and
regulation in plants with particular reference to symbiotic
systems.
Uptake of nitrogen compounds from
the rhizosphere
Nitrate uptake
Considering the relative importance of nitrate as an N
source for agricultural crops, much research has focused
on the uptake and assimilation of this substrate. It is
recognized that NO 3 acts as an important signal molecule
for growth. Arabidopsis mutants have been used to
demonstrate that there is a speci®c mechanism for sensing
NO 3 and inducing root proliferation towards local
To whom correspondence should be addressed. Fax: z44 1382 322318. E-mail: [email protected]
ß Society for Experimental Biology 2001
436
Parsons and Sunley
supplies in the rhizosphere (Forde and Zhang, 1998).
Rao and Rains showed, by using speci®c protein and
RNA synthesis inhibitors, that NO 3 stimulates its own
speci®c transport system which takes up NO 3 using
proton symporters (Rao and Rains, 1976). Research over
the last two decades has led to the cloning of genes
encoding the representatives of two families of nitrateinduced nitrate transporters (NRTs). An Nrt2 family
contributes to a highly inducible, high af®nity nitrate
uptake system. The Nrt1 group appears to account for
more general, low level nitrate uptake characterized by
constitutive, although to some extent inducible gene
expression (Crawford and Glass, 1998). Recent kinetic
analysis using Xenopus oocytes expressing cloned Chl1,
a nitrate transport gene from Arabidopsis, have identi®ed
a biphasic pattern of NO 3 transport activity with the
single transporter having Michaelis-Menten Km values of
50 mM and 4 mM (Liu et al., 1999). It is envisaged that
there is subsequent compensatory down-regulation of
these systems involving internal nitrogen metabolites as
potential signal molecules which are cycled in the phloem
(Imsande and Touraine, 1994; Marschner et al., 1997). In
effect, this mechanism enables nitrogen demand to be met
in accordance with plant growth. Reports on the molecular basis for the regulation of expression of the nitrate
transporters has recently identi®ed nitrate reductase as
a possible regulator of Nrt1 expression (Lejay et al., 1999),
and assimilatory products such as glutamine in controlling
Nrt2 expression (Zhuo et al., 1999; Vidmar et al., 2000).
Uptake of ammonia
Ammonium-speci®c transporters have now also been
isolated in plant root hairs (von Wiren et al., 2000).
LeAMT1;1 and LeAMT1;2 assessed from root hair
isolation in tomato plants were found to be differentially
regulated, high af®nity NHz
4 transporters. LeAMT1;1 is
induced by N de®ciency (interestingly coinciding with low
in planta glutamine concentrations) whereas LeAMT1;2
is positively regulated by increased NHz
4 supply. Three
different AMT1 genes have been identi®ed in Arabidopsis
with differing NHz
4 af®nities permitting regulation at
the transcriptional level (Gazzarrini et al., 1999). Work
with AMT1 expression has also demonstrated feedback
regulation by root glutamine (Rawat et al., 1999). The
collective system forms an ef®cient mechanism for root
hair NHz
4 acquisition from the rhizosphere.
Uptake of other forms of N
Some species native to cold climates with nitrogen-limited
ecosystems such as the arctic sedge (Eriophorum vaginatum)
will preferentially take up organic N forms by directly
scavenging amino acids from the soil (Chapin et al.,
1993). The speci®c transporters (AAPs) remain to be
fully characterized although functional complementation
analysis using known AAP genes from models such as
yeast have identi®ed several plant genes with potential
amino acid transport roles (Fischer et al., 1998; Schulze
et al., 1999).
Marschner has reviewed the uptake of urea and
concluded that it enters plants and is hydrolysed by
urease within cells (Marschner, 1995). Urea transporters
have been characterized in bacterial (Siewe et al., 1998)
and mammalian systems (Ripoche and Rousselet, 1996),
and the kinetics of a ureausodium symport described for
the plant Chara (Walker et al., 1993).
Conversion of different forms of N to ammonium
In non-symbiotic nitrogen metabolism the NO 3 which
has been imported into the symplast is reduced to NO 2
by nitrate reductase, another enzyme in the nitrate
assimilation pathway which is regulated by its substrate
(Beevers and Hageman, 1983). Cytosolic NO2 can induce
toxic symptoms if allowed to accumulate by inhibiting
vacuolar ATPase proton pumping (Nelson and Taiz,
1989). Consequently, it is quickly reduced to NHz
4 by
nitrite reductase and then further assimilated into organic
compounds.
As with the uptake of nitrate, all other N compounds
obtained from the rhizosphere are also chemically
converted to ammonium as a consistent start point for
plant amino acid biosynthesis. Protonation of NH3
to NHz
4 will occur at physiological pH in most plants.
Urea and amino acids that are taken up are rapidly
broken down by their respective catabolic enzymes to
yield NHz
4 .
Specialized N nutrition: plants with
physiological adaptations/associations for
nutrient acquisition
Nitrogen uptake by mycorrhizas and transfer to plants
Research on the role of mycorrhizas in plant nutrition has
concentrated on their importance for P, K and water.
Both arbuscular and ectomycorrhizas play a crucial role
in plant P nutrition, and N transfer has been demonstrated for arbuscular mycorrhizas (Barea et al., 1992)
and ectomycorrhizas (Finlay, 1996). In ericoid mycorrhizas, uptake and transfer of N from both soil
ammonium and amino acid sources has been demonstrated (Leake and Read, 1991). On heathland ecosystems
where acid mor soils dominate, fungi form a signi®cant
proportion of the soil biomass and so fungal wall
components such as chitin and hexosamines become very
valid sources of N in the soil complex. Some ericoid
mycorrhizal species can certainly degrade and utilize these
substrates. It has been shown that the N is transferred
from mycorrhiza to plant enhancing the growth of the
host (Kerley and Read, 1995).
RootÐshoot nitrogen signalling
Other ectomycorrhizal and ericoid species produce
acid proteinases and can access complex N sources via
external protein hydrolysis (Hutchison, 1990a; Finlay et
al., 1992). It has been postulated that the dominance of
ectomycorrhizal species over arbuscular species in northern hemisphere coniferous ecosystems is related to their
capability of utilizing complex forms of N when N
becomes a strongly limiting nutrient (Hogberg, 1990).
Nitrogen uptake in carnivorous and parasitic plants
Carnivory occurs in 10 families of plants and provides N,
P and other nutrients. In general, N is absorbed as amino
acids and ammonium following the release of proteases,
although there are some interesting exceptions: N absorption from insects trapped on Roridula gorgonias has
been demonstrated eloquently using natural abundance
15
N methods (Midgley and Stock, 1998). In this case it
was shown that the plant does not produce exogenous
enzymes like the morphologically similar sundews (Ellis
and Midgley, 1996) and that autolysis and microbial
breakdown of the trapped insects releases nutrients for
subsequent absorption by the plant.
Several N transporters have been isolated in Nepenthes
pitcher plants. These include NaAMT1; an ammonium
speci®c transporter on the lower digestive glands of the
pitchers, NaAAP1; an amino acid transporter expressed
in the bundle sheath cells surrounding the vascular tissue,
and NaNTR1; a peptide transporter detected in pitcher
phloem cells thought to be involved in nitrogen export
and phloem loading (Schulze et al., 1999). Indeed, it now
appears that the glands of pitcher plants are specialized
for bi-directional transport (Owen et al., 1999) although
this hypothesis has not yet been applied or tested across
the range of carnivorous families.
Similarly, parasitism has evolved many times and
occurs in 16 families of angiosperms with varying extents
of host dependence (Musselman and Press, 1995). Most
parasites form a haustorial complex with a xylem to
xylem or xylem to parenchyma connection and receive
N nutrition via an apoplastic connection with host
xylem. This can represent NO 3 or amino acid sources
that are subsequently assimilated by the parasite
(Press, 1995).
N2-fixing symbiotic systems
Symbiotic N2 ®xation has evolved in 11 families of dicotyledons and in the cycads, pteridophytes and bryophytes.
Three types of symbiotic bacteria are recognized;
rhizobia, Frankia and heterocystous cyanobacteria. In
each symbiosis, C is transferred from the host plant and
respiration of the bacteria within a specialist structure
provides energy and reductant for bacterial nitrogenase
activity. NH3 produced in this process, equilibrates to
NHz
4 and is available for transfer to the host. In general,
437
the ammonium is assimilated within the symbiotic plant
cells, which act to maintain a very low level of free
NH3uNHz
4 , thus ensuring continued N2 ®xation.
In most symbiotic systems it is generally accepted that
the symbiont gives up its ®xed nitrogen to the host as
ammoniauammonium. However, recent data has illustrated that organic N compounds such as alanine can be
excreted from N2-®xing soybean nodule bacteroids under
certain conditions (Allaway et al., 2000; Waters et al.,
1998). A candidate for a peribacteroid membrane ammonium transporter to account for the channel-like transporter system elucidated by patch clamp techniques and
described in full by Tyerman et al. (Tyerman et al., 1995)
was recently reported and characterized. Isolated from
soybean nodules, the function of GmSAT1 was based on
the ability of the protein to complement an NHz
4 transport defect in a yeast mutant (Kaiser et al., 1998). This
could have accounted for bacteroid NHz
4 export certainly
in the leguminous systems. However, on further analysis
(Marini et al., 2000) the GmSAT1 protein was found
merely to interfere with the Mep suite of NHz
4 transporters in yeast which seems to enable NHz
4 uptake. In
summary, an ammonium transporter for the peribacteroid
membrane has yet to be characterized.
Assimilation of ammonium
Irrespective of the original source of N obtained by plant
cells, NHz
4 is a key compound in many of the systems.
Ammonia assimilation is therefore a central process and it
occurs in the same fashion in all systems characterized.
The ®rst stage involves the ATPuNADPH dependent
GSuGOGAT cycle, which produces glutamine and then
glutamate in the nodules. Aspartate, asparagine and alanine are subsequently metabolized by amino and amido
transferase enzymes from glutamine and other more
complex N compounds may be synthesized from these.
The assimilated compounds are subsequently exported in
the xylem.
Xylem and phloem N transport
A wide variety of N compounds have been characterized
as the major compounds in the xylem sap of both
symbiotic and non-symbiotic plants. Amino acids such as
asparagine and glutamine are frequently the major
components, and other compounds such as citrulline,
ureides and NO 3 are all transported (Table 1). The suite
of compounds present in the xylem sap of a plant is
characteristic of the species, season and the form of the N
nutrition. The composition of N in the phloem is more
uniform, and typically a diverse range of amino acids can
be detected with asparagine, glutamine, glutamate and
aspartate the major abundant components (Table 2).
438
Parsons and Sunley
Table 1. Examples of the diversity of major xylem nitrogen compounds in different plants
Plant
Compounds translocated in the xylem
References
Lupin (Lupinus albus)
Soybean (Glycine max)
Alder (Alnus glutinosa)
Norway spruce (Piciea abies)
Parasponia (Parasponia andersonii)
Asn, gln, asp, glu
Allantoin, allantoic acid, asn gln
Cit, asp, glu
Gln, asp, arg
Gln, asn, glu, asp, 4-methylglutamate
Pate et al. (1979)
Layzell et al. (1981)
Gardner and Leaf (1960)
Weber et al. (1998)
Baker et al. (1996)
Table 2. Examples of major phloem nitrogen compounds in different plants
Plant
Compounds translocated in the phloem
References
Lupin (Lupinus albus)
Soybean (Glycine max)
Castor bean (Ricinus communis)
Norway spruce (Piciea abies)
Tomato (Lycopersicon esculentum)
Asn, gln, val, ser
Asn, gln, asp
Gln, glu, asp, ser
Arg, asp, gln
Gln, glu, thr
Pate et al. (1979)
Layzell et al. (1981)
Allen and Smith (1986)
Weber et al. (1998)
Valle et al. (1998)
Feedback control in symbiotic systems
In parallel with NO 3 uptake and regulation, symbiotic
systems require overall regulation to ensure N2 ®xation
matches plant N demand. Indeed it may be viewed as an
essential requirement of symbiotic systems that feedback regulation occurs, as symbiotic N2 ®xation has
the potential for oversupply leading to an inef®cient
system.
Sensing N status in plants
A single overriding mechanism for sensing N status in
plants is not recognized. Undoubtedly the regulation and
expression of many systems is regulated by components
of N metabolism in plants, but understanding which
chemicals are involved is crucial. The relative abundance of glutamine, glutamate and 2-oxoglutarate can be
expressed as a ratio similar to Atkinson's (Atkinson, 1968,
in Atkinson, 2000) treatment of adenylate charge, to provide a `nitrogen charge' measure within a cell. This may
represent a physiologically meaningful ratio that, via
metabolite interactions with enzymes and gene expression
directly affect N metabolism.
Nitrogen charge ˆ
1=2(2wGlnxzwGlux)
wGlnxzwGluxzw2-oxoglux
Other key compounds that may be involved in N status
recognition in cells include N rich compounds such as
arginine and citrulline. Both these chemicals are not only
components of primary N metabolism, but are also
observed to accumulate under conditions of high N
status.
Comparisons with yeast
Current understanding of N status and N signalling
in plants is likely to be led by work on yeast as the
physiology and genetics of these eukaryotes is studied
as a model system. Murray et al. have concluded that
the sensing and initial signalling of the availability or
quality of N sources in yeast is not well understood
(Murray et al., 1998). They have presented data showing
that a glutamine tRNA (isoform tRNACUG) is involved
in signalling N status for activities such as catabolite
gene expression and sporulation. Further work with this
signalling mutant (Beeser and Cooper, 1999) has supported its role in pseudohyphal growth, but questioned
its affect on catabolite repression. The involvement of
such a novel compound in N signalling pathways in
a eukaryote is fascinating and may provide important
insights into N status sensing and signalling in plants.
Autoregulation nitrate regulation and feedback
regulation of nodulation and nitrogen fixation
In parallel with other disciplines, care is required to de®ne
the actual processes identi®ed using particular keywords.
Autoregulation is taken as describing the automatic
control of further nodule development on a plant following inoculation with a compatible bacterium. This was
elegantly demonstrated (Kosslak and Bohlool, 1984)
using a split root experiment with delayed inoculation
to the second split. This resulted in very few nodules
developing on the delayed side, even after only 4 d. The
response occurred before actual N2-®xing activity, by the
nodules forming on the roots ®rst inoculated, had begun.
Mutant plants de®cient in the autoregulation signalling
pathway have been isolated (Carroll et al., 1985) and
roots of these plant become covered in nodules (supernodulation) as suppression of nodule development
does not occur. Interestingly, nodulation of these plants
is insensitive to nitrate fertilization and for the Bragg
soybean cultivar, grafting studies have shown it is the
RootÐshoot nitrogen signalling
genotype of the shoot that regulates the appearance of
supernodulation on the roots (Delves et al., 1986).
Case studies of different systems
Three case studies are presented to demonstrate current
understanding and introduce experimental systems to
investigate the role of N status and signalling in different
N2-®xing systems.
Case study 1. LotusuRhizobium
Over the last decade Lotus japonicus has become a useful
model for determinate noduled legume physiology and
molecular genetics studies whilst Medicago truncatula is
also well established as a model species of equal value
for indeterminate legume studies. Lotus is the diploid
member of the Lotus corniculatus (birdsfoot trefoil) group
of the genus with natural distributions recorded in eastern
Asia (Taiwan, Korea, and Japan) extending as far west
as Pakistan (Grant and Small, 1996). Under propagation
it is easy to germinate, self-fertile, and will complete a
seed setting life cycle in just 12 weeks when grown under
optimal conditions, making it a rapidly growing and
reliable research tool. Stougaard and Handberg at the
University of Aarhus, Denmark pioneered and developed
much of the now well-established Lotus transformation and culture methodology. Several Agrobacterium
tumefaciens- mediated transformation procedures are
now established in the literature which include hypocotyl infection and callus regeneration (Handberg and
Stougaard, 1992; Handberg et al., 1994), hairy root
infection (Stiller et al., 1997), and direct infection and
Fig. 1. Speci®c nitrogenase activity and nodule fraction (%) of Lotus
japonicus plants following a 3-week exposure to different soil nitrogen
regimes (6 plants under each treatment). N-treated plants were given
15 mg N as weekly doses of KNO3 or NH4Cl. Speci®c activity is determined by a closed system acetylene reduction assay (Parsons and Baker,
1996). Nodule fraction is an expression of the% nodule dry weight
contributing to total plant dry weight. Control plants were starved
of any additional nitrogen source. Bars represent standard errors of
means (n ˆ 6).
439
regeneration from seedling cotyledon attachment sites
(Ogar et al., 1996).
As a novel approach to understanding more about
the relative importance of cycled amino compounds in
feedback control of nitrogen ®xation, a system is being
developed for Lotus-based transformation systematically
to alter the expression of some of the genes involved in
amino acid transamination. This will enable the investigation of which compounds speci®cally act as signal
molecules of N status which are sensed at the nodules and
transduced to modulate changes in nodule turnover and
N2-®xing activity in response to N demand. Initial trials
on Lotus nodules show regulatory responses through both
nitrogenase activity and nodule development under different N regimes (Fig. 1). Nodule growth in particular was
shown to be highly inducible in Lotus under N-starved
conditions. There are obvious parallels that can be made
here to the regulation of nitrate uptake from the soil
environment so the implications of this research may
have much broader implications on plant nitrogen nutrition. An interesting observation in a report by Knight and
Langston-Unkefer noted that alfalfa plants showed
enhanced N2 ®xation when infected by a plant pathogen
which releases a glutamine synthetase (GS) inhibiting
toxin (Knight and Langston-Unkefer, 1988). This may
offer supporting evidence for a feedback response involving cycled organic N in that GS inhibition in the transamination pathway may render the subject incapable of
sensing its own N status by depleting or upsetting the
normal pool of cycled amino compounds.
Case study 2. Actinorhizal plantsuFrankia
Actinorhizal plants are characterized by the diversity of
plants that form symbiotic root nodules with Frankia.
Although there are only some 220 species, they occur
in 25 genera across eight plant families. The physiology
of the nodule symbioses is similarly diverse with a
range of nodule morphologies characterized, which in
turn are related to the regulation of oxygen diffusion to
the symbiotic Frankia (Abeysekera et al., 1990). Carbon
translocation to actinorhizal nodules will occur in the
phloem, and is frequently sucrose. Nitrogen translocation
occurs as citrulline in Alnus (Wheeler and Bond, 1970)
and Casuarina equisetifolia (Walsh et al., 1984), while in
Casuarina cunninghamiana arginine is the single most
abundant N compound (Sellstedt and Atkins, 1991).
In most other actinorhizal plants examined, including Myrica, Hippophae, Ceanothus, and Elaeagnus, the
amides glutamine and asparagine are common amino
acids (reviewed by Huss-Danell, 1990).
The form of nitrogen translocated is recognized to be
due to the genotype of the plant (Huss-Danell, 1990) and
further control of plant N status will also be plant regulated. In a manner similar to legumes, actinorhizal plants
440
Parsons and Sunley
have been shown to regulate nodule formation and
activity to match N demand (Stewart and Bond 1961;
Baker et al., 1997). However, as for legumes, understanding the exact mechanism of the plant sensing and
signalling of N status remains to be discovered. It can be
hypothesized that sensing may occur in cells present in
the shoots (where N uptake mechanisms are integrated,
free from sources of N), and that signals are returned to
nodules and root systems via the phloem. From the
observed responses of nodulated plants, it can be predicted that these signals operate in a quantitative manner,
permitting N uptake and N2 ®xation to be matched
accurately to demand.
Nitrogen ®xation in the nodules may be altered by the
plant regulating the supply of carbon to the symbiotic
Frankia. However, a limitation to current understanding
of this process, is that the form of C transferred between
plant and bacteria has not been determined in any actinorhizal system. The organic acids malate and succinate are
recognized as forming the predominant form of C transferred in legume systems (Day and Copeland, 1991) and
the sugars glucose, fructose and sucrose support Nostoc
activity in Gunnera (see below). The need for further
studies of the exchange of C in actinorhizal nodules
was highlighted (Huss-Danell, 1990), and this challenge
remains outstanding.
Case study 3. GunnerauNostoc
Gunnera species are the only angiosperms that form a
symbiosis with cyanobacteria and they exist outside the
traditional grouping of plants that form root nodules
(Soltis et al., 1995). However, the GunnerauNostoc symbiosis is a complex system in which the Nostoc exist
intracellularly within the stem tissue of Gunnera and this
is characteristic of all 40 species of Gunnera. The bacteria
appear to be non-specialized Nostoc punctiforme and
once in symbiosis their metabolism is altered to become
ef®cient symbionts. The heterocyst frequency increases
to approximately 50% and the bacteria no longer ®x
CO2 or release O2 in the light. However, chlorophyll synthesis continues and light stimulates nitrogen ®xation
of isolated bacteria (Silvester et al., 1996).
Regulation in the GunnerauNostoc symbiosis: Fixed N is
released from the bacteria as NHz
4 (Silvester et al.,
1996) and nitrogenase activity in isolated Nostoc is supported by exogenous sucrose, glucose or fructose
(Fig. 2). An investigation of the levels of sugars present
in symbiotic tissue when Gunnera was exposed to 100%
oxygen for 4 h to destroy the symbiotic Nostoc nitrogenase activity showed in excess of twice the concentration of glucose, fructose and sucrose (assayed by
GC-MS) than that in control treatments. In this system,
where oxygen diffusion is largely restricted by the
heterocyst envelope, regulation of the symbiosis must
involve plant control of metabolites, or signals to the
bacteria. It can be hypothesized that the availability of
sugars is altered to match the N requirements of the
plant. However, it is still not understood how the
cyanobacterium is induced to produce a very high ratio
of heterocysts, ®x N2 in excess and release this N as
ammonia for assimilation by the plant. Nostoc may
represent a unique symbiont, in that it may not have
additional `symbiotic' genes and it may only exist in
symbiosis because the plant hijacks its metabolism by
placing it in conditions that induce the symbiotic state.
The physical and chemical signals that bring this about
remain to be determined. Support for this hypothesis
may be obtained from molecular studies of diversity of
symbiotic cyanobacteria (Rasmussen and Svenning,
1998) which show no clear separation of symbiotic and
free-living isolates of Nostoc. Evidence to support the
hypothesis that symbiotic Nostoc does not possess any
specialist symbiotic genes will also be dif®cult to obtain.
Isolates of speci®c tagged mutants that lack the ability
to form symbiosis would permit the identi®cation of
putative symbiotic genes.
Summary
Fig. 2. Mean nitrogenase activity of isolated symbiotic Nostoc cells,
supplied with individual sugars (200 mmol l 1) assayed in the dark after
60 min. Bars represent standard error.
The regulation of root activity to supply plants with
adequate N is recognized to involve feedback systems,
such that the N status of the whole plant in¯uences root
growth, transport activity and, in the case of plants with
root nodules, nodule growth and activity. The precise
signals that carry plant N status to nodules are unknown,
but the route is almost certainly the phloem and likely
candidates are N-rich amino acids that are translocated
from the shoot. The control of root nodule activity in
response to N status is likely to be achieved in different
mechanisms in different plants. In the GunnerauNostoc
symbiosis carbon transfer to the bacteria as sugars may be
RootÐshoot nitrogen signalling
restricted when N is abundant, while in legume systems
evidence exists that O2 gas diffusion is closely regulated
and can restrict nodule activity when N is available. In
actinorhizal plants, with Frankia as a symbiont, control
may be via carbohydrate availability or O2 diffusion
depending on the symbiosis. However, before characterizing the regulation of these symbioses it is necessary to
understand how C is transferred and data have been
presented to show the authors' recent progress in this
work. Together, these examples illustrate ef®cient feedback systems that have evolved to regulate biological
activity to match demand.
Acknowledgements
We wish to thank BBSRC for studentship grant support
and Lorraine Kay for providing the data relating to the
GunnerauNostoc symbiosis. We also wish to acknowledge the
useful comments of two expert referees.
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