Journal of Experimental Botany, Vol. 57, No. 9, pp. 1971–1979, 2006
doi:10.1093/jxb/erj144 Advance Access publication 4 April, 2006
This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details)
RESEARCH PAPER
The suffulta mutation in tomato reveals a novel method of
plastid replication during fruit ripening
Daniel Forth and Kevin A. Pyke*
Plant Sciences Division, School of Biosciences, University of Nottingham, Sutton Bonington Campus,
Loughborough LE12 5RD, UK
Received 6 October 2005; Accepted 2 February 2006
Abstract
Mutant alleles at the suffulta locus of tomato dramatically affect the pattern of plastid division throughout
the plant, resulting in few, greatly enlarged chloroplasts in leaf and stem cells. suffulta plants are compromised in growth and have distinctly pale stems.
The green developing tomato fruit are generally paler
compared with the wild type, but ripe red fruit are much
more similar in colour and pigment content. By using
plastid-targeted green fluorescent protein, the underlying plastid phenotypes in the ripening suffulta fruit
reveal that enlarged chlorophyll-containing chloroplasts degenerate and give rise to a wild type-like
population of chromoplasts in ripe fruit by a process of
plastid budding and fragmentation, resulting in a heterogeneous population of plastid-derived structures
which eventually become chromoplasts. In stomatal
guard cells, plastid-derived structures lacking chlorophyll, but containing GFP, are also observed, especially in guard cells which completely lack normal
chloroplasts. How this novel ‘replication’ process in
suffulta relates to conventional plastid division and
stromule formation is discussed.
Key words: Plastid, plastid differentiation, plastid division,
tomato.
Introduction
Plastids are integral to plant metabolism and, in addition to
being the site of cellular photosynthesis, they carry out
many other essential cellular metabolic activities (Neuhaus
and Emes, 2000). Thus it is crucial for cell functionality
that plastids divide and are subsequently allocated correctly to each daughter cell during somatic cell division and
that aplastidic cells do not arise (Pyke, 1999). In higher
plants, plastid division also occurs during cell expansion
in tissues such as leaves, in which chloroplasts differentiate resulting in large cellular populations of chloroplasts.
Large numbers of small chloroplasts within the cell appear to be an advantageous strategy compared with cells
containing a single large plastid, because smaller plastids
can move more readily through the cytosol and are thus
better able to adjust their positioning to exploit fully low
light conditions or to avoid photo-oxidative damage
caused by high light (Jeong et al., 2002).
Plastids divide by a process of binary fission resulting in
two equally sized daughter plastids (Pyke, 1997). Progress
in understanding the molecular mechanism of the plastid
division process has been made by analysis of mutants in
which plastid division is perturbed and also by analysis
of higher plant homologues of prokaryotic cell division
genes. Plastids arose by an endosymbiotic event between
a proto-eukaryotic cell and a cyanobacterium-like organism (McFadden, 2001) and it appears that plastids have
inherited at least part of their division mechanism from
their cyanobacterial ancestor, since the majority of known
plastid division-associated proteins have prokaryotic
homologues. Several of these proteins were identified by
active attempts to find homologues of bacterial division
proteins in plants. These include FtsZ (Osteryoung and
Vierling, 1995; Stokes et al., 2000), minD (Colletti et al.,
2000; Fujiwara et al., 2004), and minE (Itoh et al., 2001;
Maple et al., 2002; Reddy et al., 2002). Other plastid
division proteins found to have bacterial homologues are
ARC6, which is a homologue of the cyanobacterial division protein FTN2 (Vitha et al., 2003), and an Arabidopsis homologue of the bacterial division inhibitor SulA
(Maple et al., 2004; Raynaud et al., 2004).
Some components of the plastid division apparatus appear to have a eukaryotic origin. ARC5 has characteristics
* To whom correspondence should be addressed. E-mail: [email protected]
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1972 Forth and Pyke
of dynamin, a group of proteins associated with vesicle
formation and also mitochondrial division (Robertson
et al., 1996; Gao et al., 2003) and the ARC3 protein appears to be a fusion between a homologue of FtsZ and
a protein kinase (Shimada et al., 2004). These proteins
appear to function in a stepwise series of biochemical
events that divide the plastid (Maple et al., 2004), most
prominently involving an electron dense protein ring
which appears to provide mechanical force (Osteryoung
and Nunnari, 2003).
Mutants with altered plastid division phenotypes were
identified by visual microscopic screening in Arabidopsis
(Pyke and Leech, 1991) and amongst others, yielded the
ARC6 mutant with 1–3 giant chloroplasts in leaf mesophyll
cells (Pyke et al., 1994; Robertson et al., 1995). In general,
the effect of generating giant chloroplasts in cells of a
mutant plant does not appear to have a major deleterious
effect on plant physiology or morphology since such plants
appear to grow relatively normally (Pyke et al., 1994; Pyke
and Leech, 1994). Several questions remain unanswered
regarding the functionality of giant plastids in higher plants.
Since plants with cells containing giant plastids do not
appear to produce significant numbers of aplastidic cells,
except during stomatal biogenesis (Robertson et al., 1995),
the question arises as to whether such plastids may have
an alternative mechanism by which they can replicate
during cell division or during plastid differentiation. It may
be that enlarged plastids simply break during cytokinesis
and are segregated in a rudimentary way.
Analysis of plastid division and development in tomato
has proved a useful system because a major plastid differentiation pathway, namely chloroplast to chromoplast
differentiation in the fruit, can easily be studied. Both the
exploitation of tomato mutants with altered plastid development (Cookson et al., 2003) and the use of plastidlocated green fluorescent protein (GFP) (Pyke and Howells,
2002) have provided significant insights into the morphology of plastid differentiation and chromoplast accumulation and the role of stromules in this process (Waters
et al., 2004).
The divergens mutant in tomato Solanum lycopersicum
cv. Condine Red was generated by X-ray mutagenesis
during a study on the evolutionary origin of the cultivated
tomato (Stubbe et al., 1958) and was reported to have only
1–3 giant chloroplasts per leaf mesophyll cell (Zelcer et al.,
1991). In this study divergens is shown to be a member of
an allelic series of plastid division mutants in tomato,
known as suffulta. Although the giant chloroplast phenotype is present throughout the aerial parts of the suffulta
plants, ripe fruit have a normal complement of large
numbers of small chromoplasts. Using plastid-targeted
GFP in the suffulta mutant plants has revealed a novel
method of plastid replication by budding and fragmentation
which enables normal chromoplast differentiation and
population accumulation in the ripe suffulta fruit.
Materials and methods
Plant material
Seeds of three suffulta mutants in tomato; su-1 (background cv.
Lukullus), su-2 (background cv. Rheinlands Ruhm), su-3 (background cv. Condine Red), and a wild-type line cv. Condine Red, were
obtained from the CM Rick Tomato Genetics Resource Centre,
University of California, Davis, USA (http://tgrc.ucdavis.edu/index.
html). Plant growth conditions and cultivation were as previously
described (Waters et al., 2004). Tomato plants cv. Ailsa Craig
containing a construct targeting GFP to the plastid (Köhler et al.,
1997; Pyke and Howells, 2002) were used to introduce the GFP
construct into su-3 mutant plants by crossing and F2 plants showing
the su-3 cellular phenotype and strong GFP fluorescence were
selected.
Tissue sampling and microscopy
In order to observe mesophyll cell plastids, leaf tissue from fully
expanded leaflets from the fourth to sixth leaf above the cotyledons
in 50-d-old plants and stem tissues were fixed in 3.5% (v/v)
gluteraldehyde and cells were separated after incubation in 0.1 M
Na2EDTA (ethylene diamine tetraacetate) (Pyke and Leech, 1992).
Peels from the abaxial epidermis were made from the fourth to sixth
leaf above the cotyledons of 50-d-old plants, and mounted in water
on glass slides. Samples of tissue of the pericarp and of immature and
ripening tomato fruit were hand cut with a razor blade at a thickness
of <1 mm and placed on a slide and mounted in water on a glass slide.
Samples were imaged using a Nikon Optiphot microscope (Nikon,
Kingston-upon-Thames, UK) and a Nikon DXM1200 digital camera.
Images were acquired using the Nikon ACT-1 (Version. 2.12)
software and measurements were made using the Lucia G (Version
4.6, Laboratory Imaging for Nikon) software. Fluorescence microscopy and image processing was performed using a Leica confocal
microscope as previously described (Waters et al., 2004).
Pigment quantification in tomato fruit
A section of the epidermis and pericarp was cut from an area above
a locule in a 5 mm wide strip around the equator of mature green and
ripe fruit, weighed and ground in a pestle and mortar with acidwashed sand (Sigma-Aldrich Company Ltd, UK) and a few ml of
60:40% hexane:acetone. The hexane:acetone was removed and
stored in a glass universal bottle wrapped in foil, and replaced
repeatedly with fresh liquid until no longer discoloured by grinding
of the fruit. The optical absorbance of the samples was immediately measured in a Phillips PU 8720 scanning spectrophotometer
and the chlorophyll and carotenoid contents were calculated with the
following equations: total chlorophyll mg mlÿ1 = 8.02(OD643)+
20.2(OD647) and total carotenoid mg mlÿ1 = (OD450)/0.25 (Fray
and Grierson, 1993). Individual tissue samples were taken from
8–10 fruit for each line.
Results
Mutations at the suffulta locus
The original divergens mutation of tomato (Stubbe et al.,
1958) was reclassified as a suffulta (su) mutant (Stubbe
et al., 1972) and therefore to establish fully the genetics of
the three suffulta mutants available, reciprocal backcrosses
to wild type and all permutations of crosses between mutants were made and the F1 and F2 progeny were screened
for mutant phenotypes. su-1, su-2, and su-3 mutants all
Plastid replication during tomato fruit development 1973
segregate in a recessive Mendelian manner and all crosses
between mutant plants produced mutant phenotypes in
the F1 plants confirming that they represent a series of
allelic recessive mutations at a single locus.
Suffulta mutant plastid phenotype in green tissue
Differences in the whole plant phenotype of suffulta mutant plants compared with wild type are subtle and are
manifested as a reduction in growth rate to around 60%
of wild type and slightly paler leaves. Plants of su-1 and
su-3 have very pale stems and petioles, whereas su-2 is
more normal (Fig. 1). Otherwise plants appear normal in
morphology and fertility. A more dramatic phenotype is
observed at the cellular level. Plastid division in leaf
mesophyll cells is substantially affected in all three mutants giving rise to mesophyll cells containing only a
single chloroplast which is greatly enlarged compared
with wild type (Fig. 2). A similar giant chloroplast phenotype is also observed in both leaf epidermal cells (Fig. 2),
as well as in all other aerial tissues examined (data not
shown). Counts and measurements of plastid sizes in epidermal cells of each of the three su mutants show a dramatic
reduction in plastid number per cell with a concurrent
increase in plastid size (Table 1).
Suffulta mutant plastid phenotype in tomato fruit
Mature green fruits from su-1 and su-3 plants are much
paler than su-2 and the wild type (Fig. 3A). To confirm
that this was due to an actual reduction in the level of
chlorophyll pigment present and was not an optical effect
Fig. 1. Whole plants of wild type cv. Condine Red and three suffulta
mutants of tomato 50 d after sowing. The lower panel shows colouration
of the main stem in each plant.
generated by the plastid division phenotype, the chlorophyll content was measured. This showed that the total
chlorophyll level of the mature green fruit of su-1 and
su-3 was 30% of the levels found in the wild type, whereas
su-2 contains levels similar to the wild type (Fig. 3B). The
colouration of the ripe fruit of all three alleles appears
similar to the wild type (Fig. 3A) and pigment assays
show that the total carotenoid levels in ripe fruit of su-2
and su-3 are not significantly lower than wild type,
while su-1 contains only 60% of the carotenoid level of
wild-type fruit (Fig. 3C).
Differences in fruit colouration and pigment content are
largely borne out by the underlying cellular phenotypes
within the fruit. Pericarp cells of all three su alleles contain
a greatly reduced number of green chloroplasts compared
to wild type and these plastids are enlarged, particularly
in su-1 and su-3 fruit (Fig. 4). As the fruit ripen, green
chloroplasts differentiate and produce large populations of
red chromoplasts. During this ripening process, differences
between su mutants and the wild type are lost, such that
pericarp cells from mature ripe fruit of su plants appear
normal with large populations of similar-sized chromoplast
bodies (Fig. 4). This apparent loss of a mutant plastid
phenotype during chromoplast differentiation is surprising,
since it appears that a few enlarged green chloroplasts in
mature green fruit of suffulta are capable of generating
large numbers of red chromoplasts.
Confocal imaging of fruit plastid differentiation
In order to examine how a few green chloroplasts give
rise to large populations of chromoplasts, a plastid targeted
GFP construct in transgenic wild-type tomato was used
and introduced into su-3 plants by crossing and selection.
su-3 was the chosen allele because it showed the most dramatic cellular differences between immature and ripe fruit
(Fig. 4) as well as the largest changes in pigmentation
during ripening (Fig. 3B, C). Pericarp cells in young green
fruit from wild-type plants have a large number of regularsized plastids, containing chlorophyll and GFP (Fig. 5A).
In su-3, the chlorophyll-containing plastids are greatly
enlarged but, in addition, there are several smaller bodies
containing GFP but no chlorophyll (Fig. 5B). As fruit develop to the mature green stage, the enlarged chlorophyllcontaining plastids in su-3 cells become reduced in size
(Fig. 5D), but are still much larger than wild type at the
equivalent stage (Fig. 5C). At this stage in su-3 cells there
are a substantial number of bodies, of varying sizes, that
contain only GFP (Fig. 5D). As the fruit ripens (Fig. 5E–H),
chlorophyll is lost and differences between wild-type cells
and su-3 cells become less marked such that fully ripe
pericarp cells of wild and su-3 both possess large populations of chromoplasts of a similar size, containing both
GFP and carotenoids (Figs 4, 5G, H). These observations
strongly suggest that the giant chloroplasts in su-3 green
1974 Forth and Pyke
Fig. 2. Light micrographs showing the plastid cellular phenotype in wild type and suffulta mutants in (A) leaf palisade mesophyll cells and (B) the leaf
abaxial epidermis. Scale bars are (A) 20 lm; (B) 100 lm.
Table 1. Measurements of abaxial epidermal cell size, plastid
number and plastid area in wild type (Condine red) and three
suffulta mutants of tomato
generally as ripening progresses, as previously observed
in wild-type fruit (Waters et al., 2004).
n >50 and standard errors are shown in parentheses.
Plastid distribution in suffulta stomata
Plant type
Mean epidermal cell
plan area (lm2)
Mean plastid
number
Mean plastid
area (lm2)
Condine red
su-1
su-2
su-3
2964
3143
2890
3308
12.1
1.8
3.9
1.6
14.2
58.2
50.2
89.1
(131)
(156)
(134)
(168)
(0.6)
(0.2)
(0.2)
(0.2)
(0.7)
(5.5)
(2.85)
(9.6)
fruit give rise to GFP-containing bodies which differentiate
into large populations of red chromoplasts.
High-magnification confocal imaging of this process in
the outer mesocarp cells of su-3 fruit shows that the
enlarged chloroplasts of su-3 give rise to GFP-containing
bodies by a complex process of budding and plastid
fragmentation (Fig. 6) whereby the large chlorophyllcontaining plastids (Fig. 6A, C, E) produce a population
of plastid-derived vesicles containing GFP but not chlorophyll. These vesicles appear to arise by extensive budding
from the chloroplast envelope of the giant chloroplasts
and are eventually released as distinct entities which are
able to bud further, resulting in a highly heterogenous
population of structures which containing GFP but lacking
chlorophyll (Fig. 6B, D, F, H). In addition to this budding
process, thin membraneous tubules known as stromules
(Waters et al., 2004) are also observed emanating from
both the large chloroplasts and also the GFP-containing
vesicles (Fig. 6B, D, F, H). Stromule incidence increases
Since a large plastid Arabidopsis mutant, arc6, has been
shown to give rise to stomata lacking chloroplasts
(Robertson et al., 1995), stomata were observed in leaves
of the su-3 mutant. In addition to the dramatic and novel
fruit phenotype, the replication behaviour of giant suffulta
plastids during stomatal biogenesis is also novel. Viewed
using chlorophyll fluorescence, stomatal guard cells from
suffulta leaves apparently contain either two, one or no
plastids (Fig. 7) whereas wild-type guard cells normally
contain 6–7 chlorophyll-containing plastids (Fig. 7I).
Imaging plastid-targeted GFP, however, reveals the situation to be more complex in that all cells contain plastidlike structures containing GFP but no chlorophyll (Fig.
7B, D, F, H) and in guard cells that apparently are aplastidic
as viewed by chlorophyll fluorescence, there are extensive
structures containing GFP. In the most dramatic case,
a stomata in which both guard cells lack chlorophyllcontaining chloroplasts contains extensive GFP-containing
structures as well as short stromules (Fig. 7G, H). Thus the
ability of giant suffulta plastids to give rise to achlorophyllous structures is demonstrated in stomata as well as
in ripening fruit cells.
Discussion
Mutation at the suffulta locus in tomato causes abberant
plastid division throughout the plant resulting in a few giant
Plastid replication during tomato fruit development 1975
Fig. 4. Outer pericarp cells from mature green and ripe tomato fruit
of wild type and three suffulta mutants. Scale bars are mature green
100 lm, fully ripe 80 lm.
chloroplasts per cell in green tissues, a phenotype similar
to that of the arc6 mutant in Arabidopsis (Pyke et al.,
1994; Robertson et al., 1995; Vitha et al., 2003). However,
since the suffulta mutants are in tomato, this has enabled
analysis of how such large plastids undergo the process
of chloroplast to chromoplast differentiation during fruit
ripening. Whilst the chloroplasts in pericarp cells of
green suffulta fruit are larger and fewer in number than in
the wild type, during ripening there is a spectacular fragmentation of these large plastids resulting in ripe fruit with
a normal red phenotype, near wild-type levels of carotenoids and pericarp cells containing large populations of
red chromoplasts. The process by which suffulta plastids
Fig. 3. (A) Mature green and fully ripe tomato fruit of wild type and
three suffulta mutant alleles. (B) Comparisons of chlorophyll content in
mature green fruit in respective lines. (C) Comparisons of carotenoid
content in fully red ripe fruit in respective lines.
1976 Forth and Pyke
Fig. 5. Fruit ripening sequence in wild type (A, C, E, G) and the su-3
mutant (B, D, F, H) both expressing plastid-targeted GFP. The sequence
shows outer pericarp cells of immature green fruit (A, B), mature green
fruit (C, D), breaker fruit (E, F), and fully ripe fruit (G, H). Images are
overlays of chlorophyll fluorescence and GFP fluorescence such that
structures containing only chlorophyll appear red, those containing only
GFP appear green and those containing both chlorophyll and GFP appear
orangey red/yellow. Scale bars A–D 40 lm, E–H 80 lm.
Fig. 6. Pairs of confocal images of plastid budding and fragmentation in
ripening fruit of su-3. Red chlorophyll fluorescence only (A, C, E, G) and
overlay of chlorophyll fluorescence and GFP fluorescence of the same
images (B, D, F, H). Mature green fruit (A–D), breaker fruit (E, F), fully
ripe (G, H). Scale bars=20 lm (B, D), 10 lm (F, H).
give rise to structures which become chromoplasts appears
to be a type of budding mechanism in which vesicle-like
bodies containing stromal GFP, but not chlorophyll, bud
off from the main plastid body. These bodies also have
the ability to bud subsequently giving rise to a heterogeneous population of GFP-containing structures within the
cell. The inference is that these structures have the ability
to accumulate carotenoids and become mature chromoplasts. This novel type of replication is not evident during
chloroplast to leucoplast differentiation in arc6 petals in
which giant chloroplasts simply give rise to giant leucoplasts (Pyke and Page, 1998). Similarly, over-expression
of the plastid division gene FtsZ1 in tomato gives rise to
giant plastids in mesophyll cells and also giant chromoplasts in mature ripe fruit (Cookson, 2003). Possible plastid
budding has only been reported previously in leaves of
Bryophyllum pinnatum (Kulandaivelu and Gnanam, 1985)
and has not generally been considered as a mechanism for
Plastid replication during tomato fruit development 1977
plastid division or differentiation. The situation in ripening
suffulta fruit cells is further complicated by the ability of
both large plastid bodies and the plastid-derived vesicles
to generate stromules, which themselves can break and
produce smaller vesicles. Heterogeneous GFP-containing
vesicles are also observed during wild-type tomato fruit
ripening in the outer pericarp cells, but because of their
small size most probably arise by the breaking of long
stromules rather than a distinct budding event from the
main plastid body (Waters et al., 2004). Although suffulta
fruit chloroplasts and vesicles derived from them could
both form stromules, budding from the main plastid body
or from plastid-derived vesicles produces vesicles significantly larger than those that might arise by stromule
breakage. In wild-type fruit, small vesicles containing
only GFP are observed throughout the cytoplasm of green
ripening fruit and the implication was that these have arisen
from breakage of stromules and that they also have the
potential to develop into full chromoplasts (Waters et al.,
2004). Budding from wild-type chloroplasts in ripening
fruit is occasionally observed (data not shown) but the
majority of chromoplasts in wild-type fruit probably arise
by binary fission of chloroplasts during the green fruit
stage up to and including breaker stage prior to differentiation (Cookson et al., 2003). It seems that such a budding
mechanism is only used extensively when conventional
plastid division is perturbed.
The most similar plastid division phenotype to the
budding observed in this study is seen in chloroplasts in
plants mutant for the minD gene (Colletti et al., 2000;
Fujiwara et al., 2004), which show asymmetric division
constrictions giving rise in the arc11 mutant to a heterogeneous population of plastids, (Marrison et al., 1999)
including small ones. However, a clear axis between the
two poles of the plastid is maintained in these plastids
and divisions are not initiated around the entire periphery
of the plastid, as can be observed in suffulta fruit chloroplasts (Fig. 6). Also, in leaf cells, the suffulta phenotype is
one of few giant plastids, rather than a heterogeneously
sized population of chloroplasts and thus it is unlikely
that suffulta alleles are mutant in a tomato minD gene.
Since chromoplasts are essentially a storage sac into
which carotenoid biosynthesis is directed, the requirement
for authentic plastid structure and thylakoid membrane
appears to be lost. Indeed, during chromoplast differentiation, thylakoid membrane and chlorophyll are degraded
and plastid DNA expression is minimal (Kobayashi et al.,
1990; Marano et al., 1993). Consequently, a plastid-derived
Fig. 7. Confocal imaging of stomatal guard cells (GC) from leaves of
the su-3 tomato mutant (A–H) and in the wild type (I, J). Images A, C, E,
G, I show chlorophyll fluorescence and the same images B, D, F, H, J
are overlaid with the GFP fluorescent image. (A, B) Stomata have two
large chlorophyll-containing plastids per GC, (C, D) one GC has two
chlorophyll-containing chloroplasts and the other GC has none, (E, F) one
GC has a single large chlorophyll-containing chloroplast and the other
GC has none, (G, H) neither GC contains a chlorophyll-containing
chloroplast, but extensive GFP-containing strucures are observed. In
some cases green light is reflected from the inner pore walls highlighting the stomatal pore. Scale bar =10 lm.
1978 Forth and Pyke
vesicle lacking chlorophyll and thus thylakoid membrane
is unlikely to be seriously compromised in its ability to
undergo full chromoplast differentiation.
Such novel behaviour by suffulta plastids is not confined
to chromoplast differentiation in ripening fruit since
stomatal guard cell plastids also show extensive heterogenous vesicles within guard cells which contain GFP but
no chlorophyll. It is particularly interesting that guard
cells apparantly lacking normal chlorophyll-containing
chloroplasts do contain plastid-derived vesicles containing
GFP. This suggests that at cytokinesis of the guard mother
cell during stomatal biogenesis, the giant chloroplasts can
reside in only one of the two daughter cells, but that plastidderived vesicles containing GFP can be proportioned
between both of the daughter cells. This observation explains how apparantly aplastidic guard cells in suffulta
leaves and in those of arc6 mutant plants (Robertson et al.,
1995) can be generated in what appear to be functional
stomata. Stomata in which both guard cells lack giant
chlorophyll-containing chloroplasts, but which both have
plastid-derived vesicles containing GFP, presumably arise
ocassionally when existing giant chloroplasts are segregated out during the cell divisions prior to that producing
the guard mother cell (Nadeau and Sack, 2002). It remains
unclear how mutant plants which possess giant chloroplasts, such as suffulta or arc6 (Pyke et al., 1994), avoid
generating cells lacking plastids at cell division as a result of abberant plastid segregation. It is feasible that
such a budding type mechansim could explain how
mutant plants containing giant plastids segregate plastids
during cell divisions in the shoot apical meristem and
in developing organs and thus avoid generating significant numbers of aplastidic cells.
suffulta mutant plants show reduced vigour compared
with wild-type plants and, in general, tomato appears to
be more sensitive to the plastid size manipulation than
other species, since overexpression of FtsZ in tomato has
a dramatic effect on plant morphology (Cookson, 2003). By
contrast, arc6 and arc12 mutants in Arabidopsis
grow similarly to the wild type (Robertson et al., 1995;
Yamamoto et al., 2002) and FtsZ gene manipulation in
tobacco has little effect on plant morphology except under
extreme light conditions (Jeong et al., 2002). The pale stem
phenotype most extreme in su-1 and su-3 alleles is similar
to that observed in the petioles of arc6 and arc12 plants
(personal observation) and most likely is an optical effect
as a result of there being few giant chloroplasts in the
parenchyma cells of stems and petioles, an effect which
is much less apparent in leaves. It is also apparent that su-1
and su-3 also have paler green fruit and may reflect
variation in strength of the three suffulta alleles.
In addition to the three suffulta alleles described here,
another mutant, nitida, is listed as a suffulta allele (su-ni)
in the Tomato Genetics Resource Center database. nitida
does not have a plastid division phenotype nor do allelic
crosses with existing suffulta alleles show allelism. This
agrees with Khush (1965) who suggests that the suffulta
locus is on the long-arm of chromosome 4 while two other
groups have shown nitida to be located on chromosome
8 (Rick et al., 1973; Kerr, 1974).
The molecular nature of the suffulta gene is presently
unknown and its characterization may well provide a clearer
understanding of the basis of a plastid budding mechanism
and whether budding during giant plastid chromoplast
differentiation utilizes a similar molecular mechanism to
plastid binary fission.
Acknowledgements
Thanks to Maartin Koorneef for donation of original divergens
seed, the CM Rick Tomato Genetics Resource Centre, University
of California, Davis, USA for supplying suffulta seeds, and the
BBSRC for funding DF with a CASE studentship with Syngenta.
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