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Altered Epigenesis in Allotetraploids |
Allotetraploidization refers to
the hybridization of two separate species
a process of enormous
importance both in plant speciation and in plant breeding. Although
allotetraploids inherit a complete set of chromosomes from each
parental species, allotetraploids of recent origin typically exhibit
genomic and phenotypic instability. These instabilities may result from
the hybridization of redundant and diverged
homeologous sets of genes. Such hybridization events may trigger widespread epigenetic changes involving alterations in gene
silencing, chromatin structure, and DNA methylation patterns. Indeed,
gene silencing has previously been reported to be frequent in synthetic
allotetraploids of Arabidopsis and Cardaminopsis arenosa
(Fig. 1). In this issue, Madlung et
al. (pp. 733-746) use this same synthetic allotetraploid to
explore the possible role of DNA methylation in mediating phenotypic
instability. They report that changes in cytosine methylation patterns
are more frequent in the allotetraploids than in the parents. To
test whether demethylation might restore the synthetic allotetraploids
to a stable phenotype, the authors examined the effects of an inhibitor of DNA methyltransferase on plant phenotype. This drug scarcely affected either in the parental lines or in Arabidopsis
suecica, a naturally occurring allotetraploid, but had gross
effects on the phenotypes of the synthetic allotetraploids.
Drug-induced demethylation of the genome increased the rates of
transcriptional changes in the synthetic allotetraploids. The authors
conclude that phenotypic instability in newly formed allotetraploids is accompanied by non-random changes in the methylation state of the
combined genomes and that transcriptional changes involving both gene
silencing and gene activation are involved.
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Figure 1.
C. arenosa. This species hybridizes
with Arabidopsis, forming allotetraploid offspring that are
characterized by phenotypic instability.
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Thylakoid Membrane Mutants |
Thylakoid membranes have unusual lipid compositions compared
with other organelles. For example, the highly unsaturated fatty acids
16:3 and 18:3 account for nearly 67% of all the fatty acids in
thylakoids and over 90% of the fatty acids in
monogalactosyldiacylglycerol, the most abundant chloroplast lipid. It
seems likely that these peculiarities of lipid composition may be
important for proper photosynthetic function. Two contributions in this
issue shed new light on the role of chloroplast lipids in
photosynthesis. In the first, Vijayan and Browse (pp.
876-885) examine the photosynthetic capabilities of four
Arabidopsis mutants that have reduced levels of fatty acid
unsaturation. Three of these lines are more susceptible to
photoinhibition when compared with wild type (WT), whereas the fourth
shows no difference. A triple mutant that contained no trienoic fatty
acids was the most susceptible to photoinhibition. The
photoinactivation of photosystem II was the same in this triple mutant
as in the WT, but its recovery was slower at all temperatures below
27°C. These results indicate that the trienoic fatty acids of
thylakoid membrane lipids are essential for low-temperature recovery
from photoinhibition in Arabidopsis. In the second contribution,
Xu et al. (pp. 594-604) describe a mutant of Arabidopsis
(pgp1) in which the overall content of phosphatidylglycerol
(PG) is reduced by 30%. The mutant shows an 80% reduction in
plastidic phosphatidylglycerolphosphate synthase activity. The mutant
plants show reduced photosynthesis and are pale green (Fig.
2). Photosynthetic pigments were reduced,
and there was a marked decrease in the quantum yield of linear electron transport through photosystem II. These results underscore the importance of PG for the proper structure and function of
photosynthetic membranes.
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Figure 2.
An Arabidopsis mutant (pgp1) with
reduced phosphatidylglycerol exhibits reduced photosynthesis and a pale
coloration.
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Cytokinesis and Root Hair Growth: Related Processes? |
The processes of plant cytokinesis and root hair morphogenesis
show many similarities. Both processes depend upon the directed transport of Golgi-derived vesicles bearing cell wall materials. Both
processes are directed by the cytoskeletal elements. In this issue,
Söllner et al. (pp. 678-690) identify six previously uncharacterized Arabidopsis genes required for cytokinesis. The mutants
are seedling lethal, have morphological abnormalities, and are
characterized by cell wall perturbations and multinucleate nuclei.
Phenotypic analyses of these six mutants as well as four previously
characterized cytokinesis mutants indicate that the secondary
consequences of a primary defect in cytokinesis include anomalies in
body organization, organ number, and cellular differentiation, as well
as organ fusions and perturbations of the nuclear cycle. Two of the 10 loci examined are required for both cytokinesis and root hair
morphogenesis, underscoring the mechanistic similarities between these
two processes.
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Heterotrimeric G-Proteins and Arabidopsis Seed
Germination |
Previous studies of the G-protein null mutant Arabidopsis
gpa1 revealed that G-proteins are essential components
of many signal transduction pathways, including those activated by
abscisic acid, ethylene, gibberellic acid (GA), brassinosteroid
(BR), and Glc. Because seeds integrate many of these same signals
during germination, Ullah et al. (pp. 897-907) adapted seed
germination as a model system for examining the role of heterotrimeric
G-proteins in signal cross talk and transduction. Their most
interesting results concern the role of heterotrimeric G-proteins in
controlling the interactions of GA and BR. As in most species, GA
induces seed germination in Arabidopsis. Seed germination of GA
biosynthesis mutants can be rescued by the application of either
exogenous GA or BR. Seeds carrying a null mutation in the gene encoding for the 
-subunit of the Arabidopsis G-protein (GPA1) are
100-fold less responsive to GA than are WT. Seeds that ectopically
express GPA1 are at least a million-fold more responsive to
GA, but surprisingly, still require GA for germination. The authors
conclude that GPA1 indirectly operates on the GA pathway to control
seed germination by potentiation. Because a BR response mutant and a BR
synthesis mutant share the same sensitivity to GA as gpa1
seeds, the authors propose that brassinosteroids mediate this
potentiation. Indeed, gpa1 seeds are completely insensitive
to BR rescue of germination when GA levels in the seeds are reduced.
The gpa1 mutants also have increased sensitivity to high
levels of Glc, whereas their responses to ABA and ethylene are similar
to those of the WT.
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Proteomics of GA-Induced Seed Germination |
From physiological studies on a wide variety of species, it
appears that GAs play a key role in the late stages of seed
germination. In this issue, Gallardo et al. (pp.
823-837) used two proteomic approaches toward understanding the
process of GA-induced seed germination in Arabidopsis. The first system
consisted of seeds of the GA-deficient ga1 mutant; the
second involved WT seeds incubated with paclobutrazol, a specific
inhibitor of GA biosynthesis. The results of these proteomic analyses
indicate that GAs participate in few germination processes prior to
radicle emergence. Out of 46 protein changes detected prior to radicle
emergence, only one (-2,4-tubulin) appeared to be dependent upon the
action of GA. In marked contrast, GAs appeared to be involved, directly
or indirectly, in controlling the abundance of several proteins
associated with radicle protrusion. For example, two isoforms of
S-adenosyl-Met (SAM) synthetase increased in abundance
during radicle protrusion, and the authors suggest that SAM might play
a major role in controlling metabolism during this phase of
germination. GAs also play a role in controlling the abundance of a
-glucosidase, an enzyme that is undoubtedly needed for cell
elongation and radicle extension.
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Circadian Rhythms Enhance Plant Fitness |
Previous studies have shown that the constitutive expression of
the CIRCADIAN CLOCK ASSOCIATED 1 (CCA1)
gene in Arabidopsis plants (CCA1-ox) results in the loss of circadian
rhythmicity. In this issue, Green et al. (pp. 576-584)
report that these CCA1-ox plants retain the ability to respond to
diurnal changes in light. The transcript levels of several
circadian-controlled genes, as well as CCA1 itself, oscillate robustly
if CCA1-ox plants are grown under diurnal conditions. However, in
contrast to WT plants in which the transcript levels change in
anticipation of the dark/light transitions, the CCA1-ox plants lack the
ability to anticipate this daily change in light conditions. The
authors took advantage of this defect to examine the effects of loss of circadian regulation on the fitness of Arabidopsis. CCA1-ox plants flower late, especially under long-day conditions, and are less viable
under short-day conditions compared with WT plants. These findings
demonstrate the adaptive advantage of circadian rhythms in Arabidopsis.
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Altered Epigenesis in...
Thylakoid Membrane Mutants