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Chloroplast Division |
As descendants of
endosymbiotic bacteria, plastids maintain many prokaryotic features,
including division by binary fission. During fission, a constriction
forms at the plane of division, progressively pinching the plastid into
two. In bacteria, the tubulin-like GTPase FTsZ assembles into a
circular structure (the Z-ring) at the cell center. The Z-ring serves
as a cytoskeletal framework to which other cell division proteins are
recruited and may also provide the force that powers the contractile
machinery. In vascular plants, plastid division entails the
participation of two distinct nuclear-encoded FtsZ protein families,
FtsZ1 and FtsZ2. Perhaps because plastids, unlike their bacterial
forbears, have double membranes, the plastid division apparatus is even more complicated, consisting of two distinct plastid-dividing (PD)
rings associated with the region of constriction. An outer PD ring is
localized to the cytosolic surface of the outer envelope membrane, and
an inner PD ring occurs in association with stromal surface of the
inner envelope membrane. Establishing the topological relationship
between the FtsZ1 and FtsZ2 proteins and the PD rings is essential for
understanding the molecular architecture of the plastid division
apparatus in higher plants. In this issue, McAndrew et al. (pp.
1656-1666) report that both FtsZ1 and FtsZ2 colocalize to the
stromal compartment of the chloroplast and may physically interact. The
stoichiometry between FtsZ1 and FtsZ2 appears to be an important aspect
of their function: The overexpression of FtsZ2 in transgenic
Arabidopdis inhibits plastid division in a dose-dependent manner.
A second contribution in this issue, by Itoh et al. (pp.
1644-1655), provides new insights into the function of AtMinE, another conserved component of plastid division. In bacteria, Min
proteins determine the site of Z-ring formation. A lack of MinE, which
is normally located near but separate from the Z-ring, results in a
filamentous phenotype. Itoh et al. report on the presence and location
of a MinE homolog (AtMinE1) in Arabidopsis. Histochemical studies
reveal the activation of AtMinE1 in the shoot apex, suggesting a role
for AtMinE1 in the process of proplastid division in green tissues. The
overexpression of AtMinE1 leads to the production of giant,
heteromorphic chloroplasts in transgenic plants (Fig.
1).
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Figure 1.
The overexpression of AtMinE1 in transgenic
Arabidopsis leads to giant, filamentous plastids (top) compared with
wild-type controls (bottom).
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Cyclic Nucleotides Reduce Na+ Toxicity |
Na+ uptake from the soil is a major cause of
salinity toxicity in plants, yet little is known about the mechanisms
that underlie Na+ uptake into plant roots. By the
systematic elimination of other possible candidates, more attention of
late has centered on the possible involvement of voltage-independent
cation channels (VICs) in this process. Unfortunately, little is known
about the factors, presumably chemical, that regulate the gating
properties of VICs. In this issue, Maathius and Sanders (pp.
1617-1625) report that the cytoplasmic addition of either cAMP or
cGMP causes a strong and reversible reduction in the probability of VIC
opening in Arabidopsis root cells. The authors also present
pharmacological evidence that points to a role for the cyclic
nucleotide regulation of VICs in controlling Na+
toxicity. The injury and death that typically occurs to Arabidopsis seedlings within 4 or 7 d following the application of 100 mM NaCl is delayed by the simultaneous application of
cyclic nucleotides (Fig. 2). Forskolin,
an adenyl cyclase agonist, also reduced Na+ accumulation,
whereas LY83583, a guanyl cyclase inhibitor, exacerbated Na+ toxicity symptoms.
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Figure 2.
Arabidopisis seedlings succumb within 5 d to
the application of 100 mM NaCl (top). The simultaneous
application of cAMP (100 µM) greatly delays the onset of
toxicity symptoms (bottom).
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"Castor Oil" from Arabidopsis |
Castor oil is rich in the hydroxy fatty acid ricinoleic
acid (18:1-OH), a valuable industrial raw material. The
crucifer Lesquerlla fenderli also produces a seed rich
in hydroxy fatty acids, particularly lesquerolic acid (20:1-OH).
Biochemical studies have revealed that developing L.
fenderli embryos hydroxylate oleic acid (18:1) at the
12 site to form ricoleneic acid and then
desaturate and elongate this fatty acid to form lesquerelic
acid. To identify the gene encoding the enzyme involved in hydroxy
fatty acid elongation, Moon et al. (pp.
1635-1643) screened a genomic library of L. fendleri
using the coding region of Arabidopsis fatty acid elongase (FAE1) as a
probe. A gene, LfKCS3, with a high sequence similarity to
other known very long-chain fatty acid-condensing enzymes was
isolated. LfKCS3 transcripts accumulated only in the embryos
of L. fendleri and first appeared in the early stages of
development. Seeds of Arabidopsis plants transformed with
LfKCS3 showed no change in their very long-chain fatty acid
content, but when these plants were crossed with transgenic plants
expressing the oleate 12 hydroxylase from castor bean (Ricinus
communis), significant amounts of 20-C hydroxy fatty acids
accumulated in the seed. This finding indicates that LfKCS3
specifically catalyzes the elongation of 18-C hydroxy fatty acids.
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Overexpression of Malate Dehydrogenase (MDH) Enhances Al
Tolerance |
Al toxicity to plants is a major agricultural problem in
acid soils, which compose about 40% of the world's arable land. Many Al-tolerant cultivars secrete Al-chelating organic acids into the
rhizosphere, thereby alleviating Al phytotoxicity. In an effort to
increase organic acid secretion in alfalfa (Medicago
sativa), Tesfaye et al. (pp. 1836-1844) produced
transgenic alfalfa using nodule-enhanced forms of MDH cDNAs under the
control of a constitutive promoter. They report that a 1.7-fold
increase in MDH activity in selected transgenic lines was associated
with a 5-fold increase in organic acid titer and exudation. The degree of Al tolerance by transformed plants coincided with their patterns of
organic acid exudation and supports the concept that enhancing organic
acid synthesis in plants may be an effective strategy for coping with
Al stress.
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Restriction of Tobacco Etch Virus (TEV) Movement |
Virus infection of plants is a multistep process
requiring compatible interactions between host- and virus-encoded
factors during genome expression, cell-to-cell movement via
plasmodesmata, and long-distance movement through the
vascular system. Restricted infection may result if cellular factors
required by the virus are lacking or incompatible with the virus, or if
the host responds to the virus and activates a defense response. A
process involving at least three genes (RTM1, RTM2, and
RTM3) restricts the long-distance movement of TEV through
resistant Arabidopsis ecotypes. An intriguing aspect of
this process is that it involves neither a hypersensitive response nor
systemic acquired resistance. In this issue, Chisholm et
al. (pp. 1667-1675) show that the regulatory sequence of both
RTM1 and RTM2 directs the expression of a
reporter construct exclusively in phloem-associated cells. The authors
discuss several mechanisms by which these two gene products may act to
restrict TEV movement.
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Circadian Control of Phytochrome Expression |
Many physiological and biochemical processes in plants
exhibit circadian rhythms. The circadian clock is entrained to a 24-h period by the reception of light signals by at least two classes of
photoreceptors: phytochrome and cryptochrome. In this issue, Tóth et al. (pp. 1607-1616) report on the spatial and temporal, light-regulated expression of Arabidopsis phytochromes (PHYA to PHYE) and cryptochrome (CRY1
and CRY2) using luciferase reporter constructs. The
transcription of all of the constructs (except that incorporating
PHYC) displayed circadian oscillations under constant
conditions. The circadian production of these photoreceptors may
represent an important regulatory loop involved in resetting the
circadian clock.
In a companion paper, Hall et al. (pp. 1808-1818)
examine the expression of the PHYA::luciferase
construct in intact tissues at the cellular level. They show that
rhythmic PHYA expression rapidly damps to a constant high
level in the dark, and that damping is dependent upon tissue connection
to the plant. Simply wounding intact leaves has no effect on rhythmic
expression. These results indicate that a whole plant-dependent
mechanism controls rhythmic PHYA expression in intact Arabidopsis.
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"Combinatorial Design." A Fast Track for Genomics |
For many systems in biology, genes, pathways, or
metabolites are regulated by multiple, interacting "input" signals.
In the post-genomic era, a significant challenge is to understand how whole genomes or metabolomes respond not only to single signals but
also to a matrix of interacting inputs. The study of multiple inputs
and multiple "doses" quickly leads to an intractable number of
experiments that need to be performed. Borrowing from the field of
software testing, Shasha et al. (pp. 1590-1594) discuss how
combinatorial design might be a useful strategy for reducing the number
of experiments that must be run in order to analyze the effects of
interacting inputs on genomic expression.
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Chloroplast Division
Cyclic Nucleotides Reduce Na+...