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No Role for Actin in Stem Gravitropism |
The actin cytoskeleton is
hypothesized to play a major role in gravity perception and
transduction in plant roots. To determine whether actin microfilaments
(MFs) are involved in gravitropism in stem-like organs,
Yamamoto and Kiss (pp. 669-681) studied gravitropism
in Arabidopsis inflorescence stems and hypocotyls. Localization
studies using Alexa Ruor-phalloidin in conjugation with confocal
microscopy demonstrated a longitudinally and transversely oriented actin MF network in endodermal cells of stems and
hypocotyls (Fig. 1). Latrunculin B
(Lat-B) treatment of hypocotyls caused depolymerization of actin MFs in
endodermal cells and a significant reduction of hypocotyl growth rates.
Actin MFs in Lat-B-treated inflorescence stems also were
disrupted, but growth rates were not affected. Despite
disruption of the actin cytoskeleton in these two organs, Lat-B-treated
stems and hypocotyls exhibited a promotion of
gravitropic curvature in response to reorientation. In contrast, Lat-B
reduced gravitropic curvature in roots. Thus, in contrast to prevailing
root-based hypotheses, these results suggest that actin MFs are not a
necessary component of gravitropism in inflorescence stems and
hypocotyls.
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Figure 1.
The endodermal cells of stems and hypocotyls have
an extensive network of MFs, but its disruption does not prevent
gravitropic curvature.
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Salicylic Acid (SA) and Tobacco Mosaic Virus
(TMV) |
SA is a component of the signal transduction pathway needed for
induction of systemic acquired resistance, a plant-wide enhancement of
resistance against a broad spectrum of pathogens. The trigger for the
synthesis and induction of systemic acquired resistance is the
recognition of an invading microorganism by the product of a resistance
gene. Often, this recognition is accompanied by the
hyper-sensitive response, a form of rapid programmed host cell
death in a region around the point of pathogen entry. In SA-treated
tobacco (Nicotiana tabacum), the accumulation of
TMV is inhibited at the site of inoculation. In this issue,
Murphy and Carr (pp. 552-563) take advantage of
TMV::green fluorescent protein (TMV::GFP) to
examine the effects of SA on the cell biology of viral infection.
Treatment of tobacco with SA led to the restriction of
TMV::GFP to single epidermal cell infection sites for
at least 6 d postinoculation (Fig.
2). However, microinjection
experiments, using size-specific dextrans, revealed that SA does not
inhibit TMV movement by decreasing the plasmodesmatal size exclusion
limit or by interfering with plasmodesmal gating. The mechanism by
which SA inhibits viral movement between epidermal cells remains
elusive. In the case of mesophyll protoplasts, SA acted by strongly
inhibiting TMV replication. Therefore, it appears that SA has distinct
cell type-specific effects on virus replication and movement in the mesophyll and epidermal cell layers, respectively.
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Figure 2.
False-colored epifluorescence image shows
TMV::GFP restricted to a single epidermal cell of tobacco.
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Engineering Drought Resistance |
The plant hormone abscisic acid (ABA) is important in seed
maturation and dormancy, and in the adaptation of plants to a variety of environmental stresses. It is now well established that ABA in
higher plants is derived from C40 carotenoids. Epoxidation of the C40 carotenoid, zeaxanthin, gives rise to all
trans-violaxanthin, one of the xanthophylls in higher
plants. The first C15 precursor of ABA is xanthoxin, which
is a cleavage product of C40 epoxycarotenoids. This key
regulatory step in ABA biosynthesis is catalyzed by
9-cis-epoxycarotenoid dioxygenase (NCED). In bean
(Phaseolus vulgaris), the gene encoding this cleavage
enzyme (PvNCED1) is up-regulated by water stress, preceding accumulation of ABA. Qin and Zeevaart (pp.
544-551) have genetically engineered Nicotiana
plumbaginifolia plants to overexpress the
PvNCED1 gene either constitutively or in an inducible manner. The constitutive expression of PvNCED1 resulted
in an increase in ABA and its catabolite. When the
PvNCED1gene was under the regulation of the
dexamethasone (DEX)-inducible promoter, a transient induction of
PvNCED1 mRNA and accumulation of ABA were observed after
application of DEX. In the presence of DEX, seeds from a homozygous
transgenic line showed a 4-d delay in germination. After spraying with
DEX, the detached leaves from the transgenic line underwent a drastic
decrease in their water loss relative to control leaves. These plants
also showed a marked increase in their tolerance to drought stress.
These results indicate that it is possible to manipulate ABA levels in
plants by overexpressing the key regulatory gene in ABA biosynthesis
and that stress tolerance can be improved by increasing ABA levels.
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Anaerobiosis Enhances Cyclic Electron Flow |
Cyclic electron flow around photosystem I (PS I) has
been widely described in vitro in chloroplasts or thylakoids isolated from C3 plant leaves, but its occurrence in vivo is still a
matter of debate. The plastid genome of higher plants contains
ndh genes encoding peptides homologous to subunits of
the proton-pumping NADH-ubiquinone oxidoreductase, a component of the
mitochondrial respiratory chain. Inactivation of some
ndh genes demonstrated the existence of a functional Ndh
complex and its involvement in the transient non-photochemical
reduction of the plastoquinine pool after a light-to-dark transition.
Based on the study of chlorophyll fluorescence kinetics and the effects
of inhibitors, such as antimycin, on tobacco leaf discs of a Ndh-less
tobacco mutant, it was recently suggested that the Ndh complex might be
involved in a PS I cyclic electron pathway in C3 plants. In
this issue, Jöet et al. (pp. 760-769) report on their
analysis of cyclic PS I activity in tobacco leaf discs by means of
photoacoustic spectroscopy and kinetic spectrophotometry. Only a very
weak activity was measured in air with both techniques. When leaf discs
were placed under anaerobic conditions, however, a high and rapid
cyclic PS I activity was measured. The stimulatory effect of
anaerobiosis was mimicked by infiltrating leaves with inhibitors of
mitochondrial respiration or of the chlororespiratory oxidase,
indicating that changes in the redox state of intersystem electron
carriers tightly control the rate of PS I-driven cyclic electron flow
in vivo. Measurements of energy storage at different modulation
frequencies of far-red light showed that anaerobiosis-induced cyclic PS
I activity in the leaves of a tobacco mutant deficient in the plastid
Ndh complex was kinetically slower from that of the wild type (WT).
These results suggest that the Ndh complex is required for rapid
electron cycling around PS I.
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Maternally Expressed Transcription Factor Affects
Seed Germination |
The Dof proteins are a large family of recently discovered
transcription factors unique to plants. A strongly conserved 52-amino acid domain encompassing a single CX2CX21CXC zinc finger characterizes these proteins. Outside the conserved Dof domain, these proteins diverge widely and are involved, in different species, in regulating the expression of genes encoding for proteins involved in carbon metabolism, storage protein synthesis, abscorbate oxidase, and the
auxin-inducible expression of oncogenes. Analysis of the Arabidopsis genome indicates the presence of some 40 members of the
Dof gene family in this species. The only
Dof gene for which an effect in plants has been so far
convincingly demonstrated is DAG1, which has been shown
to be involved in seed germination. A knockout mutant of
DAG1, isolated from a TDNA insertion collection,
produces seeds that do not develop dormancy, lose their dependence upon red light for germination, and are capable of germinating in the dark.
Previous studies have shown that the gene DAG1 is
expressed only in the mother plant and not in the seed at any stage of
development. The segregation pattern of the dagl mutant
seed phenotype indicates that the maternal tissues of the seed mediate
the effect of the mutation on seed germination. In this issue,
Papi et al. (pp. 411-417) report that dagl
mutant seeds are altered in their sensitivity to light and in the
stainability of their seed coats. They also demonstrate that
DAG1 is specifically expressed in the phloem of all
organs. The authors propose that DAG1 may be involved in enhancing the
expression of red light-inducible genes, or in subtly weakening the
structural integrity of the seed coat.
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ABA Lowers SA-Mediated Resistance to Botrytis |
Increased levels of endogenous ABA have often been observed in
plants in response to infection with viruses, bacteria, and fungi. In
this issue, Audenaert, De Meyer, and Höfte (pp. 491-501) show that tomato (Lycopersicon
esculentum) mutants with reduced ABA levels
(sitiens plants) are much more resistant to the
necrotrophic fungus Botrytis cinerea than are WT plants. Sitiens tomato mutants are defective in the structural
gene for ABA-aldehyde oxidase (the enzyme that converts ABA-aldehyde to ABA), and have a residual ABA level of only 8% of WT
plants. Moreover, they are unable to increase their ABA levels upon
elicitation by wounding, heat, or electrical current. The authors
report that the exogenous application of ABA restores the
susceptibility of sitiens plants to B.
cinerea and increases the susceptibility of WT plants to
this pathogen. In their initial efforts to elucidate the mechanistic
basis by which ABA promotes susceptibility to this disease, the authors
have found evidence that suggests that ABA may negatively regulate
SA-dependent defense mechanisms that are critical for
mounting an effective defense against B.
cinerea.
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No Role for Actin...
Salicylic Acid (SA) and...