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A New Class of Ethylene Response Mutants |
The isolation of
ethylene-response mutants in Arabidopsis has led to substantial
insights into the components and mechanisms of ethylene signal
transduction. Most of the ethylene mutant screens to date have utilized
saturating levels of ethylene or no added ethylene. In this issue,
Larsen and Chang (pp. 1061-1073) announce their discovery
of enhanced ethylene-response (eer)
mutants, a new class of ethylene response mutants that were isolated by screening for the induction of triple-response phenotypes in the presence of subthreshold levels of ethylene. The eer1
mutation causes a profound enhancement in both the sensitivity and
amplitude of response to ethylene. In dark-grown eer1
seedlings, the mutant phenotype is typified by the swelling of the
lower portion of the hypocotyl.
There is no change in response in the apical hook or in the
number of root hairs. The alleviation of the eer1 phenotype
by ethylene synthesis inhibitors, and its suppression by the
ethylene-insensitive mutant etr1-1, indicate that ethylene
production and binding is necessary for the expression of the
eer1 phenotype. The authors propose that the EER1 product
acts to oppose ethylene responses in an organ-specific manner in the
hypocotyl and stem of Arabidopsis.
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No Circadian Rhythm in Nuclear Calcium |
Endogenous biological rhythms entrained by daily changes in the
light regimen are known to cause numerous circadian rhythms in the
physiological processes of plants. A previous study employing transgenic tobacco plants expressing the
Ca2+-sensitive photoprotein aequorin (Fig.
1) indicated that cytosolic free
Ca2+ levels
([Ca2+]cyt) also undergo
strong circadian oscillations. Since the transcription rates of many
nuclear genes are under circadian regulation, including some that are
also regulated by nuclear free Ca2+
([Ca2+]n), it is of
interest to determine whether
[Ca2+]n also oscillates
with a circadian rhythm. By means of transgenic plants in which
aequorin has been targeted to the nucleus, Wood et al. (pp.
787-796) conclude that
[Ca2+]n does not
oscillate in a circadian manner. However, the authors do find that the
[Ca2+]cyt of particular
cells and tissues within the same plant may oscillate with distinct
differences in phase, suggesting the possibility of manifold control
mechanisms.
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Figure 1.
Transgenic tobacco plants expressing the
photoprotein aequorin reveal the complexity of circadian rhythms in
[Ca2+]cyt.
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Advances in Understanding Tracheary Differentiation |
The last stage of tracheary differentiation in Zinnia
elegans culture cells involves secondary cell wall formation and
programmed cell death (PCD). Previous studies have shown that
uniconazole, an inhibitor of brassinosteroid synthesis, specifically
inhibits the last stage of tracheary differentiation. In this issue,
Yamamoto et al. (pp. 556-563) examine the questions of
whether brassinosteroids actually do increase in differentiating
tracheids and, if so, what types of brassinosteroids are they? Gas
chromatography-mass spectrometry analysis revealed that five different
species of brassinosteroids accumulate in Zinnia during
differentiation and that there are marked differences in the
proportions of these brassinosteroids in the culture medium versus the
cells themselves. This suggests that specific brassinosteroids are
selectively secreted into the medium and may function outside the
cells. In a second contribution in this issue, Obara et al. (pp.
615-626) present their results of a vital staining and a confocal
laser scanning microscopical study of PCD during the last stage of
tracheary differentiation in Zinnia cultured cells. Their
observations indicate that nuclear degeneration is complete within 20 min after the irreversible rupture of the central vacuole and
consequent release of hydrolytic enzymes into the cytoplasm (Fig.
2). A loss of tonoplast semi-permeability
precedes vacuolar rupture. In these aspects, PCD in differentiating
tracheids is similar to the PCD that occurs during aerenchyma formation
in maize roots and during senescence of unpollinated ovaries. However,
the PCD of tracheids is markedly different from other types of
plant cell senescence in which macromolecular digestion precedes
tonoplast rupture and nuclear degeneration occurs at a very late stage.
Thus, there appears to be more than one road to plant cell
death.
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Figure 2.
Confocal microscopy reveals the rapid degeneration
of the nucleus following vacuolar rupture during tracheary
differentiation in Zinnia.
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Ozone Hole Affects Plant Growth in Antarctica |
Stratospheric ozone depletion, which is especially pronounced over
Antarctica, leads to increases in solar UV-B irradiation (UV-B)
reaching the Earth's surface. Relatively few studies have examined the
influence of solar UV-B on Antarctica biota, and the vast majority of
these have focused on marine phytoplankton; increases in solar UV-B
levels in Antarctica depress photosynthesis in these microorganisms,
thereby reducing marine productivity. In this issue, Xiong and
Day (pp. 738-751) examine the effects of natural variations in
solar UV-B levels on the growth and photosynthesis of
Colobanthus quitensis and Deschampsia antarctica, two vascular plants native to Antarctica. Specimens were potted and
grown under filters that either absorbed or transmitted solar UV-B.
Solar UV-B absorption was found to lead to an 11% to 22% decrease in
biomass production and a 24% to 31% decrease in total leaf area.
Surprisingly, the rates of photosynthetic gas exchange per leaf area
were unaffected by UV-B irradiation. How then does one explain the
decrease in biomass production? Xiong and Day observed that the
leaves on plants exposed to UV-B were denser and thicker and had higher
concentrations of photosynthetic and UV-B-absorbing pigments. The
authors hypothesize that the development of thicker and pigment-rich
leaves may allow these plants to maintain their photosynthetic rates
per unit leaf area at rates comparable to those under reduced UV-B
conditions. However, the additional costs associated with the
construction of these modified leaves may account for the reduction in
growth and biomass production that stems from UV-B exposure.
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Insect Saliva and Plant Response to Attack |
Plants respond differentially to mechanical damage as compared to
herbivore damage even when the herbivore attack is carefully mimicked.
For example, the feeding of the specialist lepidopteran herbivore
Manduca sexta upon its natural host Nicotiana
attenuata, unlike purely mechanical damage to the host, elicits a
burst of jasmonic acid (JA) production. The addition of M. sexta oral secretions to mechanical damage mimics the natural
attack and also elicits the JA burst. In this issue, a trilogy of
papers offers the first glimpse into the large transcriptional changes
that occur in N. attenuata after herbivore attack. In the
first contribution, Hermsmeier et al. (pp. 683-700)
report, in an analysis of only 5% of the insect-responsive
transcriptome of N. attenuata, the occurrence of 27 transcripts that accumulate differentially after herbivore attack.
Extrapolation of this finding indicates that the transcription of more
than 500 N. attenuata genes are affected by herbivore damage. The authors estimate that about one-half of these genes may be related to plant-pathogen interactions, a not-too-surprising fact given that the mechanical damage resulting from herbivory greatly
increases the susceptibility of plants to pathogen entrance. A
comparison of these findings to another laboratory's research concerning the responses of Arabidopsis to attack by the herbivore Pieris napae reveals little commonality, suggesting a high
degree of species specificity. More than one-half of the 27 differentially regulated transcripts were identified. Overall,
transcripts involved in photosynthesis were strongly
down-regulated, whereas those responding to stress, wounding, and
pathogens and involved in shifting carbon and nitrogen to defense were
strongly up-regulated. In the second paper, Schittko et al. (pp.
701-710) examine how a subset of the genes identified by mRNA
differential display in the first paper are affected by mechanical
wounding with or without the addition of regurgitant-derived cues. They
report that some wound-induced transcripts are systemically
down-regulated by the addition of regurgitant, whereas others are
locally up-regulated. In the third contribution, Halitschke et
al. (pp. 711-717) identify several fatty acid-amino acid
conjugates in the regurgitants of M. sexta which, when
applied to mechanical wounds at concentrations comparable to those
found in regurgitant, are sufficient to activate three
herbivore-specific plant defense responses (JA accumulation, changes in
transcript accumulation, and volatile release). The authors propose
that fatty acid-amino acid conjugates may serve as emulsifiers during
insect digestion, but that plants use these compounds to "identify"
and respond appropriately to the insect type that is feeding upon them.
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