Plant Physiol, January 2001, Vol. 125, pp. 172-173
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The Role of Microtubules in Stomatal Opening |
In contrast to most plant
cells, the cortical microtubules of guard cells remain well organized
even after microfibril deposition and cellular differentiation is
complete. This has led some researchers to speculate that microtubules
might play an important role in the regulation of stomatal function.
However, recent pharmacological studies concerning the effects of
microtubule inhibitors on stomatal opening have yielded seemingly
contradictory findings. To gain insight into the role of microtubules
in guard cell function, Marcus et al. (pp. 387-395)
employ a microtubule reporter gene construct consisting of the
microtubule-binding domain of the mammalian MAP4 protein fused to
green fluorescent protein (GFP::MBD).
Biolistically-transformed guard cells expressing
GFP::MBD display the distinctive microtubule arrays that
characterize guard cells (Fig. 1), but
no longer retain the ability to open in response to illumination (red,
white, or blue) or low [CO2]. Several
microtubule-disrupting drugs, including propyzamide, colchicine,
trifluralin, and oryzalin, also inhibit light-induced stomatal opening
(colchicine less than the others). Fusicoccin, which directly leads to
stomatal opening through its effects on H+
extrusion, was found to promote stomatal opening even in the presence
of propyzamide or GFP::MBD. These results indicate that microtubules are not necessary for events occurring after
H+ pump activation, but are required for one or
more of the signal transduction events that occur prior to
H+ pump activation. The authors speculate that a
microtubule-associated protein may be critical for light-induced
stomatal opening.
| |
Cell Division Contributes to Apical Hook Formation |
The emerging hypocotyl of a dark-grown Arabidopsis seedling
forms an apical hook about 24 h after germination. The hook is formed by differential axial growth and is maintained for about the
next 4 d after which the hypocotyl restraightens itself and the
hook is lost. Most differential growth responses, including apical hook
formation, have traditionally been assumed to arise solely from
differential cell elongation. In this issue, Raz and
Koornneef (pp. 219-226) examine the possibility that
differential cell division might also play a role in hook development.
The authors employ an expression marker, cyclin1B tagged with
-glucosidase (Cyc1B-GUS), to identify dividing cells in the
hypocotyl during hook development. Cyc1B-GUS expression in the hook
region was found to be temporally restricted to the first 2 d of
growth and spatially restricted to the subepidermal layers in the
apical part of the hook (Fig. 2). As
expected, Cyc1B-GUS expression did not occur in the apical hypocotyl
region of several hookless mutants.
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Nutrient Drain Hypothesis and Flowering in Fuchsia |
Different carbohydrate sinks in plants compete for available
Suc, the primary mobile assimilate of plants. The nutrient drain hypothesis proposes that it is the relative successes of these vying
sinks in attaining Suc that determines whether certain plants flower or
not. The strong import of Suc to the apex is thought to promote
flowering, whereas diversion of Suc away from the apex by, for example,
the application of gibberellic acid (GA), is thought to deter
flowering. The long day (LD) plant Fuchsia hybrida (Fig.
3) is an interesting candidate for
testing the nutrient drain hypothesis because its flowering is enhanced
by increased photosynthetic irradiance even in short days. Both LD- and
high irradiance-induced flowering are inhibited by applied GA. In this issue, King and Ben-Tal (pp. 488-496) use GC-MS-SIM to perform sensitive measurements of the Suc content of individual Fuchsia shoot apices. The authors found a very strong
correlation (r = 0.93) between flowering in short days
and shoot apex Suc content, indicating a florigenic role for Suc. In
contrast, LD-induced flowering at low irradiance levels induced
flowering, but there was no corresponding increase in shoot apical Suc
levels. Suc, therefore, is florigenic in this species, but it is not
the long-sought "florigen" that is produced in LDs. Consistent with
the nutrient drain hypothesis, GA inhibited LD-induced flowering and
led to a decrease in shoot apical Suc content.
| |
Salicylic Acid Activates Nuclear Protein Kinase
CK2 |
Salicyclic acid (SA) is an important secondary signal in
plants that plays a major role in the activation of defense genes in
response to pathogen attack. Two groups of SA-activated genes
early and latecan be distinguished. Among the early genes are those that
encode for glutathione S-transferases, a
multigene family involved in the detoxification of cytotoxic compounds
produced during the defense reaction. Examples of late genes are those that encode for acidic pathogenesis-related proteins. DNA promoter elements called as-1-like elements control many of the
early SA-activated genes. SA enhances the binding of tobacco nuclear
factors to as-1 sequences in a process mediated by
protein phosphorylation. In this issue, Hidalgo et al.
(396-405) present evidence that this phosphorylation step
may be mediated by nuclear protein kinase CK2. First, CK2 activity is
enhanced in nuclear extracts of SA-treated tobacco. Second, a specific
inhibitor of CK2 inhibits the activating effect of SA on the
transcription of both a -glucosidase reporter gene controlled by a
tetramer of the as-1 element as well as several glutathione
S-transferase genes. These results constitute the first
evidence for the activation of a plant CK2 kinase by a stress-induced second messenger.
Peter V. Minorsky
Department of Biology
Vassar College
Poughkeepsie, NY 12604
© 2001 American Society of Plant Physiologists
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