 |
INTRODUCTION |
The guard cells that flank the
stomatal pores of leaves and stems must integrate and respond
appropriately to a multitude of instantaneously varying stimuli. In
addition to a background circadian rhythmicity, the principle
physiological determinants of stomatal aperture are the respective
levels of blue light, CO2, and the stress-induced
hormone abscisic acid (ABA). The 1990s witnessed a blossoming in our
understanding of how guard cells work, largely because of the advent of
patch-clamp recording techniques, new ways to measure cytoplasmic free
Ca2+
([Ca2+]cyt), and the
adoption of Arabidopsis as a model organism. This month's The
Hot and the Classic presents a brief synopsis of the most cited
guard cell research contribution for each year of the 1990s.
| |
1990: Ca2+ Increase Precedes ABA-Induced
Closure |
Although there had been much indirect evidence that
Ca2+ fluxes might be involved in regulating the
responses of guard cells to ABA, McAinsh et al. (1990)
were able to
demonstrate this conclusively by application of fura-2, a fluorescent
Ca2+ indicator to Commelina communis
guard cells. Physiological concentrations of ABA caused a 2- to 10-fold
increase in
[Ca2+]cyt.
| |
1991: Multiple Stretch-Activated Channels |
Mechanosensitive ion channels in the plasma membrane of fava bean
(Vicia faba) guard cell protoplasts were studied by the patch clamp technique (Cosgrove and Hedrich, 1991).
Stretch-activated (SA) channels in outside-out patches were
analyzed for channel conductance, kinetics, and ion selectivity. Three
distinct SA channels were found that were permeable to
Cl
, K+, and
Ca2+. These SA channels may mediate ion transport
across the plasma membrane directly, as well as influence the activity
of non-SA channels via effects on membrane voltage and
[Ca2+]cyt.
| |
1992: Foreign Expression of Plant K+
Channel Gene |
KAT1 had previously been cloned from Arabidopsis by
complementation of Saccharomyces cerevisiae mutants
deficient in K+ uptake. Schachtman et al. (1992)
report that a single mRNA transcript from the Arabidopsis
KAT1 cDNA confers the functional expression of a
hyperpolarization-activated K+ channel in
Xenopus laevis oocytes. The channel encoded by
KAT1 is highly selective for K+ over
other monovalent cations and is blocked by tetraethylammonium and
Ba2+. These characteristics demonstrate that
KAT1 encodes an inward-rectifying K+ channel.
| |
1993: Control of Outward K+ Channel by
Cytoplasmic pH |
The activation by ABA of outward-rectifying
K+ channels and its dependence on cytoplasmic pH
were examined in stomatal guard cells of V. faba (Blatt and
Armstrong, 1993). ABA caused a cytoplasmic alkalinization and a
parallel rise in the outward-rectifying K+
channel current. Acid loads, imposed with external butyrate, abolished
the ABA-evoked K+ current. These results
establish a causal link between cytoplasmic alkalinization and the
activation of the outward K+ current by ABA and
thus affirm a role for H+ in signaling and
transport control in plants.
| |
1994: Vacuolar K+ Channel |
More than 90% of the K+ released from guard
cells during stomatal closure originates from the guard cell vacuole.
Ward & Schroeder (1994) report upon a novel type of
K+ channel in the vacuolar membrane of V. faba guard cells that is activated by physiological
increases in
[Ca2+]cyt. The
Ca2+, voltage, and pH dependences, high
selectivity for K+, and high density of the
K+ channels in the vacuolar membrane suggest a
central role for these channels in vacuolar K+ release. The
authors also presented a model of a possible mechanism of
Ca2+-induced Ca2+ release
involving the vacuolar K+ channel and a
previously described slow vacuolar channel.
| |
1995: Protein Phosphatase Regulation of K+
Channels |
Disruption of ABA sensitivity in wilty abi1-1 mutants
of Arabidopsis and evidence that this gene encodes a
protein phosphatase suggest that protein (de-)phosphorylation
contributes to stomatal control by ABA. Armstrong et al. (1995)
stably introduced the abi1-1 mutant allele into
Nicotiana benthamiana, and monitored its influence on ion
channel activity in guard cells under voltage clamp. Expression of the
abi1-1 gene was associated with 2- to 6-fold reductions in
an outward K+ current and the desensitization of
both inward and outward K+ currents to ABA. In
guard cells from the abi1-1 transformants, the protein
kinase antagonists H7 or staurosporine restored the normal responses of
both types of K+ channels and stomatal aperture
to ABA. These results implicate ABI1 as part of a phosphatase/kinase
pathway that modulates the sensitivity of guard-cell
K+ channels to ABA-evoked signal cascades.
| |
1996: Ca2+ and CO2-Induced
Closure |
Webb et al. (1996) used fura-2 fluorescence to measure
increases in guard cell
[Ca2+]cyt in
stomatal guard cells of C. communis
in response to increased CO2. Removal of
extracellular Ca2+ both prevented the
CO2-induced increase in
[Ca2+]cyt and inhibited
the associated reduction in stomatal aperture. These data suggest that
an influx of Ca2+ is required for stomatal response to
CO2.
| |
1997: Activation of an Anion Channel by ABA |
ABA strongly activates slow anion channels in wild-type
Arabidopsis guard cells (Pei et al., 1997). Protein phosphatase
inhibitors suppressed ABA-induced anion channel activation and stomatal
closing. ABA activation of slow anion channels and ABA-induced
stomatal closing were abolished in wilty abi1 and
abi2 mutant guard cells. These impairments in ABA signaling
were partially rescued by kinase inhibitors in abi1 but not
in abi2 guard cells. These data provide evidence that the
abi2 locus disrupts early ABA signaling, that abi1 and abi2 affect ABA signaling at different
steps in the cascade, and that protein kinases act as negative
regulators of ABA signaling in Arabidopsis.
| |
1998: Farnesyltransferase and ABA-Induced Closure |
Protein farnesylation, a posttranslational modification
process, mediates the COOH-terminal lipidation of specific cellular proteins such as Ras and G-proteins. Pei et al. (1998) report that
deletion of the Arabidopsis farnesyltransferase gene
ERA1 or application of farnesyltransferase inhibitors
resulted in ABA hypersensitivity of guard cell anion-channel activation
and of stomatal closing (Pei et al., 1998). Double-mutant analyses of era1 with the ABA-insensitive mutants abi1 and
abi2 showed that era1 suppresses the
ABA-insensitive phenotypes. Moreover, era1 plants
exhibited a reduction in transpirational water loss during drought treatment.
| |
1999: Phospholipase C and Ca Oscillations |
ABA induces oscillations in C. communis guard
cell [Ca2+]cyt
(Staxen et al., 1999). The pattern of the oscillations depended on
the ABA concentration and is correlated with the final
stomatal aperture. U-71322, an inhibitor of phosphoinositide-specific
phospholipase, inhibited both ABA-induced oscillations in
[Ca2+]cyt and stomatal
closure. An inactive analog of U-71322 was without effect. These
findings suggest a role for phosphoinositide-specific phospholipase in the generation of ABA-induced oscillations in [Ca2+]cyt and suggest the
involvement of oscillations in
[Ca2+]cyt in the
maintenance of stomatal aperture by ABA.
TOP
INTRODUCTION
1990: Ca2+ Increase Precedes...