Direct Modulation of the Guard Cell Outward-Rectifying Potassium Channel (GORK) by Abscisic Acid (original) (raw)
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TheArabidopsisguard cell outward potassium channel GORK is regulated by CPK33
FEBS Letters, 2017
A complex signaling network involving voltage-gated potassium channels from the Shaker family contributes to the regulation of stomatal aperture. Several kinases and phosphatases have been shown to be crucial for ABA-dependent regulation of the ion transporters. To date, the Ca 2+-dependent regulation of Shaker channels by Ca 2+-dependent protein kinases (CPKs) is still elusive. A functional screen in Xenopus oocytes was launched to identify such CPKs able to regulate the three main guard cell Shaker channels KAT1, KAT2, and GORK. Seven guard cell CPKs were tested and multiple CPK/Shaker couples were identified. Further work on CPK33 indicates that GORK activity is enhanced by CPK33 and unaffected by a nonfunctional CPK33 (CPK33-K102M). Furthermore, Ca 2+-induced stomatal closure is impaired in two cpk33 mutant plants.
Proceedings of the National Academy of Sciences, 1995
Abscisic acid (ABA) modulates the activities of three major classes of ion channels-inwardand outwardrectifying K+ channels (IK,i and IK,out, respectively) and anion channels-at the guard-cell plasma membrane to achieve a net efflux of osmotica and stomatal closure. Disruption of ABA sensitivity in wilty abil-1 mutants ofArabidopsis and evidence that this gene encodes a protein phosphatase suggest that protein (de-)phosphorylation contributes to guard-cell transport control by ABA. To pinpoint the role of ABIl, the abil-1 dominant mutant allele was stably transformed into Nicotiana benthamiana and its influence on IK,ing, IK,out, and the anion channels was monitored in guard cells under voltage clamp. Compared with guard cells from wild-type and vectortransformed control plants, expression of the abil-l gene was associated with 2to 6-fold reductions in IK,out and an insensitivity of both hc,in and IK,out to 20 ,uM ABA. In contrast, no differences between control and abil-1 transgenic plants were observed in the anion current or its response to ABA. Parallel
FEBS Letters, 2000
Here we report on the molecular identification, guard cell expression and functional characterization of AtGORK, an Arabidopsis thaliana goutward rectifying K+ channel. GORK represents a new member of the plant Shaker K+ channel superfamily. When heterologously expressed in Xenopus oocytes the gene product of GORK mediated depolarization‐activated K+ currents. In agreement with the delayed outward rectifier in intact guard cells and protoplasts thereof, GORK is activated in a voltage‐ and potassium‐dependent manner. Furthermore, the single channel conductance and regulation of GORK in response to pH changes resembles the biophysical properties of the guard cell delayed outward rectifier. Thus GORK very likely represents the molecular entity for depolarization‐induced potassium release from guard cells.
Current Biology, 2007
Membrane vesicle traffic to and from the plasma membrane is essential for cellular homeostasis in all eukaryotes . In plants, constitutive traffic to and from the plasma membrane has been implicated in maintaining the population of integral plasmamembrane proteins and its adjustment to a variety of hormonal and environmental stimuli . However, direct evidence for evoked and selective traffic has been lacking. Here, we report that the hormone abscisic acid (ABA), which controls ion transport and transpiration in plants under water stress , triggers the selective endocytosis of the KAT1 K + channel protein in epidermal and guard cells. Endocytosis of the K + channel from the plasma membrane initiates in concert with changes in K + channel activities evoked by ABA and leads to sequestration of the K + channel within an endosomal membrane pool that recycles back to the plasma membrane over a period of hours. Selective K + channel endocytosis, sequestration, and recycling demonstrates a tight and dynamic control of the population of K + channels at the plasma membrane as part of a key plant signaling and response mechanism, and the observations point to a role for channel traffic in adaptive changes in the capacity for osmotic solute flux of stomatal guard cells .
PLANT PHYSIOLOGY, 2001
Inward-rectifying potassium (K ϩ in) channels in guard cells have been suggested to provide a pathway for K ϩ uptake into guard cells during stomatal opening. To test the proposed role of guard cell K ϩ in channels in light-induced stomatal opening, transgenic Arabidopsis plants were generated that expressed dominant negative point mutations in the K ϩ in channel subunit KAT1. Patch-clamp analyses with transgenic guard cells from independent lines showed that K ϩ in current magnitudes were reduced by approximately 75% compared with vector-transformed controls at Ϫ180 mV, which resulted in reduction in light-induced stomatal opening by 38% to 45% compared with vector-transformed controls. Analyses of intracellular K ϩ content using both sodium hexanitrocobaltate (III) and elemental x-ray microanalyses showed that light-induced K ϩ uptake was also significantly reduced in guard cells of K ϩ in channel depressor lines. These findings support the model that K ϩ in channels contribute to K ϩ uptake during light-induced stomatal opening. Furthermore, transpirational water loss from leaves was reduced in the K ϩ in channel depressor lines. Comparisons of guard cell K ϩ in current magnitudes among four different transgenic lines with different K ϩ in current magnitudes show the range of activities of K ϩ in channels required for guard cell K ϩ uptake during light-induced stomatal opening.
Plant Physiology, 2014
Ca2 +-dependent protein kinases (CPKs) form a large family of 34 genes in Arabidopsis (Arabidopsis thaliana). Based on their dependence on Ca2+, CPKs can be sorted into three types: strictly Ca2+-dependent CPKs, Ca2+-stimulated CPKs (with a significant basal activity in the absence of Ca2+), and essentially calcium-insensitive CPKs. Here, we report on the third type of CPK, CPK13, which is expressed in guard cells but whose role is still unknown. We confirm the expression of CPK13 in Arabidopsis guard cells, and we show that its overexpression inhibits light-induced stomatal opening. We combine several approaches to identify a guard cell-expressed target. We provide evidence that CPK13 (1) specifically phosphorylates peptide arrays featuring Arabidopsis K+ Channel KAT2 and KAT1 polypeptides, (2) inhibits KAT2 and/or KAT1 when expressed in Xenopus laevis oocytes, and (3) closely interacts in plant cells with KAT2 channels (Förster resonance energy transfer-fluorescence lifetime imag...
AKT2/3 Subunits Render Guard Cell K+ Channels Ca2+ Sensitive
Journal of General Physiology, 2005
Inward-rectifying K+ channels serve as a major pathway for Ca2+-sensitive K+ influx into guard cells. Arabidopsis thaliana guard cell inward-rectifying K+ channels are assembled from multiple K+ channel subunits. Following the recent isolation and characterization of an akt2/3-1 knockout mutant, we examined whether the AKT2/3 subunit carries the Ca2+ sensitivity of the guard cell inward rectifier. Quantification of RT-PCR products showed that despite the absence of AKT2 transcripts in guard cells of the knockout plant, expression levels of the other K+ channel subunits (KAT1, KAT2, AKT1, and AtKC1) remained largely unaffected. Patch-clamp experiments with guard cell protoplasts from wild type and akt2/3-1 mutant, however, revealed pronounced differences in Ca2+ sensitivity of the K+ inward rectifier. Wild-type channels were blocked by extracellular Ca2+ in a concentration- and voltage-dependent manner. Akt2/3-1 mutants lacked the voltage-dependent Ca2+ block, characteristic for the ...
PLoS Biology, 2006
Abscisic acid (ABA) signal transduction has been proposed to utilize cytosolic Ca 2þ in guard cell ion channel regulation. However, genetic mutants in Ca 2þ sensors that impair guard cell or plant ion channel signaling responses have not been identified, and whether Ca 2þ -independent ABA signaling mechanisms suffice for a full response remains unclear. Calcium-dependent protein kinases (CDPKs) have been proposed to contribute to central signal transduction responses in plants. However, no Arabidopsis CDPK gene disruption mutant phenotype has been reported to date, likely due to overlapping redundancies in CDPKs. Two Arabidopsis guard cell-expressed CDPK genes, CPK3 and CPK6, showed gene disruption phenotypes. ABA and Ca 2þ activation of slow-type anion channels and, interestingly, ABA activation of plasma membrane Ca 2þ -permeable channels were impaired in independent alleles of single and double cpk3cpk6 mutant guard cells. Furthermore, ABA-and Ca 2þ -induced stomatal closing were partially impaired in these cpk3cpk6 mutant alleles. However, rapid-type anion channel current activity was not affected, consistent with the partial stomatal closing response in double mutants via a proposed branched signaling network. Imposed Ca 2þ oscillation experiments revealed that Ca 2þ -reactive stomatal closure was reduced in CDPK double mutant plants. However, longlasting Ca 2þ -programmed stomatal closure was not impaired, providing genetic evidence for a functional separation of these two modes of Ca 2þ -induced stomatal closing. Our findings show important functions of the CPK6 and CPK3 CDPKs in guard cell ion channel regulation and provide genetic evidence for calcium sensors that transduce stomatal ABA signaling.
The Plant Journal, 2010
Stomatal pores formed by a pair of guard cells in the leaf epidermis control gas exchange and transpirational water loss. Stomatal closure is mediated by the release of potassium and anions from guard cells. Anion efflux from guard cells involves slow (S-type) and rapid (R-type) anion channels. Recently the SLAC1 gene has been shown to encode the slow, voltage-independent anion channel component in guard cells. In contrast, the R-type channel still awaits identification. Here, we show that AtALMT12, a member of the aluminum activated malate transporter family in Arabidopsis, represents a guard cell R-type anion channel. AtALMT12 is highly expressed in guard cells and is targeted to the plasma membrane. Plants lacking AtALMT12 are impaired in dark-and CO 2 -induced stomatal closure, as well as in response to the drought-stress hormone abscisic acid. Patch-clamp studies on guard cell protoplasts isolated from atalmt12 mutants revealed reduced R-type currents compared with wild-type plants when malate is present in the bath media. Following expression of AtALMT12 in Xenopus oocytes, voltage-dependent anion currents reminiscent to R-type channels could be activated. In line with the features of the R-type channel, the activity of heterologously expressed AtALMT12 depends on extracellular malate. Thereby this key metabolite and osmolite of guard cells shifts the threshold for voltage activation of AtALMT12 towards more hyperpolarized potentials. R-Type channels, like voltagedependent cation channels in nerve cells, are capable of transiently depolarizing guard cells, and thus could trigger membrane potential oscillations, action potentials and initiate long-term anion and K + efflux via SLAC1 and GORK, respectively.