CDPKs CPK6 and CPK3 Function in ABA Regulation of Guard Cell S-Type Anion- and Ca2+- Permeable Channels and Stomatal Closure

Abstract

Abscisic acid (ABA) signal transduction has been proposed to utilize cytosolic Ca2+ in guard cell ion channel regulation. However, genetic mutants in Ca2+ sensors that impair guard cell or plant ion channel signaling responses have not been identified, and whether Ca2+-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 Ca2+ activation of slow-type anion channels and, interestingly, ABA activation of plasma membrane Ca2+-permeable channels were impaired in independent alleles of single and double cpk3cpk6 mutant guard cells. Furthermore, ABA- and Ca2+-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 Ca2+ oscillation experiments revealed that Ca2+-reactive stomatal closure was reduced in CDPK double mutant plants. However, long-lasting Ca2+-programmed stomatal closure was not impaired, providing genetic evidence for a functional separation of these two modes of Ca2+-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.

Roles of ion channels and transporters in guard cell signal transduction

“…However, due to the fact that both K + and Ca 2+ can permeate through the TPC1 channel it is still debated whether the phenotype observed is derived from altered K + fluxes or from TPC1-dependent Ca 2+ signaling. Given recent evidence for roles of CDPKs in transducing ABA regulation of anion and Ca 2+ channels at the GC plasma membrane [104], it is anticipated that new connections may also soon emerge between ABA, CDPKs and regulation of tonoplast transport. A possible link between extracellular Ca 2+ concentration and tonoplast transport is indicated by the recent observation that the generation of Ca 2þ cyt oscillation is regulated by external Ca 2+ concentration via an apoplastic Ca 2+ sensor (CAS)-mediated CAS-InsP 3 pathway in Arabidopsis [153].…”

SLAC1 is required for plant guard cell S-type anion channel function in stomatal signalling

“…4b-d). R-type anion currents 11 were activated as described 25,28 . Interestingly, no significant differences in Rtype anion currents between wild-type and slac1 guard cells were observed (Fig.…”

Plant Stress Surveillance Monitored by ABA and Disease Signaling Interactions

“…Disruption of these two genes produced defects in ABA-and Ca 2+ -activation of S-type anion channel. Additionally, ROS (reactive oxygen species)-activation of I Ca -channel was also reduced in the cpk3 cpk6 mutant suggesting CPK3 and CPK6 might control downstream of ROS and Ca 2+ signaling steps in ABA signal transduction (Mori et al, 2006) (Fig. 2).…”

“…CPK3 and CPK6 are the major CDPKs expressed in guard cells and regulate ABA induction of S-type anion channel and Ca 2+ -permeable channels (Mori et al, 2006). Single cell typebased guard cell microarray analyses identified that CPK3 and CPK6 expressions were mainly localized in guard cells.…”

“…2). Furthermore, with the pronounced phenotypes of the double mutant cpk3 cpk6 compared to the individual single mutants, functional overlaps of CPK3 and CPK6 activity in guard cell ABA signal transduction were expected (Mori et al, 2006).…”