Abstract
Stomatal movement, critical for photobiosynthesis, respiration, and stress responses, is regulated by many factors, among which abscisic acid (ABA) is critical. Early events of ABA signaling involve Ca(2+) influx and an increase of cytoplasmic calcium ([Ca(2+)](cyt)). Positive regulators of this process have been extensively studied, whereas negative regulators are obscure. ABA-induced stomatal closure involves K(+) flux and vacuolar convolution. How these processes are connected with Ca(2+) is not fully understood. We report that pat10-1, a null mutant of Arabidopsis (Arabidopsis thaliana) PROTEIN S-ACYL TRANSFERASE10 (PAT10), is hypersensitive to ABA-induced stomatal closure and vacuolar convolution. A similar phenotype was observed in cbl2;cbl3, the double mutant of CBL2 and CBL3, whose tonoplast association depends on PAT10. Functional loss of the PAT10-CBL2/CBL3 system resulted in enhanced Ca(2+) influx and [Ca(2+)](cyt) elevation. Promoting vacuolar K(+) accumulation by overexpressing NHX2 suppressed ABA-hypersensitive stomatal closure and vacuolar convolution of the mutants, suggesting that PAT10-CBL2/CBL3 positively mediates vacuolar K(+) accumulation. We have identified CBL-interacting protein kinases (CIPKs) that mediate CBL2/CBL3 signaling during ABA-induced stomatal movement. Functional loss of the PAT10-CBL2/3-CIPK9/17 system in guard cells enhanced drought tolerance. We propose that the tonoplast CBL-CIPK complexes form a signaling module that negatively regulates ABA signaling during stomatal movement.