Calcium elevation‐dependent and attenuated resting calcium‐dependent abscisic acid induction of stomatal closure and abscisic acid‐induced enhancement of calcium sensitivities of S‐type anion and inward‐rectifying K+ channels in Arabidopsis guard cells

Abstract

Summary: Stomatal closure in response to abscisic acid depends on mechanisms that are mediated by intracellular [Ca 2+] ([Ca 2+]i), and also on mechanisms that are independent of [Ca 2+]i in guard cells. In this study, we addressed three important questions with respect to these two predicted pathways in Arabidopsis thaliana. (i) How large is the relative abscisic acid (ABA)‐induced stomatal closure response in the [Ca 2+]i‐elevation‐independent pathway? (ii) How do ABA‐insensitive mutants affect the [Ca 2+]i‐elevation‐independent pathway? (iii) Does ABA enhance (prime) the Ca 2+ sensitivity of anion and inward‐rectifying K+ channel regulation? We monitored stomatal responses to ABA while experimentally inhibiting [Ca 2+]i elevations and clamping [Ca 2+]i to resting levels. The absence of [Ca 2+]i elevations was confirmed by ratiometric [Ca 2+]i imaging experiments. ABA‐induced stomatal closure in the absence of [Ca 2+]i elevations above the physiological resting [Ca 2+]i showed only approximately 30% of the normal stomatal closure response, and was greatly slowed compared to the response in the presence of [Ca 2+]i elevations. The ABA‐insensitive mutants ost1‐2, abi2‐1 and gca2 showed partial stomatal closure responses that correlate with [Ca 2+]i‐dependent ABA signaling. Interestingly, patch‐clamp experiments showed that exposure of guard cells to ABA greatly enhances the ability of cytosolic Ca 2+ to activate S‐type anion channels and down‐regulate inward‐rectifying K+ channels, providing strong evidence for a Ca 2+ sensitivity priming hypothesis. The present study demonstrates and quantifies an attenuated and slowed ABA response when [Ca 2+]i elevations are directly inhibited in guard cells. A minimal model is discussed, in which ABA enhances (primes) the [Ca 2+]i sensitivity of stomatal closure mechanisms.