Calcium Elevation- and Attenuated Resting Calcium-dependent Abscisic Acid Induction of Stomatal Closure and Abscisic Acid-Induced Enhancement of Calcium Sensitivities of S-type Anion and K+in Channels in Arabidopsis guard cells

• Published in: The Plant journal : for cell and molecular biology 2009-03, Vol.59 (2), p.207-220
• Main Authors: Siegel, Robert S., Xue, Shaowu, Murata, Yoshiyuki, Yang, Yingzhen, Nishimura, Noriyuki, Wang, Angela, Schroeder, Julian I.
• Format: Article
• Language: English
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/j.1365-313X.2009.03872.x

Stomatal closure in response to abscisic acid depends on mechanisms that are mediated by intracellular [Ca 2+ ] ([Ca 2+ ] i ), and also on mechanisms independent of [Ca 2+ ] i in guard cells. In this study we address three important questions in reference to these two predicted pathways in Arabidopsis thaliana. 1. How large is the relative abscisic acid (ABA)-induced stomatal closing response from a [Ca 2+ ] i -elevation-independent pathway? 2. How do ABA-insensitive mutants affect a [Ca 2+ ] i -elevation-independent pathway? 3. 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 in ratiometric [Ca 2+ ] i imaging experiments. ABA-induced stomatal closing in the absence of [Ca 2+ ] i -elevations above the physiological resting [Ca 2+ ] i showed only ≈30% of the stomatal closure response and was greatly slowed compared to the presence of [Ca 2+ ] i -elevations. The ABA-insensitive mutants ost1-2, abi2-1, gca2 showed partial stomatal closing responses that correlate with [Ca 2+ ] i -dependent ABA signaling. Interestingly, patch clamp experiments show that exposure of guard cells to ABA greatly enhances the ability of cytosolic Ca 2+ to both activate S-type anion channels and down-regulate inward K + channels, providing strong evidence for the Ca 2+ sensitivity priming hypothesis. The present study shows and quantifies an attenuated and slowed ABA response, while directly inhibiting [Ca 2+ ] i -elevations in guard cells. A minimal model is discussed, in which ABA increases (primes) the [Ca 2+ ] i sensitivity of stomatal closing mechanisms.