Nitric oxide regulates K⁺ and Cl^- channels in guard cells through a subset of abscisic acid-evoked signaling pathways

Abscisic acid (ABA) triggers a complex sequence of signaling events that lead to concerted modulation of ion channels at the plasma membrane of guard cells and solute efflux to drive stomatal closure in plant leaves. Recent work has indicated that nitric oxide (NO) and its synthesis are a prerequisite for ABA signal transduction in Arabidopsis and Vicia guard cells. Its mechanism(s) of action is not well defined in guard cells and, generally, in higher plants. Here we show directly that NO selectively regulates Ca²⁺-sensitive ion channels of Vicia guard cells by promoting Ca²⁺ release from intracellular stores to raise cytosolic-free [Ca²⁺]. NO-sensitive Ca²⁺ release was blocked by antagonists of guanylate cyclase and cyclic ADP ribose-dependent endomembrane Ca²⁺ channels, implying an action mediated via a cGMP-dependent cascade. NO did not recapitulate ABA-evoked control of plasma membrane Ca²⁺ channels and Ca ²⁺-insensitive K⁺ channels, and NO scavengers failed to block the activation of these K⁺ channels evoked by ABA. These results place NO action firmly within one branch of the Ca ²⁺-signaling pathways engaged by ABA and define the boundaries of parallel signaling events in the control of guard cell movements.