Calcium signals in guard cells enhance the efficiency by which abscisic acid triggers stomatal closure

During drought, abscisic acid (ABA) induces closure of stomata via a signaling pathway that involves the calcium (Ca(2+))‐independent protein kinase OST1, as well as Ca(2+)‐dependent protein kinases. However, the interconnection between OST1 and Ca(2+) signaling in ABA‐induced stomatal closure has not been fully resolved. ABA‐induced Ca(2+) signals were monitored in intact Arabidopsis leaves, which express the ratiometric Ca(2+) reporter R‐GECO1‐mTurquoise and the Ca(2+)‐dependent activation of S‐type anion channels was recorded with intracellular double‐barreled microelectrodes. ABA triggered Ca(2+) signals that occurred during the initiation period, as well as in the acceleration phase of stomatal closure. However, a subset of stomata closed in the absence of Ca(2+) signals. On average, stomata closed faster if Ca(2+) signals were elicited during the ABA response. Loss of OST1 prevented ABA‐induced stomatal closure and repressed Ca(2+) signals, whereas elevation of the cytosolic Ca(2+) concentration caused a rapid activation of SLAC1 and SLAH3 anion channels. Our data show that the majority of Ca(2+) signals are evoked during the acceleration phase of stomatal closure, which is initiated by OST1. These Ca(2+) signals are likely to activate Ca(2+)‐dependent protein kinases, which enhance the activity of S‐type anion channels and boost stomatal closure.