Open Stomata 1 (OST1) is limiting in abscisic acid responses of Arabidopsis guard cells

Summary Open Stomata 1 ( OST 1) (Sn RK 2.6 or SRK 2E), a serine/threonine protein kinase, is a positive regulator in abscisic acid (ABA) ‐mediated stomatal response, but OST 1‐regulation of K + and Ca 2+ currents has not been studied directly in guard cells and it is unknown whether OST 1 activity is limiting in ABA ‐mediated stomatal responses. We employed loss‐of‐function and gain‐of‐function approaches to study native ABA responses of Arabidopsis guard cells. We performed stomatal aperture bioassays, patch clamp analyses and reactive oxygen species ( ROS ) measurements. ABA inhibition of inward K + channels and light‐induced stomatal opening are reduced in ost1 mutants while transgenic plants overexpressing OST 1 show ABA hypersensitivity in these responses. ost1 mutants are insensitive to ABA ‐induced stomatal closure, regulation of slow anion currents, Ca 2+ ‐permeable channel activation and ROS production while OST 1 overexpressing lines are hypersensitive for these responses, resulting in accelerated stomatal closure in response to ABA . Overexpression of OST 1 in planta in the absence of ABA application does not affect basal apertures or ion currents. Moreover, we demonstrate the physical interaction of OST 1 with the inward K + channel KAT 1, the anion channel SLAC 1, and the NADPH oxidases AtrbohD and AtrbohF. Our findings support OST 1 as a critical limiting component in ABA regulation of stomatal apertures, ion channels and NADPH oxidases in Arabidopsis guard cells.