A Plasma Membrane Receptor Kinase, GHR1, Mediates Abscisic Acid- and Hydrogen Peroxide-Regulated Stomatal Movement in Arabidopsis

The plant hormone abscisic acid (ABA) regulates stomatal movement under drought stress, and this regulation requires hydrogen peroxide (H 2 O 2 ). We isolated GUARD CELL HYDROGEN PEROXIDE-RESISTANT1 ( GHR1 ), which encodes a receptor-like kinase localized on the plasma membrane in Arabidopsis thaliana . ghr1 mutants were defective ABA and H 2 O 2 induction of stomatal closure. Genetic analysis indicates that GHR1 is a critical early component in ABA signaling. The ghr1 mutation impaired ABA- and H 2 O 2 -regulated activation of S-type anion currents in guard cells. Furthermore, GHR1 physically interacted with, phosphorylated, and activated the S-type anion channel SLOW ANION CHANNEL-ASSOCIATED1 when coexpressed in Xenopus laevis oocytes, and this activation was inhibited by ABA-INSENSITIVE2 (ABI2) but not ABI1. Our study identifies a critical component in ABA and H 2 O 2 signaling that is involved in stomatal movement and resolves a long-standing mystery about the differential functions of ABI1 and ABI2 in this process.