MAP4K1 and MAP4K2 regulate ABA-induced and Ca²⁺-mediated stomatal closure in Arabidopsis

Abscisic acid (ABA)-induced stomatal closure limits water loss from plants under drought stress. To investigate the signaling pathways involved in ABA-induced stomatal closure, we performed a phosphoproteomic analysis of ABA-treated Arabidopsis guard cell protoplasts (GCPs). We found that ABA-responsive phosphorylation of MITOGEN-ACTIVATED PROTEIN 4 KINASE 1 (MAP4K1) was significantly down-regulated in SnRK2-disrupted mutants. Subsequent biochemical assays showed that Ser⁴⁷⁹ of MAP4K1 is directly phosphorylated by SRK2E/OST1, a central ABA kinase. Mutational analyses of MAP4K1 and MAP4K2 revealed that both kinases positively regulate ABA-induced stomatal closure and that Ser⁴⁷⁹ of MAP4K1 is required for this phenotype. In map4k1map4k2 double mutants, stomatal closure was induced by applying exogenous Ca²⁺ but not H₂O₂. Electrophysiological experiments showed that MAP4K1/2 is required for ABA-dependent activation of Ca²⁺-permeable channels in GCPs. Together, our results indicate that SnRK2 and MAP4K function as a signaling module to regulate the Ca²⁺-mediated pathway in ABA-induced stomatal closure.