MAP4K1 and MAP4K2 regulate ABA-induced and Ca <sup>2+</sup> -mediated stomatal closure in <i>Arabidopsis</i>
Kota Yamashita, Sotaro Katagiri, Hinano Takase, Yangdan Li, Anzu Oishi, Airi Otoguro, Yoshiaki Kamiyama, Shota Yamauchi, Yuh‐Shuh Wang, Atsushi Takemiya, Izumi C. Mori, Hannes Kollist, Taishi Umezawa
Abstract
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 Ser 479 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 Ser 479 of MAP4K1 is required for this phenotype. In map4k1map4k2 double mutants, stomatal closure was induced by applying exogenous Ca 2+ but not H 2 O 2 . Electrophysiological experiments showed that MAP4K1/2 is required for ABA-dependent activation of Ca 2+ -permeable channels in GCPs. Together, our results indicate that SnRK2 and MAP4K function as a signaling module to regulate the Ca 2+ -mediated pathway in ABA-induced stomatal closure.