GSK3-like kinases positively modulate abscisic acid signaling through phosphorylating subgroup III SnRK2s in Arabidopsis

Arabidopsis glycogen synthase kinase 3 (GSK3)-like kinases have versatile functions in plant development and in responding to abiotic stresses. Although physiological evidence suggested a potential role of GSK3-like kinases in abscisic acid (ABA) signaling, the underlying molecular mechanism was largely unknown. Here we identified members of Snf1-related kinase 2s (SnRK2s), SnRK2.2 and SnRK2.3, that can interact with and be phosphorylated by a GSK3-like kinase, brassinosteroid insensitive 2 (BIN2). bin2-3 bil1 bil2, a loss-of-function mutant of BIN2 and its two closest homologs, BIN2 like 1 (BIL1) and BIN2 like 2 (BIL2), was hyposensitive to ABA in primary root inhibition, ABA-responsive gene expression, and phosphorylating ABA Response Element Binding Factor (ABF) 2 fragment by in-gel kinase assays, whereas bin2-1, a gain-of-function mutation of BIN2, was hypersensitive to ABA, suggesting that these GSK3-like kinases function as positive regulators in ABA signaling. Furthermore, BIN2 phosphorylated SnRK2.3 on T180, and SnRK2.3(T180A) had decreased kinase activity in both autophosphorylation and phosphorylating ABFs. Bikinin, a GSK3 kinase inhibitor, inhibited the SnRK2.3 kinase activity and its T180 phosphorylation in vivo. Our genetic analysis further demonstrated that BIN2 regulates ABA signaling downstream of the PYRABACTIN RESISTANCE1/PYR1-LIKE/REGULATORY COMPONENTS OF ABA RECEPTORS receptors and clade A protein phosphatase 2C but relies on SnRK2.2 and SnRK2.3. These findings provide significant insight into the modulation of ABA signaling by Arabidopsis GSK3-like kinases.