Two E-clade Protein Phosphatase 2Cs enhance ABA signaling by dephosphorylating ABI1 in Arabidopsis.

ABA INSENSITIVE 1 (ABI1) and ABI2 are co-receptors of the phytohormone abscisic acid (ABA). Studies have demonstrated that phosphorylation of multiple amino acids on ABI1/2 augments their ability to inhibit ABA signaling in planta. However, it is currently unknown whether there exists a mechanism to regulate the dephosphorylation of ABI1/2 that enhances the plant's sensitivity to ABA. In this study, we identified two protein phosphatases, designated ABI1 Dephosphorylating E clade PP2C 1 (ADEP1) and ADEP2, that interact with ABI1/2. Mutants lacking ADEP1, ADEP2, or both (adep1/2) exhibited reduced sensitivity to ABA-inhibited seed germination, root growth and ABA-induced stomatal closure. Additionally, ABA-induced accumulation of ABI5 protein and the expression of downstream target genes were reduced in the adep1/2 mutant compared to the wild-type. These findings suggest that ADEP1/2 function as positive regulators of the ABA signaling pathway. Mass spectrometry analysis and two-dimensional electrophoresis identified Ser¹¹⁷ as a major ABA-induced phosphorylation site on ABI1 protein. ADEP1/2 can dephosphorylate Ser¹¹⁷, leading to the destabilization of ABI1 protein and increased sensitivity to ABA in plants. Moreover, ABA treatment decreases the abundance of ADEP1/2 proteins. Overall, our study discovers two novel regulatory proteins that modulate ABA signaling and provides new insights into the regulatory network that fine-tune plant ABA responses.