Unveiling the molecular mechanism underlying PSKR-mediated amplification of the ABA signaling in Arabidopsis thaliana.

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Our research identified residues in AtPSKR1 and OsPSKR15 critical for kinase activity and interaction with ABA receptors, revealing PSKRs as core ABA signaling members that phosphorylate AtPYL4 and AtPYL9 receptors. Abscisic acid (ABA) serves as a vital signaling molecule that help plants respond to various environmental stresses, ensuring their survival and adaptability. The ABA signaling pathway begins when ABA is recognized by receptors known as PYR/PYL/RCAR. Upon ABA binding, these receptors undergo structural changes, but the precise modifications occurring during post-translational stages and their impact on ABA signaling are not fully understood. In this study, we have identified and characterized the ABA receptor family as target of PSKRs in both Arabidopsis and rice. In addition, we pinpointed the critical active sites in AtPSKR1 (N865) and OsPSKR15 (N892) that are responsible for kinase activity of the respective receptors and also important for direct interaction with ABA receptors. In vitro kinase experiments demonstrated phosphorylation of ABA receptors at S99 in AtPYL4, and S79 in AtPYL9. In addition, our genetic analysis demonstrated that PSKR plays a positive role in regulating ABA-mediated physiological responses, and promotes ABA-dependent leaf senescence in Arabidopsis. Phenotypic studies and expression analysis of ABA-related genes in complementation lines (AtPSKR1:pyl9 and OsPSKR15:pyl9) suggested that the overexpression of PSKR can partially restore the insensitivity of pyl9 mutant plants to ABA. These findings underscore the critical role of PSKR in enhancing ABA signaling via phosphorylation of PYL4/PYL9 in Arabidopsis.