Together with SRK2I, key component and activator of the abscisic acid (ABA) signaling pathway that regulates numerous ABA responses, such as seed germination, Pro accumulation, root growth inhibition, dormancy and seedling growth, and, to a lesser extent, stomatal closure (PubMed:17307925). In response to ABA, phosphorylates the ESCRT-I complex component FREE1, which is required for ABA-induced FREE1 nuclear import (PubMed:30962512). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with ABI1 (PubMed:19874541). Interacts with I-2, TOPP1 and TOPP2 (PubMed:26943172). Interacts with FREE1 (via C-terminus) (PubMed:30962512). A number of isoforms are produced. According to EST sequences. Expressed in seeds, seedlings, roots (especially in tips), stems, leaves, shoots, flowers and siliques. By abscisic acid (ABA), salt, and osmotic stress (at protein level). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.