Together with SRK2D, 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] Activated by autophosphorylation of its activation loop. Interacts with ABI1 (PubMed:19874541). Interacts with I-2 and TOPP1 (PubMed:26943172). Interacts with FREE1 (via C-terminus) (PubMed:30962512). Expressed at low levels in seeds, seedlings, roots (especially in tips), stems, leaves, shoots, flowers and siliques. By abscisic acid (ABA), salt, and osmotic stress (at protein level). Autophosphorylated in vitro. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. Truncated N-terminus.