May play a role in signal transduction pathways that involve calcium as a second messenger. Involved in maintaining Ca2+ homeostasis in pollen tube tips by regulating CNGC18 (PubMed:24121288). Functions as regulator of the calcium-mediated abscisic acid (ABA) signaling pathway (PubMed:16299177). Phosphorylates ABA-responsive transcription factor ABF4 in vitro (PubMed:16299177). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by calcium. Autophosphorylation may play an important role in the regulation of the kinase activity (By similarity). Interacts with ABF4 (PubMed:16299177). Interacts with CNGC18 (PubMed:24121288). Expressed in embryos and most of the vegetative tissues. Induced by touch, wounding, and darkness exposure. Also induced by high-salt treatment. There are 3 contiguous domains conserved in the CDPK subfamily: a kinase domain, an autoinhibitory (junction) domain and a calmodulin-like domain. The autoinhibitory domain (327-357) inactivates kinase activity under calcium-free conditions (By similarity). May be due to intron retention. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDPK subfamily.