May play a role in signal transduction pathways that involve calcium as a second messenger. Functions in abscisic acid (ABA) regulation of guard cell S-type anion- and Ca(2+)-permeable channels and stomatal closure (PubMed:17032064). Phosphorylates FD (PubMed:25661797). 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 SLAC1 (PubMed:20385816). Interacts with FD (PubMed:25661797). Expressed in both guard cells and mesophyll cells (PubMed:17032064). Expressed in the shoot apical meristem (PubMed:25661797). 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 (349-379) inactivates kinase activity under calcium-free conditions (By similarity). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDPK subfamily.