May play a role in signal transduction pathways that involve calcium as a second messenger (By similarity). Functions in signal transduction pathways that positively regulate responses to abscisic acid (ABA) and salt stress (PubMed:21136139). 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. Expressed in spikelets and developing seeds. By abscisic acid (ABA) and salt stress. 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 (364-394) inactivates kinase activity under calcium-free conditions. Plants over-expressing CPK21 display enhanced tolerance to salt stress. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDPK subfamily.