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 low-nitrogen (Ref.7). Functions in multiple signaling pathways, positively regulating salt tolerance and negatively modulating rice blast fungus resistance. May promote tolerance to salt stress by negatively regulating NADPH oxidase and positively regulating reactive oxygen species (ROS) scavengers (PubMed:21883553). 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 roots, leaf blades and developing seeds (Ref.7). Expressed in vascular tissues of roots and leaf blades. Expressed in the phloem tissue of the large vascular bundle in leaf blades (PubMed:21883553). 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 (354-384) inactivates kinase activity under calcium-free conditions. Plants over-expressing CPK12 show enhanced tolerance to low nitrogen stress (Ref.7). Plants over-expressing CPK12 show increased tolerance to salt stress, increased sensitivity to abscisic acid (ABA) and increased susceptibility to rice blast fungus (M.oryzae) (PubMed:21883553). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDPK subfamily.