May play a role in signal transduction pathways that involve calcium as a second messenger (By similarity). Functions upstream of MPK5 in a signaling pathway that represses defense gene expression and negatively regulates resistance to rice blast fungus. Phosphorylates MPK5 at Thr-14 and Thr-32 and activates MPK5 independently of MAP kinase kinase (MKK) phosphorylation (PubMed:25035404). May be involved in arbuscular mycorrhizal presymbiotic phase signaling (PubMed:21595879). Phosphorylates the elicitor-responsive protein ERG1 in vitro. Phosphorylation is calcium-dependent (PubMed:23456295). 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. Interacts with MPK5. By the mycorrhizal fungus G.intraradices colonization in roots. 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 (318-348) inactivates kinase activity under calcium-free conditions. Autophosphorylated. Phosphorylated by MPK5. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDPK subfamily.