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 drought, osmotic, and dehydration stress. Regulates expression of stress-associated genes in response to drought. Involved in tolerance to drought stress by increasing proline and soluble sugars, and improving stomatal closure. Required for pollen maturation and spikelet fertility (PubMed:24884869). 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 leaf blades and stems (PubMed:15695435, PubMed:24884869). Expressed at low levels in anthers and spikelets (PubMed:24884869). By infection with the rice blast fungus (M.oryzae) (PubMed:15695435). Induced by abscisic acid (ABA), salt stress and osmotic shock (PubMed:24884869). 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 (365-395) inactivates kinase activity under calcium-free conditions. Plants over-expressing CPK9 show increased tolerance to drought, osmotic, and dehydration stresses, and improved pollen maturation and spikelet fertility under normal growth conditions. Plants silencing CPK9 show decreased tolerance to drought, osmotic, and dehydration stresses, and diminished pollen maturation and spikelet fertility under normal growth conditions. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CDPK subfamily.