Casein kinases are operationally defined by their preferential utilization of acidic proteins such as caseins as substrates. The alpha chain contains the catalytic site (By similarity). Involved in the regulation of various developmental processes (PubMed:26025542). Involved in the regulation of plant growth and flowering time (PubMed:26883224). Involved in retrograde signaling in plant responses to abscisic acid (ABA) and heat stress. May act as an enhancing factor in abiotic stress signaling through modulation of the expression of some molecular players in retrograde signaling (PubMed:24803505). Phosphorylates RuBisCo activase (RCA) at Thr-78 (PubMed:27064346). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Tetramer of two alpha and two beta chains. Expressed in root tips, lateral root primordia, cotyledons, leaf primordia, sepals, filaments, stigma, and anthers. Defects in root, hypocotyl, cotyledon and leaf development. Delayed flowering. Reduced growth, delayed flowering and hyperaccumulation of anthocyanins. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CK2 subfamily.