Casein kinases are operationally defined by their preferential utilization of acidic proteins such as caseins as substrates. It can phosphorylate a large number of proteins (By similarity). Involved in photoperiod sensitivity (PS). Increases days-to-heading under natural day (ND) and long day (LD) conditions, but not under short day (SD) conditions (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Monomer. Autophosphorylated. The Nipponbare allele of HD6 (AC Q9AQU1) contains a premature stop codon, resulting in a truncated non-functional product. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. CK2 subfamily.