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 (Probable). Can phosphorylate casein on threonine residues in vitro. Involved in the regulation of flowering time through gibberellin (GA) signaling, and independently of photoperiod. Phosphorylates the DELLA protein SLR1, stabilizing SLR1 protein and sustaining SLR1 activity as repressor of GA signaling (PubMed:20400938). Required for normal development of male floral organs and grains, through modulation of GA signaling (PubMed:26202549). Targeted and repressed by the homeobox protein HAZ1 during GA signaling (PubMed:21951842). Can phosphorylate phosvitin and SLR1 in vitro. Is not required for clock function in either the presence or the absence of light signals. Involved in a genetic control pathway for photoperiodic flowering under long day (LD) conditions that includes HD1, GHD7, HD5 and HD2. Phosphorylates and activates GHD7, a major floral repressor under LD conditions. Phosphorylation of GHD7 enhances its function in the repression of EHD1, HD3A and HD3B/RFT1, and obviously delaying flowering (PubMed:23789941). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Monomer (By similarity). Interacts with GHD7 (via C-terminus) (PubMed:23789941). Interacts with SLR1 (PubMed:20400938, PubMed:23789941). Expressed in roots, leaves and stems (PubMed:20400938). Expressed in leaf vascular bundles, and proximal regions of the shoot and roots (PubMed:23789941). Specifically expressed at stage 2 during floral development. Down-regulated by gibberellin (GA). Autophosphorylated. The cultivar Koshihikari contains a non-synonymous substitution of Ala-331 to Thr-331, which confers strongly reduced kinase activity of HD16, and results in decreased photoperiod sensitivity compared to the cultivar Nipponbare. Early flowering. Enhanced response to gibberellin (GA). Belongs to the protein kinase superfamily. CK1 Ser/Thr protein kinase family. Casein kinase I subfamily.