Suppressor of flowering in long days (LD) via the that up-regulation of HD1 and the down-regulation of EHD1. Can phosphorylate HD1 in the presence of HDR1. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with HDR1. Strongly expressed in immature seeds (PubMed:9870704). Mostly expressed in panicles, leaf sheaths and roots, and to a lower extent, in germinating seeds and leaf blades (PubMed:16087344). Accumulates transiently in the early stages of seed maturation (PubMed:9870704). Observed in maturating caryopsis 1 week after flowering (DAF). Expressed in tissues where and when starch granules appear. Detected in the pericarp at the early stage with a shift to the endosperm as the endosperm cells are formed. Confined to both aleurone layer and endosperm cells around 15 days after flowering. Also present in the basal part of leaf sheath (PubMed:16087344). Normal expression of MYBS1 and AAMY3 in response to glucose levels. Normal seed germination and seedling growth. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.