Possible role in short-day photoperiod floral induction. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] INRPK1 and INRPK1b are expressed in leaves, cotyledons, shoot tips and roots from induced and vegetative plants. The highest concentrations of INRPK1 are found in vegetative roots, and the lowest concentrations in vegetative cotyledons. INRPK1b is more abundant in roots than other tissues. INRPK1a is expressed in vegetative roots. INRPK1c is expressed in cotyledons. Produced by alternative initiation at Met-667 of isoform INRPK1. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.