Protein kinase that acts as a blue light photoreceptor in a signal-transduction pathway for photo-induced movements. Phosphorylates BLUS1, a kinase involved in stomatal opening. Mediates calcium spiking of extra- and intracellular origins in response to blue light. Involved in hypocotyl phototropism. Contributes to the chloroplast accumulation in low blue light and mediates their translocation (avoidance response) at high fluence. Regulates stomata opening and photomorphogenesis response of leaf tissue. Not involved in hypocotyl elongation inhibition, anthocyanin accumulation or cotyledon opening. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Binds 2 FMN per subunit. Exhibits a smaller absorbance peak at 350 nm. The broad fluorescence emission spectrum peaks at 490 nm. Homodimer. Interacts with PKS1, PKS2, RPT3 and PHOT1. Expressed in leaves, stems and flowers, and to a lower extent in roots. Light fluence rate-dependent induction, independent of light quality. The activation loop within the kinase domain is the target of phosphorylation (By similarity). The PAS (PER-ARNT-SIM) domains are required for the binding of FMN chromophores. Autophosphorylated in response to blue light irradiation. 2 molecules of FMN bind covalently to cysteines after exposure to blue light and are reversed in the dark. Undergoes a photocycle characterized by fluorescence and absorption changes induced by blue light. Half-time of photoproduct formation is 11 seconds and 15 seconds for dark regeneration. May be due to a competing acceptor splice site. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family.