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. Required for blue light mediated mRNA destabilization. Mediates calcium spiking of extracellular origin in response to a low rate of blue light. Also mediates rapid membrane depolarization and growth inhibition in response to blue light. Necessary for root phototropism. Involved in hypocotyl phototropism under a low rate but not under a high rate of blue light. Contributes to the chloroplast accumulation but seems not to be required for chloroplast translocation. Regulates stomata opening and photomorphogenesis response of leaf tissue. Confers sensitivity to drought. 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; disulfide-linked. Interacts with PKS1, PKS2, RPT2, RPT3, PHOT2 and BLUS1 (PubMed:10542152, PubMed:15031408, PubMed:16777956, PubMed:18585389, PubMed:20071603, PubMed:23811955). Subunit of a complex made of CAR6, PHOT1 and RPT3/NPH3 (PubMed:21367967). 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. Enhanced drought tolerance, when associated with PHOT2 disruption. Undergoes a photocycle characterized by fluorescence and absorption changes induced by blue light. Half-time of photoproduct formation is 14 seconds and 70 seconds for dark regeneration. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family.