Phytosulfokine receptor with a serine/threonine-protein kinase activity. Regulates, in response to phytosulfokine binding, a signaling cascade involved in plant cell differentiation, organogenesis and somatic embryogenesis. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Expressed ubiquitously in leaf, apical meristem, hypocotyl and root. N-glycosylated. The 36 amino-acid island present in the 18th leucine-rich repeat contains a ligand binding pocket that directly interacts with PSK. An island domain has also been found among the extracellular LRRs of the brassinosteroid receptor BRI1 and has been shown to be critical for its function. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.