Receptor with a serine/threonine-protein kinase activity. Regulates, in response to brassinosteroid binding, a signaling cascade involved in plant development, including expression of light- and stress-regulated genes, promotion of cell elongation, normal leaf and chloroplast senescence, and flowering. May be involved in a feedback regulation of brassinosteroid biosynthesis. May be also involved in the perception of systemin, a peptide hormone responsible for the systemic activation of defense genes in leaves of wounded plants (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] A 68 amino acid island between the 20th and the 21th LRR is essential for the binding of brassinosteroids. BRI1 is almost identical to SR160, a systemin receptor identified in Lycopersicon peruvianum. Competition experiments indicate that brassinosteroid and systemin are probably perceived by different regions of the receptor. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.