Receptor kinase involved brassinosteroid (BR) signal transduction. Regulates, in response to BR binding, a signaling cascade involved in plant development, promotion of cell elongation and flowering (Probable). Activates BR signaling by targeting and phosphorylating BSK3, a positive regulator of BR signaling (PubMed:26697897). Forms at the plasma membrane a receptor complex with BAK1 which is activated in response to brassinolide. Phosphorylates BAK1. Phosphorylates REM4.1, which reduces REM4.1 binding affinity to BAK1 and allows the formation and subsequent activation of the BRI1-BAK1 receptor complex (PubMed:27424498). Functions in various growth and developmental processes, such as internode elongation, bending of the lamina joint and skotomorphogenesis. Functions in internode elongation by inducing the formation of the intercalary meristem and the longitudinal elongation of internode cells (PubMed:11006334). Involved in organ development through the control of cell division and elongation. Does not seem essential for organ pattern formation or organ initiation (PubMed:16407447). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with BIP103 and BIP131 (Ref.7). Interacts with BAK1 (PubMed:19754838, PubMed:27424498). Interacts with BSK3 (PubMed:26697897). Interacts with SERK2 (PubMed:19754838, PubMed:24482436). Highly expressed in shoots. Expressed at low levels in roots. Down-regulated by brassinolide. Contains two pairs of conservatively spaced Cys (Cys pair 1 and 2) possibly involved in forming some heterodimers. Dwarf plants due to the lack of internode elongation. Insensitivity to brassinolide. Over-expression of a truncated form of BRI1 induces partial suppression of endogenous BRI1 expression, leading to an erect leaf phenotype. The estimated grain yield of the transformants is about 30 per cent higher than that of wild type at high density planting. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.