Receptor with a serine/threonine-protein kinase activity. Regulates, in response to brassinosteroid binding, a signaling cascade involved in plant development. Binds brassinolide. May be involved in cell growth and vascular differentiation. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Predominantly expressed in vascular tissues. From 7 day old seedlings, it is expressed in the columella cells of the root tip, in the vascular initials in the meristematic region of the root and in vascular tissues. After germination, it is expressed in the stele cell and in the early differentiation zone of the root, where the expression continues from the root to the hypocotyls and cotyledons following the midvein. In mature plants, it is expressed in the vasculature of the leaf, predominantly in the midvein, and in the vascular bundles of inflorescence stems. Localizes to procambial cells of the vascular bundles located between the differentiating xylem and the phloem. Contains two pairs of conservatively spaced Cys (Cys pair 1 and 2) possibly involved in forming some heterodimers. A 70 amino acid island between the 19th and the 20th LRR is essential for the binding of brassinosteroids. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.