Probable serine/threonine kinase that acts as positive regulator of brassinosteroid (BR) signaling downstream of BRI1. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with BRI1 and BSL1. Plasma membrane localization is required for its function in the regulation of brassinosteroid signaling. Induced by 24-epibrassinolide in roots and leaves. Phosphorylated at Ser-213 and Ser-215 by BRI1. Phosphorylation at Ser-215 is required for its function in the regulation of brassinosteroid signaling. Phosphorylation by BRI1 disrupts the interaction between its TPR and kinase domains, thereby increasing the binding between its kinase domain and BSL1.