Probable serine/threonine kinase that acts as positive regulator of brassinosteroid (BR) signaling downstream of the receptor kinase BRI1 (PubMed:18653891). Involved in abiotic stress tolerance. Required for salt stress and abscisic acid-mediated drought stress tolerance (PubMed:22982312). 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, BSK1, BSK6 and BSK8. Expressed in roots, shoots, leaves, stems, flowers and siliques. Induced by salt stress, drought stress, cold stress, brassinosteroid and abscisic acid (ABA). Phosphorylated by BRI1, ASK7/BIN2 and ASK9/BIL2. No visible phenotype under normal growth conditions, but mutant seedlings exhibit enhanced sensitivity to salt stress and abscisic acid (ABA). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.