Brassinosteroids (BRs) are essential steroid hormones that have crucial roles in plant growth and development.BRs are perceived by the cell-surface receptor-like kinase brassinosteroid insensitive 1 (BRI1).In the absence of BRs,the cytosolic kinase domain (KD) of BRI1 is inhibited by its auto-inhibitory carboxyl terminus,as well as by interacting with an inhibitor protein,BRI1 kinase inhibitor 1 (BKI1).