Homeostasis of brassinosteroids (BRs) maintained by the balance between their biosynthesis and inactivation is important to coordinate the diverse physiological and developmental responses of plants.Although BR signaling regulates the endogenous levels of BRs via negative feedback regulation,it remains largely unknown how the biosynthesis and inactivation of BR are triggered.Arabidopsis BAS1 (CYP734A1) inactivates the biologically active BRs via C-26 hydroxylation.