Brassinosteroids (BRs) are essential phytohormones that play crucial roles in plant growth and development.Perception of BRs in Arabidopsis requires the leucine rich repeat receptor-like kinase (LRR-RLKs) brassinosteroid-insensitive 1 (BRI1) and its homologs (BRLs).BR binding induces BRI1 heteromerization with the LRR-RLK BAK1,leading to transphosphorylation of the two RLKs.We recently solved the crystal structures of the extracellular LRR domains of BRI1 and BRL1 in complex with the most active BR,brassinolide (BL).The structures define the molecular mechanisms by which BRI1 recognizes BRs.Furthermore,the structure of BL-BRI1-BAK1 reveals that BAK1 functions as co-receptor with BRI1 for activation of the BR-induced complex.The structure-derived information can be used to design small molecules for manipulating BR signaling to benefit agricultural crops.