Brassinosteroids (BRs) are essential phytohormones that play crucial roles in plant growth and development. Perception of BRs requires an active complex of brassinosteroid-insensitive 1 (BRI1) and BRI1-associated kinase 1 (BAK1). Recognized by the extracellular leucine-rich repeat (LRR) domain of BRI1, BRs induce a phosphorylation-mediated cascade to regulate gene expression. Here we present the crystal structures of BRI1-LRR in free and brassinolide (BL)-bound forms. BRI1-LRR exists as a monomer in crystals and solution independent of BL. It comprises a helical solenoid structure that accommodates a separate insertion domain at its concave surface. Sandwiched between them, BL binds to a hydrophobicity-dominating surface groove on BRI1-LRR. BL recognition by BRI1-LRR is through an induced-fit mechanism involving stabilization of two inter-domain loops that creates a pronounced non-polar surface groove for the hormone binding. Together, our results define the molecular mechanisms by which BRI1 recognizes BRs and provide insight into BR-induced BRI1 activation.
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