MOLECULAR IDENTIFICATION AND FUNCTIONAL ANALYSIS OF BrbRI1 AS BRASSINOSTEROID RECEPTOR GENE IN BRASSICA RAPA

Abstract Brassinosteroids (BRs) are crucial phytohormones that have diverse functions in regulating plant growth and development. Brassinosteroid-insensitive1 (BRI1), an important plasma membrane-located leucine-rich repeat receptor kinase (LRR-RK), perceives BRs to initiate BR signal transduction, and is involved in altering plant architecture and increasing crop production. Here, three BRI1 proteins (BrBRI1-1, BrBRI1-2, and BrBRI1-3) from Brassica rapa exhibiting striking sequence similarity to Arabidopsis BRI1 (AtBRI1) were identified and characterized. Analysis of full-length sequences of B. rapa BRI1 (BrBRI1) proteins indicated that they contain an extracellular LRR domain, transmembrane domain, a juxtamembrane domain, and a cytoplasmic kinase domain similar to AtBRI1. The ectopic expression of these genes in the Arabidopsis bri1-5-weak mutant resulted in the full recovery of the bri1 mutant to the wild-type. Furthermore, typical BR responsiveness including hypocotyl elongation, root growth, CPD expression, and BES1 dephosphorylation to exogenous BR treatment demonstrated a restored BR signaling pathway in transgenic lines. These data confirmed that BrBRI1 proteins function as BR receptors to mediate BR signaling. Our results provide a foundation for B. rapa molecular genetic breeding by regulating the BR signal transduction pathway through BrBRI1 proteins.