BAK1, an Arabidopsis LRR Receptor-like Protein Kinase, Interacts with BRI1 and Modulates Brassinosteroid Signaling

Brassinosteroids regulate plant growth and development through a protein complex that includes the leucine-rich repeat receptor-like protein kinase (LRR-RLK) brassinosteroid-insensitive 1 (BRI1). Activation tagging was used to identify a dominant genetic suppressor of bri1, bak1-1D (bri1-associated receptor kinase 1-1Dominant), which encodes an LRR-RLK, distinct from BRI1. Overexpression of BAK1 results in elongated organ phenotypes, while a null allele of BAK1 displays a semidwarfed phenotype and has reduced sensitivity to brassinosteroids (BRs). BAK1 is a serine/threonine protein kinase, and BRI1 and BAK1 interact in vitro and in vivo. Expression of a dominant-negative mutant allele of BAK1 causes a severe dwarf phenotype, resembling the phenotype of null bri1 alleles. These results indicate BAK1 is a component of BR signaling.