Brassinosteroid-independent function of BRI1/CLV1 chimeric receptors

CLAVATA1 (CLV1) and BRASSINOSTEROID INSENSITIVE 1 (BRI1) belong to the leucine-rich repeat receptor-like kinase (LRR-RLK) family, comprising more than 200 members in Arabidopsis thaliana (L.) Heynh. and playing important roles in development and defence responses in many plant species (Diévart and Clark 2003, 2004; Shiu and Bleecker 2001a, b). To dissect the mechanisms of receptor function, we assessed the ability of chimeric proteins containing regions from two different receptors to function in vivo. Using domains from the receptor-kinases CLAVATA1 and BRASSINOSTEROID INSENSITIVE1, we tested the ability of the resulting chimeric receptors to replace CLV1 function. Receptors with the BRI1 extracellular domain and CLV1 kinase domain were able to partially replace CLV1 function. Both loss-of-function and gain-of-function mutations within the BRI1 leucine-rich repeats (LRRs) altered the extent of rescue. Chimeric receptor function was unaffected by addition of either exogenous brassinosteroids (BR) or BR biosynthesis inhibitors, suggesting that the chimeric receptors function in a ligand-independent fashion. We propose that the BRI1 LRR domain drives chimeric receptor homodimerisation, and that the BRI1 LRR domain mutations influence homodimerisation efficiency independent of ligand binding.