Perception of brassinosteroids by the extracellular domain of the receptorkinase BRI1

An assay was developed to study plant receptor kinase activation and signal ing mechanisms. The extracellular leucine-rich repeat (LRR) and transmembra ne domains of the Arabidopsis receptor kinase BRI1, which is implicated in brassinosteroid signaling, were fused to the serine/threonine kinase domain of XA21, the rice disease resistance receptor. The chimeric receptor initi ates plant defense responses in rice cells upon treatment with brassinoster oids. These results, which indicate that the extracellular domain of BRI1 p erceives brassinosteroids, suggest a general signaling mechanism for the LR R receptor kinases of plants. This system should allow the discovery of lig ands for the LRR kinases, the largest group of plant receptor kinases.