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Transmembrane receptor kinases (RKs) mediate signal transduction pathways leading to cell proliferation, growth, and diVerentiation in animals. The crucial function of RKs is recognition of an extracellular ligand, which leads to activation of the intracellular kinase domain and subsequent transduction of downstream signaling pathways. The completion of Arabidopsis genome sequencing revealed a surprisingly high number of genes (at least 610 members) encoding putative receptor kinases, which strongly suggests that the plant cells predominantly use RKs for sensing external signals and regulating gene expression. Plant RKs comprise a monophyletic group related to animal RKs. Almost all plant RKs phosphorylate serine/threonine residues, unlike animal RKs, are predominantly ligand-activated tyrosine kinases (Shiu and Bleecker, 2001a, b). The plant RKs are often referred to as ‘‘receptor-like kinases (RLKs),’’because their corresponding ligands have yet to be identified with the exception of a few.

Plant RKs are classified into several groups based on the structure of the extracellular domains. RKs containing an extracellular leucine-rich repeat (LRR) motif comprise by far the largest subfamily of plant RKs, with $222 members in the Arabidopsis genome (Fig. 1)(Shiu and Bleecker, 2001b; Yin et al., 2002b). Consistently, LRR-RKs are some of the most extensively studied and well-understood signaling molecules in plants. For instance, growing numbers of LRR-RKs whose loss-of-function mutations confer phenotypes convincingly show that they play fundamental roles in development, steroidhormone response, stress response, disease …