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INTRODUCTION Plant receptor—like kinases (RLKs) belong to the monophyletic interleukin—1 receptor—associated kinase (IRAK) or RLK/Pelle family (Shiu et al., 2004). 600 family members make this fam—ily one of the largest in Arabidopsis thaliana (Lehti—Shiu et al., 2009). Likewise, sequencing of the genomes of rice, poplar, soy—bean, or potato has revealed the presence of large RLK families in these plants. RLKs are commonly built of N—terminal ligand binding domains and C—terminal serine/threonine protein kinase domains (Morillo and Tax, 2006). Many RLKs are located in the plasma membrane. In these cases, transmembrane domains flanked by extra—and intra—cellular juxtamembrane domains sep—arate the ligand sensor/protein interaction domains and pro—tein kinase domains. The current mechanistic mode of action of RLK proteins comprises ligand binding—induced conforma—tional switches within the RLK proteins that trigger downstream signaling events subsequently activating a signal—specific cellular program (Chinchilla et al., 2009). As numerous RLKs share con—served structural features within the extracellular domains, they can be grouped into protein subfamilies (Shiu et al., 2004). A. thaliana encodes N235 RLKs with extracytoplasmic leucine—rich repeat (LRR) domains (LRR—RLK), which is the largest RLK sub—family in this plant (Lehti—Shiu et al., 2009). Forward and reverse genetic approaches have revealed various physiological functions of Arabidopsis LRR—RLKs (Morillo and Tax, 2006). Brassinos—teroid insensitive 1 (BRII), the receptor for the plant steroid hormone, brassinolide (BL …