The multifunctional leucine-rich repeat receptor kinase BAK1 is implicated in Arabidopsis development and immunity Sandra Postel, Isabell K ¨ ufner, Christine Beuter, Sara Mazzotta, Anne Schwedt, Andrea Borlotti, Thierry Halter, Birgit Kemmerling n,1 , Thorsten N ¨ urnberger 1,n Zentrum f¨ ur Molekularbiologie der Pflanzen-Pflanzenbiochemie, Eberhard-Karls-Universit¨ at T¨ ubingen, Auf der Morgenstelle 5, D-72076 T¨ ubingen, Germany article info Keywords: Receptor kinase Plant development Plant immunity Arabidopsis BAK1 PEPR1 Effector protein PAMP abstract Plant receptor-like kinases (RLKs) are transmembrane proteins with putative N-terminal extracellular ligand-binding domains and C-terminal intracellular protein kinase domains. RLKs have been implicated in multiple physiological programs including plant development and immunity to microbial infection. Arabidopsis thaliana gene expression patterns support an important role of this class of proteins in biotic stress adaptation. Here, we provide a comprehensive survey of plant immunity- related RLK gene expression. We further document the role of the Arabidopsis Brassinosteroid Insensitive 1 (BRI1)-associated receptor kinase 1 (BAK1) in seemingly unrelated biological processes, such as plant development and immunity, and propose a role of this protein as an adaptor molecule that is required for proper functionality of numerous RLKs. This view is supported by the identification of an additional RLK, PEPR1, and its closest homolog, PEPR2 as BAK1-interacting RLKs. & 2009 Elsevier GmbH. All rights reserved. Introduction Plant receptor-like kinases (RLKs) belong to the monophyletic interleukin-1 receptor-associated kinase (IRAK) or RLK/Pelle family (Shiu et al., 2004). The RLK/Pelle family is the largest gene family in Arabidopsis comprising more than 600 family members (Lehti-Shiu et al., 2009). In those cases investigated, RLK proteins have been shown to reside in the plant plasma membrane. RLK proteins are composed of extracellular ligand-binding domains that are linked to cytoplasmic serine/threonine protein kinase domains via transmembrane domains (Morillo and Tax, 2006). Ligand binding to the extracellular domain of RLKs or stimulus- induced complex formation of RLKs with other proteins (including other RLKs) is assumed to impose conformational changes on these proteins thereby initiating intracellular signal transduction (Chinchilla et al., 2009). Plant RLKs can be divided into protein subfamilies according to the nature of their extracellular domains (Shiu et al., 2004). The genome of Arabidopsis thaliana encodes 235 RLKs with extracytoplasmic leucine-rich repeat (LRR) domains (LRR-RLK), which constitute the largest subfamily of RLKs in this plant (Lehti-Shiu et al., 2009). LRR-RLKs serve various roles in plant growth and development (Morillo and Tax, 2006). For example, BRI1, the receptor for the plant steroid hormone, brassinolide (BL), is an LRR-RLK that forms heterotetrameric complexes with another LRR-RLK, BAK1 (BRI1-associated receptor kinase 1) in a hormone-dependent manner, thereby controlling developmental processes such as stem elongation, vascular differentiation, seed size, fertility, flowering time, and senescence (Li et al., 2002; Nam and Li, 2002; Wang et al., 2005). Other plant LRR-RLKs have been implicated as microbial pattern (pathogen- associated molecular pattern, PAMP) recognition receptors in basal and cultivar-specific host innate immunity (N¨ urnberger and Kemmerling, 2006). Arabidopsis FLS2 (Flagellin Sensing 2) and EFR (EF-Tu Receptor) recognise bacterial flagellin and elongation factor EF-Tu, thereby mediating activation of plant immune responses (Gomez-Gomez and Boller, 2000; Zipfel et al., 2006). Likewise, Xa21, a rice LRR-RLK mediates the activation of plant cultivar-specific immunity through recognition of the Xanthomo- nas oryzae pv. oryzae-specific avirulence factor, AvrXa21 (Song et al., 1995). The large number of LRR-RLK genes in Arabidopsis and the involvement of some LRR-RLKs in plant immunity suggest that more members of this protein family have roles in plant– pathogen interactions (Kemmerling et al., 2007). A bioinformatics approach towards the identification of LRR- RLKs implicated in plant immunity To elucidate microbial infection or microbial pattern-induced transcript accumulation in infected or infiltrated Arabidopsis Col-0 leaves, we obtained triplicate expression estimates from 30 ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.de/ejcb European Journal of Cell Biology 0171-9335/$ - see front matter & 2009 Elsevier GmbH. All rights reserved. doi:10.1016/j.ejcb.2009.11.001 n Corresponding authors. Tel.: + 49 7071 297 6657; fax: + 49 7071 29 5226. E-mail addresses: nuernberger@uni-tuebingen.de (T. N ¨ urnberger). birgit.kemmerling@zmbp.uni-tuebingen.de (B. Kemmerling). 1 Both authors contributed equally to this work. European Journal of Cell Biology 89 (2010) 169–174