Surface-localized pattern-recognition receptors (PRRs) detect microbes by recognizing conserved pathogen-associated molecular patterns (PAMPs). The Arabidopsis leucine-rich repeat receptor kinases (LRR-RKs) EFR and FLS2 recognize the bacterial PAMPs EF-Tu and flagellin, respectively. Although the overall importance of PAMP-triggered immunity (PTI) for plant defense is established, the mechanisms linking PRR activation at the plasma membrane to intracellular signaling remain largely unknown. Our long-term objective is to understand how plants perceive and signal in response to microbes to mount efficient disease resistance. The overall aim of this proposal is to decipher signaling initiation and specificity during RK-mediated innate immunity in plants. The central hypotheses of the research are that (i) the ligand-binding LRR-RKs FLS2 and EFR form dynamic complexes with multiple regulatory proteins upon PAMP binding, which are essential to signaling initiation, (ii) phosphorylation events between the ligand-binding PRRs and the regulatory LRR-RK BAK1 are key for signaling initiation and specificity, and (iii) new sensitized forward genetic screens can identify novel important immune regulators. The rationale behind this research is based on exciting new data showing that (i) the properties of the non-RD kinases FLS2 and EFR are distinct from the paradigmatic RD kinase BRI1, (ii) the function of BAK1 in immunity, growth and cell death control can be uncoupled mechanistically in a phosphorylation-dependent manner, and (iii) a suppressor screen identified several modifiers of bak1-5 (mob) mutants.To achieve our goal, we will pursue the following specific objectives:1. Decipher the composition, stoichiometry, cellular localization and dynamics of the FLS2/EFR-BAK1 complexes.2. Identify the “phosphocode” underlying signaling initiation and specificity in BAK1 signaling pathways.3. Characterize MOB proteins as novel immune signaling components.