Autoimmunity and hyperactivation of immune responses by HAE-FLS2 chimera

Plants employ cell surface receptors to perceive extracellular signals and initiate appropriate cellular responses, thereby regulating diverse physiological processes. The well-characterized leucine-rich repeat receptor-like kinases (LRR-RLK) type receptor FLAGELLIN-SENSITIVE 2 (FLS2) functions as a pattern recognition receptor (PRR) that specifically detects bacterial flagellin, activating downstream responses including MAPK signaling and ROS burst. Here, we report that engineered expression of a chimeric receptor combining the N-terminal extracellular and transmembrane domains of HAE with the cytosolic kinase domain of FLS2 (HAE-FLS2) leads to excessive activation of plant immune responses, as evidenced by dwarfism, enhanced flg22-induced ROS burst and MAPK activation, and upregulated expression of defense-related genes in transgenic plants. The chimeric transgenic plants exhibit enhanced disease resistance to bacterial and fungal pathogens. Notably, while the introduction of a kinase-dead mutation (D997N) in the FLS2 domain partially attenuated these immune responses, the persistence of the autoimmune phenotype suggests the existence of both D997-associated kinase activity-dependent and -independent signaling mechanisms. Genetic analysis revealed that HAE-FLS2-mediated immunity predominantly depends on BAK1/BKK1, as evidenced by substantial suppression of the autoimmune phenotype in the bak1-5 bkk1 mutant background. These findings establish the fundamental basis for elucidating FLS2 activation mechanisms and provide a conceptual framework for engineering plant disease resistance toward different types of pathogens through strategic manipulation of RLK.