Plasma membrane-localized pattern recognition receptors (PRRs) such as FLAGELLIN SENSING2 (FLS2), EF-TU RECEPTOR (EFR), and CHITIN ELICITOR RECEPTOR KINASE1 (CERK1) recognize microbe-associated molecular patterns (MAMPs) to activate pattern-triggered immunity (PTI). A reverse genetics approach on genes responsive to the priming agent b-aminobutyric acid (BABA) revealed IMPAIRED OOMYCETE SUSCEPTIBILITY1 (IOS1) as a critical PTI player. Arabidopsis thaliana ios1 mutants were hypersusceptible to Pseudomonas syringae bacteria. Accordingly, ios1 mutants showed defective PTI responses, notably delayed upregulation of the PTI marker gene FLG22-INDUCED RECEPTOR-LIKE KINASE1, reduced callose deposition, and mitogen-activated protein kinase activation upon MAMP treatment. Moreover, Arabidopsis lines overexpressing IOS1 were more resistant to bacteria and showed a primed PTI response. In vitro pull-down, bimolecular fluorescence complementation, coimmunoprecipitation, and mass spectrometry analyses supported the existence of complexes between the membrane-localized IOS1 and BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1)-dependent PRRs FLS2 and EFR, as well as with the BAK1-independent PRR CERK1. IOS1 also associated with BAK1 in a ligand-independent manner and positively regulated FLS2-BAK1 complex formation upon MAMP treatment. In addition, IOS1 was critical for chitinmediated PTI. Finally, ios1 mutants were defective in BABA-induced resistance and priming. This work reveals IOS1 as a novel regulatory protein of FLS2-, EFR-, and CERK1-mediated signaling pathways that primes PTI activation.
INTRODUCTIONPlants possess multilayered recognition systems that detect pathogens at various stages of infection and proliferation. Recognition of microbial invasion is essentially based upon the host's ability to distinguish between self and non-self components. Early microbial pathogens detection is performed by cell surfacelocalized pattern recognition receptors (PRRs) that sense pathogen-or microbe-associated molecular patterns (PAMPs or MAMPs) (Monaghan and Zipfel, 2012). Major examples of MAMPs are lipopolysaccharides present in the envelope of Gram-negative bacteria, eubacterial flagellin, eubacterial elongation factor Tu (EF-Tu), peptidoglycans from Gram-positive bacteria, methylated bacterial DNA fragments, and fungal cell wall-derived chitins (Girardin et al., 2002;Cook et al., 2004;Boller and Felix, 2009). MAMP recognition promptly triggers the activation of patterntriggered immunity (PTI) (Tsuda and Katagiri, 2010). Early PTI responses, such as calcium influx, production of reactive oxygen species (ROS), and activation of mitogen-activated protein (MAP) kinases, induce transcriptional reprogramming mediated by plant WRKY transcription factors as well as calmodulin binding proteins (Boller and Felix, 2009;Tena et al., 2011). In addition, Arabidopsis thaliana plants close stomata in a MAMP-dependent manner when in contact with bacteria (Melotto et al., 2006;Singh et al., 2012). Callose deposition and PTI marker gene up...