SummaryThe first layer of plant immunity is activated by cell surface receptor-like kinases (RLKs) and proteins (RLPs) that detect infectious pathogens. Constitutive interaction with the SUPPRESSOR OF BIR1 (SOBIR1) RLK contributes to RLP stability and kinase activity. As RLK activation requires transphosphorylation with a second associated RLK, it remains elusive how RLPs initiate downstream signaling.We employed live-cell imaging, gene silencing and coimmunoprecipitation to investigate the requirement of associated kinases for functioning and ligand-induced subcellular trafficking of Cf RLPs that mediate immunity of tomato against Cladosporium fulvum.Our research shows that after elicitation with matching effector ligands Avr4 and Avr9, BRI1-ASSOCIATED KINASE 1/SOMATIC EMBRYOGENESIS RECEPTOR KINASE 3 (BAK1/ SERK3) associates with Cf-4 and Cf-9. BAK1/SERK3 is required for the effector-triggered hypersensitive response and resistance of tomato against C. fulvum. Furthermore, Cf-4 interacts with SOBIR1 at the plasma membrane and is recruited to late endosomes upon Avr4 trigger, also depending on BAK1/SERK3.These observations indicate that RLP-mediated resistance and endocytosis require ligandinduced recruitment of BAK1/SERK3, reminiscent of BAK1/SERK3 interaction and subcellular fate of the FLAGELLIN SENSING 2 (FLS2) RLK. This reveals that diverse classes of cell surface immune receptors share common requirements for initiation of resistance and endocytosis.
A significant challenge for plants is to induce localized defense responses at sites of pathogen attack. Therefore, host subcellular trafficking processes enable accumulation and exchange of defense compounds, which contributes to the plant on-site defenses in response to pathogen perception. This review summarizes our current understanding of the transport processes that facilitate immunity, the significance of which is highlighted by pathogens reprogramming membrane trafficking through host cell translocated effectors. Prominent immune-related cargos of plant trafficking pathways are the pattern recognition receptors (PRRs), which must be present at the plasma membrane to sense microbes in the apoplast. We focus on the dynamic localization of the FLS2 receptor and discuss the pathways that regulate receptor transport within the cell and their link to FLS2-mediated immunity. One emerging theme is that ligand-induced late endocytic trafficking is conserved across different PRR protein families as well as across different plant species.
Summary Leucine‐rich repeat‐receptor‐like proteins (LRR‐RLPs) and LRR‐receptor‐like kinases (LRR‐RLKs) trigger immune signalling to promote plant resistance against pathogens. LRR‐RLPs lack an intracellular kinase domain, and several of these receptors have been shown to constitutively interact with the LRR‐RLK Suppressor of BIR1‐1/EVERSHED (SOBIR1/EVR) to form signalling‐competent receptor complexes. Ligand perception by LRR‐RLPs initiates recruitment of the co‐receptor BRI1‐Associated Kinase 1/Somatic Embryogenesis Receptor Kinase 3 (BAK1/SERK3) to the LRR‐RLP/SOBIR1 complex, thereby activating LRR‐RLP‐mediated immunity. We employed phosphorylation analysis of in planta ‐produced proteins, live cell imaging, gene silencing and co‐immunoprecipitation to investigate the roles of SOBIR1 and BAK1 in immune signalling. We show that Arabidopsis thaliana ( At ) SOBIR1, which constitutively activates immune responses when overexpressed in planta , is highly phosphorylated . Moreover, in addition to the kinase activity of SOBIR1 itself, kinase‐active BAK1 is essential for At SOBIR1‐induced constitutive immunity and for the phosphorylation of At SOBIR1. Furthermore, the defence response triggered by the tomato LRR‐RLP Cf‐4 on perception of Avr4 from the extracellular pathogenic fungus Cladosporium fulvum is dependent on kinase‐active BAK1. We argue that, in addition to the trans‐autophosphorylation of SOBIR1, it is likely that SOBIR1 and BAK1 transphosphorylate, and thereby activate the receptor complex. The signalling‐competent cell surface receptor complex subsequently activates downstream cytoplasmic signalling partners to initiate RLP‐mediated immunity.
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