The plant immune system is activated by microbial patterns that are detected as nonself molecules. Such patterns are recognized by immune receptors that are cytoplasmic or localized at the plasma membrane. Cell surface receptors are represented by receptor-like kinases (RLKs) that frequently contain extracellular leucine-rich repeats and an intracellular kinase domain for activation of downstream signaling, as well as receptor-like proteins (RLPs) that lack this signaling domain. It is therefore hypothesized that RLKs are required for RLPs to activate downstream signaling. The RLPs Cf-4 and Ve1 of tomato ( Solanum lycopersicum ) mediate resistance to the fungal pathogens Cladosporium fulvum and Verticillium dahliae , respectively. Despite their importance, the mechanism by which these immune receptors mediate downstream signaling upon recognition of their matching ligand, Avr4 and Ave1, remained enigmatic. Here we show that the tomato ortholog of the Arabidopsis thaliana RLK Suppressor Of BIR1-1/Evershed (SOBIR1/EVR) and its close homolog S. lycopersicum ( Sl )SOBIR1-like interact in planta with both Cf-4 and Ve1 and are required for the Cf-4– and Ve1-mediated hypersensitive response and immunity. Tomato SOBIR1/EVR interacts with most of the tested RLPs, but not with the RLKs FLS2, SERK1, SERK3a, BAK1, and CLV1. SOBIR1/EVR is required for stability of the Cf-4 and Ve1 receptors, supporting our observation that these RLPs are present in a complex with SOBIR1/EVR in planta . We show that SOBIR1/EVR is essential for RLP-mediated immunity and propose that the protein functions as a regulatory RLK of this type of cell-surface receptors.

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Plants lack the seemingly unlimited receptor diversity of a somatic adaptive immune system as found in vertebrates and rely on only a relatively small set of innate immune receptors to resist a myriad of pathogens. Here, we show that disease-resistant tomato plants use an efficient mechanism to leverage the limited nonself recognition capacity of their innate immune system. We found that the extracellular plant immune receptor protein Cf-2 of the red currant tomato (Solanum pimpinellifolium) has acquired dual resistance specificity by sensing perturbations in a common virulence target of two independently evolved effectors of a fungus and a nematode. The Cf-2 protein, originally identified as a monospecific immune receptor for the leaf mold fungus Cladosporium fulvum, also mediates disease resistance to the root parasitic nematode Globodera rostochiensis pathotype Ro1-Mierenbos. The Cf-2-mediated dual resistance is triggered by effector-induced perturbations of the apoplastic Rcr3 pim protein of S. pimpinellifolium. Binding of the venom allergen-like effector protein Gr-VAP1 of G. rostochiensis to Rcr3 pim perturbs the active site of this papain-like cysteine protease. In the absence of the Cf-2 receptor, Rcr3 pim increases the susceptibility of tomato plants to G. rostochiensis, thus showing its role as a virulence target of these nematodes. Furthermore, both nematode infection and transient expression of Gr-VAP1 in tomato plants harboring Cf-2 and Rcr3 pim trigger a defense-related programmed cell death in plant cells. Our data demonstrate that monitoring host proteins targeted by multiple pathogens broadens the spectrum of disease resistances mediated by single plant immune receptors.parasitism | secretions | SCP/TAPS proteins | hypersensitive response

Abstract. We identified a Saccharomyces cerevisiae peroxisomal membrane protein, Pexl3p, that is essential for protein import. A point mutation in the COOHterminal Src homology 3 (SH3) domain of Pexl3p inactivated the protein but did not affect its membrane targeting. A two-hybrid screen with the SH3 domain of Pexl3p identified Pex5p, a receptor for proteins with a type I peroxisomal targeting signal (PTS1), as its ligand.Pexl3p SH3 interacted specifically with Pex5p in vitro. We determined, furthermore, that Pex5p was mainly present in the cytosol and only a small fraction was associated with peroxisomes. We therefore propose that Pex13p is a component of the peroxisomal protein import machinery onto which the mobile Pex5p receptor docks for the delivery of the selected PTS1 protein.