Receptor-like kinase SOBIR1/EVR interacts with receptor-like proteins in plant immunity against fungal infection

TLDR

Plant immunity is triggered by microbial patterns detected by cell‑surface receptors, including receptor‑like kinases (RLKs) with leucine‑rich repeats and intracellular kinase domains and receptor‑like proteins (RLPs) lacking signaling domains, yet the downstream signaling mechanisms of RLPs such as Cf‑4 and Ve1 remain unclear. The study tests the hypothesis that RLKs are required for RLPs to activate downstream signaling. The tomato RLK SOBIR1/EVR interacts with Cf‑4 and Ve1, is necessary for their stability, hypersensitive response, and immunity, and acts as a regulatory RLK essential for RLP‑mediated defense.

Abstract

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.