LAUSR

Plant cells recognize microbial patterns with the plasma-membrane-localized pattern-recognition receptors (PRRs) consisting mainly of receptor kinases (RKs) and receptor-like proteins (RLPs). RKs, such as bacterial flagellin receptor FLS2, and their downstream signaling components have been studied extensively. However, newly discovered regulatory components of RLP-mediated immune signaling, such as the NLP20 receptor RLP23, await identification. Unlike RKs, RLPs lack a cytoplasmic kinase domain, instead recruiting the receptor-like kinases (RLKs) BAK1 and SOBIR1. SOBIR1 specifically works as an adaptor for RLP-mediated immunity. To identify new regulators of RLP-mediated signaling, we looked for SOBIR1-binding proteins (SBPs) in Arabidopsis thaliana using protein immunoprecipitation and mass spectrometry, identifying two G-type lectin RLKs, SBP1 and SBP2, that physically interact with SOBIR1. SBP1 and SBP2 show high sequence similarity, are tandemly repeated on chromosome 4, and also interact with both RLP23 and BAK1. sbp1 sbp2 double mutants obtained via CRISPR-Cas9 gene editing showed severely impaired nlp20-induced ROS burst, MAPK activation, and defense gene expression, but normal flg22-induced immune responses. We showed that SBP1 regulates nlp20-induced immunity in a kinase activity-independent manner. Furthermore, nlp20-induced RLP23-BAK1 interaction, though not flg22-induced FLS2-BAK1 interaction, was significantly reduced in sbp1 sbp2. This study identifies SBPs as new regulatory components in RLP23 receptor complexes that may specifically modulate RLP23-mediated immunity by positively regulating interaction between RLP23 receptor and BAK1 co-receptor. This article is protected by copyright. All rights reserved.