Pamp-Triggered Immunity Components And Lysm-Receptor-Like Kinases

Abstract

In order to identify components of Pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) pathways in Nicotiana benthamiana, a largescale forward-genetics screen using virus-induced gene silencing and a celldeath-based assay for assessing PTI was performed. The assay relied on four combinations of PTI-inducing non-pathogens and cell-death-causing challenger pathogens, and was first validated in plants silenced for FLS2 or BAK1. Over 3,200 genes were screened and 14 genes were identified that, when silenced, compromised PTI. A subset of the genes was found to act downstream of FLS2mediated PTI induction and silencing of three genes compromised production of reactive oxygen species (ROS) in leaves exposed to flg22. The 14 genes encode proteins with potential functions in defense and hormone signaling, protein stability and degradation, energy and secondary metabolism and cell wall biosynthesis and provide a new resource to explore the molecular basis for the involvement of these processes in PTI. Peptidoglycan (PGN) has been shown to trigger immune responses in Arabidopsis thaliana. However, in tomato, PGN did not trigger archetypal immune responses such as mitogen-activated protein kinase activation, ROS production or protection from subsequent bacterial infections. This lack of responses suggests that PGN is not involved in activating immunity and stopping bacterial colonization in tomato. In A. thaliana, immunity against bacteria requires LysM-receptor-like kinases (LysM-RLKs). Two tomato LysMRLKs, SlBti9 and SlLyk13, were shown to be required for resistance against both pathogenic and non-pathogenic Pseudomonas syringae pv. tomato. Flagellin-mediated responses were compromised in plants silenced for SlBti9 and SlLyk13, which could explain the increased bacterial susceptibility observed in these plants. RNAi of SlBti9 and SlLyk13 also compromised chitin perception, as was reported previously for A. thaliana LysM-RLK CERK1. Autophosphorylation seems to be required for the activity of these two tomato LysMRLKs in immunity as cell death mediated by over-expression of these proteins in N. benthamiana was abolished in kinase-inactive mutants. SlBti9 and/or SlLyk13 probably function as either pattern recognition receptors for a yet uncharacterized bacterial PAMP or are part of the receptor complex to transduce the signal once the recognition event has occurred.