Pattern-recognition receptors are required for NLR-mediated plant immunity

Abstract

The plant immune system is fundamental to plant survival in natural ecosystems and productivity in crop fields. Substantial evidence supports the prevailing notion that plants possess a two-tiered innate immune system, called pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). PTI is triggered by microbial patterns via cell surface-localized pattern-recognition receptors (PRRs), whereas ETI is activated by pathogen effector proteins via mostly intracellularly-localized receptors called nucleotide-binding, leucine-rich repeat proteins (NLRs)1-4. PTI and ETI are initiated by dist 30 inct activation mechanisms and are considered to act independently and have evolved sequentially5,6. Here we show that, contrary to the perception of PTI and ETI being separate immune signaling pathways, Arabidopsis PRR/co-receptor mutants, fls2/efr/cerk1 and bak1/bkk1/cerk1 triple mutants, are greatly impaired in ETI responses when challenged with incompatible Pseudomonas syrinage bacteria. We further show that the NADPH oxidase (RBOHD)-mediated production of reactive oxygen species (ROS) is a critical early signaling event connecting PRR and NLR cascades and that PRR-mediated phosphorylation of RBOHD is necessary for full activation of RBOHD during ETI. Furthermore, NLR signaling rapidly augments the transcript and protein levels of key PTI components at an early stage and in a salicylic acid-independent manner. Our study supports an alternative model in which PTI is in fact an indispensable component of ETI during bacterial infection, implying that ETI halts pathogen infection, in part, by directly co-opting the anti-pathogen mechanisms proposed for PTI. This alternative model conceptually unites two major immune signaling pathways in the plant kingdom and mechanistically explains the long-observed similarities in downstream defense outputs between PTI and ETI.