The immune receptor SNC1 monitors helper NLRs targeted by a bacterial effector

Plants deploy intracellular receptors to counteract pathogen effectors that suppress cell-surface receptor-mediated immunity. To what extent pathogens manipulate also immunity mediated by intracellular receptors, and how plants tackle such manipulation, remains unknown. Arabidopsis thaliana encodes three very similar ADR1 class helper NLRs (ADR1, ADR1-L1 and ADR1-L2), which play key roles in plant immunity initiated by intracellular receptors. Here, we report that Pseudomonas syringae AvrPtoB, an effector with E3 ligase activity, can suppress ADR1-L1- and ADR1-L2-mediated cell death. ADR1, however, evades such suppression by diversification of two ubiquitination sites targeted by AvrPtoB. The intracellular sensor NLR SNC1 interacts with and guards the CCR domains of ADR1-L1 and ADR-L2. Removal of ADR1-L1 and ADR1-L2 or delivery of AvrPtoB activates SNC1, which then signals through ADR1 to trigger immunity. Our work not only uncovers the long sought-after physiological function of SNC1 in pathogen defense, but also that reveals how plants can use dual strategies, sequence diversification and a multiple layered guard-guardee system, to counteract pathogen attack on core immunity functions.