Plasma membrane association and resistosome formation of plant helper immune receptors

Abstract

Intracellular plant immune receptors, termed NLRs, respond to pathogen effectors delivered into plant cells. Activation of NLRs typically confers immunity. Sensor NLRs, involved in effector recognition, are either TIR-NLRs (TNLs) or CC-NLRs (CNLs). Helper NLRs, required for sensor NLR signaling, include CCR-NLRs (RNLs) and a special class of CNLs known as NRCs. Activated TNLs produce small molecules that induce an association between the EDS1/SAG101 heterodimer and the NRG1s helper RNLs. Auto active NRG1s oligomerize and form calcium signaling channels largely localized at the plasma membrane (PM). The molecular mechanisms of helper NLR PM association and effector induced NRG1 oligomerization are not well characterized. We find that both RNLs and NRCs require positively charged residues in the second and fourth helices of their CCR or CC domain for phospholipid binding and PM association before and after activation, despite conformational changes that accompany activation. We demonstrate that effector activation of TNLs induces NRG1 oligomerization at the PM and that the cytoplasmic pool of EDS1/SAG101 is critical for cell death function. EDS1/SAG101 cannot be detected in the oligomerized NRG1 resistosome, suggesting that additional unknown triggers might be required to induce the dissociation of EDS1/SAG101 from the previously described NRG1/EDS1/SAG101 heterotrimer before subsequent NRG1 oligomerization, or that the conformational changes resulting from NRG1 oligomerization abrogate the interface for EDS1/SAG101 association. Our data provide new observations regarding dynamic PM association during helper NLR activation and underpin an updated model for effector induced NRG1 resistosome formation.