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 receptors sense pathogen effectors and form resistosomes to confer immunity1. Some sensor NLR resistosomes produce small molecules to induce formation of a heterotrimer complex with two lipase-like proteins, EDS1 and SAG101, and a helper NLR called NRG1 (refs. 2,3). Activation of sensor NLR resistosomes also triggers NRG1 oligomerization and resistosome formation at the plasma membrane4,5. We demonstrate that the Arabidopsis AtEDS1–AtSAG101–AtNRG1A heterotrimer formation is stabilized by the AtNRG1A loss-of-oligomerization mutant L134E5,6. We report structures of AtEDS1–AtSAG101–AtNRG1A L134E and AtEDS1–AtSAG101–AtNRG1C heterotrimers with similar assembly mechanisms. AtNRG1A signalling is activated by the interaction with the AtEDS1–AtSAG101 heterodimer in complex with their small-molecule ligand. The truncated AtNRG1C maintains core interacting domains of AtNRG1A but develops further interactions with AtEDS1–AtSAG101 to outcompete AtNRG1A. Moreover, AtNRG1C lacks an N-terminal signalling domain and shows nucleocytoplasmic localization, facilitating its sequestration of AtEDS1–AtSAG101, which is also nucleocytoplasmic. Our study shows the activation and inhibition mechanisms of a plant helper NLR. In plant immunity, sensor NLR resistosomes can generate heterotrimer complexes comprising the helper NLR NRG1, and investigation of cryo-EM structures shows the mechanisms of these heterotrimers in immune activation and suppression.){kind=link}