A helper NLR channels organellar calcium to trigger plant immunity.

Upon activation, plant nucleotide-binding leucine-rich repeat (NLR) immune receptors are known to assemble into oligomeric resistosomes that insert into the plasma membrane, forming calcium (Ca2+)–permeable channels and triggering immunity. Here, we found that the RPW8-like coiled-coil NLR (CCR-NLR) N requirement gene 1 (NRG1) primarily targets organelles instead of the plasma membrane. Unlike canonical CC-NLRs, activated NRG1 accumulated at the chloroplast envelope and channeled stromal Ca2+ into the cytosol. AlphaFold modeling of the NRG1 resistosome revealed an unusually long amino-terminal membrane-insertion structure that could span the double membrane of the chloroplast. Nanobody-mediated relocalization showed functional membrane specificity: Chloroplast trapping abolished activity of the canonical helper CC-NLR NRC4 but not NRG1. NRG1 orthologs, from nonflowering lineages to angiosperms, targeted chloroplasts, suggesting that organelle-centered defense dates back at least ~360 million years. We propose that CC-NLR diversification has enabled compartment-specific immune signaling to capture diverse Ca2+ stores. Editor's summary: Plant nucleotide-binding leucine-rich repeat (NLR) immune receptors form resistosome complexes thought to function primarily at the plasma membrane. Ibrahim et al. found that the helper NLR NRG1 instead targets the chloroplast envelope, channeling stromal calcium into the cytosol. AlphaFold modeling indicated that NRG1's N-terminal coiled-coil domain is longer than that of canonical coiled coil-type NLRs, enough to span the double chloroplast membrane. All tested orthologs of NRG1 across approximately 360 million years of plant evolution also targeted chloroplasts, suggesting that this compartment-specific immune mechanism is ancient. —Unnati Sonawala and Madeleine Seale [ABSTRACT FROM AUTHOR]