A pair of NLRs coordinately modulates NRC3-mediated ETI responses to facilitate age-dependent immunity

Two NLRs (Nucleotide-binding and Leucine-rich repeat Receptors) adjacent to each other on a locus, termed as paired NLRs, may act separately for effector recognition and subsequent signaling activation to mediate effector-triggered immunity (ETI) in many plants. However, it is largely unknown about their distribution and functions in Solanaceae species, in which NLR-Hs (Helpers NLR REQUIRED FOR CELL DEATHs) have been extensively studied. Here, we identified paired NLRs in Solanaceae species and found they harbor paired NLRs ranging from 6 to 100, which are significantly negatively correlated with the numbers of NLR-Hs. N. benthamiana has six paired NLRs, among which silencing of NRCX exhibits phenotypes of dwarfism and accelerated senescence. Importantly, NRCX-silencing phenotypes could be restored by simultaneously silencing its head-to-head NLR pair, thus we named it as NRCY. NRCX/Y pair is specific in Solanaceae species. NRCY contains non-canonical walker B and MHD motifs, but could not induce autoactive cell death in N. benthamiana. Instead of that, silencing NRCY impaired cell death triggered by Sw5b-Nsm and NRC3D480V, indicating NRCY is also an NLR modulator like NRCX. Furthermore, NRCX suppression of Sw5b-Nsm and NRC3-mediated cell death is dependent on NRCY. Remarkably, we found that NRCX and NRCY expressions were induced during plant senescence, while NRCY was induced more than NRCX. Accordingly, the plant resistance was stronger during maturation, indicated NRCX/Y might be involved in age-dependent resistance. Our study reveals one of the paired NLRs coordinately regulates ETI to facilitate age-dependent immunity.