Small family, big impact: RNL helper NLRs and their importance in plant innate immunity

Plants evolved a sophisticated, receptor-based, innate immune system. Cell surface localized pattern recognition (PRR) and intracellular nucleotide-binding leucine-rich repeat (NLR) receptors detect pathogen-associated molecular patterns or pathogen-derived effector molecules, respectively, and induce a range of common immune responses. These include Ca2+ fluxes, reactive oxygen species production, and mitogen-activated protein kinase activation [1]. Recent studies have demonstrated an interdependency and mutual potentiation of the 2 receptor systems [2,3]. Based on their N-terminal domains and their phylogeny, NLRs are classified in coiled-coil (CC) domain, Toll-like/interleukin-1 receptor resistance (TIR) domain, and RESISTANCE TO POWDERY MILDEW 8-like CC (CCR) domain containing NLRs, referred to as CNLs, TNLs, and RNLs, respectively [4]. In Arabidopsis thaliana (hereafter Arabidopsis), multiple PRRs and effector sensing NLRs (some CNLs and all tested TNLs) require the presence of RNLs, also termed helper NLRs, to activate full immunity [5,6]. RNLs form a small and evolutionary conserved clade comprised of 2 subfamilies, the ACTIVATED DISEASE RESISTANCE 1 (ADR1) and N REQUIREMENT GENE 1 (NRG1) families that have separated before the divergence of angiosperms [4]. The Arabidopsis genome bares 3 ADR1 and 2 NRG1 full-length genes required for full immunity [7–9]. Although RNLs represent only a relatively small part of the NLR gene repertoire in most angiosperms [4,10], they are of outmost importance for plants to fight off invading pathogens. Here, we highlight recent findings of how RNLs function during immunity and discuss mechanisms of RNL activation. © 2023 Saile, El Kasmi. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.