Plant pathogens convergently evolved to counteract redundant nodes of an NLR immune receptor network

Abstract

In plants, nucleotide-binding domain and leucine-rich repeat (NLR)-containing proteins can form receptor networks to confer hypersensitive cell death and innate immunity. One class of NLRs, known as NLR required for cell death (NRCs), are central nodes in a complex network that protects against multiple pathogens and comprises up to half of the NLRome of solanaceous plants. Given the prevalence of this NLR network, we hypothesised that pathogens convergently evolved to secrete effectors that target NRC activities. To test this, we screened a library of 165 bacterial, oomycete, nematode, and aphid effectors for their capacity to suppress the cell death response triggered by the NRC-dependent disease resistance proteins Prf and Rpi-blb2. Among 5 of the identified suppressors, 1 cyst nematode protein and 1 oomycete protein suppress the activity of autoimmune mutants of NRC2 and NRC3, but not NRC4, indicating that they specifically counteract a subset of NRC proteins independently of their sensor NLR partners. Whereas the cyst nematode effector SPRYSEC15 binds the nucleotide-binding domain of NRC2 and NRC3, the oomycete effector AVRcap1b suppresses the response of these NRCs via the membrane trafficking-associated protein NbTOL9a (Target of Myb 1-like protein 9a). We conclude that plant pathogens have evolved to counteract central nodes of the NRC immune receptor network through different mechanisms. Coevolution with pathogen effectors may have driven NRC diversification into functionally redundant nodes in a massively expanded NLR network.

An atypical NLR protein modulates the NRC immune receptor network

“…To challenge this hypothesis, we performed site-directed mutagenesis of the MHD motif predicted in the NB-ARC domain of NRCX. Previously, we showed that histidine (H) to arginine (R) or aspartic acid (D) to valine (V) substitutions within the MHD motif confer autoactivity to the NRC helpers NRC2, NRC3 and NRC4 (Derevnina et al, 2021). Therefore, we first introduced the HR and DV mutations in NRCX (Fig.…”

“…S6). It should be pointed out that although NRC2, NRC3 and NRC4 redundantly contribute to effector-activated Sw-5b hypersensitive cell death (Wu et al, 2017), we recently showed that an autoactive mutant of Sw-5b (Sw-5b D857V ) signals only through NRC2 and NRC3 (Derevnina et al, 2021). Therefore, we conclude that knocking-down of NRCX enhances the activities of NRC2-and NRC3-dependent NRC-S, but doesn't affect the activity of NRC-S that are only dependent on NRC4, as well as other NLR outside the NRC network.…”

“…Next, we investigated the extent to which NRCX affects the activities of autoactive mutants of the NRC-H, NRC2 H480R (NRC2 HR ), NRC3 D480V (NRC3 DV ) and NRC4 D478V (NRC4 DV ), which cause cell death even in the absence of pathogen effectors (Derevnina et al, 2021). We coexpressed NRC2 HR , NRC3 DV and NRC4 DV with RNAi:NRCX or RNAi:GUS by agroinfiltration in N. benthamiana leaves.…”

Parasite effectors target helper NLRs in plants to suppress immunity-related cell death

“…In this issue of PLOS Biology, the Kamoun group [7] provide some of the first evidence of parasitic effectors interfering with helper NLR-mediated cell death (Fig 1). Using an effectoromics screen with a library consisting of a total of 165 effectors from bacteria, oomycete, nematode, and aphid, they have identified 5 effectors that can suppress the cell death induced by 2 NRC-dependent sensor NLRs, Prf or Rpi-blb2, in Nicotiana benthamiana plants.…”

“…Using a combination of yeast two-hybrid (Y2H), in vitro and in planta assays, the Kamoun group [7] revealed that SS15 and AVRcap1b effectors may suppress NRC2 and NRC3 functions through distinct mechanisms. The SS15 effector directly associates with NRC2 and NRC3 through the NB-ARC domain.…”