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

Parasites target the plant immune system for successful colonization. A new study in PLOS Biology reveals that unrelated parasites have evolved effectors that specifically suppress the function of helper nucleotide-binding leucine-rich repeats (NLRs), explaining the complex plant–parasite coevolutionary dynamics.

Funding: Immunity work in SPDK laboratory is funded by US National Institute of Health grant R01GM132582. NM is funded by USDA-NIFA predoctoral fellowship grant 2021-67034-35187. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests:
The authors have declared that no competing interests exist.
Abbreviations: ADR1, Activated Disease Resistance 1; ETI, effector-triggered immunity; HR, hypersensitive response; NLR, nucleotide-binding leucine-rich repeat; NRC, NB-LRR protein required for HR-associated cell death; NRG1, N Requirement Gene 1; PAMP/MAMP, pathogen/ microbe-associated molecular pattern; PTI, 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. Among these effectors, SS15, from the cyst nematode Globodera rostochiensis, and AVRcap1b, from the potato late blight oomycete pathogen P. infestans, showed strong suppression of the cell death triggered by autoimmune mutants of NRC2 and NRC3, but not NRC4. This demonstrated that these 2 effectors act specifically at the level of the NRC2 and NRC3 helpers or their downstream pathways, independent of their upstream sensor NLR partners. In agreement with these findings, these 2 effectors can suppress sensor NLR, Rx-mediated cell death in response to PVX only if NRC4 is silenced.
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. Furthermore, SS15 can bind both inactive and autoactive forms of NRC2 and NRC3. On the other hand, AVRcap1b does not associate directly with NRCs; instead, it interacts with another host protein, NbTOL9a (Target of Myb 1-like protein 9a). Silencing of NbTOL9a enhances cell death induced by autoactive NRC2 and NRC3, but not NRC4, indicating that NbTOL9a functions as a negative regulator of cell death. These findings suggested that AVRcap1b effector co-opts NbTOL9a from the host to suppress NRC2and NRC4-mediated cell death.
The findings by Derevnina and colleagues [8] provide further insights into the evolutionary arms race between parasites and hosts. Although plants have evolved with complex NLR immune receptor networks to defend against parasites, parasites have evolved effectors that counteract central nodes of these networks. The study also reinforces the importance of using parasite effectors as probes to dissect complex immune signaling networks in plants. Effectoromics approaches involving other effectors from diverse parasites should uncover additional effectors that interfere with NRCs including NRC4 and other helper NLRs.
Additional studies are needed to understand the mechanisms by which SS15 and AVR-cap1b interfere with helper NLRs function. How does SS15 binding to the NB-ARC domain of NRC2 and NRC3 suppress cell death? Since there is no evidence so far for direct interaction between NRCs and corresponding sensor NLR partners, does SS15 binding interfere with downstream signaling steps that are required for cell death execution? Recent studies uncovered that the activated sensor NLR, ZAR1, and the helper-NLRs, NRG1, and ADR1 function as cation channels that promote calcium influx leading to HR cell death [8,9]. Since the N-terminal MADA motif of NRCs, which is sufficient to induce cell death, is functionally conserved in ZAR1 and the larger MADA-CC-NLR family [10], NRCs are more likely to function through similar mechanisms by forming cation channels. It will be interesting to test if the binding of SS15 to the NB-ARC domain interferes with channel formation and/or function, leading to the suppression of cell death. With respect to AVRcap1b effector, considering that both AVRcap1b and its interacting partner, NbTOL9a, do not associate directly with NRCs, how does the AVRcap1b-NbTOL9a complex interfere with NRC-mediated cell death? Answers to these questions should improve our understanding of the role NRCs and other helper NLRs play in ETI.