Plant “helper” immune receptors are Ca ²⁺ -permeable nonselective cation channels

Abstract

Plant nucleotide-binding leucine-rich repeat receptors (NLRs) regulate immunity and cell death. In Arabidopsis, a subfamily of “helper” NLRs are required by many “sensor” NLRs. Active NRG1.1 oligomerized, was enriched in plasma membrane puncta and conferred cytoplasmic Ca2+ influx in plant and human cells. NRG1.1-dependent Ca2+ influx and cell death were sensitive to Ca2+ channel blockers and were suppressed by mutations impacting oligomerization or plasma membrane enrichment. Ca2+ influx and cell death mediated by NRG1.1 and ACTIVATED DISEASE RESISTANCE 1 (ADR1), another “helper” NLR, required conserved negatively charged N-terminal residues. Whole-cell voltage-clamp recordings demonstrate that Arabidopsis “helper” NLRs form Ca2+-permeable cation channels to directly regulate cytoplasmic Ca2+ levels and consequent cell death. Thus, “helper” NLRs transduce cell death signals directly.

How activated NLRs induce anti-microbial defenses in plants

“…The different NLR classes are therefore also termed as TNLs (with a TIR domain), CNLs (with a CC domain) and RNLs (harboring a CC R domain). The CC and CC R domains share structural homology with the N-terminal 4-helical bundle of mammalian and plant mixed-linage kinase like (MLKL) proteins and poreforming fungal HET-s/Helo domain containing proteins [24][25][26][27][28][29]. TIR domains have been shown to be structurally and functionally conservedat least to some extentand can be found in prokaryotic and other eukaryotic organisms [30,31], where TIR domain-containing proteins have also been implicated in immunity and initiation of cell death responses [32][33][34].…”

“…( 3 ) How the EDS1/PAD4 or EDS1/SAG101 RNL components are activated by TIR domain NADase activity is not clear. Recently, it was shown that the Arabidopsis NRG1.1/NRG1A and ADR1 (both RNLs) can form activation-dependent oligomers and function also as calcium permeable cation channels at the plasma membrane [ 27 , 64 ]. Note: There is no experimental evidence yet that RNL resistosomes form ring-like structures (here a hexamere), which are similar to the pentameric ZAR1 ring-like resistosome.…”

“…Cryo-EM structures of effector-bound ROQ1 and RPP1A defined a tetramerization into a symmetric dimer of two dimers that was essential for cell death activity and immune signaling [ 38 , 43 ] ( Figure 1 ). Complex formation, self-association and also heterooligomerization was shown to be important for the activity of many other NLRs, including CNLs, TNLs and the RNLs [ 27 , 28 , 48 , 65–69 ]. Interestingly, many plant NLRs already self-associate in the absence of effector-recognition — thus, in the pre-activation state (El Kasmi lab unpublished results and [ 48 , 64 , 65 , 68 , 70 ]).…”

“…Plant NLR activation by effectors or by mutations affecting nucleotide binding leads to oligomerization and the formation of large NLR complexes, termed resistosomes [ 27 , 36 , 38 , 43 , 61 ]. Regardless of the type of NLR (TNL, CNL or CC R -NLR) oligomerization is most likely required to bring the N-terminal domains into close proximity so that these can facilitate their signaling function or the recruitment of downstream components.…”

Parasite effectors target helper NLRs in plants to suppress immunity-related 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.…”