Unexpected help: role of an N-terminally truncated helper NLR in plant immunity

Plants are constantly at war against pathogens. In order to survive, plants have evolved complex innate immune signaling pathways to detect pathogens and trigger defense responses. One key defense weapon for plants is the intracellular nucleotide-binding leucine-rich repeat (NLR) family of immune receptors, which detect effectors secreted by pathogens and activate a robust response known as effector-triggered immunity (Cui et al., 2015). These plant NLRs can be divided into two classes: sensor NLRs (sNLRs), which directly or indirectly recognize effectors, and helper NLRs (hNLRs), which help the sNLRs by serving as downstream signaling partners (Jubic et al., 2019). While sNLRs have been extensively studied, the mechanisms by which hNLRs relay signals from sNLRs are not fully understood.

Typical plant NLRs consists of an N-terminal domain, a central nucleotide-binding domain, and a C-terminal leucine-rich repeat region. Due to duplication and uneven crossover events, truncated NLRs that lack one or more of these canonical domains may arise in plant genomes. Emerging evidence indicates that these truncated NLRs also play a role in regulating plant immunity. While several C-terminally truncated NLRs have been studied, the functions of N-terminally truncated NLRs remain unknown. In this issue, Zhongshou Wu, Lei Tian, and colleagues (Wu et al., 2021) describe how an Arabidopsis thaliana N-terminally truncated hNLR, NRG1C, plays an unexpected negative role in plant immune activation.

The NRG1-type family in Arabidopsis consists of three hNLR paralogs. NRG1A and NRG1B are full-length hNLRs that play positive roles in regulating plant immunity, while NRG1C is a truncated hNLR whose biological role is unclear. The authors observed that NRG1C is upregulated in the Toll/interleukin-1 receptor (TIR) NLR (TNL) autoimmune mutant chilling sensitive 3, 2D (chs3-2D), and surprisingly, overexpression of NRG1C suppressed both the dwarf phenotype and the enhanced disease resistance of chs3-2D, as well as suppressor of npr1-1, constitutive 1 (snc1) mutant plants (see Figure). This suggests that NRG1C antagonizes chs3-2D and snc1-mediated autoimmunity. Consistently, the NRG1C overexpression lines were more susceptible to Pseudomonas syringae carrying HopQ1-1 than the wild type, at similar levels to the nrg1a nrg1b double mutant and the senescence-associated gene 101 (sag101) mutant. Because the authors also demonstrated that SAG101 works in the same pathway as NRG1A/1B, these data suggest that NRG1C may negatively regulate plant immunity by interacting with the EDS1 (ENHANCED DISEASE SUSCEPTIBILITY1)-SAG101-NRG1 complex, a major NLR immunity signaling node (Sun et al., 2021).

Figure.

NRG1C negatively regulates autoimmunity in the TNL autoimmune mutant chs3-2D. Overexpression of NRG1C suppresses the dwarf phenotype (left panel; Scale bar = 1 cm) and the enhanced disease resistance to the oomycete pathogen Hyaloperonospora arabidopsidis (H.a.) Noco2 (right) in the chs3-2D mutant. Statistical significance is indicated by different letters (P < 0.01). Error bars represent means ± SD (n = 3). Two independent experiments were carried out with similar results. Adapted from Wu et al. (2021), Figure 2.

Wu, Tian, and colleagues pursued this hypothesis by performing biochemical assays to determine if there was indeed a direct interaction between NRG1C and the EDS1-SAG101-NRG1 complex. While NRG1C was not shown to directly interact with NRG1A, a combined TurboID-based proximity labeling and co-immunoprecipitation assay revealed that NRG1C indeed directly interacts with the EDS1-SAG101 complex. Importantly, the function of NRG1C appears to be conserved in the Brassicaceae family. The authors cloned an NRG1C ortholog from canola (Brassica napus cv. Wester) and overexpressed it in the Arabidopsis chs3-2D autoimmune mutant and revealed that it also suppressed both the dwarfism and enhanced disease resistance phenotypes, similar to what was found when Arabidopsis NRG1C was overexpressed. This suggests that the function of NRG1C, and potentially other truncated hNLRs orthologs, is conserved in the Brassicaceae family.

This study revealed an interesting biological role of plant N-terminally truncated hNLRs—fine-tuning effector-triggered immunity. Even though these truncated NLRs are relatively few in number in plant genomes, they are commonly present in different plant species. It would be interesting to explore how these truncated NLRs evolved to regulate pathogen defense throughout different plant species in the future.