An atypical NLR protein modulates the NRC immune receptor network in Nicotiana benthamiana

Abstract

The NRC immune receptor network has evolved in asterid plants from a pair of linked genes into a genetically dispersed and phylogenetically structured network of sensor and helper NLR (nucleotide-binding domain and leucine-rich repeat-containing) proteins. In some species, such as the model plant Nicotiana benthamiana and other Solanaceae, the NRC (NLR-REQUIRED FOR CELL DEATH) network forms up to half of the NLRome, and NRCs are scattered throughout the genome in gene clusters of varying complexities. Here, we describe NRCX, an atypical member of the NRC family that lacks canonical features of these NLR helper proteins, such as a functional N-terminal MADA motif and the capacity to trigger autoimmunity. In contrast to other NRCs, systemic gene silencing of NRCX in N. benthamiana markedly impairs plant growth resulting in a dwarf phenotype. Remarkably, dwarfism of NRCX silenced plants is partially dependent on NRCX paralogs NRC2 and NRC3, but not NRC4. Despite its negative impact on plant growth when silenced systemically, spot gene silencing of NRCX in mature N. benthamiana leaves doesn’t result in visible cell death phenotypes. However, alteration of NRCX expression modulates the hypersensitive response mediated by NRC2 and NRC3 in a manner consistent with a negative role for NRCX in the NRC network. We conclude that NRCX is an atypical member of the NRC network that has evolved to contribute to the homeostasis of this genetically unlinked NLR network.

Ptr1 and ZAR1 immune receptors confer overlapping and distinct bacterial pathogen effector specificities

“…1a). Six NLRs were excluded from the library due to short amino acid sequences, high sequence similarities with other NLRs in the same phylogenetic groups, or retardation of growth postsilencing (Table S1; Adachi et al ., 2023). The NbNLR VIGS constructs were labeled from Com‐1 to Com‐55 and transformed to A. tumefaciens for VIGS.…”

NLR receptors in plant immunity: making sense of the alphabet soup

“…NRC4 (Adachi et al, 2023a). This points to NRCX as an immunomodulating component in the NRC network, specifically capable of regulating the functions of NRC2 and NRC3 (Fig 3).…”

“…NLRs, especially TIR-type NLRs, can show induced expression during pathogen infection, thereby enhancing general immunity or priming the immune system (Mohr et al, 2010;Yu et al, 2013;Tian et al, 2021). Similarly, Adachi et al (2023a) reported that some NRC helper genes are induced following pathogen infection. NRC2a and NRC2b were upregulated following treatment of N. benthamiana leaves with the nonpathogenic Pseudomonas fluorescens, while other NRC genes such as NRC3 and NRC4a/b and the immunomodulator NRCx remained unchanged.…”

“…RNA-sequencing of N. benthamiana roots, leaves, and flowers/buds revealed that while NRC2, NRC3, NRC4, and NRCx were expressed in all three tissues, other NRCs, such as NRC7a and NRC8b, were mainly expressed in roots and not in other tissues. (Adachi et al, 2023a). How potential tissue-or cell-type-specific expression and induction of NLR genes contribute to enhancing the robustness and efficiency of plant immune responses is not yet understood.…”

NRC immune receptor networks show diversified hierarchical genetic architecture across plant lineages

“…NRC0 often exists in a gene cluster with NRC0-dependent sensor NLRs, strengthening the hypothesis that the NRC superclade originated from an ancient helper-sensor NLR gene cluster (Wu and Kamoun, 2021). Another intriguing evolutionary aspect of the NRC superclade is the presence of noncanonical extended N-terminal domains (exNT), which exist before the CC domain in some clades of sensor NLRs (Seong et al, 2020; Adachi et al, 2023a). Although the detailed evolutionary history of the sensor NLRs is not clear, the exNT were also found in sugar beet, suggesting that the exNT of NRC-dependent sensor NLRs emerged between the common ancestor of asterids and Amaranthaceae (Caryophyllales) (Seong et al, 2020).…”