The intracellular nucleotide‐binding leucine‐rich repeat receptor (SlNRC4a) enhances immune signalling elicited by extracellular perception

Abstract

Plant recognition and defence against pathogens employs a two-tiered perception system. Surface-localized pattern recognition receptors (PRRs) act to recognize microbial features, whereas intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) directly or indirectly recognize pathogen effectors inside host cells. Employing the tomato PRR LeEIX2/EIX model system, we explored the molecular mechanism of signalling pathways. We identified an NLR that can associate with LeEIX2, termed SlNRC4a (NB-LRR required for hypersensitive response-associated cell death-4). Co-immunoprecipitation demonstrates that SlNRC4a is able to associate with different PRRs. Physiological assays with specific elicitors revealed that SlNRC4a generally alters PRR-mediated responses. SlNRC4a overexpression enhances defence responses, whereas silencing SlNRC4 reduces plant immunity. Moreover, the coiled-coil domain of SlNRC4a is able to associate with LeEIX2 and is sufficient to enhance responses upon EIX perception. On the basis of these findings, we propose that SlNRC4a acts as a noncanonical positive regulator of immunity mediated by diverse PRRs. Thus, SlNRC4a could link both intracellular and extracellular immune perceptions.

NRC proteins - a critical node for pattern and effector mediated signaling

“…NRC4 CRISPRed plants, encoding a 67 aa truncated variant, displayed intensified defense responses when challenged with EIX, and presented a higher resistance to B.cinerea. 43 Our work, together with previous publications, [41][42][43][44] positions NRC as a signaling funnel for multiple PTI and ETI sensorreceptors, demonstrating that CRISPR editing of NRCs could potentially result in agriculturally improved Solanaceae varieties possessing resistance to a broad spectrum of pathogens.…”

“…42 We have recently described tomato NRC4 as associated with an eLRR-RLP required for perception of a fungal MAMP -EIX (LeEIX2) and with an eLRR-RLK required for perception of bacterial flagellin (FLS2), enhancing defense responses mediated by them. 43 We have further shown that NRC4 is required for LeEIX2 and likely also FLS2-mediated defense responses. Furthermore, NRC4's N-terminal coiled-coil domain is sufficient to mediate the association with LeEIX2 and can enhance EIX and flagellin-elicited defense responses as efficiently as the full length protein.…”

“…Furthermore, NRC4's N-terminal coiled-coil domain is sufficient to mediate the association with LeEIX2 and can enhance EIX and flagellin-elicited defense responses as efficiently as the full length protein. 43 Earlier works have indicated that tomato NRC1 may be involved in defense responses sensed by the eLRR-RLPs Cf4, Cf9, Ve1 and LeEIX2 as well. 44 Keeping with the sensor-helper model, the eLRR receptors -Cf4, Cf9, Ve1, LeEIX2 and FLS2 can be defined as sensors, detecting effectors and MAMPs.…”

A CRISPR/Cas9 mediated 53 kb deletion of the NRC4 gene cluster of tomato does not affect bacterial flagellin-triggered immunity

“…Silencing of NRC2 and NRC3 by virus-induced gene silencing (VIGS) and RNAi reduces Cf4- and Prf-mediated cell death in N. benthamiana , indicating that NRC2 and NRC3 are involved in cell death responses activated in both pathways (Brendolise et al, 2017; Wu et al, 2016). However, the results we obtained here with our NRC4 knockout line raise questions about the precise role of NRC4a in FLS2-mediated responses (Leibman-Markus et al, 2018a; Leibman-Markus et al, 2018b). Although we cannot rule out that other NRC homologs, such as NRC2 and NRC3, contribute to FLS2-mediated ROS production, we failed to observe any measurable differences between the wild type and the NRC4 deletion line in our flagellin immunoassays.…”

“…In the Solanaceae , the NLR helpers NRC2, NRC3 and NRC4 are partially redundant but display varying degrees of specificity towards sensor NLRs that confer resistance to oomycete, bacterial and viral pathogens (Wu et al, 2017). Interestingly, a recent study linked the tomato NRC SlNRC4a to PRR-triggered immunity (Leibman-Markus et al, 2018b). Leibman-Markus et al (2018) reported that overexpression of SlNRC4a in Nicotiana spp.…”