Abstract
Plants trigger appropriate defense responses, notably, through intracellular nucleotide-binding (NB) and leucine-rich repeat (LRR)-containing receptors (NLRs) that detect secreted pathogen effector proteins. In NLR resistance genes, the toll/interleukin-1 receptor (TIR)-NB-LRR proteins (TNLs) are an important subfamily, out of which approximately half the members carry a post-LRR (PL) domain of unknown role. We first investigated the requirement of the PL domain for TNL-mediated immune response by mutating the most conserved amino acids across PL domains of Arabidopsis thaliana TNLs. We identified several amino acids in the PL domain of RPS4, required for its ability to trigger a hypersensitive response to AvrRps4 in a Nicotiana tabacum transient assay. Mutating the corresponding amino acids within the PL domain of the tobacco TNL gene N also affected its function. Consequently, our results indicate that the integrity of the PL domain at conserved positions is crucial for at least two unrelated TNLs. We then tested the PL domain specificity for function by swapping PL domains between the paralogs RPS4 and RPS4B. Our results suggest that the PL domain is involved in their TNL pair specificity, 'off state' stability, and NLR complex activation. Considering genetically paired Arabidopsis TNLs, we finally compared the PL and TIR domains of their sensor and executor sequences, respectively. While TIR and PL domains from executors present complete motifs, sensors showed a lack of conservation with degenerated motifs. We here provide a contribution to the functional analysis of the PL domain in order to decipher its role for TNL function.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.
Highlights
Plants trigger appropriate defense responses, notably, through intracellular nucleotide-binding (NB) and leucine-rich repeat (LRR)-containing receptors (NLRs) that detect secreted pathogen effector proteins
The PL domain is widely found in the TNL repertoire from the Arabidopsis thaliana genome
Among the 132 TNL-related sequences (TIR-only included), PL motifs were mainly found in TNLs that display the full canonical toll/ interleukin-1 receptor (TIR)-NB-LRR organization (Supplementary Table S1)
Summary
Plants trigger appropriate defense responses, notably, through intracellular nucleotide-binding (NB) and leucine-rich repeat (LRR)-containing receptors (NLRs) that detect secreted pathogen effector proteins. Some particular CNLs and TNLs have integrated protein domains targeted by effectors and become direct sensors of effector presence and activity These ‘sensor’ NLRs rely on one or both genetically or functionally paired executor NLRs for downstream immune signaling (Cesari et al 2013, 2014; Narusaka et al 2009). An integrated WRKY domain in RRS1 is targeted, as are many other WRKY-containing proteins, by AvrRps and PopP2 (Sarris et al 2015; Le Roux et al 2015) The sensing of those effectors by RRS1 triggers sequential reconfigurations in its intramolecular interactions, which, in turn, activates RPS4 for downstream signaling (Ma et al 2018). RRS1 is the ‘sensor’ and RPS4 the ‘executor.’ Besides RRS1/RPS4, the Arabidopsis NLRome contains eight other pairs of TNLs arranged in a similar configuration (Narusaka et al 2009), including RRS1B/RPS4B, a paralogous pair of RRS1/RPS4 (Saucet et al 2015)
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