Plants trigger appropriate defense responses notably through intracellular nucleotide-binding (NB) and leucine-rich repeat (LRR) containing receptor genes (NLRs) that detect secreted pathogen effector proteins. In NLR resistance genes, the toll/interleukin-1 receptor (TIR)-NB-LRRs (TNLs) are an important subfamily out of which approximately half 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 N. 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 provide here a first contribution to the functional analysis of the PL domain in order to decipher its role for TNLs' function.
Integrity of the post-LRR domain is required for TNLs' function