Plant-parasitic nematodes are major root pathogens that affect drastically plant development and growth. Predominant species, such as the root-knot nematodes (RKNs) Meloidogyne spp., represent a global threat for annual and perennial crops causing huge crop losses worldwide. In Europe, RKNs were mainly controlled by toxic pesticides that are getting banned. One of the most promising alternative to pesticides is to rely on biological control and on the plant innate immunity for disease resistance in crops. The plant immune system is a multi-layered network that enables the detection of pathogens using different classes of receptors. The presence/absence of such receptor can determine whether or not the plant will be resistant/susceptible to a particular disease. At INRAE Sophia Agrobiotech Institute, an immune receptor gene Ma, from the TIR-NB-LRR (TNL) family, has been cloned from the plum tree Prunus cerasifera where it provides a broad resistance against numerous RKN species. A unique feature of Ma resides in five repeated exons encoding a large C-terminal extention. Interestingly, each of those exons carries a recently characterized post-LRR (PL) domain which is found in a single copy in many other TNLs from different plant species. The PL domain’s function remains unknown but its conservation suggests an important role in TNLs. The peculiar architecture of Ma provides a great opportunity to decipher the involvement of the PL domain in TNL–mediated immunity. Combining complementary approaches, the proposed IMMUNE project provided a first description of how Ma triggers immunity in response to RKNs in plant roots and how the PL domain participates in the recognition and signaling.
We successfully assembled the Ma wild type resistant (R) and susceptible (S) alleles from plum accession P.2175 (where Ma originates from) as well as chimeric and truncated Ma alleles using the Golden Gate cloning methods. All Ma constructs were used and transferred into the RKN susceptible plum accession P.2032 producing transgenic hairy roots expressing Ma alleles. The data obtained enabled us to conclude that the regulatory regions of the Ma S are functional and that the absence of resistance to RKN conferred by this allele lies within its coding region.
I used two other TNLs, RPS4 and N, for the PL domain analysis. I generated a selection of alleles mutated within the PL domain and identified conserved amino acids shared between those two that are required for the receptor functionality. We produced a variety of PL domain swaps between RPS4 and its paralog RPS4B. Among those, we obtained auto-active as well as non-functional alleles. Altogether those actions enabled me to conclude that the integrity of the PL domain is required for TNL receptors’ function. More precisely, the PL domain is involved in TNL receptors stabilisation in resting state, TNL activation and TNL–mediated immune signaling in plants.
