Biological features, positional cloning and validation of the Ma gene for high-Ievel and complete-spectrum resistance to root-knot-nematodes in Prunus

Description

Stone fruit crops Prunus spp. grown under Mediterranean climates are severely damaged by root-knot nematodes (RKN) Me/oidogyne spp. and breeding for RKN-resistant rootstocks is a promising control alternative to nematicide ban. Resistance (R) genes that confer different spectra have been identified and mapped in plums (Ma and Rjap), peach (RMia) and almond (RMja). Thus sustainable resistance in Prunus is based on pyramiding of R genes in interspecific rootstock mate rial to take into account the risk of resistance breaking in the durable plant-nematode interaction for perennials. The Ma gene from Myrabalan plum has been shown to confer a complete-spectrum, high-Ievel and heat-stable resistance to both mitotic (M. arenaria, M. incognito, M. javanico and M. entera/obii) and meiotic (M. f/oridensis) RKN. Ma triggers a hypersensitive-like reaction (HLR) in raot apices and severe nematode attacks induce _ the development of subterminal lateral rootlets replacing primary terminal apices and providing an acive, resist,ance response to HLR damage (1). Sustainability of resistance conferred by Ma has been challenged in multi-year experiments applying a high and continuous nematode inoculum pressure by co-cultivation of RKN-infested susceptible tomato plants together with Prunus plants carrying Ma or, for comparison, with R tomato plants carrying the Mi-l reference gene. Galling and virulent nematode individuals have been observed in Mi-l resistant tomatoes but not in Ma-carrying Prunus plants. The positional cloning of the Ma locus in accession P.2175 has been performed using high resolution mapping developed in two successive steps totalling over 3000 segregants. The Ma locus interval has been reduced to a 32-kb cluster of three TIR-NB-LRR genes (TNLl to TNL3) including a pseudogene (TNL2) and a truncated gene (TNL3). Using A. rhizogenes transformed hairy raots and composite plants (2), the best candidate gene, TNLl, comprising the genomic sequence and the native promoter region (15.3 kb), has been validated as Ma as it conferred the same complete-spectrum and high-Ievel resistance as in the donor accession P.2175. The full-Iength cDNA (2048 aa) of Ma is the longest of ail R genes cloned tc-date. Its TNL structure is extended by a huge Cvterminal post-LRR (PL) region (1088 aa) comprising five repeated PL exons (3).

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