Resistance to Root Knot Nematodes in Prunus: Characterization of Sources, Marker-Assisted Selection and Cloning Strategy for the Ma Gene from Myrobalan Plum

Stone fruit crops (Prunus spp.) are mainly localized under Mediterranean climates where predominant root-knot nematodes (RKN) species Meloidogyne arenaria, M. Incognita, and M. javanica are widely spread. Selection for RKN resistance (R) in Prunus rootstock material has led to the characterization of several R sources with more or less wide spectra. Among them, some Myrobalan plum clonal accessions (Prunus cerasifera; subgenus Prunophora) express a complete-spectrum, high and heat-stable resistance to all RKN. In this plum, resistance to all tested RKN species and notably to M. arenaria, M. incognita, M. javanica, and to M. floridensis, a species developing on peach and almond sources, is conferred by the major and completely dominant Ma gene. Another complete-spectrum gene designated Rjap has been evidenced in the Japanese plum (P. salicina). In the subgenus Amygdalus, the Nemared and Shalil peaches carry the RMia gene for resistance to M. incognita and M. arenaria and the Alnem almond carry the RMja gene for resistance to M. javanica. Using RFLP and SSR markers from the reference Prunus map, the Ma and Rjap plum genes have been co-localized on LG7, and the RMia peach gene has been shown to be independent and located on LG2. Diverse hybrids between Myrobalan and Amygdalus sources have been created and are being selected to pyramid their RKN R genes and to associate their complementary soil-adaptive parental characteristics in rootstocks for Mediterranean environments. Markers for Ma have been obtained using a BSA strategy and transformed into codominant SCAR markers for marker-assisted selection. Because of the remarkable features (complete spectrum, heat-stable resistance, no virulent RKN population already known) of Ma in comparison with the Mi gene from tomato (the sole RKN gene cloned up-to-now), a cloning strategy was undergone. In this objective, codominant markers were used to build a high-resolution map at the Ma locus from segregating intra-and inter-specific crosses. A BAC library of the heterozygous resistant parent P. 2175 (Ma1 R allele) was constructed and codominant markers were used to elaborate the R and S contigs at the Ma locus. One 287 kb insert BAC, encompassing Ma1, was detected in the R contig and sequenced. Additional fine mapping, using new SSR markers and recombinants from 1700 more Myrobalan segregating seedlings, reduced this interval to a cluster of three TIR-NBS-LRR (TNL) as final candidate R genes.