A complex genetic network involving a broad-spectrum locus and strain-specific loci controls resistance to different pathotypes of Aphanomyces euteiches in Medicago truncatula

A higher understanding of genetic and genomic bases of partial resistance in plants and their diversity regarding pathogen variability is required for a more durable management of resistance genetic factors in sustainable cropping systems. In this study, we investigated the diversity of genetic factors involved in partial resistance to Aphanomyces euteiches, a very damaging pathogen on pea and alfalfa, in Medicago truncatula. A mapping population of 178 recombinant inbred lines, from the cross F83005.5 (susceptible) and DZA045.5 (resistant), was used to identify quantitative trait loci for resistance to four A. euteiches reference strains belonging to the four main pathotypes currently known on pea and alfalfa. A major broad-spectrum genomic region, previously named AER1, was localized to a reduced 440 kb interval on chromosome 3 and was involved in complete or partial resistance, depending on the A. euteiches strain. We also identified 21 additive and/or epistatic genomic regions specific to one or two strains, several of them being anchored to the M. truncatula physical map. These results show that, in M. truncatula, a complex network of genetic loci controls partial resistance to different pea and alfalfa pathotypes of A. euteiches, suggesting a diversity of molecular mechanisms underlying partial resistance.