Genetic Architecture of Pythium Root Rot Resistance in Soybean As Inferred from Genome-Wide Association Mapping.

Pythium root rot is a seedling disease of many hosts including soybean [Glycine max (L.) Merr.]. The disease was reported to be associated with yield loss. Using resistant variety is one of most efficient measure to manage this disease. To identify sources of resistance to this seedling disease and dissect the resistant genetic architecture, 430 soybean lines were evaluated for resistance to isolates of Pythium irregular in the greenhouse and genotyped with the SoySNP50k chips. Root rot score (from 0 to 5) and whole plant fresh weight data were collected for inoculated and non-inoculated (control) plants of each accession 14 days after inoculation. An index of disease rating (IDR) and a ratio of whole plant weight vs. control (RWW) were calculated. Base on the analysis of variance and test statistics, there were significant differences among the 430 lines in both IDR and RWW. Moreover, IDR and RWW also showed a normal distribution and the broad-sense heritability were 0.62 and 0.65, respectively. By means of genome-wide association mapping, a total of 5 loci underlying pythium root rot resistance were identified. The identified loci explained 27.5% of the phenotypic variance measured by RWW. One strong peak of association on chromosome 2, associated with both IDR and RWW, spanned a physical region of 210 kb around a peptidase family gene. This study identified valuable markers for developing soybean cultivars with durable resistance against pythium root rot. The candidate genes containing these SNP loci will represent promising targets for further efforts to pinpoint causal variants and to clarify how the implicated genes affect P. irregular resistance in soybean.