Identification and characterization of pleiotropic and epistatic QDRL conferring partial resistance to Pythium irregulare and P. sylvaticum in soybean

Pleiotropic and epistatic quantitative disease resistance loci (QDRL) were identified for soybean partial resistance to different isolates of Pythium irregulare and Pythium sylvaticum. Pythium root rot is an important seedling disease of soybean [Glycine max (L.) Merr.], a crop grown worldwide for protein and oil content. Pythium irregulare and P. sylvaticum are two of the most prevalent and aggressive Pythium species in soybean producing regions in the North Central U.S. Few studies have been conducted to identify soybean resistance for management against these two pathogens. In this study, a mapping population (derived from E13390 x E13901) with 228 F4:5 recombinant inbred lines were screened against P. irregulare isolate MISO 11-6 and P. sylvaticum isolate C-MISO2-2-30 for QDRL mapping. Correlation analysis indicated significant positive correlations between soybean responses to the two pathogens, and a pleiotropic QDRL (qPirr16.1) was identified. Further investigation found that the qPirr16.1 imparts dominant resistance against P. irregulare, but recessive resistance against P. sylvaticum. In addition, two QDRL, qPsyl15.1, and qPsyl18.1 were identified for partial resistance to P. sylvaticum. Further analysis revealed epistatic interactions between qPirr16.1 and qPsyl15.1 for RRW and DRX, whereas qPsyl18.1 contributed resistance to RSE. Marker-assisted resistance spectrum analysis using F6:7 progeny lines verified the resistance of qPirr16.1 against four additional P. irregulare isolates. Intriguingly, although the epistatic interaction of qPirr16.1 and qPsyl15.1 can be confirmed using two additional isolates of P. sylvaticum, the interaction appears to be suppressed for the other two P. sylvaticum isolates. An 'epistatic gene-for-gene' model was proposed to explain the isolate-specific epistatic interactions. The integration of the QDRL into elite soybean lines containing all the desirable alleles has been initiated.