Molecular markers linked to brown stem rot resistance genes, Rbs1 and Rbs2, in soybean

Brown stem rot (BSR) of soybean [Glycine mar (L.) Merr.] is caused by the fungal pathogen Phialophora gregata (Allington & D.W. Chamberlain) W. Gams and occurs in soybean production areas around the world. Brown stem rot resistance genes Rbs(1), Rbs(2), and Rbs(3) have been identified in soybean germplasm and plant introductions through traditional genetic analyses. Resistance to BSR has been shown to reduce yield losses in soybean, but selection for this trait is laborious and confounded by environmental variation. The objectives of this study were to identify molecular markers linked to BSR resistance genes Rbs(1) and Rbs(2), and map these genes in the soybean genome. Genetic families of populations segregating for Rbs(1) and Rbs(2) were evaluated in the greenhouse for BSR phenotypic reaction and identified as resistant, segregating, or susceptible. Leaf tissue collected from members of F-2:3 families was bulked and DNA simple sequence repeat (SSR) marker analysis was used to identify markers that cosegregated with BSR reaction phenotypes. Five pairs of Rbs2 near-isogenic lines were subjected to a similar analysis to verify results obtained from marker analysis conducted on the population segregating for Rbs(2). Results of marker analyses indicated that SSR markers Satt215 and Satt431 were linked to Rbs(1) and that Satt244 and Satt431 were linked to Rbs(2). Marker-assisted selection in the Rbs(1) (using Satt431) and Rbs(2) (using Satt244) populations would have correctly predicted 88 and 82%, respectively, of the BSR reaction phenotypes. The Rbs(1) and Rbs(2) loci map to Molecular Linkage Group J and lie in a region known to contain Rbs(3). This region also contains loci conditioning resistance to taxonomically diverse fungal pathogens and a locus affecting nodulation in response to a bacterial symbiont.