Molecular markers linked to brown stem rot resistance genes, Rbs(1) and Rbs(2), in soybean

Brown stem rot (BSR) of soybean [Glycine mar (L.) Merr.] is caused by the f ungal pathogen Phialophora gregata (Allington & D.W. Chamberlain) W. Gams a nd occurs in soybean production areas around the world. Brown stem rot resi stance genes Rbs(1), Rbs(2), and Rbs(3) have been identified in soybean ger mplasm and plant introductions through traditional genetic analyses. Resist ance 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 geno me. Genetic families of populations segregating for Rbs(1) and Rbs(2) were evaluated in the greenhouse for BSR phenotypic reaction and identified as r esistant, segregating, or susceptible. Leaf tissue collected from members o f F-2:3 families was bulked and DNA simple sequence repeat (SSR) marker ana lysis was used to identify markers that cosegregated with BSR reaction phen otypes. Five pairs of Rbs2 near-isogenic lines were subjected to a similar analysis to verify results obtained from marker analysis conducted on the p opulation 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) (us ing Satt431) and Rbs(2) (using Satt244) populations would have correctly pr edicted 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 know n 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.