Investigating the genetic model for brown stem rot resistance in soybean

Genetic analyses have indicated that brown stem rot (BSR) resistance in soybean is conferred by dominant alleles at three independent loci, the actions of which may be modified by linked or independent loci. A study was conducted to characterize the inheritance of BSR resistance in PI 567609, a soybean plant introduction from China. Segregating progeny from crosses of PI 567609 with BSR-susceptible and -resistant genotypes were evaluated for response to BSR-causal fungus, Phialophora gregata. Genetic analyses indicated that PI 567609 carries a single gene or cluster of linked genes for brown stem rot resistance, and that this gene (or cluster) is allelic to, or tightly linked to previously identified resistance genes, Rbs1, Rbs2, and Rbs3. Because previous allelism tests indicated that Rbs1, Rbs2, and Rbs3 were unlinked, and molecular mapping studies have indicated that Rbs1, Rbs2, and Rbs3 are linked on molecular linkage group J of soybean, a new model is proposed for BSR resistance. In this model, BSR resistance is controlled through the interaction of alleles at four independent loci, at least two of which are necessary to condition a resistance response. Functional redundancy at three of these loci allows any one of the three to interact with a fourth locus to confer resistance to BSR.