Evidence of a Susceptible Allele Inverting the Dominance of Rust Resistance in Soybean

Soybean rust (SBR) is the most threatening fungal disease in the major soybean [Glycine max (L.) Merr.] production areas around the world. In spite of the availability of chemical control with fungicides, the increase of production costs as well as operational difficulties associated with fungicide use for disease management has prompted the search for resistant genes in the soybean germplasm. Five genes (Rpp1, Rpp2, Rpp3, Rpp4, and Rpp5) have been reported as capable of conferring SBR resistance in soybean. Here we report the genetic segregation of the SBR resistance in two populations derived from crossing the SBR resistance source PI 594760B with the breeding lines TMG06_0012 (Population 1 [POP-1]) and TMG06_0011 (Population 2 [POP-2]). In both populations the resistance segregated as a single gene, but the resistance gene was dominant in POP-1 and recessive in POP-2. Molecular mapping of the phenotype placed it in the vicinity of the Rpp1 locus in both POP-1 and POP-2. When the breeding lines TMG06_0011 and TMG06_0012 where crossed with several other sources of resistance at the Rpp1, Rpp2, Rpp4, and Rpp5 loci or other putative loci, the inversion of gene action type observed with PI 594760B occurred only for the putative Rpp1 genotypes. We propose that a dominant susceptible allele, present in TMG06_0011, is causing the inversion of dominance observed in each of these crosses.