USDA reports that using state-of-the-art genomics techniques, a team of scientists from USDA’s Agricultural Research Service (ARS), Iowa State University (ISU), and Brazil have identified a cluster of soybean genes that provide resistance to the fungus Phakopsora pachyrhizi, which causes Asian soybean rust.
The discovery will help defend the $27 billion U.S. soybean crop against soybean rust through conventional breeding or biotechnological means. USDA’s report did not estimate when rust resistant soybean varieties will be available commercially.
Soybean rust was first detected in the continental United States in 2004. Although fungicide use is effective against the disease, providing growers with resistant cultivars is more sustainable, according to geneticist Michelle Graham of the ARS Corn Insects and Crop Genetics Research Unit in Ames, IA.
Genetic mapping previously linked soybean rust resistance to five DNA regions, or “loci,” within the soybean genome, named Rpp1 through Rpp5. Screening of 15,000 accessions in the ARS soybean germplasm collection revealed how uncommon resistance is: Less than 5 percent of the accessions are resistant.
Graham’s group sequenced the Rpp4 locus and identified a cluster of candidate genes that confer soybean rust resistance. Comparisons of susceptible and resistant cultivars identified a single candidate gene, Rpp4C4, thought to bestow resistance. Rpp4C4 is one of five nearly identical genes in the Rpp4 locus. Frequent “shuffling” or recombination within the cluster allowed new disease resistance genes to be formed.
For example, soybean cultivar Williams82 has three resistance genes in the cluster and lacks Rpp4C4, making it vulnerable to soybean rust. However, line PI459025B, the source of Rpp4 resistance, has five candidate genes. “Virus-induced gene silencing” studies were used to turn off the Rpp4 candidate genes in PI459025B, making it susceptible to ASR and confirming the genes’ importance.
Watch future issues of CropLife® magazine for more details on this discovery.