Researchers from USDA's Agricultural Research Service (ARS) and Brazil's Embrapa Soja have identified a cluster of genes that confer resistance to the fungus that causes Asian soybean rust (ASR). While still in the early stages of research, this discovery could go far toward developing soybean varieties resistant to ASR, which was first detected in the U.S. in 2004.

Collaborators from Embrapa Soja identified markers that were closely linked to the Rpp4 gene, says Michelle Graham, geneticist at the ARS Corn Insects and Crop Genetics Research Unit in Ames, IA. Using genomic techniques, USDA/ARS, in collaboration with Iowa State University researchers, screened pieces of the soybean genome to identify resistance sequences near the marker. Virus-induced gene-silencing technology was used to turn off the resistance genes in resistant parent plants. “If you have the right gene, the resistant parent should become susceptible,” Graham says.

These technologies enabled the researchers to identify the resistance gene, which could then be bred into a susceptible soybean line to make it resistant. The problem, Graham says, is that the genes that convey resistance are 16 times larger than the average soybean gene. This makes it difficult to transfer resistant genes into susceptible lines using an agrobacterium carrier, for example.

It is difficult to predict how soon the ARS-resistant soybean varieties will be planted in experimental trials. It could be as long as 10 years, Graham says. ARS has screened a huge number of varieties and has found that resistance to Phakopsora pachyrhizi, the fungi that causes ASR, is rare, she says. Screenings of 15,000 accessions in the ARS soybean germplasm collection revealed that less than 5% of the accessions are resistant.

This rarity in nature may mean that resistance to the fungus may come at a cost to the plant. “Once we have the gene completely identified, we will have to measure the cost,” Graham says. Yield loss from ASR is high in Brazil so developing a resistant variety might be worth the cost. Yield loss from ASR is not as high in the U.S. at this point in time.

Soybean rust has been reported in five states (Texas, Louisiana, Alabama, Georgia and Florida) to date, according to the soybean Rust and Aphids Program.