U.S. Department of Agriculture (USDA) scientists have pinpointed the location of a gene in a little-known ancient grass that could help save one of the world's most important cereal crops from an unrelenting fungus.

Agricultural Research Service (ARS) scientists Matt Rouse and Yue Jin, with the agency's Cereal Disease Research Laboratory in St. Paul, Minn., found the gene while studying the DNA of ancient grasses. They were searching for genes that could make wheat more resistant to Ug99 (Puccinia graminis), a type of stem rust that is constantly evolving. ARS is USDA's principal intramural scientific research agency, and this work supports the USDA priority of promoting international food security.

Ug99 has not yet been found in the United States, but it is spreading overseas and is considered a potential threat to up to 90 percent of the world's wheat. Genes in wheat that seem to offer immunity one growing season become susceptible to newly developed "races" the next. Ug99 was first reported by scientists in Uganda in 1999, and controlling it has since become an international priority.

Scientists often study a crop's wild relatives for genes that will confer resistance to pests and pathogens. But what makes the efforts of Rouse and Jin noteworthy is the diversity of grasses being studied. They include einkorn wheat, an ancient variety still cultivated in parts of the Mediterranean; emmer wheat, found in archeological sites and still growing wild in the Near East; and goatgrass, a wild relative of wheat with genes that breeders have tapped to boost immunity in commercial wheat varieties.

In one study, Rouse and his colleagues at Kansas State University and the University of California at Davis focused on locating a gene in einkorn wheat that confers near immunity to Ug99. They focused on locating a gene, known as Sr35, which was previously discovered in einkorn. But the exact location of this gene in the plant's vast genome remained a mystery. The wheat genome is huge, containing nearly two times more genetic information than the human genome.

To find Sr35's position, the researchers sequenced areas of the plant's genome where they suspected it was located. In one set of mutant plants, they knocked out the cloned sequences and found it made those plants susceptible to Ug99. In another set they inserted the same sequences into previously susceptible plants and found it made them resistant.

The results, published in Science in 2013, marked the first time that scientists managed to isolate and clone a Ug99 resistance gene. The achievement should make it easier to insert useful genes into wheat varieties.