A microscopic look into the genes of a Colorado wheat variety has allowed Texas AgriLife Research scientists to identify a wheat streak mosaic virus-resistance gene.

Wheat streak mosaic virus is one of the most common wheat viruses found in the 75 million acres of wheat across the U.S., says Charlie Rush, AgriLife Research plant pathologist in Amarillo. Because there are no chemicals labeled for control of the wheat curl mite, the vector for this virus, researchers must work with wheat breeders to try to find some resistance, he says.

Huangjun Lu, who was a post-doctorate research associate in Amarillo during the study, led the AgriLife Research-Amarillo team of Rush; Jackie Rudd, wheat breeder; Jacob Price, associate researcher; and Ravindra Devkota, assistant research scientist. Lu has since become an assistant professor at the University of Florida.

"Our goal was to look at the resistance in a germplasm line that was used to produce a variety of wheat in western Kansas called RonL," says Rudd. "This variety's resistance is well known but the inheritance has not been studied until now."

The first part of the study compared the resistance of a known-susceptible variety to wheat streak mosaic virus, Karl 92, with a known-resistant variety, CO960293-2, which is a parent of the RonL variety.

A Nebraska wheat variety, Mace, which showed a high level of wheat streak mosaic resistance, was included in the study, as were TAM 111 and TAM 112, two of the top varieties developed by the Amarillo wheat breeding program and grown in Texas that also show some resistance to the virus in field trials, says Rudd.

The growth chamber experiment confirmed previous fieldwork. Mace and CO960293-2 were highly resistant, while Karl 92 was highly susceptible. Both TAM 111 and TAM 112 were intermediate in resistance, with TAM 112 being slightly better than TAM 111.

For the genetic portion of the study, the Colorado line was crossed with TAM 111. Based on that cross, Lu determined the wheat streak mosaic virus resistance was due to a single dominant gene from the Colorado germplasm line. Further molecular mapping has found the location of the specific gene that provides the resistance, says Rudd.

Wheat has 21 pairs of chromosomes and this gene was mapped to chromosome 3B, "so we now know the general location, and we are developing molecular markers that can be used to track the gene in wheat breeding programs," he says. He adds that only the Mace gene with known resistance to wheat streak mosaic virus had been named previously and it is Wsm1. "Now that we have determined they are different genes, this newly identified gene will be known as Wsm2," Rudd says.

The difference, however, is that Wsm1 is on a chromosomal translocation from intermediate wheat grass, a wild relative of wheat, which means it could carry along some less-desirable characteristics such as lower yields, he says. The Wsm2 gene was identified from a bread wheat that does not have the negative traits associated with it.

"Breeders from throughout the U.S. have been using RonL and other sources of Wsm2," says Rudd. "Now that it has been identified, they can track that through marker-assisted selection."