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Stalking a cereal killer

September 24, 2010


Yue Jin, Martin Carson, and Matt Rouse.

Researchers Yue Jin, Martin Carson, and Matt Rouse are out to control the spread of the deadly Ug99 stem rust fungus.

Photo by Patrick O'Leary

On the front lines of the fight to save the world's wheat crop

By Deane Morrison

Riding the wind out of the wheat-growing areas of eastern Africa, a killer fungus has already reached Asia and now threatens up to 80 percent of the world’s wheat crop. 

Discovered in Uganda in 1999, Ug99 is a strain of “stem rust” fungus that burrows into the tissues of wheat and barley and damages or kills the plants, even some of the most rust-resistant lines. It’s a formidable foe for University of Minnesota cereal grain researchers, who are on the forefront of efforts to contain its spread.

A global reach

Today Ug99 is spreading into Iran, and variants of Ug99 have been found in southern Africa. It now threatens wheat in India and China, where a billion people depend on the grain. If it finds its way to the Americas, it will pose a threat to most of the U.S. wheat crop.

The blessing of cold

The Cereal Disease Laboratory is one of just two labs in North America (the other is in Winnipeg) allowed to study samples of this rust fungus from overseas. But only from December to February, because our cold winters will kill any spores that might escape. Yue Jin uses the different strains of the wheat stem rust fungus to screen wheat plants for resistance.

Stem rust has afflicted wheat crops since time immemorial. During World War I, legendary U of M plant pathologist Elvin Stakman and his colleagues began breeding wheat for rust resistance.

“It was because of fear that stem rust would wipe out enough of the wheat crop that the United States couldn’t feed soldiers in World War I,” says Les Szabo.

Szabo is one of about a dozen rust researchers at the Cereal Disease Laboratory (CDL) on the St. Paul campus. The lab is part of the USDA’s Agricultural Research Service, and Szabo and his colleagues also hold appointments with the U’s Department of Plant Pathology. Together they are part of a global Gates Foundation-funded project to fight the plague of stem rust.

A wily opponent

The CDL researchers are out to learn all they can about Ug99, especially the mechanism that allows it to mutate into new, virulent variants.

The name Ug99 is applied to several closely related variants within one lineage of the stem rust fungus. All Ug99 variants have the ability to infect wheat carrying the “gold standard” gene for resistance to stem rust. That gene, called Sr31, was the mainstay of crop protection for several decades.

Unfortunately, “Ug99 has mutated at least twice since its discovery,” says Martin Carson, research leader at the CDL. Each time it produced a variant with added virulence.

Stem rust on young wheat leaves.Stem rust on young wheat leaves. Photo by Patrick O'Leary

Two of the variants were discovered by CDL researcher Yue Jin and colleagues. First, they observed two wheat lines growing in Kenya, each carrying a gene for resistance to Ug99. But soon, those lines became susceptible to stem rust in Kenyan fields. Laboratory studies revealed that the rust strains attacking plants with the resistant genes were new variants of Ug99.

“[Ug99] has the potential to undo the Green Revolution led by Norman Borlaug in East Asia, because it’s capable of attacking the high-yielding wheats that he worked so assiduously to breed for resistance,” says University rust researcher Brian Steffenson. He, Jin, and other University researchers are screening wheat in heavily affected areas like Ethiopia and Kenya in a race to find long-lasting resistance to the fungus and incorporate it into wheat and barley.

More genes, please

“The way to slow down the evolution of virulence is to have multiple genes for resistance to Ug99,” says Matt Rouse, who recently completed his doctoral research on resistance to Ug99 in the Department of Plant Pathology and will soon join the CDL as a research scientist.

Rouse has already found two new resistance genes and identified molecular markers that can be easily detected and serve as flags for the presence of these genes. The markers help scientists track which plants carry the resistance genes so that only they will be used for breeding.

In another tack, Szabo is looking for the precise agents—called effector proteins—that Ug99 uses to infect plants. If these can be identified, Szabo, along with University scientists Jane Glazebrook and Fumiaki Katagiri, can test other plants, such as rice, to see if they have genes that can protect against effector proteins. These genes could possibly provide new sources of resistance to help protect wheat and barley. 

“If we understand enough, we may be able to design ‘de novo’ genes that block key points essential for this fungus to infect its host,” Szabo says.

More than anything else, Carson wants to avoid repeating the complacency that set in after the “golden” resistance gene Sr31 was bred into crops.

Because of Sr31’s success, “the effort to find new genes for resistance to stem rust was put on the back burner,” Carson says. “If we had had a fairly modest program to identify resistance [and had kept resistance genes in reserve] we could have responded much faster.”

 

 

 

 

Tags: College of Food, Agricultural and Natural Resource Sciences

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