Ug99 wheat rust’s origin may hold keys to its control

Researchers at the University of Minnesota have uncovered the biological basis for the extreme resistance characteristics of a stem rust strain known as Ug99.

“The Ug99 strain of the stem rust pathogen was widely virulent on wheat; thus, we were highly interested in what set this strain apart from others,” said Brian Steffenson, professor of cereal disease resistance at the university’s College of Food Agricultural and Natural Resource Sciences, “Moreover, the molecular and genomics tools advanced such that we could now explore this question in more detail,” said Steffenson, who co-authored a recent study.

The researchers found the strain could be traced to a phenomenon in which two different rust strains meet on a single plant, fuse together and exchange intact nuclei creating a hybrid strain with a greater host range than its parents. This is what makes Ug99 such a threat to so many wheat varieties.

“This has been demonstrated in the laboratory but examples from nature are really lacking,” he said.

The research team analyzed the DNA sequences of the two individual nuclei present in Ug99, and also looked at a more established African stem rust strain called Pgt21-0. They found that one of the two nuclei in Ug99 was almost identical to one in Pgt21-0. It is possible that it was the fusion of these two strains that created the unique and threatening hybrid.

While Ug99 appears to be a wider ranging pathogen capitalizing on an expansive host range, not all hybrids are equally aggressive. Steffenson said some hybrids might be less fit to survive in nature. However, while others like Ug99 can become more aggressive with widespread repercussions, he believes that is a rare event, not only experimentally in the laboratory but perhaps even more so in nature.

The problem with rust pathogens is that they can easily spread over continents and oceans. Ug99, first identified in Uganda in 1999 (its name origin) has spread from its initial site across Eastern Africa and into Iran and Yemen. It is now present in 13 countries, mostly along Africa’s east coast from Egypt to South Africa.

Steffenson said that, in North America, there is an annual spring migration of rust spores from winter wheat crops planted in the southern U.S. or northern Mexico.

“Spores are then blown northward on the developing wheat crops during the late spring and early summer,” he said. “Since there is virtually a contiguous “green bridge” of wheat from this region up into the northern reaches of cereal production in Canada, the rust can spread and damage a wide swath of the crop over the continent. We are very worried that this strain could be disseminated (by wind or international travellers) to other countries and cause devastating epidemics on wheat.”

Steffenson indicated wheat varieties could potentially be genetically modified as protection from Ug99, and research using GM technology is underway.

“We have inserted a construct consisting of five different stem rust resistant genes into wheat,” he said. “This construct made the wheat plant super resistant to a wide range of different stem rust pathogen strains. This was demonstrated both in the greenhouse and in the field. GM technology offers great potential in controlling this disease for the long term.”

The research team included scientists from the University of Minnesota, Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO), Australian National University, and the University of the Free State in South Africa.

The research paper was published in the journal Nature Communications.

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