When plant breeders want to give something a specific trait, they typically look to other plants that already perform the desired function. Successful breeding kept wheat stem rust – one of the most destructive grain diseases on the planet – at bay for years but new strains of the fungus are increasingly breaking through. But researchers are hopeful a newly identified plant that has even better resistance to wheat stem rust can stop the disease’s re-emergence.
Wheat is the most important staple crop in the world, providing 20% of global calories. Losses from wheat rust pathogens worldwide are estimated at $4.3 billion to $5.0 billion per year. Outbreaks are expected to become even more common based on climate change models as the fungus thrives in warm conditions. The reemergence of the disease has been particularly intense in Africa as well as Australia, but it has also become more prevalent in many parts of Europe.
Wheat stem rust, also known as black stem rust (BSR), is caused by the fungus Puccinia graminis. The Ug99 strain has been identified as a major factor behind the re-surfacing of the disease in Africa and parts of the Middle East. According to the USDA, Ug99 and its variants differ from other strains of the BSR pathogen by their ability to overcome resistance genes in wheat that has been durable against the BSR pathogen for decades. These resistance genes have been incorporated into 80% of commercial wheat varieties currently planted around the world, including North American varieties.
Brian Steffenson, professor and Lieberman-Okinow endowed chair of cereal disease resistance at the University of Minnesota, explains that stem rust has the potential to destroy vast tracts of wheat in a short time, with its spores able to be carried great distances by the wind. From its initial detection site in Uganda in 1999, Ug99 has moved to Kenya, Sudan, Ethiopia, Yemen, and, most recently in 2008, Iran. USDA predicts that if Ug99 were to strike U.S. wheat or barley, the potential yield loss could be as much as 40% to 50%.
The newly identified plant with super-wheat rust resistance is an obscure species of wild goat grass known as Aegilops sharonensis. After first mapping the genome of the goatgrass, researchers scanned the genome for mutations using the genetic map and a search-tool technique called Mutant Hunter. The search uncovered a candidate gene, which the researchers thought was responsible for protecting plants. Using molecular tweezers, they isolated the gene and transferred it into a susceptible plant, which then showed protection against all tested strains of wheat stem rust fungus.
Dr. Brande Wulff, a wheat researcher at King Abdullah University of Science and Technology (KAUST), formerly a group leader at the John Innes Centre and one of the authors of the study said: “We now have this blockbuster gene that confers amazing immunity. If I were stem rust, I would be shaking in my spore.”
The discovery now offers options for breeders to develop stem rust-resistant wheat varieties.“We can do this both by conventional breeding means and by genetic modification,” said Steffenson. But he also says genetic modification techniques would be both more efficient, precise, and a lot faster. Researchers expect that more resistance genes will be identified from populations of the goat grass and perhaps other species of wild grasses using their methods of gene discovery. Its possible goat grass has other traits of agricultural importance too, as it has elevated tolerance to heat, salinity and drought.
The international team behind the discovery includes researchers from the John Innes Center in the United Kingdom, the University of Minnesota, and King Abdullah University of Science and Technology (KAUST) in Saudi Arabia. Currently, researchers are furthering experimental work and looking at using the newly discovered gene as part of a stack of genes bred into commonly used wheat varieties using genetic modification technology. Importantly, the team has published a “reference genome”, which will support ongoing efforts to clone other resistance genes. (Sources: Western Producer, APHIS, Nature)
