Four newly identified strains of the deadly grain fungus, Puccinia graminis were reported this June at a meeting of the Borlaug Global Rust Initiative (BGRI) hosted by Cornell University and held in St. Petersburg, Russia. The experts concluded that the evolving pathogens may be an even greater threat to global wheat production than the original TTKSK pathotype, more commonly known as Ug99, which was discovered in southwestern Uganda in 1998.
Wheat at Risk from Both Invading and Mutating Fungi
Two of the new stem rust variants have been DNA fingerprinted by Botma Visser and colleagues at the University of the Free State in Bloemfontein, South Africa and the University of Sydney in Camden, Australia. The TTKSP strain has a fingerprint identical to TTKSF which is common in South Africa, whereas PTKST is similar to TTKSP but differs in the lines of wheat it can infect.
This difference suggests that TTKSP is a local mutation whereas the PTKST strain was exotically introduced into South Africa, “and highlights the vulnerability of the wheat industry to foreign invasions of these pathogens,” says Visser who stresses that “P. graminis is highly adapted to long-distance migration through wind dispersal and rain deposition of its urediniospores,” adding that accidental transport of spores on contaminated clothing or goods contributes to fungal spread.
Ninety Percent of the World's Wheat in Danger
There are now seven known Ug99 mutations able to defeat stem rust (Sr) resistance genes and 90 percent of the world’s wheat has little or no protection against them. This is important, say Ravi P. Singh and Arun Kumar Joshi from the International Maize and Wheat Improvement Center in El Batan, Mexico and Kathmandu, Nepal respectively, because wheat alone provides nearly 55 percent of the carbohydrates and 20 percent of the food calories consumed globally. According to BGRI, wheat represents approximately 30 percent of the global grain crop each year and at last count, which was in 2007, 607 million tons were produced; nearly half of which harvested in developing countries.
Notably, since its discovery, Ug99 or its variants have spread to Kenya, Ethiopia, Sudan, Yemen and Iran. Rust fungi within the Ug99 lineage have also been spotted in South Africa and Zimbabwe. Scientists are concerned that one or more of these strains will enter Punjab, one of the world's great bread baskets.
Scientists Armed and Ready to Attack Evolving Mutants
“Because of its destructiveness and the economic importance of its cereal hosts - which include barley, oats and rye in addition to wheat - P. graminis is one of the most widely studied of all plant pathogens,” according to Les J. Szabo from the US Department of Agriculture’s Cereal Disease Laboratory at the University of Minnesota in St. Paul.
The St Paul facility is one of the two research centers in the world legally permitted to work with exotic cereal rust pathogens and where experiments with live culture are allowed (only during December through February each year). The other lab is in Winnipeg, Manitoba; both are purposely located in frigid climates so if fungal spores escape it is unlikely they would survive the icy, barren neighboring fields.
Homeland Insecurity
Two Ug99 variants are particularly worrying to the US Department of Agriculture: one is able to overcome the resistant gene Sr24, a favorite of North American wheat producers, and another able to overcome the resistance gene Sr36, which is widely used to protect winter wheats in the Great Plains.
Thus Szabo and his colleagues, who have already sequenced the Ug99 genome and four different members of its lineage, including those considered especially threatening to US crops, are developing a microarray enabling researchers to dissect the effector genes involved in P. graminis pathogenicity, and monitor their expression during disease development.
The isolated effector genes could then be cloned and inserted into bacteria which would produce the proteins needed to rapidly screen breeding lines of wheat. This work is also leading to the development of advanced PCR assays able to distinguish between members of the Ug99 lineage and these will eventually be deployed to regional diagnostic centers in the US and around the world.
Ongoing Research a Necessity
“Ug99 has shown us how vulnerable wheat is,” says Robert Park at the University of Sydney Plant Breeding Institute in Camden, Australia, adding that crop vulnerability to this continually evolving fungal pathogen “is a problem that will not be solved quickly - Ug99 research, monitoring and plant breeding demand an ongoing effort; an arms race that must be supported by sustained funding.”
A Bit of History...
The Green Revolution began in the 1940s with wheat rust and the great American agronomist, Norman Borlaug, who won the Nobel Peace Prize for his efforts in 1970. After a decade of painstakingly monotonous crossbreeding, Borlaug identified a gene that when transplanted into wheat enabled it to resist P. graminis, and for decades the crop was protected by a single rust-resisting gene, Sr31. Serendipitously, this particular gene also boosted crop yields and was widely adopted by farmers.
By the 1970s it looked as if wheat rust had been wiped out. But in 1998, William W. Wagoire from the Buginyanya Zonal Agricultural Research & Development Center in Uganda discovered Ug99 in an experimental field while researching a related cereal killer, Puccinia striiformis, or stripe rust. Singh confirmed that the angry crimson pustules on Wagoire’s experimental wheat were rust and Zacharias A. Pretorius from the University of the Free State in Bloemfontein, South Africa reported that Ug99 was a new strain of P. graminis.
Details about Puccinia graminis can be found in:
Leonard, Kurt J and Szabo Les J..Stem rust of small grains and grasses caused by Puccinia graminis.; Molecular Plant Pathology: 2005; 6(2): pp 99-111.
Additional information can be found at the BGRI ; the USDA Cereal Disease Lab; and Cornell University’s ongoing plant pathogen programs
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