Agricultural researchers have unfurled the genetic maps for two aggressive plant diseases, laying the groundwork for better methods to fight pathogens in wheat and poplar trees.
Scientists at the U.S. Department of Agriculture's Agricultural Research Service (ARS) and several other institutes published their findings from investigations on two separate fungus genomes: poplar leaf rust (Melampsora larici-populina) and wheat and barley stem rust (Puccinia graminis). The study appears in the May issue of the Proceedings of the National Academy of Sciences.
"What it does is it gives you a blueprint," said Les Szabo, co-author of the study and a research geneticist with the ARS.
When a pathogen's genes mutate, the new variation is a novel threat to the host plant, whose natural resistance is unequipped to protect against the new strain. The genome map is a "toolbox" geneticists can use to measure this variation.
"If you're comparing how a part of a species is adapting, this gives you a baseline," said Szabo.
Wheat stem rust and poplar leaf rust are two fungi that devastate their hosts, with expensive consequences. Wheat is the most important food crop in the world, and fast-growing poplar trees are considered a potential source of woody biomass for renewable fuels.
How 'effectors' ravage plants
Both the poplar rust and wheat rust have about 100 million DNA base pairs in their genomes. To compare, baker's yeast has about 10 million, corn has about 700 million and humans have just over 3 billion base pairs. Even though both fungi are forms of rust, there are more 100,000 base pair changes between the poplar leaf rust genome and the wheat stem rust genome, said Szabo.
The mapped genome helps geneticists track 'effectors' -- proteins secreted by the fungi that unlock the defense genes in wheat or poplars, suppressing the plants' own immune systems before the fungus attacks.
"We know a little bit about them, but we don't know a whole lot," said Szabo of the effectors, which allow the fungi to weaken the plant without completely killing it -- keeping the plant tissue alive to be used as food.
A new strain of wheat rust, named Ug99 for the location (Uganda) and year (1999) it was discovered, has spread throughout eastern Africa and the Middle East. In much the same way some viruses can evolve to resist antiviral medication, the Ug99 strain can ravage formerly rust-resistant wheat varieties. Scientists at the U.N. Food and Agriculture Organization (FAO) estimate that 80 to 90 percent of all global wheat cultivars are susceptible, and fear the strain will travel farther into Asia, attacking India and China's fields.
Under the right conditions, it can devastate up to 70 percent of a given crop yield, according to the FAO's Global Wheat Rust Monitoring System website.
Wheat rust more prevalent with climate change
Ronnie Coffman, principal investigator of the Durable Rust Resistance in Wheat Project at Cornell University, believes that mapping the stem rust genome will help scientists develop a wheat variety that can overcome the new strain.
"Understanding the pathogen helps us manipulate the host," explained Coffman. "That helps us resist a disease that may be more prevalent because of climate change."
Adrian Newton agrees. Newton is a research scientist in the plant pathology program at the James Hutton Institute, an agricultural research center in Scotland. His work centers on the rise of plant pathogens in relation to rising temperatures, with an emphasis on Fusarium head blight (FHB)-- a distant fungal cousin of rust that affects the quality and safety of wheat. FHB has re-emerged in the United Kingdom as changes in rainfall, humidity and temperature drive its spread.
"Some of the rust on wheat and barley tends to look for warmer climates," said Newton. "In Scotland, we get very little brown rust on wheat and barley, but it's likely to get warmer in the future." If the amount of overnight dew increases, the environment will become more favorable to fungal pathogens.
"This type of sequencing work could enable us to identify resistance" by understanding the work of effectors, said Newton. If scientists can understand what makes a plant more durable, they can develop crops less likely to be overcome by pathogens.
Threat to poplar biomass
Poplar leaf rust can threaten between 40 and 50 percent of trees, said Brian Stanton, managing director of the tree improvement group at GreenWood Resources, a private company specializing in breeding poplars for timber and biomass.
Stanton touts the trees as "one of the premier candidates for lignocellulosic and ethanol fuel production," reaching maximum productivity after two to three years. By examining the leaf rust genome, Stanton expects scientists will gain information on how to breed more survivable poplar trees.
"It will help us understand the way the genome is structured, the genetics of adaptation and how we can exploit and breed for superior adaptation," he said.
While Stanton can't point to any specific instance where climate change has affected poplars, he says it is important to explore the range of adaptability to protect trees from various stresses.
"I think through careful hybridization [of poplars], it will be possible to meet those challenges, both environmental and biotic," he said.
Szabo and his team are in the process of mapping the genome for wheat stripe rust, a related pathogen that affects the leaf of the plant instead of the stem. Stripe rust has shown to be such a problem that it prompted an international conference in April to address the threat to global supply (ClimateWire, April 21).
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