AMES, Iowa -- An extensive analysis of molecular changes that occur while a plant is being infected by the Asian soybean rust fungus reveals new information that could lead to a soybean variety with broad-spectrum resistance, say the Iowa State University scientists who led the research.
Their findings are published as the cover spotlight article in the August edition of the journal Molecular Plant-Microbe Interactions.
The highly infectious Asian soybean rust can devastate a crop. In countries where it is common, the fungus can wipe out 80 percent of yields, depending on environmental conditions and fungicide use. The disease was first confirmed in the United States in 2004, but has not yet spread into the major soybean-producing states during the growing season. In 2006, the value of the U.S. soybean crop exceeded $19.7 billion.
Iowa State plant pathologists Thomas Baum, Steve Whitham and Martijn van de Mortel led the three-year research project, which is the largest molecular study of the interaction of soybean and Asian soybean rust. It was funded by the Plant Sciences Institute at Iowa State.
The experiment took place in a greenhouse at Embrapa Soja, the leading national agricultural research institution in Brazil where the fungus is endemic. Brazilian researchers Alvaro Almeida and Ricardo Abdelnoor directed the collaborative experiment.
The researchers sprayed Asian soybean rust spores on two soybean varieties - a highly susceptible variety and a resistant one in which the disease progresses slowly. Samples were taken every six hours for the first 24 hours and at greater increments of time throughout the next seven days.
Then the researchers returned to Iowa State with genetic material that provided a snap shot of the level of gene expression at the time the plants were sampled. At the university's GeneChip® ( Facility, they profiled the gene expression of more than 30,000 soybean genes in each sample. To determine which of the soybean genes changed expression significantly in response to the fungus, the group worked with Dan Nettleton, Laurence H. Baker Endowed Chair in biological statistics, and doctoral student Justin Recknor.
The analyses showed that both varieties immediately responded to the fungus as indicated by significant changes in gene expression levels. Then something unexpected happened.
"Twenty-four hours into the infection, gene expression returned to the baseline - the plant's response to the rust pathogen essentially turned off," Whitham said.
There was a lull in which gene activity calmed down for about 48 hours. Then, the activity peaked again as another response was mounted - first in the variety with resistance to the disease; a day or two later in the highly susceptible variety.
"It looked like this second burst of gene activity in the resistant plants was the real resistance response," Whitham said.
It's likely the fungus produced something the plant recognized as foreign. The fact that the response happened earlier in the variety with some resistance indicated that these genes may be involved in regulating or affecting soybean defense mechanisms.
The event pointed the scientists to genes involved in defending the soybean plant, narrowing the field from 37,500 genes to just a few hundred. Clues about how these genes act to limit Asian soybean rust growth were provided by U.S. Department of Agriculture scientist Michelle Graham. She developed labels that describe what each of the 37,500 genes does or is predicted to do.
Now the researchers are studying those genes experimentally to understand their roles in limiting the growth of the pathogen. The additional work, funded by the Iowa Soybean Association, is being done at the USDA/Agriculture Research Service's high-containment facility at Fort Detrick, Md.
The data generated by the research team is a significant genomic resource available online for researchers worldwide to access. Despite the value of this fundamental research in providing a better understanding of Asian soybean rust, the development of a soybean variety resistant to the fungus is still years away, Baum said.
"It's not something that can be solved overnight, but it will work out," Baum said. "You have to put in the time, resources and manpower to get a grip on the biology. And then you can start doing other approaches to control the disease. But first we need to understand what we're trying to control."
