Defense Spurs Resistance Hope
Rust Ready Report
September 12, 2008
New information on the Asian soybean rust fungus and how it affects a soybean plant could eventually lead to a rust resistant soybean variety, according to the Iowa State University (ISU) researchers who led the research.
Their findings have been published as the cover spotlight article in the August edition of the journal Molecular Plant-Microbe Interactions. The trio — plant pathologists Thomas Baum, Steve Whitham, and Martijn van de Mortel — extensively analyzed the molecular changes that occur while a plant is being infected by the Asian soybean rust fungus.
The three-year research project, funded by the Plant Sciences Institute at ISU, is the largest molecular study of the interaction of soybeans and Asian soybean rust.
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.
After spraying Asian soybean rust spores on two soybean varieties — a resistant one in which the disease progresses slowly, the other a highly susceptible variety — samples were taken every 6 hours for the first 24 hours and at longer time intervals during the next 7 days.
Then the researchers returned to Iowa State with genetic material that provided a snapshot 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.
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 says.
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 says.
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 indicates that these genes may be involved in regulating or affecting soybean defense mechanisms.
Study The Defense
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.
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.