Ames, Iowa
August 2, 2007
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 â 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.” |
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