Pennsylvania, USA
April 14, 2009
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Colonizing bacteria that
supply nitrogen to legumes -- rhizobia, reside in
protective nodules formed by the plant. The bright red
color of the opened nodule is an indication of healthy
rhizobia inside.
Photo credit: Jennifer Dean |
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An invasion of soybean aphids
poses a problem for soybean farmers requiring application of
pesticides, but a team of
Pennsylvania State University entomologists thinks a careful
choice of nitrogen-fixing bacteria may provide protection
against the sucking insects.
Soybeans are legumes, plants that can have a symbiotic
relationship with nitrogen-fixing bacteria -- rhizobia -- and
therefore do not need additional nitrogen fertilizer. Each type
of legume -- peas, beans, lentils, alfalfa -- have their own
rhizobia.
"Soybeans are from Asia and so there were originally no
nitrogen-fixing bacteria that would colonize soybeans in U.S.
soils," said Consuelo De Moraes, associate professor of
entomology. "The rhizobia had to be transferred here."
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Soybean aphids are a recently
introduced pest to the US, causing yield losses and
increased insecticide sprays. The aphids feed on the sap
of soybean plants.
Photo credit: Jennifer Dean |
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The soybean aphid is also not
native to North America. This pest only began to infest soybean
fields about 10 years ago but are now fully established pests
requiring pesticide applications to avoid the loss of as much as
40 percent of the crop. The researchers investigated the
relationship between the type of rhizobia colonizing soybean
plants and the plants' infestation with the aphids.
"Our results demonstrate that plant–rhizobia interactions
influence plant resistance to insect herbivores and that some
rhizobia strains confer greater resistance to their mutualist
partners than do others," the researchers report in the journal
Plant and Soil online.
They looked at soybean plants inoculated with the rhizobia
provided by the inoculant company; without rhizobia, but with
added nitrogen fertilizer, and by existing rhizobia in the soil.
"The bacteria that were used initially to inoculate the first
crops of soybeans are growing wild in the soil now," said Mark
C. Mescher. "They are now considered "naturally occurring" and
are different from the inoculants purchased with the soybean
seeds."
They become natural because they change through generations of
contact with other rhizobia. While they may not provide as much
nitrogen to the plant as commercial types, the trade off between
optimal growth and heavy insect damage may still be worthwhile.
"In most cases, the inoculant companies provide rhizobia for
inoculation that gives plants the maximum yield," said Jennifer
M. Dean, postdoctoral fellow in entomology. "Their rhizobia are
highly competitive against naturally occurring nitrogen-fixing
bacteria. The inoculant companies treat the natural rhizobia
almost as a pest."
Because of this, soybeans almost uniformly incorporate the
specially developed rhizobia rather than the natural ones.
However, the researchers found that the plants associated with
the naturally occurring rhizobia had lower aphid densities than
either the artificially fertilized plants or the plants
inoculated with commercial rhizobia. They also found the same
level of nitrogen in both soybean plants inoculated with natural
rhizobia and those inoculated with commercial varieties.
"This is the first time anyone has shown how different strains
of rhizobia can effect herbivory," said De Moraes. "This may be
another tool to use to protect plants from insect herbivory. It
may also be applicable to other legumes."
The researchers do not yet know what the natural nitrogen-fixing
bacteria do to repel aphids.
"It is really exciting to see that the nitrogen producing
rhizobia can be protective," said Dean. "Next we want to isolate
rhizobia strains from the fields and look for the specific
mechanism of how they repel the aphids."
The National Science Foundations, David and Lucile Packard
Foundation, Beckman Foundation and a Du Pont young investigator
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