PLANTS: Monsanto develops double-herbicide tolerant GE crops
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TITLE: Genetically modified crops survive weed-whacking herbicide
SOURCE: Scientific American, USA
AUTHOR: David Biello
Genetically modified crops survive weed-whacking herbicide
Crops genetically altered to resist herbicides have become so
prevalent that resistant weeds are beginning to appear, necessitating
new forms of genetic modification
Genetically modified (GM) crops have spread faster in the past decade
than any agricultural technology since the plow. Of the nearly 250
million acres of GM crops planted in 2006, about 173 million acres of
corn, cotton and soybeans, among others, have been genetically altered
to resist the herbicide glyphosate (brand name Roundup™). By splicing
in a gene that allows crops to resist this plant-killer, farmers can
apply it with abandon, cutting costs and reducing the need for
tilling. But this success has sown the seeds of its own destruction by
speeding the evolution of weeds—such as giant ragweed (Ambrosia
trifida)—into varieties that also have inborn resistance to the
Now researchers at the University of Nebraska have successfully
modified crops to resist yet another herbicide - dicamba - that would
eradicate the "pernicious weeds," researchers report in Science. "We
can now spray dicamba on a number of different plants and have no
visible symptoms at all," says plant molecular biologist Donald Weeks,
the paper's senior author. "Even if we go to exaggerated levels, it is
The researchers first isolated a soil bacterium that disposes of
dicamba, an herbicide that works by mimicking plant hormones and
causing broad-leaved greenery to grow out of control as if every one
of its cells had turned cancerous. "The plant grows itself to death,"
Weeks explains. "You get gnarled leaves and stems all twisted. The
organization becomes so disrupted that the plant can no longer nourish
The bacterium Pseudomonas maltophilia (strain DI-6) quickly breaks
down dicamba, which is why farmers of grassy plants, such as corn and
wheat, have long used this herbicide to control broad-leaved weeds; it
does not build up in the soil. But because broad-leaved weeds and
broad-leaved crops are so similar, it could not be used with plants
like soybeans, until now.
Researchers used a virus that afflicts peanut plants to carry the
dicamba-disarming gene into the cells of tobacco plants, among others.
Tobacco plants are extremely sensitive to this herbicide, succumbing
to just one one-thousandth of a pound per acre. But Weeks says that
with the new gene producing the enzyme that breaks down dicamba,
tobacco plants resisted as much as 20 pounds per acre.
More importantly, researchers for the past three years have been able
to grow dicamba-resistant soybeans and other broad-leaved crops in
Nebraska. The plants carry the genetic information in their
chloroplasts (the organelles in cells responsible for photosynthesis),
which seems to make the enzyme produced extremely effective in
disposing of dicamba. "The enzyme is able to degrade any dicamba that
is sprayed onto the plant and [that] penetrates into the individual
cells," Weeks says. But "just why it is so efficacious is something
we're still working on."
The technology has been licensed to agriculture technology firm
Monsanto and could be available to farmers within as little as three
years (pending approval by the U.S. Department of Agriculture, the
Environmental Protection Agency and the Food and Drug Administration).
When it does reach fields, it will likely do so as part of a so-called
"stacked" seed, one with a variety of built-in genetic modifications.
"It is highly likely that [Monsanto] would stack our gene with the
Roundup™ resistance gene," Weeks says, thus allowing farmers to rotate
between the two herbicides and kill off any weeds developing
resistance to one or the other. And it will likely not be alone.
"We're going to see drought resistance, new insect resistance and
improved nutrition in plants," he adds. "All of those will have to be
stacked into the crop plants."
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