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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
soybean



COURTESY OF UNIVERSITY OF NEBRASKA/BRETT HAMPTON
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 herbicide.

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 still fine."

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 itself properly."

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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