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Weed Wars: A new way to fight an old enemy.

Enemies no more? The rise of glyphosate-resistant 'superweeds' has spurred a high-stakes hunt for alternative herbicides.

enemies no more? The rise of glyphosate-resistant “superweeds” has spurred a high-stakes hunt for alternative herbicides.

scientists at monsanto patented glyphosate herbicide, an amazingly efficient killer of grasses and broadleaf weeds, just over 40 years ago. Today, marketed under the brand name Roundup, it is, simply put, the world’s most essential weed slayer.

While glyphosate and Monsanto have their critics, experts with both the World Food Program and the Bill and Melinda Gates Foundation have staunchly defended both the chemical and the company. Roundup and other biotechnological solutions, they say, are key to achieving the “Green Revolution 2.0” that might avert a global food crisis.

Unfortunately for the world’s hungry, however, there are signs that glyphosate’s admirable run may be nearing its end. Farmers have recently reported an alarming increase in so-called “superweeds,” plants that have developed resistance to Roundup and other glyphosate-based products. In Missouri, Palmer amaranth and tall waterhemp are the chief culprits.

When the superweeds arrive, rich-nation farmers resort to plowing, pulling and spraying alternative herbicides to clear their fields — a situation that costs time and money. In developing countries, resistant weeds can cripple farmers’ ability to produce surplus food for sale, thus making it more difficult for them to move beyond subsistence agriculture.

For now, thankfully, superweeds remain more threat than scourge, affecting a relatively small percentage of the world’s cropland. But scientists aren’t waiting to wage war on them.

Zhanyuan Zhang, a research associate professor of plant sciences and director of MU’s Plant Transformation Core facility, has been collaborating with a team at Dow AgroSciences in the effort. The project, led by Dow AgroSciences, involves pinpointing two bacterial enzymes that appear to confer resistance to a Roundup alternative called 2,4-D, an effective superweed killer found in a variety of lawn products.

These enzymes, called AAD-1 and AAD-12, attracted the research team’s attention because they are similar to a previously identified 2,4-D-resistent enzyme in genetically engineered cotton. When the scientists inserted the new enzymes into the genome of the model plant Arabidopsis, the resulting plants had no problem handling 2,4-D. When the team introduced the enzymes into corn and soybean plants, those too passed the 2,4-D test with no negative effects on yield or plant development. This promising outcome is particularly welcome, the researchers say, because 2,4-D is cheap to produce, breaks down quickly in the environment, and has a low toxicity to humans and wildlife.

“Unlike glyphosate, which targets amino acid synthesis, 2,4-D is a hormone regulator. Because it has a different mode of action, 2,4-D is an ideal herbicide to deal with glyphosate-resistant weeds,” said Zhang, who managed the soybean transformation portion of the study and contributed to some data analysis.

Zhang is not suggesting, however, that 2,4-D or any chemical herbicide will alone solve the world’s weed worries. Instead, he suggests, farmers should explore how “integrated weed management” planning — the use of chemical and manual methods in tandem —can help them be more productive and cost-efficient. “The less chemicals farmers use in the field, the less money they spend on production,” says Zhang. “That leads to less cost for the consumer, as well as improved food safety and environmental safety.”

The study was published in the November 2010 edition of The Proceedings of the National Academy of Sciences.

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