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New enzyme may broaden herbicide tolerance in crops

First there was Bt (Bacillus thuringiensis), which didn't exactly take the crop world by storm in its earlier insecticide incarnations. The pathogen, first discovered in 1911 in Germany, was used as a commercial insecticide in 1938 in France and in the U.S. in the 1950s. These early products were replaced with more effective ones in the 1960s, but still saw only limited use in agriculture.

With the advent of genetic engineering and the introduction of the Bt gene into plants, control of key crop pests became possible from the inside out. Not only could farmers get more effective control, the technology insured that only target pests would be exposed to the toxin, leaving beneficial insects unharmed. It was a boon for cotton farmers, who eagerly adopted varieties with the Bt gene.

And then came the blockbuster, the Roundup Ready gene, which allowed crop plants containing the gene to resist glyphosate, by then the world's most widely-used herbicide. A field could be sprayed with glyphosate to kill the weeds, while leaving the crop unharmed.

The technology took the crop world by storm; it is now estimated that some 80 percent of U.S. soybean and cotton acres are planted with glyphosate-resistant varieties. Worldwide, about 75 percent of genetically modified crops are tailored for herbicide resistance.

Other transgenic traits have been successfully introduced into other crops, although for some market acceptance has been less enthusiastic (FlavSavr tomatoes, NewLeaf potatoes, and more recently, Roundup Ready wheat).

A new gene in the pipeline, GAT (glyphosate N-acetyltransferase) may offer growers another option for herbicide resistance, according to an article by Erik Stokstad in the May 21 edition of the prestigious research journal, Science.

The issue also contains a technical paper by a team from Verdia, Inc., and Maxygen, Inc., both in Redwood City, Calif., and Pioneer Hi-Bred International, Johnston, Iowa, describing the detoxifying enzyme that allows plants to resist glyphosate.

“If the plants make it to market,” Stokstad writes, “they could heat up competition, lower the price of genetically modified crops, and stimulate further innovation.” Stephen Duke, USDA, University of Mississippi, is quoted as saying the new approaching to finding the enzyme was “brilliant work.”

According to the Science article, the team's search for a glyphosate-resistant enzyme went through 11 rounds of selection, finally coming up with an enzyme that “was nearly 10,000 times more efficient.” In a test, corn plants that incorporated the gene “tolerated six times the concentration of glyphosate that farmers normally apply, with no apparent effect on health or reproduction — more than enough for commercial acceptance, says Verdi's Linda Castle (research team leader).” Preliminary studies indicate that the enzyme's byproduct is as non-toxic to mammals as glyphosate, and Castle says GAT should work in crops other than corn.

The article quotes a scientist as saying it will take at least five years before GAT plants can be stacked up against glyphosate-resistant crops. But, he says, if the new technology pans out, it will spur agricultural biotech companies to come up with even more genetic traits to improve crop production.

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