Resistance Is Fruitful

Battlefield? A new kind of herbicide-tolerant crop, shown here in field trials with corn, might one day compete with the market leader.

Genetic engineering has had a huge economic success with crops that can withstand herbicides, particularly glyphosate, which is safe, cheap, potent, and environmentally friendly. Monsanto has a monopoly on glyphosate-tolerant plants, but rivals are trying to get into the game. In the 21 May issue of Science, a team describes a new way of making plants resist glyphosate. If the plants make it to market, they could lower the price of genetically modified crops and stimulate further innovation.

Glyphosate inhibits a key enzyme that plants use to make amino acids. Monsanto engineered resistance by adding the gene for a similar microbial enzyme that isn't affected. The technology has been phenomenally successful in several crops, particularly soybeans and cotton. Hoping to find another way to protect plants, researchers with Verdia Inc. and Maxygen Inc., both in Redwood City, California, and Pioneer Hi-Bred International Inc. in Johnston, Iowa, took a cue from a different technology--one in which a microbial enzyme is used to modify a herbicide called glufosinate.

First, the researchers searched for an enzyme that would detoxify glyphosate. After growing several hundred strains of common microbes, they determined that the most effective was a soil microbe called Bacillus licheniformis. The team identified three related genes encoding the enzyme, called glyphosate N-acetyltransferase (GAT). To speed the search for the best enzyme, the team fragmented the genes, shuffled the pieces, and added them back to bacteria. Then they selected those more effective at acetylating glyphosate. After 11 rounds of selection, the enzyme was nearly 10,000 times more efficient. Corn plants outfitted with 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 potential, says Verdia's Linda Castle. Preliminary studies suggest that the enzyme's byproduct is as nontoxic to mammals as is glyphosate, Castle says. She adds that GAT should work in other crops as well.Stephen Duke of the U.S. Department of Agriculture in University, Mississippi, says the approach to finding the new enzyme was fast and effective--“brilliant work." But it will take at least 5 years before these plants can be stacked up against Monsanto's glyphosate-tolerant crops, predicts Jonathan Jones of the John Innes Centre in Norwich, U.K. But if the new technology does pan out, he says, it will spur agbiotech companies to come up with even more genetic traits that improve crop production.

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