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Why genetically engineer herbicide resistance?
How does GE herbicide resistance work?

Synthetic chemical herbicides are frequently used in agriculture to control weeds. Weeds growing in the same field with crop plants can significantly reduce crop yields because the weeds compete for soil nutrients, water, and sunlight. Herbicides are classified by the kinds of plants they kill. Broad-spectrum herbicides kill all (or most) plants, and thus cannot be applied directly onto a field of crop plants. Broad-spectrum herbicides are most often applied pre-emergence-- that is, the field is sprayed to kill all of the weeds before newly-planted crops emerge in the field. Weeds that appear after the crop plants are growing cannot be killed with broad-spectrum herbicides, and (until recently) were often controlled mechanically, either by cultivation or hoeing.

In the last several decades, with the increased adoption of no-till farming practices (to reduce soil erosion) and the development of many new post-emergence herbicides, many farmers now control weeds by spraying herbicides directly onto the crop plants. Because post-emergent herbicides generally have a more narrow spectrum of plants they can kill (if they didn't, they would kill the crop plants, too), many farmers apply mixtures of multiple herbicides to control weeds post-emergence. For example, cereal farmers often apply several broad-leaf herbicides (which kill plants with "broad leaves," or dicots) to their fields throughout the season.

Researchers realized that if a crop plant is genetically engineered to be resistant to a broad-spectrum herbicide, weed management could be simplified to an application of a single herbicide without concern of damaging the crop plant itself, which could reduce the number of herbicide applications. It is often argued that these GE varieties reduce soil erosion, because they make adoption of soil-conserving practices like "no-till" easier.
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There are several different specific kinds of GE herbicide resistance, but they all employ one of the following strategies (or a combination of both) to make the plant resistant to the applied herbicide:

• the GE plant produces a new protein which detoxifies the herbicide, or
• the protein in the plant which is normally the target of the herbicide's action is replaced by a new protein which is unaffected by the herbicide.

Resistance to four synthetic herbicides has been genetically engineered into various plants:

Follow the links in the table above to learn more about how genetic engineering confers herbicide resistance to these crops.
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Glyphosate resistance
The herbicide glyphosate (marketed as Roundup, Rodeo, Accord, etc) affects an important metabolic pathway in plants by blocking an enzyme called "3-enolpyruvylshikimate-5-phosphate synthate" abbreviated EPSPS. When the function of EPSPS is blocked by glyphosate, the plant dies. Glyphosate is a broad-spectrum herbicide-- that is, all green plants are killed by glyphosate. Resistance to glyphosate is engineered into a plant by adding a gene from a soil bacterium (Agrobacterium sp) that encodes a glyphosate-resistant version of the EPSPS.

Some glyphosate-resistant plants have an additional gene genetically engineered into them that also aids glyphosate resistance. They contain the gene for glyphosate oxidoreducatase (GOX), an enzyme from the bacterium Achromobacter that detoxifies glyphosate. The advantage of GOX is during the process of genetic engineering rather than to the GE plants themselves. The GOX gene is used mainly as a selectable marker in the process of genetic engineering rather than providing additional glyphosate resistance to plants in the field.

Which crops have glyphosate resistance?
To date, Monsanto has developed all of the glyphosate resistant crops that are now on the market, usually labeled as "Roundup Ready," including corn, soybeans, cotton, and canola (rapeseed). Monsanto also has developed a variety of Roundup Ready sugarbeets, but they have not been marketed yet.
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Glufosinate resistance
Glufosinate is marketed under the names Basta, Liberty, Ignite, etc. The active ingredient in glufosinate herbicides is phosphinothricin, a compound similar in structure to the amino acid glutamine. Because of this similarity, phosphinothricin kills plants by blocking a plant enzyme called glutamine synthase (GS), an enzyme needed by the plant for nitrogen metabolism and to detoxify ammonia, a byproduct of plant metabolism. To genetically engineer resistance to glufosinates in plants, researchers use an enzyme called phosphinothricin acetyltransferase (PAT) from a strain of Streptomyces bacteria. PAT detoxifies phosphinothricin and prevents it from blocking the GS enzyme.

Which crops have glufosinate resistance?
Initially, this form of herbicide resistance was used as a selectable marker to assist in the genetic engineering of other desirable traits. But many of these GE varieties are now marketed for their glufosinate resistance alone. Varieties of GE glufosinate resistant corn, soybeans, canola (rapeseed), rice, and sugar beets have been approved, but the latter two are presently not on the market. These are all marketed by Aventis (formerly AgrEvo), usually under the brand name "LibertyLink."
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Bromoxynil resistance
The herbicide bromoxynil (marketed as Buctril) is applied post-emergence to kill broadleaf weeds, and works by inhibiting photosynthesis in plants. Bromoxynil nitrilase (BXN), a gene from the bacterium Klebsiella pneumoniae, detoxifies bromoxynil in genetically engineered plants.

Which crops have bromoxynil resistance?
Only cotton (a broadleaf plant) has been engineered with bromoxynil resistance (Monsanto's "BXN cotton"). The EPA initially ruled that it would not extend its bromoxynil tolerance levels to allow greater application to GE cotton, limiting the plantings of bromoxynil-resistant cotton to about 1% of total US cotton acres. Although bromoxynil is applied to a variety of other crop plants (especially cereals), the EPA estimated that the application of bromoxynil to cotton (which isn't normally sprayed with bromoxynil) could push total environmental bromoxynil exposure above the level that is considered to be safe. The EPA later increased the number of allowed BXN cotton acres to about 10% of the US crop.
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Sulfonylurea resistance
Sulfonylurea is the active ingredient in the herbicide Staple (and others). Sulfonylureas are actually a family of compounds which kill broadleaf plants by blocking the plant enzyme acetolactate synthase (ALS), an enzyme important to the plant for the synthesis of some amino acids (leucine, isoleucine, and valine). Working with an ALS gene from tobacco, researchers developed a new version of the ALS gene resistant to sulfonylurea, and then genetically engineered it into plants to confer resistance to the herbicide.

Which crops have sulfonylurea resistance?
Cotton (Monsanto) and flax (University of Saskatchewan) have been engineered to express sulfonylurea resistance. Sulfonylureas are not broad-spectrum herbicides, and are often applied by cereal farmers to control broadleaf weeds. Broadleaf crops (like flax and cotton) are also susceptible to the herbicide, and are not normally sprayed with sulfonylureas. However, because many sulfonylureas have the property of persisting in the soil, in some conditions this even prevents rotation of broadleaf crops into fields that have been previously sprayed with the herbicide. The Researchers at the University of Saskatchewan developed their GE flax (named "CDC Triffid") explicitly so that it could be grown on soils contaminated with sulfonylurea residues, but the GE flax is not currently being grown by farmers.

In 1993, DuPont first marketed varieties of sulfonylunrea resistant soybeans, the "STS Soybeans," which were developed by conventional plant breeding (not genetic engineering). The STS Soybeans utilize a naturally occuring mutation in the ALS gene which renders the soybean plant resistant to the herbicide.
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