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How does Bt-based insect resistance work?
Which plants have been genetically engineered to contain Bt-based insect resistance?

One of the most widely-used genetic engineering strategies is the development of Bt-based resistance to insects. "Bt" is short for Bacillus thuringiensis, a common soil bacterium that produces an insect toxin. Applications of the Bt bacteria in powder form have been used to kill insects in agriculture for many years, and Bt bacterial insecticides have been a particularly important insect-control tool to organic farmers. In the last few years, several crops have been genetically engineered to produce their own Bt toxins, making them resistant to specific groups of insects.

Genetically engineered (GE) plants with Bt-based insect resistance produce an insect toxin in all of their tissues. Bacillus thuringiensis bacteria produce proteins called delta-endotoxins that are toxic to insects when ingested. When an insect consumes the protein, protease enzymes in the insect's digestive system cut the normally non-toxic protein into a smaller piece that is highly toxic to insects. The smaller, activated form of the delta-endotoxin binds to a specific receptor on the surface of cells lining the insect's gut, causing a disruption of electrolyte balance, leading to death. Bt-toxins have been considered very safe for human consumption because the intestinal walls of mammals do not have the endotoxin receptor necessary for the toxic effect, and the proteins are degraded quickly in the stomach.

There are actually many variants of Bt-toxins found in nature. One of the unique features of this family of insect toxins is that different toxins affect different groups of insects. For example, the Bt-toxin most commonly used in genetic engineering, named CRYIAb, kills only moths and butterflies (Lepidoptera), but not insects in other insect families. The various delta-endotoxins are given names that begin with "CRY" because the endotoxins normally exist in a crystalline form. All Bt-plants to date have only one version of endotoxin each. Eventually, combining multiple versions of Bt-toxins in the same plant could provide resistance to several different families of insects at the same time. This strategy could also be used to prevent insects from developing their own resistance to Bt: it is much more difficult for insect populations to evolve resistance to 2 or 3 different Bt-toxins at the same time.
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Corn
Bt-corn varieties provide resistance to the European Corn Borer, a moth larva that damages corn by burrowing into stalks. Monsanto and Novartis license the Bt technology to several seed companies which market Bt-corn hybrids, often sold under names like "YieldGard" or "Maximizer." Monsanto and Novartis are currently in litigation over patent disputes related to these products. Most Monsanto varieties also include genes for glyphosate tolerance. Until recently, Aventis marketed a variety of Bt-corn called "StarLink" (which also included resistance to glufosinate herbicides).
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Cotton
Monsanto markets a variety of Bt-cotton called "Bollgard." This variety of cotton provides resistance to several major insect pests of cotton, including the cotton bollworm, tobacco budworm, and the pink bollworm (all are moth larvae).
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Potato
The "NewLeaf" lines of Bt-potatoes are marketed by Monsanto's NatureMark, and provide resistance to the Colorado Potato Beetle, a pest causing leaf damage to potato plants. This version of delta-endotoxin, CRYIIIC, is toxic to a specific group of Coleopteran insects (beetles). Two additional varieties, NewLeaf Plus and NewLeaf Y, include the Bt gene plus genetically engineered resistance to viral diseases (caused by the Potato Leaf Roll Virus and Potato Virus Y, respectively). The "Lew Leaf" lines were discontinued by the developer before the 2001 growing season and have not been marketed since.
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Tomato
Monsanto has received clearance from the USDA for marketing of Bt-tomatoes, but they have not yet been marketed.
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