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Chapter: Biotechnology Applying the Genetic Revolution: Transgenic Plants and Plant Biotechnology

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Food Safety Assessment and Starlink Corn

Many controversies exist over the use of genetically modified crops. The first point to make is that all crops have been genetically modified in some way.


Many controversies exist over the use of genetically modified crops. The first point to make is that all crops have been genetically modified in some way. Even the oldest variety of edible corn bears no resemblance to its ancestor, teosinte. These changes have occurred through cross-pollination and selective breeding. Thus, using the term transgenic crop instead of genetically modified crop is more accurate.


A main concern with transgenic crops is the health issue. Does the transgenic crop include toxins or allergens, change the nutrient level of the food, or promote antibiotic resistance in humans or cattle? The spread of antibiotic resistance due to reporter genes or selective markers is no longer an issue because new transgenic varieties no longer contain these marker genes. The nutrient level of transgenic food is strictly controlled and evaluated before any transgenic crop is released to the public.


The allergenic potential of transgenic crops has caused much controversy. In 2000, an unapproved transgenic corn called Starlink was detected in taco shells found in the grocery store. Starlink corn has two transgenes. One makes it resistant to the European corn borer by encoding the toxin from Bacillus thuringiensis (see earlier discussion). This Bt transgene is the Cry9C isoform. The second transgene, from Streptomyces hygroscopicus, makes the corn resistant to a commonly used broad-spectrum herbicide.


The Cry9C isoform of Bt toxin is much more resistant to stomach acid. Also, after cooking and processing, Starlink corn had a higher concentration of Cry9C protein than expected, suggesting that this protein is more stable than other isoforms of Bt toxin. (In contrast, cooking, processing, and digestive enzymes readily break down the Cry1A isoform of Bt toxin.) Because the findings for Cry9C protein came from only one study, the EPA demanded more tests to ensure that it would not cause an allergic reaction if consumed by the public. The companies that developed Starlink corn pushed the EPA for some sort of approval. The EPA responded by giving split approval—that is, Starlink corn could be grown, as long as it was only used to feed livestock. What the EPA failed to realize is that after corn is grown, it is hauled to the nearest grain elevator. The corn is then mixed with all the other corn in the region and shipped to processing centers. So the company and farmers were following the EPA guidelines in good faith, but the next step in the process made it impossible to keep the Starlink corn separate from all the other varieties.

In September of 2000, a coalition of groups opposed to genetically modified foods announced they had detected traces of Starlink in taco shells. Further studies confirmed this and all the products were taken off the shelves. The Centers for Disease Control (CDC) examined all the people who complained of an allergic reaction to the contaminated taco shells. They first determined the type of antibody that the body would produce in response to Cry9C protein. Next they took blood samples and coded them. The blood samples were examined for the presence of Cry9C antibodies by both the CDC and an outside lab. Both concluded that none of the samples contained antibodies to Cry9C. This suggested that any allergic reactions were to some other component in the meals eaten.

After this, the company offered to buy back all remaining Starlink corn, providing the farmer with a premium price, so that no more food became contaminated. In addition, all Starlink seed was pulled from the market to prevent its future growth. In all, Starlink was on the market for only 2 years, 1999 and 2000. In 1999, the amount of Starlink grown in the United States represented only 0.4% of the corn crop and in 2000, 0.5%. Because this was such a small percentage of the overall crop, the Cry9C in the taco shells was massively diluted by other varieties of corn. Starlink is no longer grown anywhere in the world, and the EPA has revoked all approval.

Assaying allergic potential is critical to development of transgenic crops. In another example, soybean plants were transformed with a gene from the Brazil nut. The gene was intended to increase the methionine content of soybeans, which would improve them as cattle feed. Because many people are allergic to Brazil nuts, the FDA ordered tests for allergenicity by skin prick tests and immunoassays. This transgene was found to cause allergic reactions. The work was discontinued, and none of the transgenic plants were ever released to the public.


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