Types of genetically modified crops

Herbicide Tolerant

Producing plants that are tolerant to specific herbicides is one of the largest uses of plant genetic engineering. Herbicide tolerant crops "will allow nonpersistent herbicides (e.g. glyphosphate) to be more widely used and will permit postemergence spraying of herbicide-resistant crops." (Snow et. al, 1997) Herbicides work by effecting a single enzyme, which causes a metabolic change in the plant. There are three methods by which a plant can convey herbicide resistance (OCDE, 1999):

1. Producing an enzyme which detoxified the enzyme
2. Producing an altered target enzyme which is not affected by the herbicide
3. Producing physical or physiological barriers to the uptake of the herbicide

Plants have been genetically engineered to be tolerant of a wide variety of herbicides. For the simplicity of this paper, glyphosphate-tolerant plants will be used as an example. Glyphosphate is a synthetic herbicide and is the active ingredient in Monsanto’s herbicide Roundup®. Glyphosphate works by inhibiting the enzyme 5-enolpyruvyl-3-phosphoshikimic acid synthase (EPSPS), resulting in a disruption of the plants’ biosynthesis and ultimately death. A two-fold method has been used to produce crops that are glyphosphate-resistant. One part of the method uses recombinant DNA techniques to introduce plants that encode a glyphosphate-resistant EPSPS enzyme and the other introduces an enzyme that inactivates glyphosphate, glyphosphate oxidoreductase (GOX). (OCDE, 1999) Since crops are highly sensitive to glyphosphate, it was normally used as a pre-crop emergence herbicide. These new resistant cultivars will allow application both before and after crops emerge, with little to no crop damage.

Plants that have been field-tested include beets, corn, cotton, lettuce, poplar, potato, rapeseed, soybean, tobacco, tomato, and wheat.

There is a variety of other herbicide tolerant plants that exist or are currently being developed for similar use or for use as selectable markers to identify transformed plants. Other types of herbicide tolerance that has reached field-testing stages in the U.S. are listed in Table 1.

Table 1. Herbicides and herbicide-tolerant cultivars (adapted from Snow et. al, 1997)

Herbicide	Herbicide-tolerant plant
Butricil	Cotton, potato, tobacco
Phosphoinothiricin	Alfalfa, Arabidopsis, barley, beet, corn, creeping bentgrass, melon, peanut, poplar, rapeseed, rice, soybean, sugar cane, sweet potato, tobacco, tomato, wheat
Sulfonylurea	Corn, cotton, grape, rapeseed, tobacco, tomato

Insect resistance

Devastation to crops by pests has been dealt with historically by the use of chemical pesticides. However, many of these chemicals have proven to be either ineffective or toxic. Therefore, a new strategy was needed and the miracle of recombinant technology answered the call. Plants have been produced that contain natural plant toxins that kill pests. The most common type of genetically engineered plant, the Bt plant, will be used here as an example for explanation purposes. Bt plants are created by inserting into a host plant’s genome the gene for Bacillus thuringiensis, a soil bacterium known to be a natural endotoxin. Bt toxins work by damaging the membrane or the pest’s midgut, then causing massive water uptake and eventually death. Bt toxin, however, is not harmful to humans or other invertebrates. It has been used naturally as an external pesticide, but breaks down quickly, especially in water. Transgenic Bt plants provide constant doses of the toxin and can kill pests in a single feeding. (Snow, et. al, 1997) Monsanto Corporation has recently developed a Bt corn plant to combat infestations by corn rootworms. A more widely known Bt corn exists that aids in resistance to corn borers. (Ferber, 2000) Monsanto also produces a Bt tomato and potato, while Ciba-Geigy, Mycogen Corporation, Northrup King, and Genetique SARL have their own Bt corn products. (Steinbrecher, 1996) Other insect-resistant plants have been made to produce lectins, which disrupt midgut epithelial cells, and inhibitors of certain digestive enzymes. However, none are as effective as transgenic Bt crops.

Disease resistance

Crops are susceptible to a variety of viral, bacterial, and fungal diseases. For example, a major invader of corn is Aspergillus flavus, which can be a threat to farm animals that eat contaminated feed. A. flavus gives off a carcinogenic by-product called aflatoxin, known the cause hepatitis, cirrhosis, and death in many countries. (Brown, 1999) For this reason as well as many others, recombinant technology has developed genetically engineered plants with disease resistance. By inserting genes that code for viral coat proteins into a host cultivar, plants have been shown to have immunity to certain viral pathogens. Consequently, a variety of constructs are needed to provide resistance against a broad spectrum of viral diseases since one type of coat protein will only provide resistance to one virus or very close relatives. Fungal diseases, such as rust, mildew, and wilts, have been difficult to combat in the past. Transgenic plants carrying genes for chitinases or glucanases have been produced, which can break down chitins and carbohydrates found in fungal cell walls. This method has been introduced into tobacco, corn, potato, lettuce, squash, melon, and petunias. (Snow et. al, 1997)

Other transgenic traits of value

There are a handful of other genetically engineered cultivars out that have helped with environmental stress tolerance or improved product quality. Some genes have been found to increase cold tolerance or drought tolerance in plants that suffer physiological stress from these factors. In addition, plants have been produced that provide better tasting or better looking product, products with increase shelf lives, altered nutritional value, or easier harvesting methods. Some transgenic plants may even be used to produce pharmaceuticals and marketable compounds.

Available in: http://www.nyu.edu/classes/jaeger/genetically_modified_foods.htm

Entry author: Astrid Espinosa Sánchez.