Potential Adverse Effects

The risks associated with transgenic plants stem from, but are not directly caused by, the nature of the transformation process. First, transgenic methods enable traits to be expressed that have never before been expressed in a plant. This widened range of traits creates potential risks that should be evaluated. Second, the present transgenic methods cannot incorporate foreign DNA into precise locations in the plant genome. Because expression of genes can depend on where the gene occurs in the genome, and because the incorporation can be complex, the expression of the transgene cannot be predicted completely. The scope of potential traits and the uncertainty associated with trait expression create the circumstances requiring the evaluation of risks to human health or the environment.

Because transgenes code for proteins, human health risks associated with these proteins and products produced by these proteins are possible. These potential risks include creation of novel toxicants, possible shifts in the nutritional content of food, and the possible creation of novel allergens. Most of the scientific attention has focused on allergens, because they are difficult to assess and there has been an increase in the incidence of food allergies. Novel proteins and their products can be altered after synthesis by alterations in amino acid sequence and by reactions with other chemicals, such as glycosylation. Assessing each of these possibilities will be challenging.

Environmental risks stem from several types of potential effects: (1) effects associated with the movement of the transgene itself and its subsequent expression in a different organism or species, (2) effects associated directly or indirectly with the transgenic plant as a whole, (3) nontarget effects associated with the transgene product outside of the plant, (4) resistance evolution in the targeted pest populations, and (5) indirect effects on human health that are mediated by the environment. The European Union (EU) recognizes affects on genetic diversity as a separate category of environmental effect in the modified 90/220 directive. The United States government has not recognized this as an environmental effect because it believes that it is the effects of altered genetic diversity, such as increased extinction rate, a compromised genetic resource, inbreeding depression, or increased vulnerability to environmental stresses, that are the actual environmental hazards. The EU recognizes this category as a precautionary measure, because the effects of movement of the transgenes are uncertain and are at present incompletely characterized. By recognizing the more easily measured, intermediate effects on genetic diversity as a potential effect, the EU risk analysis will address all of the effects caused by movement of transgenes without having to assess them specifically.

Hazards Associated with Movement of the Genes

Horizontal transfer is the nonsexual transfer of genetic material from one organism into the genome of another. Although there are no cases of transgenes moving horizontally from plants to any other organism at rates higher than normal, new discoveries could change the assessment and significance of this risk. Pollen dispersal provides an opportunity for the sexual transfer of transgenes to relatives of the crop, including other varieties of that crop, related crops, and wild relatives. Potential effects include the evolution of increased weediness (i.e., more vigorous agricultural weeds, more invasive plants) or increased risk of extinction of native species by hybridization.

Hazards Associated with the Whole Plant

The transgenic plant itself may become an environmental hazard if the traits it receives improve its fitness and ecological performance. Although many crop plants may pose little hazard, insofar as they are unable to survive without human assistance, most crops have weedy and/or wild populations in some part of their global distribution. In these areas, transgenes that improve fitness could increase weediness of the crop. In addition, because transformation includes forage grasses, poplars, alfalfa, sunflowers, wild rice, and many horticultural species, the risk of invasiveness may increase.

Nontarget Hazards

Nontarget organisms are any species that are not the direct target of the transgenic crop, and consequently, the list of potential nontarget species is very long. These organisms can be grouped conveniently into five categories: (1) beneficial species, including natural enemies of pests (lacewings, ladybird beetles, parasitic wasps, and microbial parasites), and pollinators (bees, flies, beetles, butterflies and moths, birds, and bats); (2) nontarget pests; (3) soil organisms, which usually are difficult to study and identify to species; (4) species of conservation concern, including endangered species and popular, charismatic species (monarch butterfly); and (5) biodiversity, which is the entire group of species in an area.

Hazards of Resistance Evolution

Resistance evolution can occur in pests that are targeted for control by or associated with the transgenic crop. If the pest becomes resistant, then alternative, more environmentally damaging controls may be used. Insects, weeds, and microbial pathogens all have the potential to overcome most control tactics used against them. Insect resistance to Bt crops is considered inevitable, and efforts are being made to manage resistance evolution to these transgenic crops.

Indirect Hazards

Transgenic crops can have indirect environmental impacts, especially when scaled up for commercial production. Many of these effects are associated with changes in production practices or cropping systems. For example, transgenic maize resistant to corn rootworms may lead to an expansion of continuous corn (corn planted after corn) and its attendant environmental risks, such as soil erosion. In addition, it is possible that crops transformed to produce pharmaceutical or other industrial compounds might mate with plants grown for human consumption with the unanticipated result of novel chemicals in the human food supply.

See Also the Following Articles

Biotechnology • Genetic Engineering • Insecticides • Pathogens of Insects

Further Reading

Andow, D. A. (2001). Resisting resistance to Bt corn. In "Genetically Engineered Organisms: Assessing Environmental and Human Health Effects" (D. K. Letourneau and B. E. Burrows, eds.), pp. 99-124. CRC Press, Boca Raton, FL. DellaPenna, D. (1999). Nutritional genomics: Manipulating plant micronutrients to improve human health. Science 285, 375-379. Ellstrand, N. C., and Elam, D. R. (1993). Population genetic consequences of small population size: Implications for plant conservation. Annu. Rev. Ecol. Sys. 24, 217-242. Georghiou, G. P. (1986). The magnitude of the resistance problem. In "Pesticide Resistance: Strategies and Tactics for Management," pp. 14-39. National Academy Press, Washington, DC. Green, M. B., LeBaron, H. M., and Moberg, W. K. (eds.) (1990). "Managing Resistance to Agrochemicals: From Fundamental Research to Practical Strategies." American Chemical Society, Washington, DC. Hansen, L. C., and Obrycki, J. J. (2000). Field deposition of Bt transgenic corn pollen: Lethal effects on the monarch butterfly. Oecologia 125, 241-248.

Hilbeck, A., Moar, W. J., Pusztai-Carey, M., Filippini, A., and Bigler, F. (1998). Toxicity of Bacillus thuringiensis CrylAb toxin to the predator Chrysoperla carnea (Neuroptera: Chrysopidae). Environ. Entomol. 27, 1255-1263.

Losey, J. E., Rayor, L. S., and Carter, M. E. (1999). Transgenic pollen harms monarch larvae. Nature 399, 214.

Munkvold, G. P., Hellmich, R. L., and Rice, L. G. (1999). Comparison of fumonisin concentrations in kernels of transgenic Bt maize hybrids and nontransgenic hybrids. PlantDis. 93, 130-138.

National Research Council (2002). "Environmental effects of transgenic plants: The scope and adequacy of regulation." National Academy Press, Washington, DC.

Saxena, D., Flores, S., and Stotzky, G. (1999). Insecticidal toxin from Bacillus thuringiensis in root exudates of transgenic corn. Nature 402, 480.

Shintani, D., and DellaPenna, D. (1998). Elevating the vitamin E content of plants through metabolic engineering. Science 282, 2098-2100.

Snow, A. A., and Moran-Palma, P. (1997). Commercialization of transgenic plants: Potential ecological risks. BioScience 47, 86-96.

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