Genetic Engineering

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Genetic Engineering

Humans have been modifying the genetic constitution (genomes) of crop plants for thousands of years, since the very beginning of agriculture. In the past, modifying the genomes of crop plants was accomplished by selecting seeds from those individual plants that produced the most grain, were most resistant to diseases, or were most tolerant of environmental stresses (e.g., drought). These seeds were then used to plant the next year's crop. This approach is sometimes referred to as classical plant breeding, and it has been extraordinarily successful at producing improved crops. However, this approach is subject to a major limitation in that it only allows for the selection of genes (and the associated genetic traits) that are already present in the genome of the crop plant. Although many potentially useful genes are present in the genomes of other organisms, they are not always present in the genomes of crop plants.

The limitations of classical plant breeding can be partially addressed by facilitating sexual reproduction between a crop plant and a wild plant species. This approach can be used to introduce new genes into a crop plant genome (e.g., genes for disease resistance). Unfortunately, sexual reproduction between crop plants and other species is restricted to closely related plant species, limiting the pool of potential genes that could be added to the crop plant genome. For example, commercially grown tomatoes (Lycopersiconesculentum ) may be able to reproduce sexually with wild tomato relatives (other members of the genus Lycopersicon ), but they would not be able to reproduce sexually with any wild (or domestic) member of the grass family (species within the family Poaceae). In addition, no human-directed mechanism for introducing genes into plants from other types of organisms (e.g., bacteria, fungi, mammals, etc.) was available prior to the advent of genetic engineering.

Plant genetic engineering has been developed to circumvent the limitations of classical plant breeding, allowing the addition of genes (i.e., segments of deoxyribonucleic acid [DNA]) from any organism to the genomes of crop plants. Plants to which one or more genes have been added through a means other than sexual reproduction are called genetically modified organisms (GMOs) or transgenic plants. The genes that are introduced into the plants are referred to as transgenes.

The Process of Genetically Engineering Plants

In order to achieve genetic engineering of plants, a series of tools and technologies are necessary. The ability to identify, isolate, and replicate specific genes is required. This series of steps is referred to as gene (or DNA) cloning. (A complete description of cloning is beyond the scope of this entry, but may be found in most biology textbooks.) A method to insert the cloned gene(s) into the genome of the crop plant is required, as is a method for identifying those cells that have incorporated the gene. The ability to regenerate entire plants from transgenic cells is essential, as is a means of verifying the presence of the transgene in the GMO.

Once a target gene has been cloned it is typically introduced into the genome of a crop plant by one of two means: Agrobacterium -mediated transformation or particle bombardment (biolistic) transformation. Agrobacterium tumefaciens is a very interesting soil bacterium that is a nonhuman "genetic engineer." Under natural circumstances, Agrobacterium has the ability to cause crown gall disease in some plants. The cause of the disease stems from Agrobacterium 's ability to insert some of its own genes into the genome of the plant. This results in the formation of a gall (tumorlike growth) on the plant and in the production of a food source, of which only the bacteria can make use. Deletion of the gall-inducing genes has allowed Agrobacterium to be used to produce many types of transgenic crop plants, including tomato, potato, and cotton.

Unfortunately, many important crop species are not easily susceptible to Agrobacterium -mediated transformation. The globally important cereal grains (such as corn, wheat, and rice) are prime examples of such species. Therefore, other approaches for introducing genes were developed. The most widely used is the particle bombardment technique. This technique involves accelerating small DNA-coated gold particles to a high velocity such that they are able to penetrate the cell walls of plant cells. Once inside the cell wall, some of the DNA is able to separate from the gold particles and integrate into the genome of the plant cell.

Regardless of whether the new DNA (transgene) is introduced by Agrobacterium or by particle bombardment, it is essential to understand that not all of the plant cells will contain the new gene(s). Therefore, a means of screening for the presence of the new DNA is required. Commonly, this is accomplished by including additional genes in the introduced DNA whose protein products confer resistance to a toxin that would normally kill plant cells. For example, the phosphinothricin acetyltransferase (PAT) gene confers resistance to the herbicide Basta. If a population of plant cells is bombarded with DNA that includes the PAT gene, and subsequently the cells are grown on a medium containing Basta, only those cells that contain the newly introduced transgenes will be able to grow.

The transgenetic cells must then be able to be regenerated into entire plants. This is accomplished through a series of steps referred to as tissue culture and makes use of plant hormones to stimulate the production of roots and stems from the transgenic cells. Various methods can be used to verify the continued presence of the transgene(s) in the regenerated plants. One method is to specifically amplify (thereby allowing easy detection) the DNA of the transgene using a technique called polymerase chain reaction (PCR).

An Example of a Genetically Engineered Crop Plant

To see how genetic engineering may be used to improve crops, let's look at a specific example. In the United States corn (maize) is attacked by an introduced insect pest called the european corn borer (ECB). This pest is a moth, and it is during the larval stage (the caterpillar stage) that it actually feeds on corn plants. Under some circumstances this pest can cause major damage to corn crops. There is also a naturally occurring species of bacterium (Bacillus thuringiensis ) that possesses a gene encoding a protein toxin (Bt-toxin) that is quite effective in killing many species of caterpillars. In addition, the Bt-toxin protein is not toxic to mammals, birds, and most other animals. Clearly, this Bt-toxin gene cannot be introduced into corn via sexual reproduction, as corn plants and bacteria are unable to reproduce sexually. However, genetic engineering provides an alternate approach to introduce the Bt-toxin gene into the genome of the corn plant.

The Bt-toxin gene has been cloned from Bacillus thuringiensis and introduced into the genome of corn plants using the particle bombardment approach. The resulting transgenic corn plants are resistant to ECB, and are, in fact, capable of killing ECB larvae that feed upon them. In essence, these transgenic corn plants are producing their own internal insecticide, allowing the farmer to plant corn without needing to subsequently apply chemical insecticides to prevent damage caused by ECB.

Potential Benefits and Concerns Regarding the Widespread Use of Genetically Engineered Crops

What are the potential benefits of planting transgenic corn producing the Bt-toxin? Potentially, the yield of corn may increase as corn plants are protected from ECB. Depending on market conditions and the cost of buying the Bt-toxin producing corn seed, this may or may not represent an economic benefit for corn growers. However, if the corn crop had been routinely sprayed with chemical insecticides to control ECB (true in some parts of the corn belt), then that economic cost to the farmer would be eliminated. In addition, the environmental costs of spraying a chemical pesticide, which can result in extensive killing of nontarget species, would be minimized. There have been, however, various concerns raised about the widespread use of Bt-toxin-containing corn, so we should also consider the potential dangers of applying this technology.

The potential concerns with widespread usage of Bt-toxin containing corn fall into three primary categories: direct health impacts of Bt-toxin on humans, selection for Bt-toxin resistant populations of ECB, and unintended environmental impacts. Bt-toxin-containing insecticides have been used for many years, and there have been no indications of direct toxic effects on humans. Nonetheless, there is the possibility that some individuals could develop allergic reactions to this protein. Widespread human use of transgenic corn containing Bt-toxin could potentially expose a much larger number of people to this protein.

Development of Bt-toxin-resistant populations of ECB is also certainly a major area of concern. Such insects would be able to feed on Bttransgenic corn and could potentially lead to increased populations of ECB. Other insects have demonstrated a remarkable ability to develop resistance to various insecticides (DDT-resistant mosquitoes, for example). Current approaches to minimize development of Bt-resistant ECB include planting mixtures of Bt-toxin containing corn and nontransgenic corn. The idea is to allow those corn borers that are susceptible to Bt-toxin to continue to reproduce, maintaining the presence of the susceptibility gene(s) in the ECB population. A related concern is that decreasing the population of ECB through widespread use of Bt-toxin-containing corn will result in a decline of the populations of other insects (principally types of wasps) that prey on ECB, reducing the potential for natural control of the ECB population.

The area of unintended environmental impacts is also of major concern. It is important to recognize that, while the Bt-toxin does not affect mammals or birds, it is toxic to many species of insects, not just ECB. Current commercially available Bt-toxin-containing corn varieties produce the toxin throughout the entire body of the plant, including the pollen grains. When a pollen grain is released from an anther it is dispersed by the wind in an attempt to reach the stigma (silk) of a female flower, but the vast majority of the pollen grains never reach a silk; they land somewhere else. If that somewhere else is a plant leaf that serves as a food source for another species of insect, that nontarget insect may be harmed or killed by the Bt-toxin. This concern has been specifically raised with regard to the monarch butterfly, whose caterpillars feed on milkweeds in and near corn fields; but there are, of course, many other species of moths or butterflies that could be potentially affected. Further studies are in progress to assess the impact of pollen-born Bt-toxin on monarch or other butterfly or moth species. One possibility to minimize this problem is to eliminate production of the Bttoxin in the pollen grains, while continuing production in those plant parts most affected by ECB, principally the stems and ears. This approach will necessitate the development of new varieties of Bt-toxin-containing transgenic corn that include the appropriate gene-regulating elements.

Clearly, the use of genetic engineering to modify crop plants has the potential to greatly benefit humans. Increased crop productivity could be used to feed the still-increasing population of humans on our planet. Increased production might also allow preservation, or restoration, of some natural areas that would otherwise be required for agricultural purposes. Other plant genetic engineering strategies may allow production of crops with improved nutritional qualities, the ability to synthesize industrial feed-stocks, resistance to various types of environmental stresses, or even the ability to produce drugs used in human medicine. However, as the Bt-toxin-containing corn example demonstrates, it is essential that we investigate, as thoroughly as possible, what unintended consequences may arise from use of the technology prior to widespread adoption of a particular type of transgenic crop plant.

see also Breeding; Genetic Engineer; Tissue Culture; Transgenic Plants.

James T. Colbert