PLANT BREEDING AND TESTING
Making a transgenic plant is a relatively small step; evaluating and testing the transformed plants is the most time consuming part of the whole process. The expression level of the transgene may vary considerably, depending on the number of integrated transgenes and their location. The term event refers to each independent case of transgene integration. For example, if one copy of the transgene inserted into chromosome 2 of the first transformant, this would be referred to as event 1. If, in the same experiment, a separate transformed plant received the same transgene, but integrated into chromosome 4, that would be a second event. The location of integration affects the expression of the transgene. If the transgene in event 1 integrated into a region of heterochromatin, the gene would probably be silenced and never be expressed at all, even if provided with a strong promoter. In contrast, if in event 2, the transgene integrated just downstream of a very active chromosomal gene, it would probably show high expression levels. The number of integrated transgenes can also vary. Often a single transformed plant will gain multiple copies of the same transgene.
The first issue to address is whether the transgene causes any harmful side effects to the plant. Does the transgene function as expected? Does the transgene affect the crop quality? Does the transgene affect the ecosystem? The answer to these questions depends on the individual transgene being used (see later discussion for specific examples).
If no harmful effects are found, then the transgene must be transferred from the experimental plant to one with a much higher yield. Most transgenic plants are made from old varieties that are good for work in laboratories, but do not make a lot of seeds per acre or are very susceptible to diseases. Furthermore, as discussed earlier, the regeneration of plants through tissue culture may itself cause mutations. In order to overcome these problems, the transgene is moved by traditional cross-breeding into high-yielding varieties that farmers are already using. First, the pollen from the plant with the transgene is harvested and put onto the corn silk or stigma of the high-yielding variety. The seeds from this cross are harvested and grown. This is the F1 generation, and the plants containing the transgene are selected. For example, if the transgene makes the plant resistant to an herbicide, the F1 generation is sprayed to kill the plants without the transgene. The pollen from the F1 plants that survive is back-crossed to the original high-yielding parent. The seeds are grown, plants with the transgene are selected, and the whole process is repeated about four or five times. This crossing scheme will ensure that about 98% of the genes in the final plant are from the high-yielding variety, and the remaining genes are from the original transgenic plant. Because it takes an entire summer for one generation of corn, soybeans, or cotton to grow, this backcrossing scheme can take many years to complete.
Once the transgene is back-crossed into a suitable variety, field tests are performed to determine how the transgene affects the growth, yield, disease resistance, and other important traits of the plant. These field tests must be done over many different locations so that soil type, terrain, rainfall, and other factors can be allowed for. The field tests may also take many years. Different amounts of rain from one year to the next can greatly affect crop yields. The plant breeder selects only the plants that consistently have the highest yield with the best disease resistance. The other plants are never grown again.
The other issue in releasing transgenic plants to the public is passing the tests of government regulatory agencies. These agencies regulate all stages of the transgenic construction process. An Institutional Committee for Biosafety regulates how the transgene is handled when making the transgenic plant, whether in E. coli, Agrobacterium, or the plant itself. These committees are usually associated with the university or company where the work is done, but they all follow guidelines from the National Institutes of Health (NIH). The guidelines regulate the environment in which the transgenic plant may be grown (laboratory, greenhouse, etc.). In order to test the transgenic crop in the field, the Animal and Plant Health Inspection Service of the U.S. Department of Agriculture must be notified and must approve the plan. The scientist must provide extensive data on the transgene, its potential effect on the plant, the ecosystem, and any other crops similar or related to the transgenic plant.
Two other agencies must also approve the transgenic crop. If the transgenic plant gives a food product such as corn, the Food and Drug Administration (FDA) must do rigorous testing for possible allergies to the transgenic plant. The potential toxicity of the transgenic crop and whether or not the nutritional quality of the product is affected by the transgene are also tested. The Environmental Protection Agency also evaluates the transgenic crop for potential effects on the environment and on animals or insects that also inhabit the farmers’ fields. These are just the beginning of the regulations since anything sold overseas must also satisfy the regulatory commissions from all the countries in which the product is sold. At this time, overseeing transgenic technology is a hot issue that is constantly changing. As transgenic technology becomes more common and more information becomes available, the regulatory issues will change and adapt to the type of crops being produced.