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Steps Heal Cuts

base pairing and DNA ligase

Clonning Bacteria Steps

Figure 14.10 Gene cloning by bacteria. The gene of interest from another organism is cut out of its genome by a restriction enzyme and inserted into a bacterial plasmid that has been cut with the same restriction enzyme. Sticky ends of the foreign fragment bind to the complementary sticky ends of the plasmid, followed by the formation of new sugar-phosphate bonds through the action of the enzyme DNA ligase. Finally, the altered plasmid is taken up by the bacteria, which make many copies of the foreign gene by duplicating the plasmid genome during reproduction. From Moore, Clark, and Vodopich, Botany, 2nd edition. © 1998 The McGraw-Hill Companies. All rights reserved.

Figure 14.10 Gene cloning by bacteria. The gene of interest from another organism is cut out of its genome by a restriction enzyme and inserted into a bacterial plasmid that has been cut with the same restriction enzyme. Sticky ends of the foreign fragment bind to the complementary sticky ends of the plasmid, followed by the formation of new sugar-phosphate bonds through the action of the enzyme DNA ligase. Finally, the altered plasmid is taken up by the bacteria, which make many copies of the foreign gene by duplicating the plasmid genome during reproduction. From Moore, Clark, and Vodopich, Botany, 2nd edition. © 1998 The McGraw-Hill Companies. All rights reserved.

into their chromosomes. Precisely how this process works is still a mystery, but through the use of this technique, foreign genes have already been permanently inserted into crop plants. In fact, transgenic corn containing a bacterial insect resistance gene is now widely grown in the United States.

It is difficult to control the number of gene copies inserted during transformation. In addition, genes are inserted at random locations in plant chromosomes during this step. Sometimes, the foreign genes may be inserted in areas of the

Tomato Chromosome
Figure 14.11 A crown gall, caused by the bacterium Agrobacterium tumefaciens, on a tomato plant. Courtesy Terese Barta.

genome that are not expressed by the plant, or they may insert into a critical portion of a plant gene. In addition, considerable effort is often needed to get the foreign gene expressed at suitable levels in appropriate tissues. Transformation and gene expression, therefore, are the most challenging aspects of transgenic plant production.

Transgenic corn, soybean, cotton, potato, and canola varieties containing herbicide and insect resistance are grown extensively in North America (Fig. 14.14). Traits such as disease, insect, and herbicide resistance are important for the producers of those crops, but you would not notice them as a consumer. The new generation of transgenic crops focuses on traits that will be more obvious. Transgenic plants can act as bioreactors to create pharmaceuticals. These plants would provide inexpensive access to vaccines and other medicines, especially in parts of the world where medical facilities are not readily available. People would simply take their medicine by eating potatoes or carrots. For example, transgenic plants are being tested for the production of vaccines against hepatitis B, rabies, cholera, tuberculosis, malaria, acute diarrhea, and even dental caries. Transgenic plants have also been made to produce a protein that can prevent or delay the onset of insulin-dependent diabetes mellitus. At one time, all of our drugs were derived from plants, but we have learned to synthesize many

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Agrobacterium tumefaciens DNA from

Agrobacterium tumefaciens DNA from

Examples Transgenic Plants

H HI —DNA of plant v-,-' chromosome inserted plasmid DNA carrying plasmid gene from source organism

Figure 14.12 Inserting foreign genes by using the Agrobacterium Ti plasmid. First, the bacterium is transformed by the insertion of a foreign gene into its Ti plasmid, which carries genes for inserting itself into plant chromosomes. Then a plant cell that is infected with the plasmid receives the foreign gene into its genome. From Moore, Clark, and Vodopich, Botany, 2nd edition. © 1998 The McGraw-Hill Companies. All rights reserved.

of them in laboratories. Now, we are completing the circle by letting the plants once again create our drugs.

Nonedible transgenic plants are also showing potential in some novel ways. Plants are being created to produce biodegradable polymers in order to reduce our dependence on plastics made from nonrenewable resources. Plants are also being engineered to sequester heavy metals. These transgenics could be grown in contaminated soils where they will pick up wastes such as copper or mercury. Then, they could be harvested and disposed of properly.

Finally, transgenic technology is providing us with ornamental plants never before seen in nature. Most popular cut flowers do not produce blue hues, and efforts are presently being directed toward the development of blue carnations, chrysanthemums, and roses. For example, a violet carnation named Moonshadow contains a petunia gene for blue color (Fig. 14.15). Genetic engineers are also trying to extend the shelf life of cut flowers, mainly by blocking genes for ethylene synthesis. In addition, "flowering-time" genes are being used to develop plants that flower under day-length conditions that would normally prevent flowering; plant architecture genes are being used to produce either compact or vine-type plants; and fragrance genes have been identified in an effort to restore fragrance quality to roses and carnations.

Pros and Cons of Transgenic Plants

In October 2000, activists called the Green Streets destroyed a test plot of genetically engineered corn at a University of California test facility. In January 2001, a group called the Nighttime Gardeners destroyed a greenhouse containing genetically engineered wheat in Albany, California. And, in February 2001, a group called the Earth Liberation Front burned a research cotton gin in Visalia, California, to protest the development of genetically engineered cotton. In less than two years, over 40 antigenetic engineering acts of vandalism have occurred in North America. Why are people so concerned about transgenic crops?

There are approximately 109 million acres of transgenic crops grown worldwide, 68% of which are in the United States. The most common transgenic crops are soybean, corn, cotton, and canola. Most often, these plants contain a gene making them resistant to the herbicide glyphosate, commercially sold as Roundup or an insect resistance gene that produces a protein called Bt toxin (because it is derived from the bacterium Bacillus thuringiensis).

On the positive side, proponents of transgenic crops argue that transgenic crops are environmentally friendly because they allow farmers to use fewer and less noxious chemicals for crop production. For example, a 21% reduction in the use of insecticide has been reported on Bt cotton. In addition, when glyphosate is used to control weeds, then other, more-persistent herbicides do not need to be applied.

On the negative side, opponents of transgenic crops suggest that there are many questions that need to be answered before transgenic crops are grown on a large scale. One question deals with the effects that Bt plants have on "nontarget" organisms such as beneficial insects, worms, birds, and even humans who consume the genetically engineered crop. Monarch caterpillars feeding on milkweed plants near Bt cornfields will eat some corn pollen that has fallen on the milkweed leaves. Laboratory studies indicate that the caterpillars can die from eating Bt pollen. However, field tests indicate that Bt corn is not likely to harm monarchs. Remember, too, that application of pesticides (the alternative to growing Bt plants) has been demonstrated to cause widespread harm to nontarget organisms.

Another unanswered question is whether herbicide resistance genes will move into populations of weeds. Crop plants are sometimes grown in areas where weedy relatives also live. This is especially true outside of the United States. (Remember that most of our crop plants did not originate in the United States, and they do not have wild relatives here.) If the crop plants hybridize with weedy relatives, then this herbicide-resistance gene will be perpetuated in the offspring. In this way, the resistance gene can make its way into the weed population. If this happens, a farmer can no longer use glyphosate, for example, to kill those weeds. This scenario is not likely to occur in many

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Plant Breed ing and Propagation 263

Urinal Outlet

Figure 14.13 Steps in making transgenic plants. Differentiated cells from a plant tissue are put into cell culture where they grow into an undifferentiated mass of cells. The culture is transferred to a liquid medium and infected with T plasmids carrying a gene for herbicide resistance. Cells that survive the herbicide treatment are transgenic. Plants that are regenerated from these cells are also transgenic, making them resistant to the herbicide in the field. From Moore, Clark, and Vodopich, Botany, 2nd edition. © 1998 The McGraw-Hill Companies. All rights reserved.

instances because there are no weedy relatives growing near the crop plant. However, in some cases, it may become a serious problem. For example, canola readily hybridizes with mustard weed species and could transfer its herbicide-resistance genes to those weeds. In addition, Canadian farmers have reported that herbicide-resistant canola has invaded wheat fields like a weed and, of course, cannot be killed with a major herbicide.

We know that evolution will occur when transgenic plants are grown on a large scale over a period of time. Of special concern is the development of insect populations resistant to the Bt toxin. This pesticide has been applied to plants for decades without the development of insect-resistant populations. However, transgenic Bt plants express the toxin in all tissues throughout the growing season. Therefore, all insects carrying genes that make them susceptible to the toxin will die. That leaves only the genetically resistant insects to perpetuate the population. When these resistant insects mate, they will produce a high proportion of offspring capable of surviving in the presence of the Bt toxin. Farmers are attempting to slow the development of insect resistance in Bt crops by, for example, planting non-transgenic border rows to act as a refuge for susceptible insects. These insects may allow Bt susceptibility to remain in the population.

Perhaps the most serious concern about the transgenic crop plants currently in use is that they encourage farmers to head farther away from sustainable agricultural farming practices. Transgenics, at least superficially, simplify farming by reducing the choices made by the manager. The planting of a glyphosate-resistant crop commits a farmer to the use of that herbicide for the season, probably to the exclusion of all other herbicides and other weed-control practices. In the long run, though, it may be in the best interest of the farmer and the land to use more integrated, sustainable weed-control approaches. Farmers who use Bt transgenics may not feel they need to follow through with integrated pest-management practices that use beneficial insects and timely application of pesticides to control insect pests. In fact, a farmer must decide whether to plant Bt corn even before he knows whether the European corn

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How Use Pesticides StepsPotato Plants Flowering

Figure 14.14 Transgenic potato plants expressing the Bt gene for insect toxin (left). The photo on the right is the same variety but has been defoliated by the Colorado potato beetle because it has not been transformed with the toxin gene. Courtesy Jeffrey Wyman.

borer will be a problem during the growing season. In contrast, a more sustainable approach would be to plant non-transgenic corn, monitor the fields throughout the growing season, and then apply a pesticide only if and when it is needed.

The jury is still out on the long-term effects of transgenic plants on our agricultural and natural environments. The "Catch 22" is that we will not know whether transgenic crops cause serious negative consequences until we grow them on a large scale. However, if we do grow them on a large scale and find that the concerns are valid, then we cannot reverse what has been done. Transgenic pollen has been released, and populations of everything from weeds to corn borers have been altered.

In addition to questions regarding environmental safety, human health issues must be addressed. The major concern over the consumption of foods derived from transgenic crops is the potential for the transgene protein product to cause an allergic reaction. For example, a Brazil nut gene was added to soybean to increase its methionine content. Methionine is an amino acid commonly added to animal feed. However, this gene produced a protein that caused an allergic reaction in some people. Although the transgenic soybean was being developed as an animal feed, there was concern that it might find its way into the human food chain, and consequently, it was dropped from production. How likely is it that a crop designed for animal feed will end up in human food products? Recently, Starlink corn was found in taco shells, tortilla chips, and corn dogs. Starlink is a variety of transgenic corn containing a gene for Bt toxin and was developed as an animal feed. It has not been approved for human consumption because of concerns that it might cause an allergic reaction. So far, no antibodies to the Starlink protein have been found in people who have experienced a potential allergic reaction after eating these products. The Starlink incident is just the tip of an iceberg faced by the food industry as it tries to keep track of transgenic products.

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