Chapter 13 plant biotechnology and genetically modified organisms: an overview.
Biotechnology is used today in a variety of areas including hydroponics, tissue culture used for micropropagation, the production of specialty chemicals, and the use of single cells as a source of genetic variability for plant improvement. In addition, foreign genes are transferred via different methods to produce genetically engineered plants. Through genetic engineering, crops are improved and biopesticides are produced. Common questions about genetically engineered plants include the following: Will genetically engineered plants be safe to eat? Is it possible for genetically engineered plants to become weeds or can they transfer their genes to other plants and thereby make them weeds? What impact do genetically engineered plants have on agricultural practices? Opposition to genetically engineered foods comes from ethical considerations, safety considerations, anticorporate arguments, sustainability considerations, and philosophical considerations. Many genetically modified crops are produced in the United States and the world. The United States regulates genetically modified food and agricultural biotechnology products through several agencies.
After reading this chapter, you should be able to
* provide background information on biotechnology.
* discuss several examples of biotechnology currently used today.
* provide background information on genetically engineered plants.
* provide answers to commonly asked questions about genetically engineered plants.
* provide reasons why there is opposition to genetically engineered foods.
* provide a description of genetically modified crops produced in the United States and the world and the way in which the United States regulates genetically modified food and agricultural biotechnology products.
genetically modified organism (GMO)
This chapter provides an overview of plant biotechnology and genetically modified organisms. By the year 2100, some estimate that Earth's population will reach 12 billion. The main goal of agricultural research is to maximize crop yields while minimizing adverse effects on the environment. Biotechnology is the manipulation of living organisms, or substances from organisms, to make products that benefit humanity. Plant biotechnology is changing the ways in which food crops, pharmaceuticals, and chemicals are currently produced.
The simplest form of plant biotechnology is hydroponics, which is a method of growing plants that provides nutrients needed by the plant via a nutrient solution. Work in this area began in the late 1800s. When hydroponics was first used for commercial applications, plants were grown in large containers with aerated nutrient solution that was regularly flushed with fresh solution. This method has since been replaced by the nutrient film technique, which uses a thin film of nutrients dissolved in water that continually flows past the root system. Hydroponics require much capital and energy, so only high-value crops such as tomatoes are grown using this method.
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Another form of biotechnology is tissue culture, which is a method of growing new plants from single cells and plant parts on artificial media under sterile conditions (Figure 13-1). Some commonly used methods of tissue culture include callus culture, cell suspension culture, embryo culture, meristem culture, and anther culture. These methods can be used for micropropagation, for production of specialty chemicals, as a source of variability for plant improvement, and for genetically engineering plants.
The capability to regenerate plants via tissue culture is an important step for producing genetically engineered plants because after the gene is inserted into a cell or tissue, it must be regenerated into a plant for future propagation. Genetic engineering involves the transfer of genes between related and unrelated organisms. Gene transfer into plants can be done in three ways: via Agrobacterium tumefaciens, particle bombardment, or electroporation.
Transformation via Agrobacterium occurs when A. tumefaciens attaches to a wound site and transfers its genes into the plant cell. When genes are transferred from a single bacterium to a single plant cell and are integrated into the chromosome of the plant cell, the cells are defined as being transformed. Although A. tumefaciens is useful in a wide range of plants, many plant species still cannot be transformed using this method. To provide an alternative to A. tumefaciens, researchers developed a method called particle bombardment as a means of introducing genes into plant cells.
The particle bombardment method uses a particle gun to shoot DNA-coated particles with enough speed to penetrate the first cell layer of plant tissue. After insertion of DNA into plant cells, the DNA is transcribed into RNA, which is translated into protein. When the transformed cells divide, they pass on the DNA that had been introduced to its progeny.
Another commonly used method to introduce DNA into plant cells is electroporation. In electroporation, plant protoplasts are exposed to a sudden electrical shock that opens up pores in the plant cell and enables DNA to enter. The DNA that enters the cell is then incorporated into the plant's chromosome and can be passed on to its progeny. One of the drawbacks of this method is that many plant species cannot be regenerated into whole plants via protoplasts.
Researchers are currently looking for new and improved methods for inserting DNA into plants; however, at present, the previously mentioned three methods are commonly used.
The main goal of genetic engineering is to improve crop plants by introducing foreign genes into them. Many desirable characteristics can be put into plants by inserting a single gene with currently available technology. In theory, inserting a gene into a plant is not very different from what plant breeders have been doing for years. One of the main problems with improving plants via gene transfer is that many useful genes have not been identified precisely. Researchers are currently looking for useful genes that can be used for crop improvement. Another major technical problem with using gene transfer for crop improvement is the capability to regenerate plants from transformed cells. Scientists worldwide are currently developing better methods for regenerating plants from transformed cells.
Although many advantages are associated with genetically engineering plants, there are many concerns with their use for human consumption. Some commonly asked questions include the following:
* Will genetically engineered plants be safe to eat?
* Is it possible for genetically engineered plants to turn into weeds or transfer their genes to other plants thereby making them weeds?
* What effect will genetically engineered plants have on the environmental impact of agricultural practices?
Many countries, groups, and individuals oppose the use of genetically modified organisms. The reasons for this opposition are broken down into five categories: ethical considerations, safety considerations, anticorporate arguments, sustainability considerations, and philosophical considerations.
The use of genetically modified organisms for food and in agriculture has generated a considerable amount of interest and controversy in the United States and around the world. Many people are strong supporters of gene transfer technology, although others raise questions about environmental and safety issues. Approximately 670 million acres of land are under cultivation worldwide, of which 16 percent was used for GMOs in 2000. Four countries that grew 99 percent of the global GMO crop as of the year 2000 included the United States with 74.9 million acres, Argentina with 24.7 million acres, Canada with 7.4 million acres, and China with 1.2 million acres. South Africa, Australia, Mexico, Romania, Bulgaria, Spain, Germany, France, Uruguay, and Indonesia also had significant acreage of GMOs, although much less than the four major countries.
Biotechnology products in the United States are regulated under the same laws that govern the health, safety, efficacy, and environmental impacts of similar products derived by more traditional methods such as plant breeding. The three major agencies that have primary responsibilities for regulating biotechnology products in the United States are the Food and Drug Administration (FDA), the United States Department of Agriculture (USDA), and the Environmental Protection Agency (EPA). Currently the system regulating biotechnology products appears to be working; however, only time will tell whether additional regulations will be necessary.
GENERAL BACKGROUND ON BIOTECHNOLOGY
The production of food over the past several decades has kept pace with the increases in population thus far. The main goal of all agricultural research continues to be to increase crop yields with minimal adverse effects on the environment. Plant biotechnology is changing the ways in which food crops, pharmaceuticals, and plant derived chemicals are currently being produced.
Biotechnology is described as the manipulation of living organisms, or substances from these organisms, to make products that benefit humanity. Although biotechnology is a fairly new term, its origins go back to the beginning of human civilization. Instead of gathering food from the wild, people started domesticating animals and plants for food. The grower selected for beneficial characteristics year after year and unknowingly modified them. Over the past 100 years, the modification of useful organisms has increased dramatically. Plant breeders have selected crops that can grow under a wide variety of conditions while maximizing their yields and quality characteristics. In addition, plants that produce valuable chemicals, such as Taxus plants for the production of taxol (an antitumor compound) and peppermint plants for mint oil, have been modified to maximize the production of these important compounds. Other biotechnological advances have been made by industrial microbiologists who have selected for microorganisms that produce drugs such as penicillin or enzymes that are added to laundry detergents for use in the food industry (Sonnewald, 2003).
Today with the advent of genetic engineering technology, dramatic modifications of living organisms is possible. Prior to the introduction of this technology, plant breeders were generally limited to genetic exchanges within and between species. Now genes can be transferred between very different organisms. The ability of scientists to transfer genes among humans, plants, and bacteria has revolutionized biotechnology. Although genetically modifying organisms is an important tool for biotechnological advancement, other technologies-such as plant tissue and organ culture for the mass production of crops-are vital components to the excitement in the field of biotechnology.
Probably the simplest form of biotechnology is hydroponics, which is a method of growing plants that provides nutrients needed by the plant via a nutrient solution. Research on hydroponics began in 1860 when two German plant physiologists found that many plants could be grown in aqueous solutions containing four salts, which were calcium nitrate, potassium dihydrogen phosphate, magnesium sulfate, and a small amount of iron sulfate. Interestingly, nearly 50 years later, it was found that the salts used for these experiments were impure and supplied other trace elements that are required for plant growth. Today, we know that 16 elements are essential for normal plant growth and development.
Early research in the field of hydroponics has led to biotechnological applications today. When hydroponics was first used for commercial applications, plants were grown in large containers of aerated nutrient solutions or in gravel beds, which were regularly flushed with nutrient solution. Today, these methods have been replaced by the nutrient film technique (Figure 13-2). This technique uses a thin film of nutrients dissolved in water that continually flows past a root system, which is small as compared to plants grown in soil. The continuous flow of nutrients ensures the proper concentration and availability to the plant. The use of hydroponics is expensive and requires a high amount of energy, so only high-value crops such as tomatoes, peppers, and lettuce are grown using this method.
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Tissue culture is a method for producing new plants from single cells, tissue, or pieces of plant material called explants on artificial media under sterile conditions. Some commonly used methods of tissue culture are callus culture, cell suspension culture, embryo culture, meristem culture, and anther culture, which are described in Chapter 7 of this text. These methods can be used for micropropagation, for production of specialty chemicals, as a source of genetic variability for plant improvement, and for genetically engineering plants.
Micropropagation of plants in tissue culture has become a large industry. Meristem culture is used to produce large numbers of plants. This method uses a small shoot apex consisting of a meristem (a mass of dividing cells) with a few embryonic leaves. When put under the proper conditions, the meristem can be used to grow a whole plant in tissue culture. If a small amount of the plant hormones cytokinin and auxin is added at the proper concentration, multiple shoots will emerge from a single shoot apex. This method of multiplying plants is known as clonal propagation. Each of the plants produced by this method is called a clone, which is a plant that is genetically identical to its parent. Within a year, one plant can produce millions of cloned plants identical to the parent plant using this method, which has led to the emergence of a new worldwide micropropagation industry.
More than a thousand different plant species have been propagated by tissue culture. Initially most of the work in this area was with ornamental plants, many of which are sold in supermarkets or other retail outlets that sell in quantity. However, this method has now been extended to strawberries, potatoes, medicinal plants, trees, and others because of the many associated benefits. One of the major benefits of tissue culture as a method of propagating plants is that elite specimens, especially in trees, can be reproduced quickly and efficiently without fear of any changes in the plant's genetic makeup. Obtaining disease-free plants is another major advantage of using tissue culture as a means of propagating plants. Viruses live in plant cells and can be transmitted from one generation to another, causing major problems. Interestingly, generally no viruses are found in plant meristems; therefore, starting plants from a meristem ensures that all derived plants will lack viruses. Commercial strawberry and potato growers start every year with new virus-free planting material using meristem culture. The use of tissue culture is now a major method of micropropagating plants.
Propagating plants via seeds is the major means of reproduction found in nature and agriculture. For some agricultural uses, it would be advantageous if the ease of handling seeds could be combined with the benefits of clonal propagation using in vitro somatic embryogenesis. Somatic embryogenesis is a pathway to differentiation in plants that are induced in undifferentiated cell, tissue, or organ cultures by appropriately controlling nutritional and hormonal conditions, which results in the formation of organized embryo-like (embryoid) structures. Under appropriate cultural conditions, these organized structures can develop to form plantlets and eventually whole plants. Somatic embryos can be produced from callus growing on a solid medium or in a liquid suspension culture. The use of callus for the widespread production of somatic embryos is not economically feasible due to the high cost. However, the production of somatic embryos in liquid culture can be scaled up in bioreactors, and millions of embryos can be produced at one time. A variety of food and ornamental crops can now be propagated quickly and efficiently. In addition, the transfer of plantlets from liquid solution culture to trays containing a sterile soil mix can now be done using machines at a rate of 8,000 plantlets per hour (Chrispeels & Sadava, 1994).
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To use embryos as functional seeds, they would have to be individually packaged to facilitate their handling, transport, storage, and dispersal into the field for growth into plants. At present, embryos are encapsulated in a protective hydrated gel. The way an embryo is encapsulated depends on the species, agricultural application, and the physiological state of the embryo (hydrated or dehydrated, dormant or nondormant). The material used to encapsulate the embryo must provide physical protection and can carry nutrients, growth regulators, and fungicides or bactericides, thereby helping the seedling become established in the field. Currently, synthetic seeds are not used widely due to the technical problems of establishing seedlings in the field.
Production of Specialty Chemicals by Plants
When small sections of leaves, stems, or roots are excised from the plant, surface sterilized, and placed on agar under aseptic conditions with the proper ratio of auxins and cytokinins, they will form callus. Callus is an undifferentiated mass of cells (Figure 13-3). The cells in callus proliferate and grow very rapidly in tissue culture and can be used for the production of specialty chemicals. Rapidly growing callus can be used to produce a cell suspension culture, which is a group of single cells or small clumps of cells grown in liquid culture (Figure 13-4). Plants are a source of specialty chemicals for application in the food, pharmaceutical, flavor, and fragrance industries. Some compounds from plants are used in pure form whereas others are used as mixtures. Sometimes plant compounds are further processed chemically to produce new structures, for example, steroid hormones. Approximately 119 chemical substances used as drugs are extracted from plants. So far, only a small percentage of all plants have been screened for useful compounds with the aid of modern scientific tools. Some estimate that only 5 to 10 percent of all plant species have been partially screened for some form of biological activity. The plant kingdom has been called the sleeping giant for drug development.
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The great potential has been confirmed in recent years by finding totally new drugs such as taxol (Figure 13-5), a compound that is produced by the yew plant (Figure 13-6). Taxol is a novel antitumor drug that is highly effective in treating breast cancer and a variety of other types of cancer. Taxol, like many other important compounds produced by plants, is typically found in low amounts in plants. Because the chemical synthesis of taxol is complicated and not cost effective, large-scale extraction from wild or cultivated plants is required. However, because the concentrations of these important specialty compounds are very low in plants, difficult, labor-intensive extraction procedures, as well as dependence on seasonal and geographical factors, make it difficult to obtain large quantities of a given compound. The use of plant tissue culture offers a viable alternative for the efficient production of specialty compounds using bioreactor technology (Figure 13-7). Although bioreactor technology for plant cells is not used widely on a commercial scale, its use for taxol and vinblastine (Figure 13-8) is expected to be on a major scale in the future.
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In addition to taxol, many plant species--especially tropical plants--produce unusual chemicals that can be used for a variety of drugs, cosmetics, flavorings, or agrichemicals. Until now, these compounds have been extracted from plants grown in open fields, often in tropical or semitropical regions. The development of new bioreactor technology for the growth of cells in tissue culture may in the near future replace farms with plant-cell factories. This process with plants imitates the advances in microbial biotechnology that took place over the past 50 years.
Use of Single Cells as a Source of Genetic Variability for Plant Improvement
Small pieces of tissue put under the proper conditions (aseptic and others) can be used to produce callus. After callus is formed, manipulating the ratio of the plant hormones auxin and cytokinin promotes callus, roots, shoots or roots, and shoots. With this in mind, scientists from Germany, Japan, and the United States discovered that a whole plant can be grown from a single cell given the correct nutritional and hormonal environment. Making an entire organism requires the correct expression of at least 30,000 genes. The capability of a single mature plant cell to produce an entire organism is called totipotency. Because plant cells are all interconnected by cell walls, isolating a single cell is difficult. Isolating a single cell is also tedious with variable yields, so protoplasts, which are plant cells without cell walls, are commonly used.
Protoplasts are obtained by digesting the cell walls of plant tissue with enzymes. Although protoplasts are very fragile, they can be used to produce entire plants. Protoplasts are cultured in liquid nutrient media. After a protoplast is produced, the following series of events takes place: first a complete cell wall is produced, followed by cell division, which results in a small clump of cells. From the small clumps of cells, there are two ways to regenerate a complete plant; the best way usually depends on the species. One way is to transfer these small clumps of cells to a solid medium where they will grow into callus that can form shoots. The shoots are cut off and transferred to a medium that induces root formation, thereby regenerating a new plant. A second way to regenerate a plant is to use these small clumps to produce somatic embryos. Somatic embryos resemble the embryos that normally arise as a result of the growth of a fertilized egg cell. With certain systems, thousands of embryos can be produced in liquid culture, whereas in other systems, embryos or embryolike structures are formed at the surface of the embryogenic callus growing on a solid medium. Such embryogenic callus are formed when tissues are obtained from developing seeds used to start the new tissue culture. By removing the embryos from the surface of the callus, they can be grown into complete plants. The three different routes for regenerating plants in tissue culture are from meristems, mature plant parts, and single cells, as shown in Figure 13-9.
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When somatic embryos are derived from single cells and are grown into mature plants, the plant's characteristics exhibit some variability called somaclonal variation. When scientists first observed this variation, they were puzzled because plants regenerated from single cells were thought to be exactly alike. However, rearrangements of genetic material and mutations occur as plant cells divide before giving rise to an entire plant. When plant breeders discovered that somaclonal variation existed, they exploited it as a new source of genetic variation for crop improvements. This method is still used today for this purpose.
GENETICALLY ENGINEERING PLANTS
Genetic engineering is the isolation, introduction, and expression of foreign DNA in the plant. The first and most critical step involved in genetically engineering a plant is to identify and isolate a specific gene or genes responsible for a given trait or desirable plant attribute. After the gene or genes are identified and isolated, they must be packaged into a recombinant plasmid, which can produce more of the desirable DNA. After the recombinant plasmid is produced, it must be transferred into the plant, which can be done in three different ways. Plants produced by any of these three processes are called transgenic plants or genetically modified organisms (GMOs), which are organisms containing a foreign gene or genes. Some examples of traits that have been genetically modified in plants include insect resistance, disease resistance, herbicide resistance, modified fruit quality, and enhanced nutritional content.
Methods of Transferring Foreign Genes into Plants
The ability to regenerate plants from protoplasts, cells, and pieces of plant tissue is an important technology for producing genetically engineered plants. A designated crop can often be improved by introducing a single gene or multiple genes into a plant. Gene transfer into plants can be done via Agrobacterium tumefaciens, particle bombardment (biolistics), or electroporation.
Transferring via Agrobacterium tumefaciens
One way to transfer genes into plants is via the soil bacterium Agrobacterium tumefaciens, which causes tumors, also called galls, in many plants. Molecular biologists from the United States, Belgium, and the Netherlands were the first to show that A. tumefaciens attaches to a wound site and transfers its genes to plant cells. They showed that these genes carried information necessary to produce auxin and cytokinin. The abnormally high concentration of these hormones found in tissues as a result of this gene transfer led to uncontrolled growth. This resembled the situation in plant tissue culture where auxin and cytokinins are in continuous supply in the growth medium to promote cell proliferation and callus formation. When the genes are transferred from a single bacterium to a single plant cell and integrated into the chromosome of the plant cell, the cell is transformed. When the transformed cells containing the bacterial genes divide, all the cells produced contain the bacterial genes incorporated into their DNA. After molecular biologists realized that the bacterium could transfer its genes into plants, they quickly found that they could substitute other genes for the genes that the bacterium transferred to the plant cell. This enabled them to introduce any gene into plants. In addition, they could regenerate a whole new plant from a transformed cell and have a transformed plant in which every cell carried the gene. The steps involved in Agrobacterium transformation of a plant are shown in Figure 13-10.
Transferring via Particle Bombardment
Although A. tumefaciens is very useful in the transformation of a large number of plants, many plant species, especially grasses and cereal grains, are not susceptible to infection by Agrobacterium. To overcome the problems associated with Agrobacterium transformation, J. C. Stanford and his colleagues at Cornell University developed a microprojectile gun to deliver DNA directly into plant cells by shooting it through the cell wall and the cell membrane. They coated small tungsten particles with DNA and made an apparatus that could accelerate the particles with enough speed to penetrate the first cell layer of the plant tissue. After insertion into the plant cells, the DNA was transcribed into RNA, and the RNA was translated into protein, showing that the introduced DNA was genetically active. When the transformed cells divide and pass on their DNA, the introduced DNA is also present in all progeny of the cell that was originally transformed. A modified device used to accelerate the microprojectiles into a plant's tissue is shown in Figure 13-11.
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Transferring via Electroporation
Electroporation is a totally different way of introducing DNA into plant cells than Agrobacterium or microprojectile bombardment. In electroporation, plant protoplasts are exposed to a sudden electrical discharge that opens up pores in the plant cell membrane and enables DNA to enter. After the electroporation process has taken place, the next step is to culture the protoplasts and try to regenerate them into plants. This is the most difficult step because many plant species cannot be regenerated easily from a single protoplast. Recently, scientists have used electroporation to insert genes into organized meristems. Using normal selection procedures, they successfully obtained transformed shoots that could be regenerated into plants containing the inserted gene. This method holds great promise for transforming important crop species that cannot be regenerated readily using single protoplasts.
Crop Improvements Through Genetic Engineering
The main goal of plant genetic engineering is to improve crop plants by introducing foreign genes into them. Current technology enables many desirable characteristics to be added by inserting a single gene. In theory, inserting a gene into a plant is not very different from what plant breeders have been doing for many years. When plant breeders want to make a wheat plant resistant to a fungus that infects it, they search for a wild wheat variety that is resistant to the fungus. By making the necessary crosses, they can successfully transfer the resistance gene from the wild wheat to the cultivated wheat. When this is done, as opposed to using genetic engineering technology, hundreds of genes are transferred at the same time as the resistance gene. The extra genes usually do no harm to the plant, but they are not needed by the new strain.
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What are some of the problems associated with improving plants via gene transfer? One of the main problems is that many useful genes have not been identified precisely. The power of genetic engineering is that there are no restrictions on which genes can be transferred between very different organisms. Therefore, useful genes can be found anywhere, not just in plants. For example, the bacterium Bacillus thuringiensis (Bt) carries a gene that makes an insecticidal protein. When the Bt gene is transferred into plants, the plant's cells make the insecticidal protein and any insect larvae that eat the leaves or roots die. The Bt gene is an example of a gene that has been identified and can be used effectively for crop improvement. However, in many cases, scientists have no idea which gene(s) would have to be isolated to promote important crop improvements, such as disease resistance (fungal, bacterial, and viral), stress tolerance (cold, heat, drought, and salinity), or improved nutrient, flavor, and postharvest qualities.
Another major technical problem for producing genetically engineered plants is the ability to regenerate plants from transformed cells. Growing a whole plant from a single transformed cell is very easy for tobacco or tomatoes. Transforming rice, cotton, or canola plants is not difficult; however, it is very difficult to transform corn, wheat, or soybeans.
Production of Biopesticides
As you know, fungal diseases, insects, nematodes, and weeds adversely affect the quality and quantity of crops produced. Currently, solid Integrated Pest Management (IPM) programs using cultural, mechanical, biological, chemical, and genetic methods are used to control pest problems. However, in recent years, larger doses of chemicals have been used in many countries. The use of pesticides in the United States has declined, but this overall statistic may be deceiving because there has been a dramatic drop in the use of pesticides in cotton, a crop that in the past has required very large amounts. On a worldwide scale, the use of pesticides is increasing continually, which is detrimental to the environment. Biotechnology and gene transfer can be used to select strains of natural enemies or genetically modified strains that are effective in controlling pests. The following are examples of biopesticides:
* Mycoinsecticides. More than 400 known species of fungi produce diseases in insect pests such as caterpillars, aphids, grasshoppers, and a variety of insect larvae. Several mycoinsecticides are used effectively in third-world countries; however, they act very slowly.
* Nematode-bacteria complexes. There are 24 known species of nematodes that contain symbiotic bacteria and can be used as biopesticides. Nematodes used as biopesticides can penetrate openings in the insect body where the bacteria is released. The bacterial infection kills the insect, and the nematodes multiply in the insect carcasses. These nematodes are highly specific to insects and do not harm plants or mammals.
* Bacteria. Bacillus thuringiensis (Bt) is the success story in this area, as few species of bacteria have shown potential as microbial insecticides. B. thuringiensis produces toxic proteins that act as poisons in the gut after the bacteria is eaten by insect larvae that feed on plants with this bacteria growing on them. The gene that encodes for the toxin produced by B. thuringiensis has been cloned and is incorporated into plants such as corn.
* Viruses. Seven major groups of viruses infect insects and have the potential to be developed as biological control agents. The known insect viruses typically have high specificity and are harmless to plants, mammals, and other animals. This specificity is very different from the broadspectrum insecticides that are used to control insects. Some estimate that use of baculoviruses, which is the best studied of the insect viruses, could reduce the use of chemical insecticides in California by 60 percent and in Central America by 80 percent. Cost-effective control has been obtained with 30 different baculoviruses against lepidopteran pests. Although baculoviruses are safe and effective, they are not widely used due to their narrow host range.
CONCERNS RESULTING FROM GENETICALLY ENGINEERING PLANTS
Agricultural biotechnology using genetic engineering technology is focused on crop improvement to produce a large amount of a low-value product for human consumption. Plants that have been genetically engineered are called transgenic plants or genetically modified organisms (GMOs), which are organisms containing a foreign gene or genes. Some examples of traits that have been genetically modified in plants are insect resistance, disease resistance, herbicide resistance, modified fruit quality, and enhanced nutritional content. Because genetically engineered plants are in many cases to be used for human consumption, many questions arise.
Will Genetically Engineered Plants Be Safe to Eat?
Currently, no solid scientific evidence suggests that genetically engineered plants are not safe to eat. Genetically modified plants will have an additional gene or several genes present in all cells. On a daily basis, people now eat about 100,000 genes that are efficiently broken down by the human intestinal tract. Scientific evidence shows that genes added to plants by gene transfer are also efficiently digested. In addition to the foreign gene, the plant will also contain a new protein encoded by that particular gene. In many cases, the gene product may not be present in the part of the plant that people eat. For example, if the Bt toxin gene is expressed with a root-specific control region, then the protein is only expressed in the roots. This controls insects that feed on the roots of tomato or corn plants. In other cases, the protein is intended to be in the part of the plant that is eaten to increase nutritional value, such as increased protein or amino acid content. The detrimental effect of proteins produced by genetically engineered plants on humans must be evaluated on a case-to-case basis. Proteins produced in genetically engineered plants may also be produced naturally and already be consumed in large quantities by humans. For example, the gene for alpha amylase inhibitor found in beans is transferred to other legumes to inhibit the development of bruchid beetles. This inhibitor blocks human alpha amylase as well as insect alpha amylase. However, this protein is already eaten in large quantities by millions of people all over the world and its transfer into other crops is probably safe as long as the foods are cooked prior to being eaten. In fact, beans need to be cooked well before eating precisely because they contain alpha amylase inhibitor, as well as phytohemagglutinin, which is another plant-defense protein.
Although there is no reason to believe that GMOs are harmful if eaten, this does not mean that they will be accepted readily by consumers. Currently, there is debate as to whether or not genetically modified plants should be labeled. The FDA made its policy public in May 1992. This policy states that food obtained from genetically modified plants does not need to be labeled as such. The FDA has specified that what is important to the consumer is the actual content of the food, such as the nutritional content, allergenic responses, pesticides used, and others, not the process used to generate the plants. For example, wheat flour is not labelled based on breeding processes used to generate common wheat lines or the sources of genes in improved lines. Rather, consumers need to know the flour's protein content, presence of gluten and/or allergens, bran content, and added ingredients such as vitamins.
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One of the main concerns of the FDA and consumers are foods that produce allergic reactions in humans. Genetically engineered food must be tested, and the company that wants to market the new food must demonstrate that it is not allergenic. The FDA has a specific set of safety assessment procedures that must be followed, as shown in Figure 13-12. This FDA assessment chart applies to foods from new plants produced by traditional or molecular techniques.
Can Genetically Engineered Plants Become Weeds or Transfer Genes to Other Plants to Make Them Weeds?
Weeds cause problems by competing with crop plants and spreading diseases and insects, thereby reducing yields. Weed biologists have identified 13 characteristics that make a plant a weed, and the most serious weeds have 11 or 12 of the 13 characteristics. Crop plants typically have only between 5 and 6 of these characteristics. The addition of a single gene that is totally unrelated to weediness is unlikely to turn a crop plant into a problematic weed; however, there are cases where crop plants may become weeds. This becomes a problem when several crop species are made resistant to the same herbicide, and these crop species are used sequentially in a crop rotation. There is considerable danger that "volunteers" from the first crop will become weeds in the subsequent crop.
This problem already occurs on a small scale; however, it would be greatly aggravated by herbicide-tolerant crops. One possible solution to this problem is to use crops tolerant to different herbicides in crop-rotation programs.
Another potential problem is the transfer of herbicide resistance genes to wild-type relatives, which creates problematic weeds. At present, it is difficult to say whether gene transfer to wild relatives will be a real problem. Even for herbicide resistance, it can be argued that past practices have already led to herbicide-resistant weeds. The only real way to control herbicide-resistant plants is to use alternative cultural practices and/or alternative herbicides. Therefore, herbicide-resistant weeds arising from gene flow to wild relatives may not be any more trouble than the weeds that we already have; however, only time will tell.
Genetically Engineered Plants' Environmental Impact on Agricultural Practices
Agricultural practices can negatively impact the environment due to the use of chemicals. Genetically engineered plants may overcome or reduce the need for chemicals. For example, producing plants that are resistant to insects, fungi, bacteria, viruses, or other pathogens would reduce the need for the use of chemicals. In addition, plants could be made to use nitrate from the soil more efficiently or to dissolve rock phosphate without needing to convert it to superphosphate in a chemical process. This reduces nitrate contamination in the groundwater and also lessens the need to use large amounts of energy to produce and transport nitrate and phosphate fertilizers. Another example of a potential area that would benefit from genetic engineering is the transfer of symbiotic nitrogen fixation capability from legumes to cereal grains. This reduced need for nitrogen fertilizers would reduce the production of nitrogen fertilizers, which is an energy-intensive process. Transferring symbiotic nitrogen fixing capacity form a legume plant to a cereal plant, such as corn, would require transferring a large number of genes and is unlikely to occur in the near future; however, the potential still exists.
Although the production of GMOs has the potential to reduce the need for chemicals, the potential for increased use of chemicals in agriculture also exists. Companies produce and market herbicide-resistant crop plants that tolerate specific herbicides, such as Monsanto's Roundup[R]-resistant crop plants, Hoechst's Basta[R]-resistant crops, and Dupont's Glean[R]-resistant plants. These crops make it attractive for the growers to use those specific herbicides as the most convenient method of weed control. This approach to weed control could lead to an increased need for chemicals in agriculture. Scientists at large companies have argued that plants made resistant to biodegradable herbicides such as Monsanto's Roundup[R] will lessen the impact of the chemicals that are needed. This is not always the case, however, because several companies are making plants resistant to herbicides that are not biodegradable and are very toxic. The government also gives substantial incentives that push agricultural practices in one direction or another, making it important to elect responsible individuals who will not promote excessive use of chemicals.
Opposition to Genetically Engineered Foods
A number of countries, groups, and individuals oppose the use of GMOs. This opposition is directed toward microorganisms, plants, and animals that either have a direct or indirect role in food production. The reasons for this opposition can be broken down into five general categories, which are summarized in the following sections (Brandt, 2003).
Those opposing the use of GMOs state that gene transfer between organisms not from the same species is unethical because human beings should not alter an organism in this way. The counterpart to this argument is the theory that humanity has had a profound effect on the evolution of many species, including gene transfer between species through the use of plant breeding. In addition, gene transfer between unrelated organisms occurs in nature all the time.
One safety concern is that releasing GMOs into the environment could have many unforeseen ecological consequences. Others say that moving organisms between continents could create more problems than GMOs because there is no known biological control to bring them back into equilibrium when these organisms are relocated. Another concern is that GMOs may not be safe to eat, which is possible. However, others say that when a GMO is produced, we know exactly which gene is being introduced so it is highly unlikely that a problem will arise, as compared to traditional plant breeding. When new plants are produced via traditional plant breeding, large segments of DNA with many genes are transferred between plants, which can cause many unknown problems. The possibility exists that the GMOs used for food will contain novel allergens; however, this can be readily tested for prior to making GMOs available to the general public.
Opponents of GMOs state that the purpose of corporations is to make money, not to protect the welfare of the general public. They also say that the general public should not rely on corporations to give them correct information about GMOs and that any information presented will be twisted to fit the corporations' needs. Although true in some regard, this does not present the full story because more often than not corporations promote human welfare by making new products. In fact, they change their products in response to demand, which may be for better nutrition or health. Therefore, to say all biotechnology companies are socially irresponsible is an incorrect statement.
Those who are against biotechnology say that it is driving toward high-input agriculture and will never contribute to making agriculture sustainable. Opponents of GMOs say that GMOs will probably lead to an increase in chemicals by introducing herbicide-resistant plants. Supporters of GMOs say that a combination of technology, government regulations, and tax laws determines the direction of agriculture, not the technology alone.
Opponents of GMOs say that we must return to an ecologically based stewardship of Earth instead of exploiting it as is the current trend. Although this sounds reasonable, remember that 5 billion people must be fed currently and 10 billion more must be fed in the future; therefore, we must work toward a world that uses all resources wisely and efficiently.
GENETICALLY MODIFIED CROPS PRODUCED IN THE UNITED STATES AND THE WORLD
The use of GMOs for food and in agriculture has generated a considerable amount of interest and controversy in the United States and around the world. Many people are strong supporters of gene-transfer technology, although others raise questions about environmental and safety issues. Crop varieties developed via genetic engineering were first introduced into commercial production in 1996. Today, these crops are planted on more than 109.2 million acres worldwide. As shown in Figure 13-13, the United States accounts for over two-thirds of all GMOs produced globally. The principal states growing the major genetically modified crops of corn, soybean, and cotton are shown in Figure 13-14. Genetically modified food crops grown by farmers in the United States include corn, cotton, soybeans, canola, squash, and papaya. Other major producers of GMOs are Argentina (primarilyGMOsoybeans) and Canada (canola).Worldwide, about 670 million acres of land are under cultivation, of which 16 percent was used for GMOs as of the year 2000. The four countries that grow 99 percent of the global GMO crop as of 2000 are the United States with 74.9 million acres, Argentina with 24.7 million acres, Canada with 7.4 million acres, and China with 1.2 million acres. Other countries with significant acreage in GMOs, although much less than the four major countries, which are South Africa, Australia, Mexico, Romania, Bulgaria, Spain, Germany, France, Uruguay, and Indonesia (Emrich, 2003).
[FIGURE 13-14 OMITTED]
U.S. REGULATION OF GENETICALLY MODIFIED FOOD AND AGRICULTURAL BIOTECHNOLOGY PRODUCTS
The regulation of biotechnology products in the United States is under the same U.S. laws that govern the health, safety, efficacy, and environmental impacts of similar products derived by more traditional methods such as plant breeding.
The FDA, the Department of Agriculture, and the EPA have the primary responsibility for regulating biotechnology products. The FDA is responsible for the safety of food and animal feed and for the safety and efficacy of human and animal drugs and biological materials. Within the FDA, the four centers with responsibilities for biotechnology products include the Center for Food Safety and Applied Nutrition (CFSAN), the Center for Veterinary Medicine (CVM), the Center for Drug Evaluation and Research (CDER), and the Center for Biologics Evaluation and Research (CBER).
The EPA is responsible for regulating the use of pesticides, setting allowable levels (tolerances) of pesticide residues in food, and regulating nonpesticidal toxic substances, including microorganisms.
The USDA is responsible for the safety of meat, poultry, and egg products. It also regulates potential agricultural plant pests, noxious weeds, and the safety and efficacy of animal biologics. Within the USDA, the Animal and Plant Health Inspection Service (APHIS) has the major responsibility for regulation of biotechnology products, with additional potential responsibilities for the Food Safety and Inspection Services (FSIS). The major statutes under which the FDA, EPA, and USDA have been given regulatory or review authority include the following:
* The Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) (EPA).
* The Toxic Substances Control Act (TSCA) (EPA).
* The Food, Drug, and Cosmetics Act (FFDCA) (FDA and EPA).
* The Plant Protection Act (PPA) (USDA).
* The Virus Serum Toxin Act (VSTA) (USDA).
* The Public Health Service Act (PHSA) (FDA).
* The Dietary Supplement Health and Education Act (DSHEA) (FDA).
* The Meat Inspection Act (MIA) (USDA).
* The Poultry Products Inspection Act (PPIA) (USDA).
* The Egg Products Inspection Act (EPIA) (USDA).
* The National Environmental Policy Act (NEPA).
You now have a general appreciation of plant biotechnology and the use of GMOs in agriculture. You learned about different types of biotechnology used today, including hydroponics, tissue culture used for micropropagation, the production of specialty chemicals, and the use of single cells as a source of genetic variability for plant improvement. In addition, you learned about genetically engineering plants--including methods for transferring foreign genes into plants such as Agrobacterium tumefaciens, particle bombardment, and electroporation--crop improvements through genetic engineering, and the production of biopesticides. You realize the pros and cons of genetic engineering by addressing commonly asked questions such as, Will genetically engineered plants be safe to eat? Is it possible that genetically engineered plants will become weeds or will transfer their genes to other plants thereby making them weeds? What impact do genetically engineered plants have on agricultural practices? Opposition to genetically engineered foods comes in the form of ethical considerations, safety considerations, anticorporate arguments, sustainability considerations, and philosophical considerations. In addition, you learned about genetically modified crops produced in the United States and the world, and you learned that now the United States regulates genetically modified food and agricultural biotechnology products.
Review Questions for Chapter 13
1. What are five commonly used methods of tissue culture?
2. What are four uses for tissue culture?
3. What are three methods for transferring foreign genes into plants?
4. What are five general categories for the opposition to genetically modified foods?
5. What are the three main agencies that are responsible for the regulation of biotechnology products in the United States?
Define the following terms:
genetically modified organism (GMO)
True or False
1. A commonly used commercial method for growing plants hydroponically is the nutrient film technique.
2. The growth of single cells in tissue culture can be used as a source of genetic variability for plant improvement.
3. Somaclonal variation occurs when somatic embryos are derived from single cells and are grown into mature plants.
4. Agrobacterium thuringiensis is an example of a bacterium that is used to produce an insecticidal protein that kills any insect larvae that eat the leaves or root of that plant.
5. The use of genetically engineered plants has the potential to increase the need for more chemicals in agriculture.
1. The use of hydroponics requires a large amount of capital and energy; therefore, only high-value crops such as -- and -- are grown using this method.
A. tomatoes and peppers
B. potatoes and corn
C. wheat and barley
D. None of the above
2. After callus is formed, the manipulation of the ratio of the plant hormones auxin and cytokinin can be used to promote
C. roots and shoots.
D. All of the above
3. Which of the following countries accounts for two-thirds of all genetically modified crops globally?
C. United States
Fill in the Blanks
1. The main goal of all agricultural research is to increase --.
2. -- is second in the production of GMOs on a worldwide basis.
Now that you understand plant biotechnology and GMOs, you will have the opportunity to explore this exciting area in more detail. In this activity, you will search Web sites that contain information about plant biotechnology and GMOs, including pros and cons. Select three sites and write a description for each site that includes the following information:
* the Web site address.
* the purpose of the site.
* an explanation of the area of plant biotechnology and/or GMOs.
* the pros and cons of plant biotechnology and/or GMOs, plus any other interesting information that you want to share in this area.
Brandt, P. (2003). Overview of the current status of genetically modified plants in Europe as compared to the United States. Journal of Plant Physiology, 160; 735-742.
Chrispeels, M. J., & Sadava, D. E. (1994). Plants, genes, and agriculture. Boston: Jones and Bartlett Publishers International.
Emrich, R. (2003). Discussion of current status of commercialization of plant biotechnology in the global marketplace. Journal of Plant Physiology, 160; 727-734.
Sonnewald U. (2003). Plant biotechnology: From basic science to industrial applications. Journal of Plant Physiology, 160; 723-725.
Figure 13-13 Breakdown of genetically modified crops in the United States and the world United States Soybeans, corn, (68%) cotton, & others Argentina Soybeans (23%) Canada Canola (7%) China Cotton (2%) Others (<1%) Note: Table made from pie chart.
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|Author:||Arteca, Richard N.|
|Publication:||Introduction to Horticultural Science|
|Date:||Jan 1, 2006|
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