2. Biotechnology in Plant Agriculture Pest Management

Crop breeders have traditionally considered the development of pest resistance in crops and animals to be of primary importance. Genetic engineering can speed up the process of developing resistant varieties and potentially can introduce genes that will provide new ways for plants to withstand pests.

Insect Control

There has been much interest in using genetic engineering for insect control. The interest in developing insect-resistant varieties results from the desire to reduce the extent of pesticide use and to avoid the development of pesticide resistance in some of the important insect pests. The development of crops with insect resistance is not a new idea. Classical breeding programs have been developing varieties with insect and disease resistance or tolerance for many years. The problem has been that insects are generally able to overcome the resistance in 2 to 10 years.

Another aim of biotechnology in insect control is to provide materials that are selective, will not affect nontarget species (including humans), and to which insects will not easily develop resistance. Much of the work to date has focused on applications of Bacillus thuringiensis (Bt), an insect control bacterium, but other approaches are being explored as well.

Applications Involving Bt. Bt has been used as a biocontrol agent for 20 years. It is a naturally occurring bacterium that produces protein crystals that are toxic to insects. These proteins are very specific insecticides with activities against some species of caterpillars, mosquitoes, flies, and beetles.

Recent research on Bt has followed two approaches. The first is the selection and development of Bt strains that are specific for other pests or that produce higher concentrations of the compounds. Different strains of Bt to control plant- and animal parasitic nematodes, animal-parasitic liver flukes, protozoan pathogens, and mites have been identified.

The second approach is to move the gene that controls the production of the Bt proteins into crop plants. Bt genes for control of an insect species have been incorporated into tomatoes, tobacco, cotton, and corn with good results. Bt proteins are highly specific to particular insect species. Therefore, to achieve control of several different insect species at one time, many different gene codes would have to be incorporated into the plant's genetic makeup.

The incorporation of Bt genes into plants causes concern that insects will likely develop resistance to the Bt strain. The probability is especially high in current cases where the gene is expressed throughout the plant. For example, if three to four generations of an insect feed on the plant leaves or fruits throughout the growing season, the insect would be constantly exposed to the chemical. It is likely that the insect will develop resistance to the chemical. It may become possible to direct the expression of the Bt products to only the fruit tissues, meaning that only one generation of insects would be exposed. It may also become possible to incorporate the genes in such a way that they are expressed only after an insect begins to feed on the plant. Finally, several different Bt genes may be introduced at one time, making the development of resistance more difficult.

Other Approaches to Insect Control. Research has been conducted on developing insect resistance mechanisms that can be used in different plant species. This work has centered on a chemical complex known as the cowpea trypsin inhibitor (CpTI). Results obtained so far suggest that CpTI will control a wider range of insects than the specific Bt products. In experiments on tobacco, good control of foliage feeding caterpillars was achieved. There appears to be no adverse affect on humans because CpTI comes from the cowpea and has not appeared to cause health problems when cowpeas are eaten raw or cooked.

Another area of interest is the incorporation of insecticide resistance into the natural predators or parasites of major and secondary insect pests. Secondary insect pests are those that are controlled by natural predators until insecticides are applied to control a major insect pest. The insecticides also kill the natural predators, allowing the secondary insect pests to flourish. Resistant types of the predators and parasites are being sought so they can be used for control. Modern genetic techniques will help in understanding the mechanisms of insect resistance and in the development of greater resistance in the future.

Weed Control

Herbicide Tolerance. One of the first commercial uses of biotechnology involves the genetic improvement of crop tolerance to herbicides. Herbicide tolerance is a plant's ability to endure the effects of a herbicide at the rate normally used in agricultural production. Herbicide resistance is the ability of a plant to be unaffected at any feasible rate of herbicide application. Most crops are resistant to one or more herbicides. For example, corn is naturally resistant to atrazine, corn and soybeans are tolerant to alachlor (Lasso) and metachlor (Dual), but soybeans are not tolerant to atrazine.

Biotechnology has provided plant scientists with additional tools to determine the chemical and genetic modes of action of many of these herbicides and also the mechanisms that account for a plant's natural tolerance or resistance to herbicides. As a result, scientists will use this knowledge to incorporate herbicide tolerance into crop plant species.

Several different methods have been used successfully to develop herbicide-resistant crop varieties. One method is to find a closely related species that has herbicide tolerance or resistance and then, through classical breeding techniques, to incorporate that tolerance into the desired plant. This process has been successfully applied to canola (oilseed rape) using a related species (bird's rape) that was found to be resistant to atrazine. This new atrazine-resistant variety of canola is currently being cultivated in Canada.

A second method has been the use of cell or tissue culture to test many different lines of plants for tolerance to a specific herbicide. As a result, Pioneer Hi-Bred has been able to develop three corn hybrids for use with Pursuit, a herbicide that typically kills conventional corn hybrids.

A third method has been to determine the specific gene or genes within a plant or microbe that allow tolerance or resistance to a specific herbicide. This gene is inserted into the plant of interest, which is then tested for tolerance to the herbicide. This method has made it possible to develop cotton tolerant to the herbicide bromoxynil (Buctril) and to soybeans tolerant of glyphosate (Roundup).

Bioherbicides. In the near future, biotechnology will probably influence weed control in at least four ways. The first is through the production of bioherbicides. Bioherbicides are fungal or bacterial products selected for their ability to cause disease in specific plants, such as weeds, without harming desirable plants. They may be applied in the same manner as conventional herbicides.

At present, two bioherbicides are being marketed for the control of specific weeds that are normally hard to control: DeVine for control of strangler vine in Florida citrus and Collego for control of northern jointvetch in rice and soybeans in Arkansas, Louisiana, and Mississippi. DeVine has been so successful in destroying strangler vine that the market for the product has almost been lost. The reason for its great effectiveness is that the product remains in the soil and gives 95 to 100 percent control for 6 to 10 years after a single application.

There are other bioherbicides in various stages of research and development for such things as control of prickly sida in cotton and soybeans, control of sicklepod in cotton and soybeans, control of spurred anode in cotton, control of velvetleaf, and growth suppression of water hyacinth. Biotechnology will play a major role in helping overcome problems in manufacturing these bioherbicides by the development of better fermentation processes, as well as assisting in the isolation of the genetic determinants of virulence, specificity, sporulation capacity, toxin production, and tolerance to climatic stresses.

Natural Control Compounds. A second application of biotechnology that shows promise is the development of microbial and secondary plant products for use as herbicides. Much effort has been devoted to determining the actual compounds associated with allelopathy (the ability of one plant to influence the growth of others nearby by releasing chemical compounds). Many of these compounds have limited selectivity and a lack of stability. However, one herbicide (Herbiaceae) derived from chemicals found in a naturally occurring microbe is being marketed in Japan. Herbiaceae exhibits strong herbicidal activity against a wide spectrum of grass and broadleaf weeds when it is applied to foliage.

A more important role for these compounds is to provide models for the development of new chemicals that could be produced as commercial herbicides. It may be possible to produce synthetic derivatives of these chemicals that are more stable under field conditions, have greater selectivity than the natural chemicals, or have other advantages over the original chemicals. Several companies have developed chemicals based on this natural herbicide chemistry. It is believed that the naturally occurring herbicides will be safer for the environment because many of them are degraded rapidly in the soil.

Potential Concerns. Concerns have been raised about these developments. One is that development of herbicide-resistant or herbicide-tolerant crop varieties may lead to overdependence on herbicides. Many of the herbicides being used for the development of crop herbicide resistance are broad spectrum, low-mammalian-toxicity chemicals that are thought to be safer for the environment than conventional herbicides. However, attempts are also being made to develop varieties with resistance to certain other compounds, such as atrazine, that are more persistent and have been found in groundwater in certain areas of the United States. Continued use of these older chemicals, ones that tend to persist in the soil and may move into groundwater, could cause environmental problems.

Associated with the exclusive use of one herbicide for weed control in each crop in a rotation is the possible development of herbicide-resistant weeds. There are already over 100 species of weed plants known to have developed resistance to one herbicide or another. Much of this resistance has resulted from the continual use of one type of herbicide (for example, triazines) increasing the probability of developing resistant plants within the weed population. The use of a comprehensive weed control program, including rotating chemicals based on their mode of action, should allow the successful use of the new, "safer" herbicides.

Another concern is the possible movement (outcrossing) of the "engineered" genes from the host plant to related weed species. This possibility has not received much attention in the United States because in most cases our weed species are not closely related to crop species. An exception is the vegetable industry in California because many of the vegetable crops have closely related weed relatives in the wild. On the other hand, the possibility of outcrossing may not be very likely, considering the lengths to which plant breeders had to go to accomplish interspecific hybridization before genetic engineering was introduced.

Disease Control

Control of disease is a subject of great interest for biotechnologists. The majority of advances have been in control of viral diseases. Because most viruses are spread mechanically or through insect vectors, control efforts have traditionally revolved around control of the vectors and destruction of diseased plant material.

Viruses are composed of two parts--the viral DNA and a coat protein that surrounds the viral DNA. Researchers have known about the phenomenon of cross-protection: that infection of a plant by a mild strain of virus can often protect the plant from a serious infection by a more virulent related strain. The researchers have recently discovered that it is the presence of the coat protein that restricts the infection by the virus in cross-protection. By incorporating the genes for the coat protein into the plant it is possible to have the plant itself produce low levels of the coat protein. These low levels of the coat protein delay or restrict infection of the plant by the virus.

An example is the incorporation of genes for the production of the coat protein of the tobacco mosaic virus (TMV) into tomato plants. Untreated tomato plants infested with TMV showed up to 60 percent loss in yield, whereas resistant plants showed no yield decrease after inoculation with the virus. The level of the viral coat protein in transgenic plants (those that contain genetic material from a different organism) is lower than that found in plants infected with endemic strains of the virus.

Cross-protection via gene transfer offers a number of advantages. The first is that the protection is essentially permanent, similar to that afforded animals by vaccines. The need for the use of chemicals to control insect vectors is also reduced.

Other possible aspects of disease control in which biotechnology may play a role include the use of plant disease biocontrol fungi and the development of fungal resistance in plants. Plant disease biocontrol fungi are naturally occurring organisms that are antagonists for certain soil-borne plant pathogens. Biotechnology will play a role in better understanding the mechanisms for host specificity and virulence of these fungi and may lead to the production and formulation of control products.

Fungal resistance is an area of interest, but no great progress has been made yet. Biotechnology will enhance our understanding of the mechanisms that control a plant's ability to recognize and defend itself against disease-causing fungi.

Other Agricultural Applications of Biotechnology

Although biotechnology efforts have focused primarily on pest management, there are other potential applications to crop production. The possibilities are highly varied, and new applications are developing so quickly that it is difficult to keep up with all of them. Here are a few examples of current biotechnology research on plants.

Enhancing the Value of Plant Products

Plant breeders have long been interested in modifying components of crop products, such as the amount of various amino acids contained in the proteins of corn used for human and animal feeds. Traditional breeding has developed cultivars with high concentrations of lysine, an essential amino acid. The increase in lysine content, however, was accompanied by a 10 percent decrease in yield. Scientists are working to incorporate a gene in corn that will increase the amount of lysine without the associated yield decrease. They have also developed corn with a higher-than-usual oil content for use as a source of vegetable oil and feed.

Oilseeds are another crop of interest. Researchers are seeking to improve the nutritional qualities of vegetable oils along with the type and concentration of these oils within the plant seeds. Work is under way to develop varieties of sunflowers and canola with high oleic acid content. Researchers have also produced a low-palmitic-acid soybean, making the oil lower in saturated fats and therefore more comparable nutritionally to canola oil.

Bioengineers have recently been able to alter canola so that it can also produce lauric acid, a key raw material for the soap, detergent, oleochemical, personal care, and food industries. Currently, the only commercial sources of lauric acid are coconut and palm kernel oils.

Improving Fruit Quality

Food producers and processors are interested in the ripening of fruit and in the processing quality of fruits and vegetables. An example is improving the processing quality of tomatoes, a major food crop and a significant source of vitamin C. The characteristics of most interest to processors are percentage of solids and consistency. It has been estimated that an increase of 1 percent in tomato solids would save the processing industry between $70 and $100 million annually. Another objective of tomato research is the desire to produce a better-tasting "vine-ripe" tomato that has a greater shelf life and will resist bruising during shipment. Currently, supermarket tomatoes are picked green, then artificially ripened just before shipment. This process has ensured that the tomatoes resist bruising, but flavor is affected.

Biotechnology has been used to develop the Flavr Savr tomato (bottom), which resists spoilage longer than conventional tomatoes (top).

Studies have shown that two different mechanisms within the tomato appear to control the ripening process. The first is the production of ethylene within the fruit. Researchers have been able to isolate the compound responsible for ethylene production and to regulate this compound, lengthening the ripening period. Understanding of the second mechanism has been used to develop the Flavr Savr tomato. The researchers found an enzyme that is responsible for degrading tomato flavor and fruit firmness. Genetic engineering techniques have allowed researchers to block this enzyme, which has led to a tomato that can remain on the vine to ripen (so that it will have better flavor) and still can be shipped with a minimum of bruising and spoilage. Processed juice made from these tomatoes has shown greater consistency and a higher percentage of solids than that made from the conventional varieties.