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Summary

During the last 30 years, production of the main food crops has doubled. This increase of production has mainly been achieved by introduction of high-yielding varieties, irrigation, and the use of fertilizers and pesticides. Due to this increase, the share of people in developing countries with insufficient average food supply has decreased from 74 % in 1962 to 6 % in 1988, representing 230 million people. In many regions of the world, the intensification of crop production has led to deterioration of soil fertility, erosion, salinization, reduction of biodiversity, and other deleterious side-effects. The use of pesticides has more than tripled since 1970 and is a growing concern especially in developed countries. Despite the intensive use of chemical crop protection methods, the losses due to pests, pathogens and weeds are more than 40 % of attainable production, representing a value of more than 240 billion US$.

Genetic engineering offers new possibilities for the breeding of plant varieties with increased resistance to pests and pathogens. New resistant varieties may lessen the dependence on pesticides and help securing sufficient crop yields in the future.

In plant genetic engineering, genes from different organisms (other plants, bacteria, viruses, etc.) are transferred into the genome of a plant cell¹. The bacterium Agrobacterium tumefaciens is frequently used as a vehicle for the introduction of foreign DNA into the plant genome. In nature, these bacteria transfer some of their genes into the plant genome, thereby inducing a plant disease which leads to production of compounds used by the bacteria. In genetic engineering, the genes causing disease are replaced by genes conferring other traits. For some plants, e.g. wheat or maize, Agrobacterium-mediated gene transfer is difficult or not possible. In these cases, a technique called particle bombardment is often applied. In this method, gold or tungsten particles of about 5 µm in diameter are coated with DNA and shot into plant cells, where the DNA is released and incorporated into the plant genome. After incorporation of the foreign gene, a plant is regenerated from an engineered cell, and the traits coded for by the transferred gene are expressed by the plant.

Resistance of transgenic plants to insect pests or diseases has been achieved in more than 20 different crops, including maize, potato, squash, cotton, soybean, oilseed rape, tomato, tobacco, alfalfa, rice, barley and others. Very high levels of resistance to insect pests and viral diseases have been reached, while examples of successful protection to bacterial and fungal diseases are still scarce.

Insect resistance has mostly been obtained by using a gene derived from the common soil bacterium Bacillus thuringiensis. This bacterium produces a protein called Bt toxin which is toxic for certain insects. Intensive investigations have led to a detailed knowledge of the mechanism and specificity of toxin activity. In several studies, no effect of Bt toxin on humans, other mammals, and most non-target insects could be shown. Transgenic plants expressing Bt toxin were found to be protected against repeated heavy infestations of the target insect pest which totally devastated non-transgenic control plants. Other approaches to insect resistance focus on the use of genes which are part of the natural defense system of plants. The products of these genes interfere with insect digestion. For example, plant-derived protease inhibitors prevent protein degradation, and amylase inhibitors block starch- degrading enzymes in the insect midgut. Some of these strategies have proven to be effective and may soon be used in the development of commercial varieties.

Virus resistance is mostly achieved by introducing gene sequences derived from pathogenic viruses into the crop genome. The introduction of genes coding for viral coat proteins has been very successful. During the last years, this strategy has led to a number of crop varieties resistant to important plant viruses. More recently, also other viral genes were found to confer resistance, e.g. replicase genes, defective viral genes or antisense coat protein genes. The mechanisms of resistance are not yet completely understood.

Strategies applied to achieve fungal resistance make use of plant genes acting on different levels of the plant defense system against pathogens. Several of these strategies have led to increased resistance, but so far the level of protection was mostly to low to be of agronomic importance. Chitinase and glucanase genes coding for enzymes which break down fungal cell walls have been used in several crops including rice and have led to significant protection in some cases. The growing understanding of plant defense mechanisms is expected to lead to increased levels of protection in the near future.

Also methods investigated to obtain resistance to bacteria have not led to high levels of protection yet. Reduction of disease development in tobacco was achieved by transferring a cecropin gene derived from the Giant silk moth. Cecropins are produced by insects to fight pathogen attack and had a similar effect in some plants. Other partially successful strategies make use of genes which code for toxindetoxifying enzymes or plant genes involved in the response to pathogen attack.

Besides genetically engineered plants, also viruses and bacteria have been genetically altered in order to develop new crop protection methods. Baculoviruses are insect pathogens which have been used as a biological pesticide since the 1930s. As these viruses may take weeks to kill their host after infection, their usefulness has been limited. By transferring genes coding for insect-specific toxins, insect hormones and insect enzymes into the virus genome, the killing time has been reduced by up to 50 %, which is not enough to achieve sufficient protection. Bacillus thuringiensis (Bt) toxin genes have been introduced into different bacteria for Bt toxin delivery to insect pests. In one approach, transgenic bacteria expressing Bt toxin are killed and then sprayed on the crop plants like a pesticide. Another approach uses bacteria living inside of plants for Bt toxin delivery.

The safety aspects of transgenic organisms have been discussed and investigated since the first successful gene transfer in the early 1970s. The release of transgenic plants is subject to different legal regulations. Before a transgenic crop may be released, potential hazards like the possibility of gene transfer to other plants or microorganisms, weediness of the engineered crop, and the expression of undesirable traits resulting from secondary effects of the gene insertion are examined. Also possible toxic and allergenic effects are analyzed, especially if the engineered plant is destined to serve as a food crop. So far, no deleterious effects of transgenic plants or other organisms have been reported.

Between 1986 and 1993, more than 1000 field releases of transgenic plants were conducted in 32 countries, and the number is increasing rapidly. The USA and Canada account for more than half of all trials recorded. Thirty-eight different plant species have been tested. Potato, oilseed rape, tobacco, maize, tomato and sugar beet constitute together 70 % of all trials. Among these crops, about one third of all trials evaluated pest and disease resistance (16 % virus resistance, 13 % insect resistance, 3 % fungal resistance and 1 % bacterial resistance). The trait most commonly tested was herbicide tolerance (34 %). Other traits examined were quality improvement (20 %) and marker genes (10 %).

In the USA, permits for commercialization of 10 transgenic crop varieties have been granted as of July 1995: Insect resistant potato, maize and cotton expressing Bt toxins, virus resistant squash expressing viral coat proteins, two tomato varieties with extended shelf-life, one tomato variety with enhanced process value, oilseed rape with altered oil composition, and herbicide tolerant cotton and soybean. In Canada, herbicide tolerant flax and oilseed rape have been commercialized. In the European Union, a herbicide tolerant tobacco variety has obtained a permit for market introduction.

Several other transgenic crops are approaching commercialization. In the field of pest and disease resistance, it is likely that more insect resistant crops expressing Bt toxins or virus resistant crops engineered with viral genes will enter the market in the near future. Within some years, varieties with enhanced resistance against fungal and bacterial pathogens may also become available. Other applications of transgenic plants which may reach the marketplace within some years include e.g. cotton with altered fibers, crops with improved nutritional value and plants producing biodegradable plastic, cheap vaccines and pharmaceuticals.

¹ See glossary for basic terms of genetics.