Biotechnology: A new era for plant pathology and plant protection

Plant biotechnology ushers in a new era for plant sciensts working to maintain healthy plants, opmize crop yields, and minimize pescide usage. One of the ulmate aims of agricultural biotechnology is to feed an expanding world populaon.

During the last 30 years, producon of the main food crops has doubled. This increase of producon has mainly been achieved by introducon of high-yielding variees, irrigaon, and the use of ferlizers and pescides.

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, represenng 230 million people. In many regions of the world, the intensificaon of crop producon has led to deterioraon of soil ferlity, erosion, salinizaon, reducon of biodiversity, and other deleterious side-effects.

The use of pescides has more than tripled since 1970 and is a growing concern especially in developed countries. Despite the intensive use of chemical crop protecon methods, the losses due to pests, pathogens and weeds are more than 40 % of aainable producon, represenng a value of more than 240 billion US$.

Genec engineering offers new possibilies for the breeding of plant variees with increased resistance to pests and pathogens. New resistant variees may lessen the dependence on pescides and help securing www.africaagricultureinsight.com sufficient crop yields in the future. In plant genec engineering, genes from different organisms (other plants, bacteria, viruses, etc.) are transferred into the genome 1 of a plant cell . The bacterium Agrobacterium tumefaciens is frequently used as a vehicle for the introducon 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 producon of compounds used by the bacteria. In genec 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 parcle bombardment is oen applied. In this method, gold or tungsten parcles 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. Aer incorporaon 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, coon, 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 protecon to bacterial and fungal diseases are sll 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 invesgaons have led to a detailed knowledge of the mechanism and specificity of toxin acvity. 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 infestaons 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 digeson. For example, plant-derived protease inhibitors prevent protein degradaon, and amylase inhibitors block starch- degrading enzymes in the insect midgut. Some of these strategies have proven to be effecve and may soon be used in the development of commercial variees. Virus resistance is mostly achieved by introducing gene sequences derived from pathogenic viruses into the crop genome. The introducon of genes coding for viral coat proteins has been very successful.

During the last years, this strategy has led to a number of crop variees resistant to important plant viruses. More recently, also other viral genes were found to confer resistance, e.g. replicase genes, defecve viral genes or ansense coat protein genes. The mechanisms of resistance are not yet completely understood. Strategies applied to achieve fungal resistance make use of plant genes acng 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 protecon was mostly to low to be of agronomic importance. Chinase 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 protecon in some cases. The growing understanding of plant defense mechanisms is expected to lead to increased levels of protecon in the near future. Also methods invesgated to obtain resistance to bacteria have not led to high levels of protecon yet. Reducon 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 aack and had a similar effect in some plants. Other parally successful strategies make use of genes which code for toxindetoxifying enzymes or plant genes involved in the response to pathogen aack.

Besides genecally engineered plants, also viruses and bacteria have been genecally altered in order to develop new crop protecon methods. Baculoviruses are insect pathogens which have been used as a biological pescide since the 1930s. As these viruses may take weeks to kill their host aer infecon, their usefulness has been limited. By transferring genes coding for insectspecific toxins, insect hormones and insect enzymes into the virus genome, the killing me has been reduced by up to 50 %, which is not enough to achieve sufficient protecon. 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 www.africaagricultureinsight.com sprayed on the crop plants like a pescide. Another approach uses bacteria living inside of plants for Bt toxin delivery The safety aspects of transgenic organisms have been discussed and invesgated since the first successful gene transfer in the early 1970s. The release of transgenic plants is subject to different legal regulaons. Before a transgenic crop may be released, potenal hazards like the possibility of gene transfer to other plants or microorganisms, weediness of the engineered crop, and the expression of undesirable traits resulng from secondary effects of the gene inseron are examined. Also possible toxic and allergenic effects are analyzed, especially if the engineered plant is desned to serve as a food crop. So far, no deleterious effects of transgenic plants or other organisms have been reported.

Several other transgenic crops are approaching commercializaon. 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, variees with enhanced resistance against fungal and bacterial pathogens may also become available. Other applicaons of transgenic plants which may reach the marketplace within some years include e.g. coon with altered fibers, crops with improved nutrional value and plants producing biodegradable plasc, cheap vaccines and pharmaceucals.