Genetic Engineering to Improve Phytoremediation

Plant productivity is controlled by many genes and is difficult to promote by single gene insertion. Genetic engineering techniques to implant more efficient accumulator genes into other plants have been suggested by many researchers (Cunningham and Ow 1996; Brown et al. 1995; Chaney et al. 2000). Implanting more efficient accumulator genes into other plants that are taller than natural plants increases the final biomass. Zhu et al. (1999) genetically engineered Brassica juncea to investigate rate-limiting factors for glutathione and phytochelatin production; they introduced the Escherichia coli gshl gene. g-ECS transgenic seedlings showed increased tolerance to Cd and had significantly higher concentrations of phytochelatins, g-GluCys, glutathione, and total nonprotein thiols as compared to wild-type seedlings. The potential success of genetic engineering may be limited by anatomical constraints (Ow 1996).

The effectiveness of transgenic plant varieties at increasing production and lowering production costs has been demonstrated in the cases of virus-, insect-, and herbicide-resistant plants, in which average increases in production of 5-10% and savings in relation to herbicides of up to 40% and to insecticides of between $60

and $120 per acre were reported in 1996 and 1997 (James 1997). However, these increases in total yield, impressive as they are in terms of their economic and environmental value, will have a limited impact for global food supply. In fact, most of the developments in transgenic crops are aimed either at reducing production costs in agricultural areas that already have high productivity levels, or at increasing the value added to the final product by improving, for instance, oil quality. This trend has been used by developed countries to limit the production of key products, like cereals, meat, and dairy products due to the reductions in the international prices of these products, and also to reduce the intensive use of fertilizers and pesticides because of their harmful effects on the ecosystem.

At a global level, a more effective strategy would be to increase productivity in tropical areas, where an increase in food production is required and where crop yields are significantly lower than those obtained in developed countries. In tropical areas, the losses caused by pests, diseases, and soil problems are exacerbated by climatic conditions that favor high levels of insect pests and vectors, and by a lack of the economic resources needed to purchase insecticides, fertilizers, and high-quality seeds. In addition to low productivity levels, postharvest losses in tropical areas are very high, again because of climatic conditions that favor fungal and insect infestation and because of the lack of appropriate storage facilities. Despite efforts to prevent pre- and postharvest crop losses, pests destroy over half of all crop production worldwide. Preharvest losses caused by insects, the majority of which occur in the developing world, are calculated at around 15% of the world's production.

Using biotechnology to produce transgenic plants that better withstand diseases, insect attack, or unfavorable soil conditions is not a simple task. There are an estimated 67,000 species of insects worldwide that damage crops, and a similar or even higher number of plant pathogens. For instance, in the case of Phaseolus vulgaris, over 200 diseases and 200-300 species of insects can affect bean productivity (Van Schoonhoven and Voysest 1980). These numbers give an idea of the complexity of the task that scientists face in increasing productivity. There are of course a certain number of diseases and insect pests that can be singled out as the most important constraints on the production of each crop. However, it is also true that when a particular disease or insect pest is controlled, others that were originally considered to be minor pests can then flourish and become major productivity constraints themselves.

One of the major advantages of plant biotechnology is that it can generate strategies for crop improvement that can be applied to many different crops. In this sense, genetically engineered virus resistance, insect resistance, and delayed ripening are good examples of strategies that can benefit many different crops. Transgenic plants of over 20 plant species that are resistant to more than 30 different viral diseases have been produced using different variations of the pathogen-derived resistance strategy. Insect-resistant plant varieties that use the D-endotoxin of Bacillus thuringiensis have been produced for several important plant species, including tobacco, tomato, potato, cotton, walnut, maize, sugarcane, and rice. Of these, maize, potato, and cotton are already under commercial production. It is envisaged that these strategies can be used for many other crops that are important for developing countries. Genetically engineered delayed ripening, although tested only on a commercial scale for tomato, has an enormous potential application in tropical fruit crops, which suffer severe losses because they ripen rapidly (in many developing countries there are neither appropriate storage conditions nor adequate transportation systems to allow their efficient commercialization).

To date, most of the developments in plant gene transfer technology and the different strategies for producing improved transgenic plant varieties have been driven by the economic value of the species or the trait. These economic values are in turn mainly determined by their importance to agriculture in the developed world, particularly the United States and Western Europe. This economical emphasis is understandable, because important investments are needed to develop, field test, and commercialize new transgenic plant varieties. However, in terms of global food production, it is necessary to ensure that this technology is effectively transferred to the developing world and adapted to the local crops and/or local varieties of crops for which it was originally developed. Developing improved transgenic versions of local varieties or local crops is not a trivial issue; in most, if not all, cultures, the use of specific crops has a deep social and/or religious meaning. Cultural preservation is just as important as environmental preservation. Cultural aspects of technology transfer need to be considered because simply replacing crops to increase productivity could have an enormously negative effect on certain cultures, and new introductions may not be accepted easily for human consumption.

It is unfortunate that most developing countries do not have sufficient resources to implement the biotechnological capacity needed to solve the major problems that limit agricultural productivity, at least not in the time frame that is required to cope with the increasing demand for food. However, it is in the developing world that biotechnology could have its biggest impact in increasing crop production, especially in the areas of the world where yields are low because of the lack of technology. Plant genetic engineering could be considered a neutral technology that in principle does not require major changes in the agricultural practices of farmers in developing countries. Perhaps more importantly, it has the potential to bring about great benefits to the small farmers who lack the economic resources to purchase agrochemicals or prevent postharvest losses because of the lack of storage facilities.

Whether there is time to increase agricultural productivity in the developing world is a question with a complex answer, because there are many factors that need to be taken into account to make this happen. We need to identify and establish mechanisms of technology transfer from developed countries, from both academic institutions and the private sector, to the developing world; there is a need to create a sufficient number of research centers with the capacity to acquire this technology, adapt it to local crops, and develop their own technologies. Seed production facilities must be improved, and an effective mechanism implemented to reach subsistence farmers with this new technology.

To meet these requirements, several economic, political, and social issues must be dealt with to ensure the general application of plant biotechnology to the agriculture of developing countries. The discussion of these issues goes beyond the scope of this chapter. However, it is our opinion that it will not be technological limitations but rather political and/or economic constraints that will determine how successful we are at supplying food to the hundreds of millions of people who will be malnourished in the next millennium.

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