Genetic Engineering

Because humankind has been highly inventive of xeno-biotic molecules (e.g. DuPont's ''better living through chemistry'') and has unsustainably used concentrated energy to smelt metals and create other toxins, some genetic engineering of plants and microbes will be necessary to achieve more sustainable waste management. Some xenobiotic substances such as polychlorinated biphenyls (PCBs), dichloro-diphenyl-trichloroethane (DDT), and several others have been partially or completely banned because of increasing accumulations in the environment. The lack of sufficient analogs among the 200 000 primary and secondary plant metabolites are the reason for these accumulations, but some microbiological and botanical transformations of these persistent pollutants do occur. Unfortunately, unmanaged ecosystems may not have all the transformation processes available in the same locations at the right times to achieve sustainable and safe degradation of persistent pollutants over the times scales consistent with most human endeavors. In addition to the need to develop better-managed ecosystems in order to treat and control wastes, some metabolic and genetic engineering of organisms will be necessary to clean up persistent organic pollutants and to properly manage xenobiotic compounds invented in the future.

Plants are the most likely candidates for most genetic engineering to support sustainable waste management. While microbial enzymatic activity is better adapted to mineralization of organic pollutants, microorganisms lack the degree of self-engineering that plants exhibit. Most animals, especially mammals, may be too mobile and too charismatic to genetically engineer for waste management based on the ethical values of most contemporary societies. In contrast, plants are easier to control and manage because the most likely candidates are rooted in place and propagation can be controlled with single-sex clones and harvesting before reproduction occurs. Fencing, netting, and other techniques control plant exposure to other organisms. Compared to animal husbandry, humankind has more experience in agricultural control and production of plants for food and fiber. Because of the role of sunlight as the primary energy source for synthesis, plant management is generally less intensive and less expensive than animal husbandry and unit process production of microbial enzymes.

In general, the control of genetically modified organisms seems to favor large plants, rooted in place and capable of controlling groundwater, soil redox, and many natural insults. Microorganisms are not easily detected in real time, are difficult to control in the environment, and lack the genetic diversity of plants and animals. Animals are much more mobile and these ranges of movement are not always known, especially in response to environmental insults. As precedents for genetically engineering plants for waste management, the use of genetically modified soybeans to tolerate gly-phosate-type herbicides, genetically modified corn for animal feed, and modified tomatoes are standard practices in the US and some countries, but not in Europe and elsewhere.

Already important research on the development of transgenic plants (genetically engineered deoxyribonu-cleic acid (DNA) with genes from microorganisms or animals) provides insight into what is possible and what other research is needed. The initial work includes testing and developing

• transgenic Arabidopsis thaliana, yellow poplar (Liriodendron tulipifera), tobacco (Nicotiana spp.), Indian mustard (Brassica juncea), and eastern cottonwood (P. deltoides) to tolerate and 'phytovolatilize' (see Table 1) mercury and selenium (but any application to mercury contamination can only be done with a site-specific assessment to establish that the atmospheric release of mercury does not present an increased risk; Table 1);

• transgenic plants to tolerate, accumulate, or speciate arsenic, cadmium, cooper, zinc, nickel, lead, and iron;

• transgenic tobacco (Nicotiana spp.) expressing cyto-chrome P450 from the human gene 2E1 that breaks down TCE 640 times faster than control plants;

• transgenic tobacco (Nicotiana spp.) expressing the Enterobacter cloacae nitroreductase nsfI to tolerate and more rapidly transform explosives like TNT;

• transgenic poplar (Populus spp.), Indian mustard (B. juncea), tobacco (Nicotiana spp.), rice (Oryza sativa), and potato (Solanum tuberosum) enhanced tolerance and metabolism of herbicides;

• transgenic flowering plants to remove atmospheric nitrous oxides in major cities around the world;

These case studies establish what is possible. But so far these pioneering studies do not establish that genetic engineering is useful and applicable in waste management. Merely speeding up the degradation of explosives or TCE is not useful until proof is available that genetic engineering is less expensive and more sustainable that planting up to 640 more wild-type plants or taking 640 times longer to remove TCE from a site. By contrast, the lack of natural plants that tolerate and phytovolatilize mercury and arsenic are justification for genetic research on mercury and arsenic management. Because only microorganisms transform nitrous oxides, there is also a high likelihood of applicability of transgenic plants for atmospheric cleanup in large cities.

The studies so far do not establish that transgenic plants efficiently maintain transgenes from one generation to another. This further reduces the risks of transgenes being dispersed in the environment and favors longer-living trees as the best organism to engineer. Applications are much more expensive, and perhaps unsustainable if each generation of plants must be re-engineered genetically.

Genetic engineering is not only necessary to sustain-ably manage persistent organic pollutants and some new xenobiotic chemicals, but perhaps for sustainable phyto-accumulation of heavy metals, metalloids, and other elements as well. In addition, transgenic plants can be engineered to take advantage of a wider range of specialized genes in bacteria to mineralize versus transforming contaminants to more or less toxic by-products. Transgenic plants might also express more active enzymes from mammals. These transgenic enhancements can be put into phreatophytes, fast-growing crops, long-living trees, or other specialized plants to achieve much wider control of wastes in the environment. However, a super-transgenic plant to control and manage every witches' brew of hazardous chemicals at all contaminated sites is highly unlikely.

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