Plastics from Plants

Experimental varieties of transgenic mustard, corn, and cotton have been genetically engineered to produce the world's first plant-synthesized plastic compounds. It was hoped that such plastics, properly industrialized, might provide a green alternative to traditional petrochemical plastics such as polystyrene, polyethylene, and polypropylene, because the crop-based plastics would come from a renewable resource (living plants, rather than fossil material), and they would biodegrade relatively quickly after disposal.

The first plastic biopolymer to be coaxed from transgenic plants was polyhydroxybutyrate (PHB), a biodegradable high-molecular-weight polyester with chemical and physical properties similar to polypropylene. This polyester is produced naturally by Alcaligenes eutrophus, a bacterium from which genetic engineers borrowed the transgenes that were delivered to host plants. The initial experiments were conducted in 1992 on Arabidopsis thaliana, but the GM mustards turned out to be sickly due to overactivity of the transgenes. The first such experiments on cotton (Gossypium hirsutum) were conducted in 1996, with more promising results.

In native cotton, the commercially valuable fibers are seed hairs with two walls, one composed of cellulose and the other of proteins, waxes, and polysaccharides. Variation in the molecular composition and microstructure of the walls influences the cotton fibers' characteristics such as strength, thermal properties, and water-absorption profiles. In their experiments, the scientists isolated two bacterial genes (encoding the enzymes acetoacetyl-CoA reductase and polyhydroxyalkanoate synthase) that form part of the metabolic machinery by which Alcaligenes bacteria convert acetyl-CoA to PHB. The microbes store and use this plastic material in much the same way that animals use fats.

Using a gene gun (see "Galls and Goals" in the appendix), the researchers literally fired these bacterial genes into the cells of cotton embryos. Once incorporated and activated, the transgenes synthesized PHB from acetyl-CoA already naturally present in these plants. The resulting plastic granules, deposited between the cellular walls, measurably increased the insulating properties of the resulting cotton fibers. These were not plastic plants like artificial

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flowers and trees, but true living creatures otherwise showing normal growth. However, the plastic content of the seeds was minuscule—less than four-tenths of 1% of the total dry weight of the cotton fibers.

To be of genuine value to the textile industry, at least two problems will have to be overcome: First, the PHB content of cotton fibers must be increased manyfold (an outcome that might adversely affect the plants). Second, PHB naturally biodegrades when exposed to sunlight and water, so measures will have to be taken to ensure that this does not occur in any fabrics derived from the transgenic cotton. These challenges are being tackled but have not yet been overcome.

What about other potential applications? The fact that plant-produced (or microbe-produced) bioplastics are renewable and biodegradable would argue strongly in their favor, compared to their synthetic analogues made from petroleum products. Manufacturing conventional plastics consumes more than 270 million tons of oil and gas worldwide every year, depleting petroleum reserves that otherwise fuel much of the world's transportation and energy production. And, due to their abundance and resistance to degradation, traditional plastics are also a major source of environmental pollution. In 1999, President Bill Clinton issued an executive order insisting that researchers devote more energy toward finding ways to replace fossil resources with plant materials, both as raw materials and fuels.

With regard to producing biodegradable plastics from transgenic crops such as corn, the Monsanto Corporation already had been on the job for several years, but then abandoned their efforts for economic reasons. Although corn plants were engineered to produce rather high levels of plastic compounds in their stover (leaves and stems), extracting and processing these proved prohibitively expensive. Indeed, when all the financial expenses were tallied— those of harvesting and drying the stover, isolating the raw compounds with factory solvents, separating and recycling the solvents, and purifying and blending the plastics to produce a usable resin—they vastly exceeded costs of producing plastics the old-fashioned petrochemical way.

Some hidden environmental costs also made plant-produced plastics less appealing. First, with existing technologies, the processing steps listed above actually consume a greater weight of fossil fuel than is saved in usable bioplastics. Second, burning fossil fuels to produce bioplastics would contribute to greenhouse gases responsible for global warming. Thus, Monsanto and others decided it was uneconomical, for now, to pursue the production of bioplastics from GM crops. Of course, this situation could change if petrochemicals become more expensive or if governments offered economic incentives for research and development on biodegradable plastics.

A related but distinct biotechnology is being pursued by Cargill Dow, an agribusiness giant. Rather than engineering GM plants to produce plastic substances directly in their tissues, its researchers are exploring ways to manufacture new kinds of plastics from plant sugars. In 2001, the company began efforts to mass produce the first plastic of this sort (NatureWorks) that may find application in candy wrappers and other kinds of packaging.

Artificial plastic plants are manufactured and sold widely as lasting decorations for homes and cemeteries. Unfortunately, real-life transgenic plants that produce plastic compounds internally are not yet commercially successful and remain problematic as a viable economic enterprise.

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