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Environmentally Minded Plastic Plants

27 September 1999 7:00 pm
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Autumn brings heaps of apples, pumpkins, and other crops. Now another harvest is on the horizon, more akin to the plastic fruit on your grandmother's sideboard: Scientists have engineered plants to grow a biodegradable plastic from nothing but sunshine, water, and CO2. If the yield can be boosted, plastics from plants might one day turn up in everything from soda bottles to diapers.

With oil reserves dwindling, some companies are looking to biodegradable plastics produced from renewable sources--such as living organisms. Many kinds of bacteria naturally make plasticlike polymers for their energy reserves--"the bacterial form of fat," as biochemist Kenneth Gruys puts it. It wouldn't be cost-effective to harvest these polymers, called polyhydroxyalkanoates (PHAs), mainly because the bacteria need expensive nutrients. So a few years ago scientists decided to go "green" by transferring the necessary metabolic genes from bacteria to plants, which only need to be watered. In the early 1990s the first plant-produced PHA, polyhydroxybutyrate, saw the light of day. But polyhydroxybutyrate is not very useful because "it's brittle and hard to process," says Gruys.

So Gruys and his team at Monsanto in St. Louis, Missouri, set out to trick two plants--canola and a mustard plant called Arabidopsis--into producing a more practical plastic consisting of two building blocks instead of just one. Because one of the units is usually not synthesized by the plants, Gruys and his colleagues had to introduce four bacterial genes into plant cells, one more than for the production of polyhydroxybutyrate. When the researchers checked for plastic production, they found the composite polymer in the leaves, seeds, and other organs of about 25% of the manipulated plants. Although the mixture of polymer building blocks was in the right range for commercial applications, as the team reports in the October Nature Biotechnology, yield never topped 3% of the plant's dry weight. "We've got to do more than that," says Gruys. He estimates that to be economically viable the plastic should amount to about 15%.

But the research is already a big leap, says plant biotechnologist Yves Poirier of the University of Lausanne, Switzerland. Any production of polymer with commercial applications in plants is a good start, given the complexity of the endeavor, he says. Producing plastics in plants "is not something that can be done in 3 or 4 years, but [biopolymers] nevertheless have an enormous potential." Apparently not enough for Monsanto, which abandoned its research into bioplastics early this year.

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