Plastic-Producing Plants: The Cash Crop of the Future
November, 2000: Stanford, California
From water bottles to artificial heart valves, plastics are essential to our everyday lives. Plastics are synthetic resins made of large, organic polymers refined from valuable natural fossil fuels (oil, coal, petroleum, or natural gas). Normally, plastics take years to degrade and are a major component of landfill and waste areas. However, Dr. Chris Somerville, director of the Carnegie Institute of Washingtons Department of Plant Biology, and other institute scientists have discovered a method for engineering plants to make plastic that would biodegrade more easilyand not use up natural resources in the production process.
Alcaligenes eutrophus, as well as other bacterial species, produce the organic polymer polyhydroxyalkanoate as a carbon reserve. Moreover, these same microorganisms produce the enzyme necessary to break down the polymer into monomers, metabolized as a carbon source. This natural source of plastic polymer is therefore biodegradable. With the aid of modern molecular tools, three genes within the DNA sequence of A. eutrophus have been identified as enabling this organism to manufacture the plastic polymer. An additional gene was eventually revealed to code for the polymers to sequester specifically in plastids. Armed with the genes to code for the making and storing of the plastic polymer, the scientists used gene-splicing (genetic engineering) techniques to insert these codes into the DNA of the plant Arabidopsis thaliana. Until the organelle-specific gene was revealed, plastic polymers were detected in the nucleus, vacuole, and cytoplasm of A. thaliana. These polymer accumulations were associated with decreases in both plant growth and propagule production. However, polymer accumulation in the plastids had no obvious negative effects on either the growth or fertility of the plants.
Bacteria in general lend themselves to this type of genetic transfer because they are a kingdom of prokaryotes (Monera). As such, their DNA is not housed within a nucleus but is found in a single strand and in easier-to-work-with circular fragments called plasmids. The plant-produced plastics are harvested in a series of chloroform extractions in which the plant material is separated from the plastic polymer. Transferring the genes into plants, as opposed to harvesting the plastic from A. eutrophus, is more cost-effective. It is estimated that plastic extracted from the bacteria would cost $4 per pound, while plant-extracted plastic would cost about $1.50 per pound. (This is still more, however, than petroleum-based plastic, which costs about $ .50 per pound.)
A. thaliana, a member of the Cruciferaceae, has a diploid number (2n) = 10. Also known as Brassicaceae, this "mustard family" includes the genus Brassica, represented by broccoli and cauliflower. Arabidopsis is well suited for genetic research because its life cycle is completed within a month.
While it has been suggested that plastic-producing plants can be commercially grown on agricultural land as an additional cash crop for farmers, the ecological ramifications for organisms that might feed on the engineered plants and ingest potentially lethal doses of plastics is forcing the scientists to seek alternative, perhaps enclosed, methods of agriculture. And just as we have come a long way since 1862 when Alexander Parkes synthesized the first man-made plastic (known as pyroxylin) for use in photography, Monsanto, a biotechnology company based in St. Louis, Missouri, still has a ways to go before marketing the biopolymers. A spokesperson for the biotech giant, which purchased the rights to this polymer breakthrough in 1994, estimates that plastic-producing plants will be available to farmers around the year 2004.
In 1976, the U.S. federal government passed the Resource Conservation and Recovery Act (RCRA) to promote the conservation and recycling of materials such as plastics. However, collecting and sorting used plastics is expensive and time-consuming, even though in 1988 the Plastic Bottle Institute of the Society of the Plastics Industry established a system for identifying containers by plastic type. Since 1990, the plastics industry, individual companies, and organizations such as the American Plastics Council (APC) have invested more than $1 billion to support increased recycling and education in U.S. communities. These organizations maintain that because plastic is lightweight, durable, and easily molded into various shapes, the use of plastic often conserves more resources during a product's life than other materials.
While the biopolymers may be a few years off, biodegradable biopolymers could mean fewer noxious gases (some plastics give off poisonous fumes when burned), less debris in landfills, less dependence on fossil fuel, independence from raising oil prices, and the creation of an oil surplus for use in other venues.
References, Websites, and Further Reading
Nawrath, C., Y. Poirier, and C.R. Somerville. 1994. Targeting of the polyhydroxybutyrate biosynthetic pathway to the plastids of Arabidopsis thaliana results in high levels of polymer accumulation. Proc. Natl. Acad. Sci. USA 91:1276064.
Poirier, Y., D.E. Dennis, K. Klomparens, and C.R. Somerville. 1992. Production of polyhydroxybutyrate, a biodegradable thermoplastic, in higher plants. Science 256:52023.
Poirier, Y., C. Nawrath, and C.R. Somerville. 1995. Production of polyhydroxyalkanoates, a family of biodegradable plastics and elastomers in bacteria and plants. Biotechnology 13:14250.
http://carnegiedpb.stanford.edu/ Carnegie Institution of Washington
http://www.monsanto.com/monsanto/default.htm Monsanto homepage
Stern, Introductory Plant Biology, 8th Edition
Chapter 1: What Is Plant Biology?
Chapter 2: The Nature of Life
Chapter 3: Cells
Chapter 12: Meiosis and Alternation of Generations
Chapter 14: Plant Propagation and Biotechnology
Chapter 17: Kingdom Monera and Viruses
Chapter 24: Flowering Plants and Civilization
Chapter 25: Ecology
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