Issue Date: August 18, 2008
SCIENTISTS at Cambridge, Mass.-based Metabolix have genetically engineered switchgrass to produce significant amounts of a biodegradable polyester within the plant's cell walls (Plant Biotechnol. J. 2008, 6, 663). The ability to coproduce a bioplastic along with a biofuel derived from the plant's cellulose boosts the prospects for using switchgrass and other nonfood crops as renewable feedstocks for biorefineries that one day are expected to economically produce fuels and chemicals.
Some bacteria naturally make polyhydroxyalkanoates (PHAs) for energy storage, much in the way animals use fat. Metabolix has previously optimized this process by incorporating a series of genes from PHA-producing bacteria into other microbes. The genes express enzymes that convert sugars or oils into PHAs via a multistep process within the bacterial cells. Metabolix has partnered with Archer Daniels Midland through a joint venture called Telles for fermentation production of Mirel brand biodegradable PHAs for use in fibers, films, and molded goods.
Now, a Metabolix research team led by Kristi D. Snell has rewired the genetic sequence of switchgrass to enable production of polyhydroxybutyrate. The polymer accumulates in beadlike granules inside chloroplasts of the plant's cells, primarily in leaves but also in stalks. In greenhouse studies, the engineered plants produced leaves with PHA content up to 3.7% dry weight. The company projects that polymer levels will need to hit 5.0 to 7.5% dry weight for commercial production to be viable.
Switchgrass PHA could be used directly as a polymer, or it could be depolymerized to form hydroxy acids for use as chemical feedstocks. Residual plant material could be burned to produce electricity, or the cellulose could be converted into liquid fuels.
The paper "represents a very good research result," says Michigan State University's Mariam B. Sticklen, an expert in genetically modified biomass crops. Sticklen stresses that coproducing bioplastics in plants is not new, but because of patent licensing the early efforts have primarily been of academic interest and have yet to pan out commercially. The Metabolix development is promising, she says, but due to regulatory hurdles it could still be years before bioplastics from engineered plants are commercially available.
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