Researchers have genetically engineered transgenic corn plants that produce enzymes that can turn their leaves and stems into sugar by breaking down cellulose. The plants could lower the cost of creating ethanol from these sources, making such biofuel more competitive with that produced from corn kernels, the primary source of ethanol in the United States today.
Cellulosic sources of ethanol, such as waste biomass and switchgrass, are attractive because they are cheap and abundant. But converting cellulose, a complex carbohydrate, into sugars that can be fermented to make ethanol is more expensive than converting the starch in corn grain into sugar: breaking down the cellulose typically requires expensive enzymes extracted from genetically engineered microbes.
Now Mariam Sticklen, professor of crop and soil science at Michigan State University, and her colleagues have genetically engineered corn to produce the same enzyme that the transgenic microbes produce. The plant-grown cellulase could save about 30 to 50 cents per gallon of ethanol produced, Sticklen says.
Key to Sticklen’s advance was engineering the corn so that the enzymes would not break down cellulose while the plants were still alive. Part of the solution was to use an enzyme found in bacteria that live in hot springs. The enzyme is only active at high temperatures–higher than those that a plant’s cells would reach while it is alive. As a result, the enzyme remains dormant until it is heated to about 50 ºC.
Sticklen’s transgenic corn is “one of several promising approaches to address the central obstacle impeding establishment of a cellulosic-biofuels industry: the absence of low-cost technology to overcome the recalcitrance of cellulosic biomass,” says Lee Lynd, professor of engineering and biology at Dartmouth College, in Hanover, NH. But he adds that in-plant production of enzymes comes with its own challenges.
One of these challenges, according to James McMillan, a principal group manager at the National Renewable Energy Laboratory, in Golden, CO, is ensuring that the transgenic plants don’t have negative environmental effects. For example, if plant matter containing these enzymes was left in the field, it could change ecosystems by making sugar more readily available to microorganisms.
If the researchers are able to overcome some of these challenges, the biotech corn could lead to more-efficient production of ethanol, starting by making better use of corn kernels. Much of the corn kernel contains cellulose that isn’t converted into ethanol in conventional ethanol plants. Some developers are considering adding equipment to existing ethanol plants for processing this corn-grain cellulose. In Sticklen’s transgenic corn, this cellulose in the kernel would contain enzymes for converting cellulose into sugar, which could simplify the process.
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