Modified switchgrass reduces enzyme use, boosts ethanol yield

By Kris Bevill | February 16, 2011

Researchers at The Samuel Roberts Noble Foundation in Oklahoma have successfully modified switchgrass to reduce levels of lignin in the plant, making it easier to process for conversion to ethanol. Fermentation studies conducted at the U.S. DOE’s Oak Ridge National Laboratory showed that the modified switchgrass produced more sugars than traditional switchgrass and not only produced approximately one-third more ethanol but also required less severe pre-treatment and fewer enzymes during the conversion process.

Richard Dixon, plant biology division director at the Noble Foundation, said the research that led to this discovery was originally focused on modifying forage crops, such as alfalfa, to make them more easily digestible for ruminant animals. About three years ago, the team realized that reducing levels of lignin in the cell walls of these plants would also improve the ability to extract sugars from the cell walls for biofuel production. “We basically have been translating the technology that has been developed for forage crops into bioenergy crops,” he said. “The trick is that switchgrass is not that easy to genetically transform, so this has been a long project. It takes about a year and a half to get the plants back after you start to do the genetic transformation.”

Zeng-Yu Wang, lead researcher on the project, said the most important aspect of this discovery is that by increasing yield per acre, growers can reduce the amount of land requirements by one-third, which will also reduce herbicide, land management, transportation and storage requirements for the feedstock. He also noted the importance of reducing the need for enzymes during the ethanol production process. “The transgenic lines require lower temperature pre-processing and only one-quarter to one-third the level of enzymes for equivalent ethanol fermentation compared to the unmodified switchgrass,” he said. “This significantly lowers the cost of biofuels and biochemicals from this switchgrass.”

Because lignin serves to strengthen plant cell walls and act as a barrier to protect against such things as disease and insects, it would be expected that reduced amounts of lignin would cause issues related to plant strength and disease. However, the research conducted at The Noble Foundation has proved otherwise. “In our experience, the opposite is true,” Dixon said. “The alfalfa plants appear to have enhanced resistance.” Field trial data on alfalfa, which has been engineered in exactly the same way as switchgrass is now being modified, has been collected for six years. None of the alfalfa plants have displayed negative agronomic impacts and the lignin-modified version of alfalfa is currently being commercialized, Dixon said. While the modified switchgrass has not yet been grown in fields, Wang said plants grown in the greenhouse have not displayed any negative effects. Bioenergy crop developer Ceres Inc. is a partner with The Noble Foundation and is currently generating the modified switchgrass in greenhouses. Dixon said The Noble Foundation’s research has put plans on the fast track for commercial plantings and he expects the variety to be available for field release in time for the 2012 growing season. The modifications made to switchgrass could apply to any energy grass, Dixon added, and may also be effective for poplar trees.

Funding for the switchgrass research was provided by the USDA and the DOE Office of Science through ORNL’s BioEnergy Science Center. The BioEnergy Science Center was established in 2007 and is one of the DOE’s three bioenergy research centers. Dixon said the research centers have one year remaining in their initial funding contracts, but he is optimistic that funding will be renewed for an additional five years. A main focus of the centers is to reduce the cost of producing cellulosic ethanol to $1 per gallon.