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switchgrass, poplar and other environmentally friendly crops.
Writing in the Aug. 15 online edition of the journal Science, the team of scientists from VIB and GhentUniversity in Belgium, the University of Dundee and The James Hutton Institute in the U.K., and the University of Wisconsin-Madison described a gene with a heretofore-unknown role in the development of lignin, a major component of secondary cell walls in plants. Unmasking this hidden gene, called caffeoyl shikimate esterase (CSE), will "necessitate revision of currently accepted models of the lignin biosynthetic pathway," the authors wrote.
The scientists also discovered that deleting the CSE gene quadruples the amount of sugar that can be extracted from the cell wall - a potentially significant finding for the biofuels industry.
"This exciting, fundamental discovery provides an alternative pathway for altering lignin in plants and has the potential to greatly increase the efficiency of energy crop conversion for biofuels," said Sally M. Benson, director of Stanford University's Global Climate and Energy Project (GCEP), which co-funded the study. "We have been so pleased to support this team of world leaders in lignin research and to see the highly successful outcome of these projects."
In addition to lignin, plant cell walls also contain large amounts of cellulose, a substance made entirely of the simple sugar, glucose. Lignin acts like a cement that embeds the cellulose in place, giving firmness and rigidity to the plant. The evolutionary emergence of lignin allowed the development of vascular land plants, enabling the transport of water and nutrients from the roots to the leaves, the authors noted. “Thanks to lignin, even very tall plants can maintain their upright stature,” they said.
Glucose can be fermented into ethanol, a renewable biofuel, and other alcohols. Most of the glucose used to make ethanol comes from starch- and sugar-based feedstocks, such as edible corn and sugar cane. But using valuable cropland to produce fuel instead of food has raised a number of environmental and social concerns.
As an alternative, researchers have turned to so-called “cellulosic feedstocks,” such as grass and wood, which can be grown on abandoned lands with relatively little water and fertilizer. However, lignin’s tight grip on cellulose has been a significant barrier to the development of biofuels from cellulosic plants. Gaining access to glucose in these plants requires removing the lignin cement that holds the cellulose in place via a pretreatment process that is energy-consuming and environmentally unfriendly.
In the Science study, the research team proposed a genetic approach that could lead to more efficient processing of cellulosic crops on an industrial scale. Using the model plant Arabidopsis thaliana, the researchers discovered that the CSE gene produces an enzyme with a central role in lignin biosynthesis. Removal of the gene resulted in a smaller plant with 36-percent less lignin per gram of stem material than a normal Arabidopsis, and caused the lignin to loosen its cement-like grip on the cellulose.
As a result, the researchers achieved a four-fold increase in the conversion of cellulose to glucose - a 78-percent conversion rate compared to 18 percent for a normal Arabidopsis plant. This dramatic increase in glucose extraction required no pretreatment of the cell walls, a potentially expensive and caustic process.
Plants with lower amounts of lignin, or with lignin that is easier to break down, can be a real benefit for the biofuel, bioplastics and paper industries, the authors said. As a result of the study, scientists can now screen wild populations of cellulosic energy crops - such as poplar, eucalyptus, switchgrass and other grass species -to see if they carry a mutated or non-functional CSE gene. This screening process could contribute to a more efficient conversion of biomass to energy, they said. Alternatively, scientists might be able to genetically engineer the CSE gene to reduce the amount of lignin in the cell walls of these crops.
The co-lead authors of the study are Ruben Vanholme and Igor Cesarino of VIB and Ghent University. Other authors are from the following institutions:
The work was supported by the multidisciplinary research partnership, Biotechnology for a Sustainable Economy, at Ghent University; the U.S. Department of Energy Great Lakes Bioenergy Research Center; and the following three awards from GCEP:
November 19, 2013
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