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Crop Biotech Update

Scientists Develop "Lignin-Lite" Switchgrass for Use as Dedicated Bioenergy Crop

October 14, 2011
(complete access to journal article may require payment or subscription)
http://www.pnas.org/content/early/2011/02/04/1100310108

Switchgrass (Panicum virgatum L.) is a perennial prairie grass, considered as a "leading" (lignocellulosic, "second generation") biofuel feedstock for "cellulose-ethanol"production in the United States. Among the positive features of this feedstock are: (1) high biomass yields, (2) "broad cultivation range", and (3) low agricultural inputs. The biomass is usually processed by extracting the complex carbohydrates from the biomass (i.e.the cellulose and hemicellulose fractions), breaking them down (i.e. "saccharification"or "hydrolysis") into simple sugars, and eventually fermenting these sugars to ethanol. As with most lignocellulosic feedstocks, however, the main challenge against reducing the production cost, is "biomass recalcitrance". This is the property of the biomass (attributed to lignin) which makes the "extraction" of the carbohydrates in the biomass difficult. Lignins are "tough" molecules (resisting chemical attack) that tightly wraps around the cellulose/hemicellulose fractions. This "tight wrapping" prevents the carbohydrate portions of the biomass from being saccharified into simple sugars for ethanol fermentation.

Pretreatment methods to "delignify" the biomass (and to break biomass recalcitrance) often involve extreme conditions of thermal and/or chemical treatment, and this contributes a large portion of the production cost. Instead of focusing on development of more cost-effective pretreatment methods, one approach is to focus on the plant by addressing the lignin associated with biomass recalcitrance. Molecular biology methods can be used to develop low-lignin plants which can be used as "dedicated bioenergy crops".

Scientists from the Samuel Roberts Noble Foundation, Georgia Tech and Oak Ridge National Laboratory (United States) report the development of a "lignin-lite" transgenic switchgrass, with a biomass recalcitrance(indicated by lignin content) reduced by about one-eighth. They used a concept called "downregulation" , where the production of a key cellular component is reduced by genetic engineering techniques. Here, they targeted on a key enzyme involved in lignin biosynthesis called, "caffeicacid 3-O-methyltransferase" (COMT). By downregulating the COMT gene, the researchers were able to decrease the plant's lignin content by one-eighth, and increased ethanol production by about 33 percent. The full report of their method is published in the journal, Proceedings of the National Academy of Sciences (PNAS) (URL above).