Development of Low-Lignin Switchgrass Biofuel Feedstocks by Downregulation of a Key Lignin Biosynthesis Gene

Switchgrass (Panicum virgatum) is one of the lignocellulosic grasses, which is considered to be a potential feedstock for the production of "second-generation" biofuel ethanol. In the conversion of lignocellulosic feedstocks (such as switchgrass) into bioethanol, the pretreatment step (for the removal of the tight lignin wrapping around the biomass) is often the economic "bottleneck" of the process. Delignification (i.e. lignin removal) is necessary in order to facilitate the processing of the carbohydrate fractions of the biomass into ethanol. Expensive and energy-intensive thermochemical methods (high temperature with strong chemical agents) are often used for delignification. Rather than focusing on effective technical processes for lignin removal, another approach is to look at the feedstock itself and develop "low-lignin" plants through molecular biology techniques.

The development of low-lignin switchgrass plants is seen to be an alternative for reducing the cost of bioethanol production from the feedstock. Scientists from the Samuel Roberts Noble Foundation and the BioEnergy Science Center (both in the United States) looked into the "downregulation" of an important gene which controls lignin biosynthesis in switchgrass. They focused on the gene which expresses the enzyme, cinnamyl alcohol dehydrogenase (CAD), which catalyzes the final step of the biochemical pathway which produces the precursors for lignin synthesis. They produced transgenic plants with a "CAD RNAi gene construct under the control of the maize ubiquitin promoter". The transgenic lines were found to have reduced CAD expression levels, reduced enzyme activities, reduced lignin content, and altered lignin composition. The "downregulation" of the CAD gene resulted in improved sugar release, improved digestibility and increased saccharification efficiency, which would be useful for lowering the cost of cellulose ethanol production. The full results of the study is published in the journal, Bioenergy Research (URL above).

Related information on cinnamyl alcohol dehydrogenase: http://www.rngr.net/publications/tree-improvement-proceedings/sftic/1989/characterization-and-genetic-control-of-cinnamyl-alcohol-dehydrogenase-in-loblolly-pine/?searchterm=loblolly.