Boosting galactan sugars could boost biofuel production

By Susanne Retka Schill | January 10, 2013

Researchers at the U.S. DOE’s Joint BioEnergy Institute continue to publish the results of their basic research aimed at improving biofuel production. In the most recent announcement, an international collaboration led by scientists at JBEI has identified the first enzyme capable of substantially boosting the amount of galactan in plant cell walls.

Among the key challenges to making advanced biofuels cost competitive is finding ways to maximize the amount of plant cell wall sugars that can be fermented into fuels. “We have confirmed the identity of the GT92 enzyme as the first enzyme shown to increase the biosynthesis of galactan,” explained Henrik Scheller, vice president for JBEI’s Feedstocks Division and director of its Cell Wall Biosynthesis group. “This identification galactan synthase provides an important new tool for the engineering of advanced bioenergy fuel crops.”

“Galactans are composed of hexoses, which in contrast to pentoses, are easily utilized by fermenting microorganisms for the production of biofuels and other compounds,” Scheller said. “It would be advantageous to develop plants with increased galactan content instead of hemicelluloses consisting largely of pentoses.”

Galactans are polysaccharide components of pectin, the sticky sugar substance that binds together the individual cells in plant cell walls and is used to make jellies and jams. Building upon the understanding of galactans found in tension wood, the researchers investigated the function of the GT92 proteins in the model plant, arabidopsis, and succeeded in producing plants with 50 percent more galactan content. Scheller pointed out that since galactan is an “ancient invention,” the discovery of the key enzyme involved in galactan expression should be applicable to switchgrass, miscanthus, poplar and other plants being considered as crops for advanced biofuels. “We do not anticipate any difficulty in being able to over express GT92 genes in these plants.”

The galactan research adds to the body of knowledge being developed at JBEI to enhance the biofuel conversion potential in plants. The research teams published two papers late last year that detailed their work to genetically modify the model plant’s expression of xylan and lignin, seeking to increase the proportion of fermentable sugars while maintaining the integrity of the plant.

Scheller, who also holds an appointment with DOE’s Lawrence Berkeley National Laboratory, is the corresponding author of a paper in the journal Plant Cell that describes this work. The paper is titled “Pectin Biosynthesis: GALS1 in Arabidopsis thaliana is a β-1,4-Galactan β-1,4-galactosyltransferase.” Co-authors were JBEI’s April Liwanag, Berit Ebert, Yves Verhertbruggen, Emilie Rennie, Carsten Rautengarten, and Ai Oikawa, plus Mathias Andersen and Mads Clausen of the Technical University of Denmark.