An international collaboration led by scientists at the US Department of Energy (DOE)’s Joint BioEnergy Institute (JBEI) has identified the first enzyme capable of substantially boosting the amount of galactan in plant cell walls. The GALS genes governing the enzyme may become important tools for developing bioenergy crops, the researchers suggest.
Among the key challenges to making advanced biofuels—i.e., drop-in bio-hydrocarbon fuels—cost-competitive is finding ways to maximize the amount of plant cell wall sugars that can be fermented into fuels. Galactan, which is a polymer of galactose, a six-carbon sugar that can be readily fermented by yeast into ethanol, is a target of interest for researchers in advanced biofuels produced from cellulosic biomass.
We have confirmed the identity of the GT92 enzyme as the first enzyme shown to increase the biosynthesis of galactan. This identification of the first β-1,4-galactan synthase provides an important new tool for the engineering of advanced bioenergy fuel crops.—Henrik Scheller, vice president for JBEI’s Feedstocks Division and director of its Cell Wall Biosynthesis group
Scheller, who also holds an appointment with DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab), is the corresponding author of an open access paper in the journal Plant Cell that describes this work.
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. The β-1,4-galactan component of pectin is especially abundant in the “tension wood” that forms in cell walls in response to mechanical stress from wind or snowfall.
Galactans are composed of hexoses, which in contrast to pentoses, are easily utilized by fermenting microorganisms for the production of biofuels and other compounds. It would be advantageous to develop plants with increased galactan content instead of hemicelluloses consisting largely of pentoses.—Henrik Scheller
GT92 is a family of glycosyltransferase proteins whose genes are found in all plants that have been genetically sequenced. An increased expression of GT92 genes has been observed in studies of tension wood. This observation combined with the knowledge that tension wood is rich in β-1,4-galactan led Scheller and his colleagues to investigate the function of GT92 proteins in Arabidopsis thaliana, a small flowering relative of mustard that serves as a model organism for plant studies.
Arabidopsis has three members of GT92, which Scheller and his colleagues designated as GALACTAN SYNTHASE 1,2 and 3 (GALS1, GALS2 and GALS3). While loss-of-function mutants in all three genes were found to be galactan deficient, Scheller and his colleague isolated and tested GALS1.
Overexpression of GALS1 resulted in plants with 50-percent higher β-1,4-galactan content and no adverse phenotype. We expect that the results for GALS2 and GALS3 overexpressors will be similar though we have yet to test them.—Henrik Scheller
Given that all three Arabidopsis GALS genes showed overlapping but not identical expression, Scheller and his colleagues are now combining mutations of GALS genes to better understand the role of β-1,4-galactan in plants. They’re also carrying out basic studies on these enzymes, including crystallization and structural analysis. In addition, they’re overexpressing the GALS proteins in different combinations to determine if even higher production of β-1,4-galactan results.
Scheller also suggests that since β-1,4-galactan is an ancient invention, the function of GT92 as a galactan synthase in Arabidopsis should also 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 overexpress GT92 genes in these plants.”
This research was funded by the DOE Office of Science, and by the Danish Strategic Research Council.
April Jennifer Madrid Liwanaga, Berit Ebert, Yves Verhertbruggen, Emilie A. Rennie, Carsten Rautengarten, Ai Oikawa, Mathias C.F. Andersen, Mads H. Clausen and Henrik Vibe Scheller (2012) Pectin Biosynthesis: GALS1 in Arabidopsis thaliana Is a β-1,4-Galactan β-1,4-Galactosyltransferase. Plant Cell. doi: 10.1105/tpc.112.106625