August 27, 2013 | 1 Comment
Research findings published online in Science Express show a way for new initiatives supporting a bio-based fuel economy. An international collaboration of plant scientists from VIB and Ghent University (Belgium), the University of Dundee (UK), The James Hutton Institute (UK) and the University of Wisconsin (USA) identified a gene in the biosynthetic pathway of lignin, a major component of plant secondary cell walls that limits the conversion of biomass to energy.
Sugars derived from the grain of agricultural crops can be used to produce biofuel but most of these crops grow in fertile soils needed for food and feed production. Alternative fast growing plants such as poplar, eucalyptus, or various grasses such as miscanthus and switchgrass, residues such as corn stover and sugarcane bagasse would not compete and can be a sustainable source for biofuel.
But the plant’s leaves and stems contain considerable lignin and cellulose whereas the grain seeds are rich in sugars and starches with little lignin and cellulose. The processing systems are vastly different – and so far lignin and cellulose are not economically viable at scale.
The collaborative team’s findings show a way for new initiatives supporting a lignin and cellulose fuel production economy. Sally M. Benson, director of Stanford University’s Global Climate and Energy Project said, “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. 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.”
To begin the background view, a plant cell wall mainly consists of lignin and the sugar molecules making up the cellulose. Cellulose can be converted to glucose that can then be used in a classical fermentation process to produce alcohol, similar to beer or wine making.
The lignin is a kind of cement that embeds the sugar molecules together giving firmness to plants. Thanks to the lignin very tall plants can maintain their upright stature. Unfortunately, the lignin severely reduces the accessibility of sugar molecules for biofuel production. The lignin cement has to be removed via an energy-consuming and environmentally unfriendly process. Plants with lower amounts of lignin or with lignin that is easier to break down can be a real benefit for biofuel and bioplastics production. The same holds true for the paper industry that uses the cellulose fibers to produce paper.
For many years researchers have been studying the lignin biosynthetic pathway in plants. Increasing insight into this process can lead to new strategies to improve the accessibility of the cellulose molecules. Using the model plant Arabidopsis thaliana the team has now identified a new enzyme in the lignin biosynthetic pathway. This enzyme, caffeoyl shikimate esterase (CSE), fulfills a central role in lignin biosynthesis. Knocking-out the CSE gene, resulted in 36% less lignin per gram of stem material. Additionally, the remaining lignin had an altered structure. As a result, the direct conversion of cellulose to glucose from un-pretreated plant biomass increased four-fold, from 18% in the control plants to 78% in the CSE mutant plants.
These new insights can now be used to screen natural populations of energy crops such as poplar, eucalyptus, switchgrass or other grass species for a non-functional CSE gene. Alternatively, the expression of CSE can be genetically engineered in energy crops. A reduced amount of lignin or an adapted lignin structure can contribute to a more efficient conversion of biomass to energy.
The research was co-financed by the multidisciplinary research partnership ‘Biotechnology for a sustainable economy’ of Ghent University, the U.S. Department of Energy’s Great Lakes Bioenergy Research Center and the ‘Global Climate and Energy Project’ (GCEP) based at Stanford University. The GCEP is a worldwide collaboration of research institutions and private industry that supports research on technologies that significantly reduce emissions of greenhouse gases, while meeting the world’s energy needs.
Sounds good, but will production species of plants with their CSE gene knocked out still grow, stand up, and be productive? There is a lot more to learn.