A plant cell wall mainly consists of lignin and sugar molecules such as cellulose. Lignin embeds the sugar molecules and thereby gives firmness to plants. However, it also severely reduces the accessibility of sugar molecules for biofuel production. It currently has to be removed via an energy-consuming and environmentally unfriendly process. Therefore, growing plants with a lower amount of lignin or with lignin that is easier to break down could potentiall revolutionise biofuel production.
Using the model plant Arabidopsis thaliana, the Hutton researchers identified the enzyme caffeoyl shikimate esterase (CSE). This enzyme fulfils a central role in lignin biosynthesis, and taking 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.
“This finding was quite unexpected because the lignin pathway has been widely examined and, it had been thought for the past decade or so, completely mapped,” said Professor Claire Halpin, of the University of Dundee. “However, we have now uncovered this enzyme which represents a new step on the pathway and a very important one.
“It looks like it could be very useful in trying to manipulate plant biomass to generate biofuels and other chemicals from non-food crops. Our studies showed that in the plant we studied – Arabidopsis – those with mutated CSE were able to release around 75% more sugars from cellulose without needing harsh chemical treatments.”
The research was co-financed by the multidisciplinary research partnership Biotechnology for a sustainable economy of Ghent University, the DOE Great Lakes Bioenergy Research Center and the Global Climate and Energy Project (GCEP) based at Stanford University.
“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," said Sally M. Benson, director of Stanford University's Global Climate and Energy Project.