Madison, Wisconsin, USA
August 15, 2013
For nearly a decade, scientists have thought that they understood how plants produce lignin - a compound that gives plant tissues their structure and sturdiness, but can limit their use as a source of biofuels.
Now, thanks to a collaboration involving the U.S. Department of Energy Great Lakes Bioenergy Research Center (GLBRC) and several international institutions, researchers have identified a new gene responsible for producing a previously unknown enzyme that is central to lignin synthesis. The breakthrough, which was recently published in Science, could improve the conversion of cellulosic - or nonfood - biomass to biofuels.
"This is the first new gene in the [lignin] pathway that's been discovered in ten years," says John Ralph, a professor in the University of Wisconsin-Madison departments of biochemistry and biological systems engineering and the leader of the GLBRC Plants Research Area.
"Any time you find a new gene, you not only better understand the biochemical pathways in which the gene is involved; you also discover new ways of perturbing those pathways," Ralph says.
One might assume that a single gene could have only a minor impact on overall plant development; however, it turns out that the newly discovered gene and the enzyme it produces - caffeoyl shikimate esterase, or CSE - play a crucial role in lignin formation.
In fact, when collaborating plant scientists at VIB/Ghent University in Belgium grew mutant Arabidopsis plants that had the gene in question "knocked out," the amount of lignin in the plants' stems was reduced by fully 36 percent. Furthermore, the lignin that did remain was quite chemically different from the lignin of a normal plant.
At UW-Madison, Ralph and GLBRC Associate Scientist Hoon Kim examined the mutant plants using cutting-edge nuclear magnetic resonance (NMR) technology housed in the Wisconsin Energy Institute building to understand the precise impact of the CSE gene on lignin development, and to explore what that could mean for biofuel production.
Because lignin plays such an important role in biomass growth and development, breaking it apart to access the simple sugars stored within plant tissues is a difficult task that often requires a combination of heat and harsh chemicals.
Knocking out the gene responsible for CSE, however, made it significantly easier to deconstruct the mutant plants because their lignin was less abundant, and their cell walls were more digestible. This suggests that reducing and altering lignin content and structure by targeting CSE could lower the time, energy and cost of processing cellulosic biomass into fuels.
"In principle, this discovery should make biofuel processing faster and cheaper for industry," says Ralph.
Plants research at the GLBRC focuses on answering basic science questions about plant cell wall structure in order to develop new technologies that will form the basis for advanced cellulosic biofuel production.
"We benefit from having John Ralph - a world-class lignin expert - and his innovative research group at the GLBRC," says Director Tim Donohue, a UW-Madison professor of bacteriology. "The discovery of CSE has the potential to make game-changing improvements in biomass processing and conversion strategies."
In addition to GLBRC and VIB/Ghent University, the international project involved researchers from the James Hutton Institute and University of Dundee in Scotland. The work was jointly funded by Ghent University, GLBRC, and Stanford University.
For the scientific community, the discovery of CSE and the gene that controls it will forever change how lignin production in plants is understood. For Ralph and his colleagues at the GLBRC, another major benefit of this work is the opportunity to create another route to commercial-scale cellulosic ethanol production.
"In the past, the wrong substrates and products were assumed to be involved in several steps of lignin biosynthesis, but now the pathway is becoming more completely understood," says Ralph. "In addition, this and other discoveries are providing bioenergy researchers in particular with new approaches that have significant potential for improving biofuel processing."
New possibilities for efficient biofuel
Limited availability of fossil fuels stimulates the search for different energy resources. The use of biofuels is one of the alternatives. Sugars derived from the grain of agricultural crops can be used to produce biofuel but these crops occupy fertile soils needed for food and feed production. Fast growing plants such as poplar, eucalyptus, or various grass residues such as corn stover and sugarcane bagasse do not compete and can be a sustainable source for biofuel. 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 new gene in the biosynthetic pathway of lignin, a major component of plant secondary cell walls that limits the conversion of biomass to energy. These findings published online in this week’s issue of Science Express pave the way for new initiatives supporting a bio-based economy.
"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. "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."
Lignin as a barrier
To understand how plant cells can deliver fuel or plastics, a basic knowledge of a plant’s cell wall is needed. A plant cell wall mainly consists of lignin and sugar molecules such as cellulose. Cellulose can be converted to glucose which can then be used in a classical fermentation process to produce alcohol, similar to beer or wine making. Lignin is a kind of cement that embeds the sugar molecules and thereby gives firmness to plants. Thanks to lignin, even very tall plants can maintain their upright stature. Unfortunately, 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 a lower amount 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 fibres to produce paper.
A new enzyme
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, an international research collaboration between VIB and Ghent University (Belgium), the University of Dundee (UK), the James Hutton Institute (UK) and the University of Wisconsin (USA) has now identified a new enzyme in the lignin biosynthetic pathway. This enzyme, caffeoyl shikimate esterase (CSE), fulfils 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, published this week online in Science Express, 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 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, the Global Climate and Energy Project is a worldwide collaboration of premier research institutions and private industry that supports research on technologies that significantly reduce emissions of greenhouse gases, while meeting the world’s energy needs.
Partners involved.
VIB
VIB is a non-profit research institute in the life sciences in Flanders, Belgium, with 1200 scientists conducting strategic basic research on the molecular mechanisms that are responsible for the functioning of the human body, plants, and micro-organisms. Through a partnership with four Flemish universities – Ghent University, the Katholieke Universiteit Leuven, the University of Antwerp, and the Vrije Universiteit Brussel − and a solid funding program, VIB unites the forces of 72 research groups in a single institute. Through its technology transfer activities, VIB strives to convert the research results into products for the benefit of consumers and patients. VIB develops and disseminates a wide range of scientifically substantiated information about all aspects of biotechnology. For more information, please visit www.vib.be.
Ghent University
After more than twenty years of uninterrupted growth, Ghent University is now one of the most important institutions of higher education and research in the Low Countries. Ghent University yearly attracts over 30,000 students, with a foreign student population of over 2,200 EU and non-EU citizens. Ghent University offers a broad range of study programs in all academic and scientific fields. With a view to cooperation in research and community service, numerous research groups, centers and institutes have been founded over the years. For more information www.UGent.be.
Universiteit van Dundee (Verenigd Koninkrijk)
The University of Dundee is internationally recognised for its excellence in life sciences and medical research with particular expertise in cancer, diabetes, cardiovascular disease, neuroscience, skin diseases and plant sciences. The University has a top-rated medical school with research expanding from "the cell to the clinic to the community", while the College of Life Sciences is home to some of the world's most cited scientists and more than 800 research staff from 60 different countries. Dundee was voted best in the UK for student experience in the 2012 Times Higher Education Student Experience Survey. See www.dundee.ac.uk for further details.
Het James Hutton Instituut (Verenigd Koninkrijk)
The James Hutton Institute is a world-leading scientific organisation encompassing a distinctive range of integrated strengths in land, crop, waters, environmental and socio-economic science. It undertakes research for customers including the Scottish and UK Governments, the EU and other organisations worldwide. The institute has a staff of nearly 600 and 120 PhD students. The Institute organises its research through seven principal themes: Safeguarding Natural Capital, Enhancing Crop Productivity and Utilisation, Delivering Sustainable Production Systems, Controlling Weeds, Pests and Diseases, Managing Catchments and Coasts, Realising Land’s Potential and Nurturing Vibrant and Low Carbon Communities. The James Hutton Institute operates commercial subsidiaries. Macaulay Scientific Consulting (MSC) Ltd is a leading environmental consultancy centre offering unparalleled experience in soil and water consultancy, and land evaluation. Mylnefield Research Services (MRS) Ltd undertakes contract research, especially plant breeding, licenses plant varieties internationally and delivers analytical services. The Institute takes its name from the 18th century Scottish Enlightenment scientist, James Hutton, who is widely regarded as the founder of modern geology and who was also an experimental farmer and agronomist.
Universiteit van Wisconsin-Madison (Verenigde Staten)
The University of Wisconsin-Madison is a public, land-grant institution located in Madison, Wisconsin. Recognized as one of America’s great universities in both achievement and prestige, UW-Madison offers a complete spectrum of liberal arts studies, professional programs and student activities. The university is the only academic institution to host one of the three U.S. Department of Energy (DOE) Bioenergy Research Centers, which were funded to make transformational breakthroughs that will form the foundation of new cellulosic biofuels technology. The Great Lakes Bioenergy Research Center (GLBRC) is led by UW-Madison with Michigan State University as the major partner. GLBRC’s additional scientific partners include DOE National Laboratories, other universities and a biotechnology company. For more information, please visit http://www.glbrc.org.
Topics
Species
