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EMERGING RESEARCH FRONTS - 2008

Wout Boerjan, John Ralph, & Marie Baucher talk with ScienceWatch.com and answer a few questions about this month's Emerging Research Front in the field of Plant & Animal Science.
Wout Boerjan Title: Lignin biosynthesis
Authors: Boerjan, W;Ralph, J;Baucher, M
Journal: ANNU REV PLANT BIOL, 54: 519-546 2003
Addresses: State Univ Ghent, Flanders Interuniv Inst Biotechnol, Dept Plant Syst Biol, K L Ledeganckstr 35, B-9000 Ghent, Belgium.
State Univ Ghent, Flanders Interuniv Inst Biotechnol, Dept Plant Syst Biol, B-9000 Ghent, Belgium.
USDA ARS, US Dairy Forage Res Ctr, Madison, WI 53706 USA.
Free Univ Brussels, Lab Biotechnol Vegetale, B-1160 Brussels, Belgium.

Why do you think your paper is highly cited?

The paper is published in Annual Reviews of Plant Biology. This book series is famous for its reviews on subjects of general scientific interest. The book series has an impact factor of 19 which is very high for Plant Sciences. The reviews are also relatively short and are meant for a broad readership.

Of course the subject is also particularly hot. Lignin is one the most abundant polymers in plants. It is present mainly in secondarily thickened cell walls making them strong so that plants can grow upward and can transport water and solutes from the roots up to the aerial parts. For certain agro-industrial uses of plants, however, lignin is a negative factor. For example, it needs to be extracted from wood chips in the production of pulp and paper, which is a costly and environmentally unfriendly process.

John Ralph
Marie Baucher

Lignin also limits the digestibility of forage crops. Researchers have studied the biosynthesis of lignin for decades and have been able to engineer the amount and structure of lignin in plants, including poplar trees, so that lignin became more easily extractable. More recently, lignin has become an even more active research subject because it is the most important limiting factor in the conversion of plant biomass to liquid biofuels.

Does it describe a new discovery, methodology, or synthesis of knowledge?

It is a review, but it is different from many other reviews in that the authors have a strongly different scientific background, all the while working on the same subject. Marie Baucher and Wout Boerjan are biotechnologists, whereas John Ralph is lignin chemist.

It is probably the first lignin review that summarizes results from genetic engineering of the lignin pathway, along with structural details of the lignin polymer. This combination provides a basis for not only understanding the biosynthetic ramifications of up- and down-regulating the various lignin pathway genes, but also for understanding the effects on lignin structures and their interactions with other cell wall components. Explaining, and often predicting, the altered processing properties of the plant is predicated on such knowledge.

How did you become involved in this research and were any particular problems encountered along the way?

The research on lignin biosynthesis at VIB started in the frame of an EU-funded project back in 1992, aimed at improving wood quality for the pulp and paper industry. In a European consortium, several of the genes involved in the biosynthesis of lignin were cloned and their expression down-regulated in transgenic plants, including tobacco, alfalfa, and poplar. The results were very exciting; when the last enzymatic step in the biosynthesis of lignin monomers was inhibited, the new lignin incorporated the substrates of this enzyme instead of its products, and this resulted in lignin with better properties for the pulp and paper industry. Indeed, lignin in these transgenic plants could be extracted using less noxious chemicals. In alfalfa, forage digestibility was increased.

Then, in a second EU-funded project, field trials with these lignin-modified transgenic trees were established in the UK and also in France. These trials were undertaken to demonstrate that the beneficial effects on pulping efficiency, which were observed with greenhouse-grown plants, were maintained under field conditions where the trees were exposed to wind, rain, pathogens, and winter periods. It turned out that the plants grew normally and they maintained the improved properties for paper-making. In the summer of 1999, however, eco-terrorists destroyed the field trial and delayed progress in making plants that allowed a more environmentally friendly pulping process.

Although the European groups were well experienced in engineering plants and in determining the amount and composition of lignin in the cell wall, more details on the altered structure of the lignin polymer in the transgenic plants became essential to understand why the new lignin had better properties.

John Ralph had worked on lignin chemistry in the context of chemical pulping and forage digestibility. His group had been developing new analytical methodologies for lignins; 2D and 3D NMR methods were particularly valuable for the rich structural detail they provided. At the same time, his group and others were discovering new structures in the lignin polymer, implicating new lignin monomers—the building blocks for the polymer—and a greater array of biosynthetic pathways. Early examinations of mutant and transgenic plants had also begun to demonstrate the rather remarkable metabolic plasticity of the lignification process allowing predictions for improving processing by redesigning lignins as early as 1997.

Where do you see your research leading in the future?

As already mentioned, lignin is the main limiting factor in the conversion of plant biomass to liquid biofuels such as bioethanol. We need to find ways to modify lignin composition such that it becomes extractable using much less chemicals and energy. This is the challenge for the future.

The positive thing is that research from past decades has taught us that the structure of the lignin polymer in the cell wall is not fixed, but seems to be malleable; there are two main units in the polymer and a lot of units that are minor in abundance, but the plant allows large shifts in the relative abundance of these units.

"Lignin is one the most abundant polymers in plants. It is present mainly in secondarily thickened cell walls making them strong so that plants can grow upward and can transport water and solutes from the roots up to the aerial parts."

Hence, we can engineer plants with large differences in their lignin composition. Some of these new lignin types are much easier to process. We are now trying to make plants which have entirely novel lignin structures that do not normally occur in nature, but yet are much more easily degraded —the so-called second-generation biofuel crops.

Do you foresee any social or political implications for your research?

Biofuels are high on the political agenda worldwide. Rocketing oil prices and concerns about global warming have made plants valuable alternative raw materials for making liquid biofuels. Biofuels such as bioethanol, biomethanol, and dimethylfuran are derived from the fermentation of simple sugars, which are abundant in plant tissues in the form of polymers.

Nowadays, most bioethanol is derived from the starch (a glucose polymer) present in corn kernels, but growing corn for bioenergy competes with growing corn for the food chain and, hence, making bioethanol from corn is not sustainable in the context of a growing world population that will reach nine billion people by the year 2050. Therefore, second-generation biofuel crops are being developed, from which the sugars in the cell walls (celluloses and hemicelluloses) can be converted to biofuel. These crops do not compete with the food chain, can be grown on marginal land and require much less fertilizer and insecticides than the first-generation crops.

Poplar trees have been advocated as a promising second-generation bioenergy crop, as they grow very fast, can be grown on various soil types in the temperate regions of the world without fertilizer, and also because poplar is the most important model tree for experimental biologists. We have already made transgenic trees that make less lignin and that are easier to convert to bioethanol. By inhibiting the expression of just one gene of the ~45,000 present in the poplar genome, 50% more ethanol can be made from wood of the modified trees as compared to the non-engineered wild types.

There are tremendous opportunities to further improve trees by a combination of breeding and genetic engineering. Many research programs worldwide promote these developments. Sadly, several lobby groups try to ban all research that uses genetic engineering technology, with irrational arguments, thus preventing the development of a sustainable agriculture.

Wout Boerjan
Professor
Department of Plant Systems Biology, VIB
Department of Molecular Genetics, Ghent University
Ghent, Belgium
Web

John Ralph
Department of Biochemistry
University of Wisconsin-Madison
Madison, WI, USA

Marie Baucher
Laboratoire de Biotechnologie Végétale
Université Libre de Bruxelles (ULB)
Gosselies, Belgium

Keywords: lignin , polymers, cell walls, roots, agro-industrial, poplar trees, plant biomass, liquid biofuels, biosynthetic ramifications, pathway genes, VIB, tobacco, alfalfa, greenhouse-grown plants, eco-terrorists, bioethanol, biomethanol, dimethylfuran, experimental biologists.

 



2008 : June 2008 : Wout Boerjan, John Ralph, & Marie Baucher