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
Title: Lignin biosynthesis
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
Free Univ Brussels, Lab Biotechnol Vegetale, B-1160
Why do you think your paper is highly
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
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
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
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
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
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.
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
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.
Department of Plant Systems Biology, VIB
Department of Molecular Genetics, Ghent University
Department of Biochemistry
University of Wisconsin-Madison
Madison, WI, USA
Laboratoire de Biotechnologie Végétale
Université Libre de Bruxelles (ULB)
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,