Biology Reference
In-Depth Information
constituents. This organization may be assimilated to a composite material
made of rods of crystalline cellulose macrofibrils glued into a matrix of
lignins, hemicelluloses and pectins.
Lignin is a major component of wood, ranging from 15% to 36% of its dry
weight (
Zobel and Buijtenen, 1989
): it is, after cellulose, the second most
abundant natural polymer on the earth. Lignin is deposited in the cell wall at
the end of xylem cell differentiation. Lignin is important for twomain functions
of wood in trees, that is, xylem sap conduction andmechanical support. Indeed,
lignin deposition reinforces plant cell walls, providing rigidity, impermeability
to water and also protection against pathogens. Therefore, lignin biosynthesis
appears in vascular plants as a critical innovation for plant adaptation to an
aerial environment and further to the development of trees (
Kenrick andCrane,
1997
). Lignin is a highly variable molecule, whose synthesis appears influenced
by environmental cues as well as by physiological parameters such as tree age,
cell type or cell wall layer (
Campbell and Sederoff, 1996
). Therefore, lignin
synthesized by a young tree grown for several months in the sub-optimal
conditions of a greenhouse can be rather different from the lignin synthesized
by the same tree after several years of growth in the field.
Lignin is a complex and variable aromatic polymer resulting from the
polymerization of elementary units, namely p-hydroxycinnamyl alcohol
monomers or monolignols, differing by the number of methoxylations on
their aromatic cycle: this is why subsequently, in accordance to
Sarkanen and
Ludwig (1971)
, we prefer to use the term ''lignins'' indicative of an array of
different polymers, in place of the term ''lignin'' indicative of a single lignin
molecule. In gymnosperms, lignins result mostly from the incorporation of
coniferyl alcohol (producing guaiacyl (G) units) and a small proportion of
p-coumaryl alcohol producing p-hydroxyphenyl (H) units. In angiosperms,
lignins are composed approximately equally of G and syringyl (S) units
resulting from the incorporation of sinapyl alcohol. The proportion between
the different elementary units affects their linkage within the polymer and in
consequence the structure and properties of the lignin polymer. In accor-
dance, lignins will be more or less condensed, and in consequence, more or
less resistant to chemical attacks, natural (pathogen) or artificial (delignifica-
tion during pulp production).
Lignin biosynthesis has been extensively studied over the last 50 years
(
Sarkanen and Ludwig, 1971
and for recent reviews see
Boerjan et al.,
2003; Bonawitz and Chapple, 2010
). In poplar, most if not all genes involved
in the biosynthesis of lignins have been identified, even before the publication
of the poplar genome sequence (
Tuskan et al., 2006
). The availability of the
poplar genome sequence enabled the complete identification of all members
of each lignin gene family, even though it still remains to be elucidated
