Page 379 - Biosystems Engineering

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cell but may give the bulk of wood many specific characteristics, such
as resistance to natural degradation, color, and odor.

Lignin
Lignin is the most abundant organic compound in the earth after
cellulose and makes up 15 to 35 percent of the dry weight of trees. It
is an important component in all vascular plants and occurs in the
secondary cell walls of fibers, xylem vessels, and tracheids, provid-
ing them with mechanical support and helping in the plant’s defense
against pathogens (Boudet et al. 1995). There is a very limited mar-
ket for lignin. The majority of lignin is used in pulp mills as energy
sources. Lignin is an obstacle to efficient pulp and paper production
because the lignin must be removed in order to extract the cellulose
from the wood. This process is energy consuming and requires the
use of polluting chemicals. It is of great interest to engineer trees to
have a lower lignin component or a lignin type that is easily extracted
without reducing tree growth rates or bole form (Pilate et al. 2002).
An extensive review by Baucher et al. (2003) provided detailed
information regarding lignin genetic engineering and the impact on
pulping. The lignin polymer is produced by the dehydrogenative
polymerization of essentially three different cinnamyl alcohols
(p-coumaryl, coniferyl, and sinapyl alcohol) that differ in the degree
of methoxylation at the C3 and C5 positions of the aromatic ring. The
terms H-type, S-type, and G-type lignin refer to lignin containing
hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, correspond-
ingly. Determining the amount, structure, or monomeric composition
of lignin in a plant is extremely difficult because of the heterogeneity
of the polymer and the high proportion of covalent bonds linking
different monomers. Moreover, during isolation, lignin undergoes
secondary modifications, such as condensation, oxidation, addition,
or substitution. Therefore, a combination of several methods has
to be used to obtain reliable information on lignin structure. Pyrolysis
gas chromatography-mass spectrometry (pyrolysis GC-MS), Fourier
transform infrared (FTIR) spectroscopy, ultraviolet and IR spectros-
copy, and nuclear magnetic resonance spectroscopy have been used
to investigate lignin content and composition. Many different trans-
genic plants and a few mutants are now available with altered lignin
content, altered lignin composition/structure, or both.
Introducing antisense O-methyltransferase into a hybrid poplar
reduced the syringyl:guaiacyl ratio (a direct result of a reduction in
the level of syringyl and an increase in the level of guaiacyl groups)
(Van Doorsselaere et al. 1995). Furthermore, a novel lignin compo-
nent (a 5-hydroxyguaiacyl residue) was identified. Both the increase
in guaiacyl units and the production of 5-hydroxyguaiacyl were
hypothesized to result from an alternative pathway in the synthesis
of guaiacyl units. Although no reduction in total lignin content was