Biology Reference
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coniferaldehyde and dihydroconiferyl alcohol (
Ralph et al., 1997; Wagner
et al., 2005, 2009
).
b
-Aryl ether (
b
-O-4), phenylcoumarans (
b
-5), resinols
(
-O-4) are among the most abundant inter-
unit linkage types identifiable in conifer lignin (
Brunow and Lundquist, 2010;
Wagner et al., 2007, 2009
). However, the proportion of
b
-
b
) and dibenzodioxocins (5-5/
b
-aryl ethers is signifi-
cantly lower in softwood lignin compared to hardwood lignin and that
of condensed units such as biphenyls substantially higher (
Brunow and
Lundquist, 2010
). The majority of the aromatic groups in monolignols are
etherified and only a minor fraction of the resulting units remain free-phenolic
(
Lai and Guo, 1991
). The differences in lignin structure between softwood and
hardwood species is a direct consequence of the varying monolignol composi-
tion. The high degree of condensation in conifer lignin contributes to the
difficulty in processing lignocellulosic material from these species.
Lignin in conifers may form covalent bonds with pectic substances and
non-cellulosic polysaccharides contained within the secondary cell wall. The
nature of these lignin carbohydrate complexes are largely influenced by the
cell wall composition in different wood types (
Timell, 1986
). Sugar residues
conjectured to be covalently linked to lignin in conifers include galactose,
glucose, mannose, arabinose, xylose and rhamnose (
Timell, 1986
).
b
D. MOLECULAR ASPECTS OF LIGNIFICATION
Our molecular understanding of lignification has increased substantially
within the last decade. The biosynthesis of the monolignols involved in
lignification in conifers has been largely elucidated (
Anterola et al., 2002;
M
¨
ller et al., 2005; Wadenb
¨
ck et al., 2008; Wagner et al., 2007, 2009, 2011
,
unpublished results). However, large knowledge gaps still remain and in-
clude: regulatory cascades that trigger lignification, metabolic connections
between monolignol biosynthesis and other metabolic pathways, the cellular
biology of monolignol biosynthesis, the transport of monolignols to the
apoplast, the role of monolignol glucosides in lignification, the process of
lignin initiation, and the interaction of lignin with other cell wall polymers
such as non-cellulosic polysaccharides.
1. Transcriptional regulation of lignification
The transcriptional regulation of lignification in angiosperm species is com-
plex and involves a large number of transcriptional regulators, which can
either function as repressors or as activators (
Zhao and Dixon, 2011
). This
complexity is in part because lignin biosynthesis can be triggered by a variety
of cues including gravitropism, wounding, pathogen challenge, sugar con-
tent, plant hormones and circadian rhythm. The vast majority of
