Biology Reference
In-Depth Information
into S lignin as indicated by mutant and over-expressing lines. Here, we summarize
our current understanding of the phenylpropanoid pathway, describe the phenotypic
consequences of miss-regulating the rate limiting cytochrome P450s involved in the
pathway on lignin structure and quantity, and discuss the flexibility of plants in
channelling carbon through the phenylpropanoid grid.
I. INTRODUCTION
Lignin monomers, or monolignols, are synthesized from the aromatic amino
acid phenylalanine (Phe) through the phenylpropanoid pathway. Lignin is
quantitatively the most important final product of the pathway. But both the
starting compound and many intermediates of the pathway play crucial roles
both in primary metabolism, for example protein biosynthesis, and specialized
or secondary metabolism. The latter includes the wide array of soluble phe-
nolics such as flavonoids and stilbenes, soluble hydroxycinnamoyl conjugates,
and benzoate derivates, which serve diverse and vital functions in plant
development and chemical ecology, for example, in pollinator attraction, UV
protection, herbivore deterrence, and pathogen defence (
Dixon, 2001; Vogt,
2010
). But especially in woody species, the vast amount of Phe produced is
channelled through the phenylpropanoid pathway to produce lignin. Indeed,
the term lignin—introduced by de Candolle in 1819—is derived from the Latin
word lignum, meaning wood. Lignin is the second most abundant biopolymer
on earth constituting 30% of non-fossil organic carbon (
Boerjan et al.,2003
). It
is an aromatic heteropolymer that is incorporated into cell walls during the
secondary thickening of the cell wall. Integration of this hydrophobic polymer
into the cellulose network of the cell wall causes the immense mechanical
strength and hydrophobicity of secondary cell walls that allows high-pressure
water transport and enables the erect growth of land plants (
Chabannes et al.,
2001; Jones et al., 2001; Sarkanen and Ludwig, 1971
). Thus, the ability of lignin
biosynthesis contributed largely to the conquest of land by plants about 430
million years ago. However, the origin of lignin, or at least phenylpropanoid
biosynthesis, predates land plant evolution, because most lignin biosynthetic
genes are present in the bryophyte Physcomitrella patens and because lignin-like
aromatic polymers have been identified in green and even red algae (
Delwiche
et al., 1989, Gunnison and Alexander, 1975, Martone et al.,2009
). For more
details on the evolutionary history of lignin, the reader is referred to Chapter
9
.
At least a part of the pathway apparently has evolved independently multiple
times within the plant lineage, as discussed in the following in more detail.
The complex racemic aromatic heteropolymers found in lignin are mainly
derived from three hydroxycinnamyl alcohol monomers differing in their degree
of methoxylation: 4-coumaryl, coniferyl, and sinapyl alcohols. Incorporated
