Biology Reference
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to p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units, respectively.
S lignin is traditionally considered to be angiosperm-specific although there is
a growing amount of evidence for its occurrence in certain species of lyco-
phytes, ferns and gymnosperms (
Weng and Chapple, 2010
). Lignin content and
composition are highly variable among taxa, species, tissues of the same plant
as well as within a single cell depending on the developmental stage and the
influence of environmental stresses (
Campbell and Sederoff, 1996
). In angios-
perms, S lignin is predominantly localized in fibres whereas the vessels are more
G-rich. S lignin is also produced in response to pathogen attacks in grasses.
Most of the genes of the lignin biosynthetic pathway have been character-
ized but the pathway has been revisited several times and is still a matter of
debate (
Boerjan et al., 2003
). The biosynthesis of lignin involves the general
phenylpropanoid pathway starting with the deamination of phenylalanine
and leading to the hydroxycinnamoyl-CoA esters. The enzymes involved in
this short sequence of closely related reactions are
L
-phenylalanine ammonia-
lyase (PAL), cinnamate 4-hydroxylase (C4H) and 4-coumarate: CoA ligase
(4CL). Although lignin is the most abundant phenylpropanoid product
derived from the hydroxycinnamoyl-CoA esters, the latter are also the pre-
cursors of a wide range of end products which vary according to cell-type and
environmental signals (
Dixon and Paiva, 1995
). In order to produce mono-
lignols, hydroxycinnamoyl-CoA esters undergo successive hydroxylation of
their aromatic ring, followed by phenolic O-methylation and the conversion
of the side-chain carboxyl to an alcohol group (
Boerjan et al.,2003
).
Tremendous progress has been made in the last decade regarding the tran-
scriptional regulation of the biosynthesis of lignin, and this area has been
intensively reviewed (
Demura and Fukuda, 2007; Umezawa, 2009; Wang and
Dixon, 2012; Zhao and Dixon, 2011; Zhong and Ye, 2007, 2009; Zhong et al.,
2010a
). The intent of this chapter is to highlight the most significant and recent
results in this rapidly evolving area, which illustrate the high complexity of the
transcriptional regulation of the lignin pathway and from our point of view,
pave the way to promising lines of future research.
Table 1
gives a comprehen-
sive overview of all the transcription factors (TFs) described in this review.
II. PHENYLPROPANOID AND LIGNIN
BIOSYNTHETIC GENES CONTAIN AC-RICH
CIS-REGULATORY ELEMENTS
Despite the apparent complexity of the pathway that leads to the synthesis of
lignin, the transcriptional regulation of genes involved in this metabolic
process is mediated by a rather limited subset of cis-acting motifs. One
group of AC-rich elements identified as being important for the regulation
