Biology Reference
In-Depth Information
transcriptional regulators identified in angiosperms belong to the MYB
family, but LIM, NAC and KNOX transcription factors that impact lignifi-
cation have also been identified (
Zhao and Dixon, 2011
).
The expression of genes associated with monolignol biosynthesis in con-
ifers seems to be regulated in a coordinated fashion, as the genes required
for monolignol biosynthesis are up-regulated in concert during lignification
(
Anterola et al., 2002; Koutaniemi et al., 2007; Wagner et al., 2007
). This
suggests that the expression of lignin-related genes in conifers is controlled
by a common set of transcriptional regulators, most likely including, as in
angiosperm species, MYB transcription factors. This has recently been ex-
perimentally verified by qRT-PCR experiments in Picea glauca and Pinus
taeda and over-expression experiments in P. glauca (
Bedon et al., 2007;
Bomal et al., 2008
). The identified MYB transcription factors are likely to
activate expression of lignin-related genes in conifers by binding to AC
elements present in the promoters of lignin-related genes (
Bedon et al.,
2009; Li et al., 1999; Wagner and Walter, 2004
). However, the complexity
of the transcriptional regulation of lignin-related genes identified in angios-
perms strongly suggests that our current understanding of transcriptional
gene regulation of lignin-related genes in conifers is far from complete.
2. Monolignol biosynthesis in conifers
The biosynthesis of monolignols in coniferous gymnosperms can be expected
to follow the same sequence of enzymatic steps previously identified in angios-
perms, as has been reviewed elsewhere (
Boerjan et al., 2003; Bonawitz and
Chapple, 2010; Boudet, 2007; Chiang, 2006; Halpin, 2004; Higuchi, 2006;
Rastogi and Dwivedi, 2008; Umezawa, 2010; Vanholme et al.,2008
). This
hypothesis is supported by genomic, proteomic and genetic studies investigat-
ing monolignol biosynthesis in conifers (
Anterola et al.,2002;Koutaniemi
et al., 2007; Mast et al.,2010;M¨ ller et al., 2005; Wadenb¨ck et al.,2008;
Wagner et al., 2007, 2009, 2011
, unpublished results). A key exception
concerns the biosynthesis of sinapyl alcohol, as conifers do not contain
S-lignin. Genes required for the biosynthesis of sinapyl alcohol, such as
coniferaldehyde-5-hydroxylase (CAld5H) and 5-hydroxyconiferaldehyde
O-methyltransferase (CAldOMT) are therefore likely to be missing.
Monolignols such as coniferyl alcohol can be glycosylated to form con-
iferin in conifers. The glucosyl transferase supporting this enzymatic step in
conifers has not been isolated to date. However, a glucosyl transferase
involved in syringin formation has been identified in Arabidopsis thaliana
(
Lanot et al., 2006; Lim et al., 2001
), which could help to identify the
homologue gene in conifers. It has been speculated that coniferin represents
a storage compound that is transported to the vacuole, where it can reach
