Biology Reference
In-Depth Information
TE cultures are quite versatile because they are independent of seasonal
effects and are amenable to metabolic manipulations that impact lignin
biosynthesis (
K¨rk¨nen and Koutaniemi, 2010; M¨ ller et al., 2005
).
A. MANIPULATIONS OF MONOLIGNOL BIOSYNTHESIS IN CONIFERS
Published studies investigating monolignol biosynthesis in conifers include
the suppression of the lignin-related genes 4CL, HCT, CCoAOMT, CCR and
CAD (
M
¨
ller et al., 2005; Wadenb
¨
ck et al., 2008; Wagner et al., 2007, 2009,
2011
). CAD suppression experiments are complemented by studies focusing
on the analysis of the P. taeda mutant cad-n1 (
MacKay et al., 1997; Ralph
et al., 1997
).
1. Suppression of 4-coumarate-CoA ligase
4-coumarate-CoA ligase (4CL) is key to the general phenylpropanoid path-
way and participates in monolignol biosynthesis through the production of
p-coumaroyl-CoA, a precursor for the biosynthesis of p-coumaryl alcohol
and coniferyl alcohol in conifers (
Fig. 2
). Importantly, p-coumaroyl-CoA
also serves as a precursor for the production of other plant metabolites
including stilbenes and flavonoids (
Boudet, 2007
). 4CL is encoded by a
small gene family in conifers (
Friedmann et al., 2007; Koutaniemi et al.,
2007
), which opens up the possibility for different 4CL unigenes to be
associated with different metabolic pathways. In conifers, expression of the
lignin-related 4CL gene is stimulated during compression wood formation
(
Zhang and Chiang, 1997
), which implies that elevated levels of 4CL activity
are required for increased lignin production in compression wood.
Suppression of 4CL in pine caused the most substantial reductions in
lignin content observed in gene suppression experiments targeting lignin-
related genes in conifers published to date (
Table I
). Quantitative acetyl
bromide-soluble lignin (ABSL) assays revealed a lignin content of 10.3%
(w/w) in P. radiata TEs with severely suppressed 4CL expression levels, a
63% reduction over non-transformed controls (Wagner et al., unpublished
results). This significant reduction in lignin content might imply that manip-
ulations at the entry level of the monolignol pathway in conifers have the
potential to restrict monolignol biosynthesis more substantially than manip-
ulations further downstream in this pathway (
Table I
). This seems to be
plausible, as metabolites can more easily be shunted into pathways other
than monolignol biosynthesis when genes early in the monolignol pathway
are suppressed. A phenotypic trend that is consistent with this observation
has also emerged from lignin-related studies in angiosperms (
Peter and
Neale, 2004
).
