Lignification and Lignin Manipulations in Conifers - Botanical Research - page 54

for the biosynthesis of H-lignin in conifers and for the key role, along with
C3H, of HCT in the pathway towards coniferyl alcohol ( Fig. 4 ). A similar
change in lignin composition was observed in angiosperms, where consider-
able elevations in H-lignin were observed in HCT suppression experiments
and where the biosyntheses of both G- and S-lignin were severely restricted
( Besseau et al., 2007 ). HCT therefore represents the metabolic entry point
leading to the biosynthesis of all methoxylated phenylpropanoids in angios-
perms and coniferous gymnosperms.
2D-NMR experiments with purified pine TEs revealed that changes in
monolignol composition also caused modifications in the lignin interunit
linkage distribution. In particular, an increase in resinols, a reduction in
dibenzodioxocins, and the presence of glycerol end groups could be observed
( Wagner et al., 2007 ). The reduction of dibenzodioxocins in HCT-RNAi
lines most likely reflected reduced levels of G-lignin in transgenic TEs.
Interestingly, the array of H-containing dibenzodioxocins observed in
Medicago sativa with high H-levels ( Ralph et al., 2006 ), was not observed
in the transgenic pine TEs.
p-Coumaroyl-CoA, the substrate of HCT, not only serves as a precursor
for the biosynthesis of monolignols, but also as a precursor for the biosyn-
thesis of flavonoids ( Fig. 4 ). It is therefore not very surprising to find an
accumulation of flavonoids such as kaempferol-, quercetin- and cyanidin-
derivatives in angiosperms with suppressed HCT levels ( Besseau et al., 2007 ).
Pine TE lines transformed with a HCT-RNAi construct displayed the same
phenotypic trend. An accumulation of flavonoids, most likely anthocyanins,
which caused TE cultures to turn red during differentiation, was observed in
HCT-RNAi lines with suppressed steady-state RNA levels for HCT (Wagner
et al., unpublished results).
3. Suppression of caffeoyl coenzyme-A 3-O-methyltransferase
Caffeoyl coenzyme-A 3-O-methyltransferase (CCoAOMT) is involved in the
biosynthesis of methoxylated phenylpropanoids in angiosperms and conifer-
ous gymnosperms ( Do et al., 2007; Marita et al., 2003; Meyermans et al.,
2000; Wagner et al., 2011; Zhong et al., 2000 ). The preferred substrate for
CCoAOMT is caffeoyl-CoA, which is converted into feruloyl-CoA ( Fig. 2 ).
CCoAOMT suppression in angiosperm species caused a 20-45% reduction
in lignin content ( Chen et al., 2006; Do et al., 2007; Marita et al.,2003;
Meyermans et al., 2000; Nakashima et al., 2008; Zhong et al.,2000 ). The
majority of the studies reported a reduction of G- and S-lignin ( Do et al., 2007;
Marita et al., 2003; Meyermans et al., 2000; Zhong et al.,2000 ), indicating that
caffeoyl-CoA is the precursor for both lignin types in angiosperms. However, a
number of studies also reported that suppression of CCoAOMT had virtually