Biology Reference
In-Depth Information
4. Suppression of cinnamoyl-CoA reductase
Cinnamoyl-CoA reductase (CCR) represents the enzymatic entry step that
leads into the monolignol-specific branch of the phenylpropanoid pathway in
angiosperms (
Lacombe et al., 1997
) and this also applies to conifer species
(
Fig. 2
;
Wadenb¨ck et al., 2008
).
Suppression of CCR in angiosperms resulted in lignin reductions of up to
50%, providing evidence for the crucial role of CCR in monolignol biosyn-
thesis (
Goujon et al., 2003; Lepl´ et al., 2007
). CCR suppression in angios-
perms also caused the incorporation of ferulate into the lignin polymer
(
Dauwe et al., 2007; Lepl
´
et al., 2007; Mir Derikvand et al., 2008; Ralph
et al., 2008b
) and changes in the metabolite profile. In a species-dependent
fashion, elevated production of ferulate, feruloyl glucose, feruloyl malate,
ferulic acid hexoside, vanillic acid glucoside and sinapic acid glucoside were
recorded (
Chabannes et al., 2001; Dauwe et al., 2007; Lepl
´
et al., 2007; Mir
Derikvand et al., 2008
). The improved enzymatic hydrolysis of cell wall
polysaccharides observed in CCR-deficient angiosperms raised interest in
this approach from a biotechnological perspective (
Boudet et al. 2003
).
Suppression of CCR in the gymnosperm species P. abies was reported
recently (
Wadenb¨ck et al., 2008
). An up to 35% reduction in CCR steady-
state mRNA levels was recorded in this study, which caused only small
reductions in lignin content (
Fig. 6
;
Wadenb¨ck et al., 2008
). Changes in
lignin composition were not recorded, but an NMR signal believed to be
related to fatty acids was identified (
Wadenb¨ck et al., 2008
). It has been
speculated that this signal reflects suberin-like material, but this speculation
awaits experimental verification.
The small reduction in lignin content reported for spruce CCR-antisense
plants is unlikely to reveal what impact CCR has on lignin biosynthesis in
conifers. The phenotypic limitation observed in this study might have been
the consequence of using an antisense approach to suppress CCR, which is
less efficient than RNAi for gene suppression in conifers (
Wagner et al.,
2005
). However, even the relatively small reductions in lignin content ob-
served in this study were able to compromise plant vigour and performance
in spruce, as stem widths were significantly decreased in CCR-antisense
plants (
Wadenb
¨
ck et al., 2008
). This indicated that conifers are very suscep-
tible to reductions in lignin content.
5. Suppression of cinnamyl alcohol dehydrogenase
Cinnamyl alcohol dehydrogenase (CAD) catalyses the reduction of hydro-
xycinnamaldehydes to the corresponding alcohols, the last step in the bio-
synthesis of monolignols (
Fig. 2
).
