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activity levels seem therefore to have very little impact on lignin content in
conifers, which differentiates CAD suppression/mutant studies from most
other lignin-related suppression studies in conifers ( Table I ). However, lignin
composition in the cad-n1 mutant did change significantly. Among the most
obvious changes was a more than 30-fold increase in coniferaldehyde, the
CAD substrate, and substantially elevated levels of dihydroconiferyl alcohol
in the lignin polymer ( MacKay et al., 1997; Ralph et al., 1997 ). The incor-
poration of those metabolites into pine lignin can be explained by the
position and function of CAD in the monolignol pathway ( Fig. 2 ), although
dihydroconiferyl alcohol production requires a currently unknown aldehyde
reductase ( Sederoff et al., 1999 ).
CAD-RNAi experiments with P. radiata TEs and plants containing ap-
proximately 20% residual CAD activity did not result in changes in lignin
content or composition compared to wild-type controls according to Klason
lignin, thioacidolysis, DFRC, or pyrolysis-GC/MS experiments ( M ¨ ller
et al., 2005 , unpublished results). In addition, histological investigations of
pine CAD-RNAi plants using confocal microscopy and TEM did not reveal
any changes on an anatomical level compared to wild-type controls (Wagner
et al., unpublished results). TEs containing a CAD-RNAi construct accumu-
lated metabolites such as dihydroconiferyl alcohol and dihydro-p-coumaryl
alcohol only when the phenylpropanoid pathway was artificially stimulated
using elicitors in combination with lignin precursors ( Fig. 7 ; M¨ ller et al.,
2005 ). Feeding experiments with P. taeda cell suspension cultures revealed
that CAD is far from being rate-limiting in the biosynthesis of monolignols in
conifers. Indeed, very high non-physiological concentrations of coniferalde-
hyde were necessary to cause a transient intracellular accumulation of con-
iferaldehyde ( Anterola et al., 1999 ).
All these data suggest that low CAD activity levels are sufficient in conifer
species to maintain a wild-type like lignin content and composition. CAD
differs in this respect from most other lignin-related genes tested in conifers
by reverse genetics approaches. One possible explanation for this difference is
that few metabolic opportunities exist in conifers to channel phenylpropa-
noids in pathways other than lignin biosynthesis once phenylpropanoids
have been converted to aldehydes.
B. LIGNIN DESIGN STRATEGIES FOR CONIFERS
Natural variation in lignin content in conifer species seems to be relatively
narrow, ranging between 26% and 30% (w/w) in normal wood of pine species
( Campbell and Sederoff, 1996 ). Recombinant studies in conifers showed that
small to moderate reductions in lignin content can already compromise plant
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