Biology Reference
In-Depth Information
In another study, 7-month-old greenhouse-grown transgenic poplars
(cv Ogy P. deltoides
Populus nigra) with very low CAD activity (10%
residual CAD activity) were analysed: the lignins of these plants presented
the same characteristics as in the previous studies but were exacerbated
(
Lapierre et al., 2004
). Preliminary small-scale assays indicated that more
than 50% of the lignin content was released after very mild alkaline treatment
(2N NaOH at 37
C) indicating very promising pulping characteristics
for this wood. Unfortunately, these CAD-downregulated plants appeared
affected in their growth and development even in favourable greenhouse
conditions (A. D
´
jardin, N. Boizot, M.C. Lesage-Descauses, F. Laurans,
J.C. Lepl
´
, G. Pilate, unpublished results).
The results obtained from CAD-downregulated poplars are comparable to
the analyses performed on a natural cad mutant in loblolly pine. This mutant
cad allele (cad-n1) resulted in severely decreased CAD gene expression, in
homozygous mutant trees, modified wood colour and lignin composition
(
MacKay et al., 1997
). cad-n1 homozygotes had nearly no detectable CAD
activity whereas heterozygous plants had half the wild-type CAD activity.
Free phenolics, mainly coniferaldehyde—the preferred substrate of CAD in
conifers—and vanillin showed substantial increases in content (10 times)
compared to wild-type wood. The wood of the cad-n1 homozygotes, howev-
er, was distinctly brown, whereas pine typically has a white wood. Lignin
content was not different from wild type in heterozygous plants and only
slightly reduced (nearly 10% reduction in the homozygous plants), whereas
significant amounts of coniferaldehyde appeared to be incorporated into the
lignin polymer of mutant plants leading to a lignin polymer with more
condensed inter-unit linkages (such as 5-5 bonds) and lower levels of
8
-O-4
bonds. Chemical pulp assays indicated that an efficient delignification of
wood from the homozygous mutant trees could be obtained with low sulfide
levels. Pulping studies with heterozygous wood from juvenile trees found a
lower Kappa number, but this did not seem to be maintained in mature trees
(
Scott et al., 2002
).
Under greenhouse conditions, the morphology and growth of cad-n1
homozygote pine plants could not be distinguished easily from that of
other genotypes. However, it was later observed that homozygous cad mu-
tant pine trees had extremely low growth rates and a high rate of death (
Scott
et al., 2002
), whereas growth of heterozygous trees was clearly enhanced with
a 14% increased volume mostly resulting from increased radial growth (
Wu
et al., 1999
). In fact this genotype was selected sometime ago in Southern
United States breeding programs for its increased growth rate. Despite
the obvious differences between angiosperm and gymnosperm lignins, it is
worthy of noting the numerous similarities in the effects of different levels of
CAD downregulation between natural mutants and GM trees.
b
