Biology Reference
In-Depth Information
conversely, a lower frequency of labile
-O-4 bonds (
Lapierre et al., 1999
).
The lignin content was not affected by the genetic modification whereas the
active xylem consistently showed a pale rose colouration. Interestingly,
comparison of lignin characteristics between 4-month-old and 2-year-old
plants indicated that lignins deposited at the early stage of lignification had
fewer labile
b
-O-4 bonds in both wild-type and transgenic trees, whereas
COMT downregulation resulted into a twofold increase in resistant 5-5
biphenyl structures (
Lapierre et al., 1999
). Likewise, the reduction in S/G
ratio in 2-year-old transgenic poplars (2-3 times) was not as important as in
younger plants (6-7 times). Overall, the effects of COMT downregulation on
lignin structure proved to have a detrimental impact on the pulping perfor-
mances of 2-year-old transgenic poplars (
Lapierre et al., 1999
). This was
further confirmed by pulping evaluation performed on wood harvested
from 4-year-old trees grown in the field (
Pilate et al., 2002
). Field-grown
transgenic trees were not altered in their growth and development. Interest-
ingly, COMT activity and lignin characteristics appeared less affected in
these older trees.
Transgenic poplars with extremely reduced COMT activity were further
produced from the same clone (INRA 717-1B4 P. tremula
b
P. alba)bygene
silencing resulting from the introduction of a sense poplar COMT sequence
under the control of the CaMV double 35S promoter, which confers a stronger
constitutive expression than the 35S promoter (
Jouanin et al., 2000
). Six-month-
old greenhouse-grown trees with a residual COMT activity lower than 3% of the
control level exhibited a substantial reduction in lignin content (17%) with an
almost complete disappearance of S units that makes its lignin polymer compa-
rable to pure G conifer lignins and also with an important incorporation of
5-hydroxyguaiacyl units (11%). Their wood presented a different brownish
colouration while tree growth and development were not affected. Kraft pulping
assay realized on this material indicated an increased pulp yield (10%), but a
high kappa number indicative of high residual lignin content in the pulp,
showing that the delignification treatment was rather inefficient on this material.
Finally, another study reported the analysis of transgenic aspen (P. tremu-
loides Michx.) with downregulated xylem COMT activity via cosuppression by
a homologous sense full-length COMT transgene (
Tsai et al., 1998
). The
woody stems of transgenic aspen harboured a mottled reddish-brown coloura-
tion, somewhat similar to the observation of
Jouanin et al.(2000)
. The authors
demonstrated that the colouration was due to the incorporation of abnormal
amounts of cinnamyl aldehyde residues in lignins. Here, the genetic modifica-
tion did not affect lignin content, but the S/G ratio appeared strongly de-
creased, as in the previous studies. The patchy pattern of colouration suggests
that the COMT gene silencing did not occur evenly along the stem.
