Biology Reference
In-Depth Information
residual enzyme activity (
Baucher et al., 1996
). The developing xylem of these
plants displayed a strong red colouration, whereas plants with weaker down-
regulation exhibited patchy phenotypes, sometimes with a radial pattern.
This colouration was shown to result from the presence of conjugated
aldehydes in the lignins. The modified plants did not exhibit any visible cell
wall alterations. Preliminary analyses on 3-month-old plants did not find
important differences in lignin content and composition (S/G ratio) (
Baucher
et al., 1996
). Nevertheless, chemical-pulping tests revealed that the transgenic
wood was easier to delignify (up to 22% Kappa number reduction), while the
DP for cellulose, pulp yield and fibre length was preserved. These results
obtained on very juvenile plants were later confirmed on 2-year-old plants
(
Lapierre et al., 1999
) and finally in 4-year-old field-grown trees planted in
two sites, Ardon (France) and Jealott's Hill (United Kingdom), under differ-
ent climate and soil conditions (
Pilate et al., 2002
). Unfortunately, the
English field trial had an early termination due to its destruction by environ-
mental activists. Nevertheless, analyses revealed that, in these older plants,
the genetic modification resulted in a slight, but consistent, decrease in lignin
content (less than 20% compared to wild type). Further in-depth lignin
analyses revealed that CAD downregulation effects on the structure of the
lignin polymer were not due to any changes in the frequency of labile ether
bonds or guaiacyl units in lignins, but resulted from an increased proportion
of syringaldehyde and diarylpropane structures and more importantly in
an increased frequency of free phenolic groups (
Lapierre et al., 1999
). This
higher content in free phenolic units facilitated lignin solubilization and frag-
mentation during Kraft pulping. The most affected antisense line (ASCAD21)
presented increased pulping properties: it appeared easier to delignify using
less chemicals, leading to a better pulp yield with less cellulose degradation.
Regarding growth and development of transgenic trees in the field, no differ-
ence in bud burst was observed between transgenic and wild-type lines. Like-
wise, none of the antisense lines exhibited any significant differences in height
or trunk diameter. Nevertheless, the only cosuppressed line planted in the field
revealed a significant reduction both in primary and secondary growth (G.
Pilate, A. Yoshinaga, J. Toillon, N. Boizot, M.C. Lesage-Descauses, F. Laur-
ans, A. D
´
jardin, J.C. Lepl
´
, C. Lapierre, unpublished results). As this was the
only cosuppressed line present in the field trial, we were not able to conclude
on a specific effect of cosuppression. At the end of the field assay, 12-year-old
poplar wood was collected and analysed in order to determine whether the
promising results obtained with the ASCAD21 line will be confirmed on
mature wood at the age of commercial harvest. The red colouration in the
developing xylem was still present, indicative of the stability of the CAD
antisense suppression. In addition, the evolution of lignin characteristics will
be determined by analyses to be performed within the different annual rings.
