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and a strong constitutive promoter. Six- to nine-month-old greenhouse-
grown plants exhibited a 40% reduction in Klason lignin content, associated
with both a 70% reduction in CCoAOMT activity and an orange coloura-
tion of the active xylem. Further analyses using FTIR indicated that the
reduction in lignin content led to less condensed and less cross-linked lignin
structure in wood. Lignin modifications in the transgenic poplar plants
did not result in any abnormal growth pattern even though anatomical
observations revealed the occurrence of cell wall distortions for a small
proportion of the vessels.
In a recent paper, Wei et al.(2008) reported the evaluation of 3-year-old
field-grown transgenic poplar (P. tremula
P. alba) withdepressedCCoAOMT
activity. Overall, lignin modifications appeared similar to those observed by
Meyermans et al.(2000) . In the Kraft pulping assay, improved pulp character-
istics and an increased pulp yield were observed with the transgenic wood when
compared to the wild-type wood. The authors did not observe any obvious
adverse effects of CCoAOMT downregulation on the growth and development
of the lignin-modified trees in field conditions.
In conifers, a 90% reduction of CCoAOMT expression in P. radiata
tracheary element cultures resulted in a 20% reduced lignin content due to
depletion of G units, an altered lignin polymer with a 10-fold increase in H/G
ratio and the presence of catechyl units due to incorporation of caffeyl
alcohol in the lignin polymer ( Wagner et al., 2011 ).
G. CAFFEIC ACID O-METHYLTRANSFERASE
Caffeic acid O-methyltransferase (COMT) was previously thought to be
a bi-functional enzyme, catalysing the methylation of both caffeic and
5-hydroxyferulic acids. However, in vitro COMT enzymatic assays in the
presence of a mix of its different potential substrates later demonstrated that,
in in vivo conditions, the preferential substrates were 5-hydroxyconiferaldehyde
and also possibly 5-hydroxyconiferyl alcohol, giving rise to sinapaldehyde
and sinapyl alcohol, respectively ( Osakabe et al., 1999; Parvathi et al., 2001 ).
COMT-downregulated transgenic poplars (INRA 717-1B4 P. tremula
P. alba) were produced with a P. trichocarpa
Populus deltoides COMT
antisense sequence under the control of the pCAMV 35S ''constitutive''
promoter ( van Doorsselaere et al., 1995 ). Three-month-old greenhouse-
grown plants with a 95% reduction in COMT activity showed a very strong
S/G ratio reduction (from 1.5 in control plants to 0.25 in the most severely
affected line) and an appreciable amount of 5-hydroxyguaiacyl units: the
strong increase in the proportion of G in the lignin polymer strongly alters its
structure, with an increased frequency of resistant interunit bonds and,
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