Biology Reference
In-Depth Information
modification in natural conditions. Unfortunately, they failed to reproduce the
initial results entirely.
In one of these studies, new 4CL-downregulated transgenic poplar lines were
produced using the same Pt4CL1 coding sequence, but using the Pt4CL1
promoter, which specifically drives gene expression in the developing xylem,
in place of the CaMV 2x35S promoter used in the former study. In these
conditions, similar modifications were obtained for lignin and cellulose content
(
Hancock et al., 2008; Li et al.2003a
). However, the transgenic plants did not
exhibit any increased growth, suggesting that the growth enhancement observed
in the former study resulted from pleiotropic effects linked to the use of the
CaMV 2x35S promoter known to confer a strong, constitutive expression.
A similar construct was also transferred into triploid Chinese white poplar
(Populus tomentosa) with again no visible differences in tree growth and
development (
Jia et al., 2004
). Repression of 4CL expression resulted in a
major reduction of lignin content (up to 41%) in transgenic poplars, but with
no significant difference in polysaccharide content between transgenic and
wild-type poplars leading the authors to question whether 4CL is really a
control point between lignin and carbohydrate biosynthesis. After peeling
the bark, the developing xylem presented a red-brown phenotype.
Recently,
Wagner et al.(2009)
produced and analysed 2-year-old Pinus
radiata plants with strong reduction in 4CL activity: transgenic plants exhibited
large decreases in lignin content ranging from36% to 50% reduction, associated
with a decrease in G/H ratio, and also some alterations in carbohydrate
metabolism, notably a twofold increase in galactose content. Both the de-
creased G/H ratio and increased galactose content were probably related to
an increased formation of compression wood. These important modifications
at the molecular level translated at the cellular/tissue level in more or less
collapsed tracheids, and the appearance of bands of axial parenchyma inserted
within xylem tracheids. At the plant level, the genetic modification resulted in
severely dwarfed plants.
In another study, the Pt4CL1 promoter-driven antisense copy of Pt4CL1
gene was introduced into the hybrid Populus tremula
Populus alba INRA 717-
1B4 clone and 14 different transgenic lines with reduced 4CL expression were
planted in the field (
Voelker et al., 2010
). Analyses were performed on trees after
2 years of growth. Most transgenic trees appeared affected in their ligninmetab-
olism with lignin reductions ranging from less than 10% to up to 50% with a
reduced S/G ratio and a small increase in H units incorporated into lignins.
Transgenic trees appeared diversely affected in their growth and development: it
seemed that, after reaching a threshold in lignin reduction, the trees developed
strong phenotype alterations. The most affected trees exhibited a red-brown
wood phenotype, which was stronger at the basis of the tree, associated with
