Biology Reference
In-Depth Information
observed by atomic force microscopy. Analyses of mechanical properties
revealed a slight decrease in axial stiffness in the wood of transgenic plants.
The C4H gene has also been incorporated in sense and antisense orienta-
tions in an elite tree of a commercially valuable eucalyptus species, Eucalyptus
camaldulensis (
Chen et al., 2001
), and 100 plants transferred to the field; the
authors have not yet reported on the evaluation of the field-grown transgenic
trees. Recently, ArborGen also announced the field evaluation of 2-year-old
eucalyptus with C4H reduced activity. Field-grown transgenic trees with a
20% lignin reduction presented normal or near-normal growth. The polysac-
charide content was not modified in transgenic trees, but lignin reduction in
transgenic trees correlated, in technological tests, with a better release of
sugars, resulting in an increased pulp yield with less chemicals and a greater
accessibility of cellulose for biofuel production (
Hinchee, 2011
).
C. 4-HYDROXYCINNAMOYL-COA LIGASE
4-Hydroxycinnamoyl-CoA ligase (4CL) is responsible for the CoA esterifica-
tion of p-coumaric acid in p-coumaroyl CoA (
Bonawitz and Chapple, 2010
).
Hu et al. (1998)
characterized two genes encoding 4CL in Populus tremuloides
and, according to their pattern of expression, identified one of them, Pt4CL1,
as specific for lignin biosynthesis in developing xylem tissues. The promoter
of this gene was further used to target expression of the transgene to xylem
cells at the lignification stage.
This Pt4CL1 gene was mobilized in a transformation vector, under the
control of a strong constitutive promoter, to produce by antisense inhibition
transgenic P. tremuloides with downregulated 4CL expression (
Hu et al., 1999
).
Ten-month-old greenhouse-grown transgenic trees exhibited more than 90%
inhibition of 4CL activity, leading to a 40-45% lignin reduction, with no
important modification in its composition. Lignin reduction appeared to be
compensated by a 15% increase in cellulose. Therefore, the 4CL downregula-
tion in the most severely affected lines resulted in a cellulose/lignin ratio of 4,
twice as much as in the wild-type plants; however, this material was not further
analysed for technological properties. The authors hypothesized that 4CL was
involved in a regulationmechanism, specific to trees, coordinating cellulose and
lignin deposition. Interestingly, stems, leaves and roots of transgenic plants had
enhanced growth when compared to wild-type plants. Anatomical studies
indicated that it was caused by increased cell proliferation activity rather than
increased cell size. Due to the very promising results reported in this first study,
a number of other experiments targeting the 4CL gene were undertaken and led
to the installation of different field trials to evaluate the effects of 4CL genetic
