Biology Reference
In-Depth Information
for many the specific function of each of these members, whether they are
specifically expressed in a given tissue, a cell type, a stage of development
or in response to a stress, or if they encode enzymes with different substrate
specificities.
Briefly, monolignol synthesis derives from the phenylpropanoid pathway,
which is initiated by deamination of phenylalanine by phenylalanine ammo-
nia-lyase (PAL). The lignin building blocks result from a series of hydroxyl-
ation and O-methylation reactions on the aromatic ring and the conversion
of the side-chain carboxyl to an alcohol group (
Boerjan et al., 2003
). For a
long time, it was thought that the modifications on the aromatic ring oc-
curred at the level of free hydroxycinnamic acids, but recent discoveries led to
a reformulation of the pathway (see for details
Bonawitz and Chapple, 2010
).
The monolignols are then transported through the plasma membrane to the
extracellular matrix where it is believed that peroxidases and/or laccases
initiate their polymerization. At the present time, little is known about the
modalities of monolignol transport and polymerization (
Li and Chapple,
2010
). In addition, we do not yet really know whether this synthesis is strictly
cell autonomous or if lignin precursors can be translocated from living
adjacent cells, such as xylem ray parenchyma cells (
Feuillet et al., 1995
).
For the last 20 years, the study of transgenic plants modified for genes
coding for enzymes of the lignin biosynthetic pathway has efficiently com-
plemented our previous knowledge acquired through biochemical experi-
ments, providing invaluable information on the lignin biosynthetic
pathway and illustrating the high plasticity of the lignin polymer. These
studies were performed on tobacco and Arabidopsis, but also on trees,
merely on poplar but also, more recently, on a few other species such as
eucalyptus and pine. We focus here on the results obtained from transgenic
trees with modified lignins with an emphasis on studies involving field trials.
Field trials present several advantages when compared to greenhouse experi-
ments (
Pilate et al., 1997
). In a field trial, tree development is not limited by
physical constraints such as pot size for the root or greenhouse height for the
stem. Running field trials allow tree growth and development to be evaluated
in more natural conditions (
Figure 1
) and also make possible, after several
years, the production of wood in sufficient quantity for technological assess-
ment of potential modifications to properties. Stability of modifications can
be monitored on long-term field trials by regular measurements of easily
scorable effects of the genetic modification (e.g. a coloured phenotype). It is
also possible to study the potential effects of the genetic modification on
some elements of the ecosystem, such as trophic interactions with herbivo-
rous insects or fungal pathogens, or the effect of wood decomposition rate on
carbon flux.
