Page 372 - Biosystems Engineering

Evaluation of Transgenic Wood for Pr oductivity & Quality 349

wood characteristics and trunk structure, increasing its growth rate,
and altering its shape. Additional breeding targets include improving
the root system and tree canopy (leaf) performance, pest resistance,
and tolerance to abiotic stresses (Tzfira et al. 1998).
Herbicide-resistant transgenic crops are considered to be one of
genetic engineering’s major successes. Direct herbicide detoxification
and reduction of target-enzyme sensitivity to the applied herbicide
are the two main approaches that have been used for engineering
plants with herbicide resistance (Tzfira et al. 1998). Most tree species
are susceptible to the herbicide used for weed control. These chemi-
cals interfere with key metabolic pathways of trees, hampering their
normal growth and having great impact on their commercial value in
forestry. Thus, introducing the herbicide-resistance trait in trees has
great economic potential. It will allow the establishment of tree plan-
tations in weed-infested sites that would otherwise be economically
unsuitable to plant (Walter et al. 2002).
Important losses caused by defoliating insects occur in various tree
species. The damage often translates into a reduction of tree growth
and survival, as well as in alterations in tree shape or fruit quality. In
practice, the use of insecticides is rather limited in forestry, in part due
to the large forest areas and tree sizes and to the development of resis-
tance by insects and the environmental impact of insecticides. Genetic
engineering currently allows the production of plants resistant to a
wide range of insects through the induction of transgenes derived from
plants, microorganisms, and mammals. These transgenes code for an
extensive range of biomolecules that attack the insects’ digestive sys-
tems using different mechanism (Schuler et al. 1998).
In general, most natural forest-tree species are well adapted to
their environment, exhibiting high ecological competence. However,
forestation with plantation-improved or imported exotic tree species
will probably reveal their sensitivity to several ecological factors.
Cold, drought, salinity, and heavy-metal toxicity are the main stresses
specifically affecting trees, which are subjected to many annual
changes during their life cycle (Tzfira et al. 1998). Genetic engineering
for cold tolerance has been evaluated in several transgenic plants
expressing an ice-nucleation gene from bacteria, antifreeze genes
from fish, and altered lipid composition in their cell membranes
(Tzfira et al. 1998). Cold tolerance in trees would enable the use of
cold-sensitive species in northern areas, as well as providing better
protection of native plants from chilling damage.
Phytoremediation, the use of transgenic plants to remove con-
taminants from soil or water, has a potential impact on environmen-
tal pollution and, in the long term, the preservation of natural forestry
(Herschbach and Kopriva 2002). Rugh et al. (1998) found that overex-
pression of the bacterial mercuric reductase in yellow poplar resulted
in transgenetic plants that were resistant to toxic levels of mercuric
ions and were able to release elemental mercury.