Environmental Engineering Reference
In-Depth Information
engineered to improve agronomic traits (growth, yield, and resistance to biotic and abiotic stress)
and to make it more amenable to pretreatment and saccharification through modification of cell
wall properties.
3.4.6 p
oplar
Poplar is discussed as the representative tree biofuel crop because it is a model crop. Poplar is
the model tree biofuel feedstock because of its wide range of adaptation, available genome, fast
growth, clonal propagation ease, sexual compatibility with other species, and available transfor-
mation techniques (Davis 2008; Yuan et al. 2008). These trees are found throughout the northern
hemisphere, are shade intolerant (grow best with complete weed control), grow in moist areas, and
have medium to short life spans. Woody biomass plants have several advantages, including flex-
ible range of harvesting time and good environmental impacts (Davis 2008). Yields for poplar are
between 5 and 20 Mg/yr or 10 and 30 Mg/yr based on genotype, site, region, etc. Poplar is used for
cellulosic ethanol or for co-firing with coal, but the costs for harvesting and chipping are the major
disadvantages to using poplar for bioenergy needs. Areas of genetic improvement would include
genes influencing growth, branching, stem thickness, light response competition, plant height, and
cell wall makeup (Rubin 2008).
Poplar is highly amenable to genetic transformation (Cseke et al. 2007). Therefore, poplar has
been variously genetically engineered to improve its growth and wood properties (Park et al. 2004;
Cseke et al. 2007; Baba et al. 2009). Expression of carbohydrate-binding modules (CBMs) has been
shown to increase cell growth in poplars transformed with cell-wall-targeted
Clostridium cellulo-
vorans
CBM (Shani et al. 1999). Xyloglucanse was overexpressed in poplar to enhance the growth
and yield in poplar. The overexpression of xyloglucanse reduced crosslinks in their cell wall, which,
in turn, increased the plasticity under turgor pressure during growth (Park et al. 2004). Similarly,
the expression of
Arabidopsis
endo-(1-4)-ß-glucanase gene (
Cel1
) in poplar trees resulted in longer
internodes compared with their wild-type control (Shani et al. 2004). Accelerated growth of pop-
lar expressing expansin has been reported recently (Gray-Mitsumune et al. 2008). Hu et al. (1999)
produced lines of transgenic poplar (
Populus tremuloides
Michx), which exhibited a 45% reduction
of lignin and an increase of 15% in cellulose. This altered cell wall composition enhanced leaf,
root, and stem growth without affecting the structural integrity of the transgenic plant, one of the
key requirements of successful modification. Introduction of tyrosine-rich peptides to poplar trees
through genetic transformation resulted in the formation of wood that is more susceptible to protease
digestion than wild-type plants. This genetic modification caused greater release of sugar from lingo-
cellulose complex during saccharification (Liang et al. 2008). Transgenic poplar resistant to heavy
metals such as mercury, cadmium, arsenic, and lead have been developed. Poplars were engineered
with
merA
,
merB
,
gsh1
,
CYP2E1
,
MnP
,
cys1
,
and
PsMT
A
1
genes. The introduced bacterial genes
merA
and
merB
conferred resistance to organomercurial pollutants (Yadav et al. 2010). Genetic
engineering of trees in relation to their application for forestry was recently reviewed (Harfouche
et al. 2011). This review reported the issue of 25 permits for field trials of GM poplars in Europe
since 1991. These GM poplars are being evaluated for altered wood composition, altered wood
properties, sterility, lignin modification, herbicide tolerance, faster growth, and phytoremediation.
3.4.7 E
ucalyptuS
Eucalyptus
is a large genus of trees and large shrubs of more than 700 species. Although many
species of eucalyptus are naturalized across the globe, they originated in Australia. Eucalyptus
comprises multipurpose trees used for timber and firewood or for the extraction of high-value chem-
icals (Eldridge 1993; Ladiges et al. 2003; Domingues et al. 2011). Many species of
Eucalyptus
grow
fast and generate large biomass suitable for bioenergy production (Stricker et al. 2000). Eucalyptus
hybrid (
E. grandis
×
E. urophylla
) has proved to be ideal for forestry because of their growth
