Environmental Engineering Reference
In-Depth Information
that reaches a domestic well may present a risk to those who
ingest the TCE-contaminated well water. A phytoremedia-
tion effort to limit the spread of the dissolved phase plume
using a transgenic plant may also have its risk of entry into
the native population. A cost-benefit analysis at such sites
will need to be done. Fortunately, at many phytoremediation
sites transgenic usage occurs typically at great distances
from food crops.
There are examples of the potential usefulness of trans-
genic engineering for increased biodegradation of common
groundwater contaminants, such as TCE (see Chap. 13) with
phytoremediation applications, as was shown in a study by
Shim et al. (2000). TCE is stable in the presence of oxygen,
on account of its oxidation during chlorine-atom substitu-
tion. TCE tends to only undergo extensive degradation if it
serves as an electron acceptor in the absence of oxygen.
TCE can, however, undergo degradation in the presence of
oxygen during co-metabolism. It has been shown that the
toluene
ortho
-monooxygenase (TOM) also oxidizes TCE
completely to CO
2
and Cl
. The genes that lead to the
production of TOM can be added to the chromosomes
of gram-negative bacteria, as was done to wheat root
rhizospheric bacteria that demonstrated the ability to remove
TCE from contaminated soils (Yee et al. 1998). In a separate
study, James et al. (2007) reported that transgenic tobacco
(
Nicotiana tabacum cv. Xanthii
) they developed expressed
genes for cytochrome P-450 that could enhance degradation
of compounds such as TCE, vinyl chloride, and benzene
above that of native tobacco.
Another beneficial application of transgenic plants is the
increased removal of TNT from explosives-contaminated
soils. Travis et al. (2007) reported that transgenic tobacco
that express bacterial nitroreductase genes can increase the
rhizosphere degradation capacity of the transgenic plant
relative to native tobacco. Moreover, the transgenic plants
had more and deeper roots and a larger rhizosphere.
Natural rhizospheric bacteria collected from the roots of
poplar trees were genetically engineered to have the TOM
gene (Shim et al. 2000). Although recombinant bacteria
from various plant rhizosphere added to poplar roots
indicated a decrease in the recombinant TCE oxidizers
over time to near 100% loss, the recombinant bacteria from
tree colonizers were more competitive, with between 39%
and 79% survival. The investigators concluded that this
increased competitiveness was a function of the source of
the bacteria from the rhizosphere of the host plant or similar
surrogates, relative to unrelated plants or soils.
The opponents of transgenic plants for phytoremediation
purposes also logically oppose the use of any plant that is not
native to the contaminated area. This is because in some
areas of the United States, or in upland contaminated areas
of the humid east not supportive of riparian vegetation, the
introduction of any non-native plant, even a hybrid, is not
without some potential negative consequences. For example,
what will happen to planted trees after a phytoremediation
project is no longer funded? Should the plants be cut down?
If so, they will grow back? Will they “escape” the site to
populate other areas?
Populus
can form hybrids with other
Populus spp.
nearby.
If hybrids escape, there always is the potential for
other areas to be overrun by the hybrid, and biodiversity
will decrease. The best example of this potential escape is
the gradual replacement of cottonwood-poplar riparian
ecosystems by salt cedar in the southwestern United States.
In order to assess the threat of introducing non-native hybrid
plants into a contaminated part of a local ecosystem,
Rotteveel et al. (2006) provide a decision tree to aid in risk
evaluation. The key assesses the biological hazard, such as
extent of invasiveness, through sexual propagation, or fast
growth rates, and then helps to determine the extent of real
risks, either at the planted site or offsite. Some of these
concerns can be alleviated by planting sterile male plants
as the hybrid sex of choice. Not only will there be no seeds
formed by these trees within themselves, they also will not
be able to sexually interact with native poplars.
In the same species gene flow is vertical. In different
species, gene flow is horizontal. Natural gene flow occurs
all the time. Gene flow can be defined as the incorporation of
genes of one or more populations into the gene pool of other
populations. It happens between domesticated crops and
their native counterparts (Ellstrand et al. 1999)—the result
is a hybrid. The hybrid can be fertile, and produce viable
seeds itself, or sterile. As such, gene flow is a driver of
evolution, sometimes of much larger impact than mutation
or selection (Ellstrand et al. 1999).
Whether or not gene flow occurs in phytoremediation
examples is not the question; rather, the question is, will
the hybrid be undesirable? Gene containment such that
phyto-plant genes do not leave the site is done by integrating
the transgene in the plasmid genome. Also, much concern
could be overcome using plant-generated genes that can
produce the detoxification enzymes rather than relying on
mammalian genes inserted into plants. There may also be
less potential for transgenic-modified plants or plant-
associated microbes to do harm if the work is done on
endophytic bacteria rather than ectomycorrhizae.
It has been shown that some chemicals when added to
plants themselves or their intermediate byproducts behave as
plant activators. This describes a process where mutagenic
compounds are activated from benign plant promutagens
(Plewa and Wagner 1993). The potential exists for the
activated mutagen to cause cancer in certain individuals.
The acceptance of transgenic plants for the use in the
phytoremediation of contaminated groundwater has and
will undoubtedly face the same challenges that faced the
use of these techniques in the production of food or other
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