Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 305

The potential effect of these endophytic microbes
on the fate of contaminants in the transpiration stream of
plants at phytoremediation sites is a topic of great interest.
Although there is some ambient biotransformation of the
solutes in the transpiration stream, it is envisioned that either
naturally present endophytes with contaminant degradation
characteristics or genetically modified bacteria with the
capability to degrade specific contaminants will be added
to plants in contaminated environmental systems. One such
application would lead to the decrease of water-soluble
contaminants, such as toluene or TCE, that are preferentially
taken up and that have high enough vapor pressures that they
exit the stomata into the atmosphere.
A potential advantage of this process would be to
decrease the contaminants or byproducts that are essentially
bound into plant tissue. For example, TCE is partially
degraded in the xylem to the intermediate TCAA. If
TCAA-degrading bacteria can be inoculated into plants
growing in a TCE-contaminated system, TCE could be
removed at the avoidance of the accumulation of TCAA in
the plants. Van Aken et al. (2004b) reported that possibly the
transformation of nitramine explosives in poplar trees could
be due to a bacterial endophyte.
Siciliano et al. (2001) examined the question of the ability
of plants to selectively enhance the number and types of
endophytic bacteria that contain genes that code for enzy-
matic degradation of specific contaminants. They were able
to document that genes that encode for the degradation of
hydrocarbons, alkane monooxygenase ( alkB ) and naphtha-
lene dioxygenase ( ndoB ), were two to four times more
abundant in bacteria contained inside the plant roots relative
to the bulk soil. Similarly at sites contaminated by
nitroaromatics, the gene that encodes for nitrotoluene degra-
dation, 2-nitrotoluene reductase ( ntdAa ) and nitrotoluene
monooxygenase ( ntnM ), were 7-14 times more abundant
in these intra-plant bacteria.
Taghavi et al. (2005) inoculated hybrid poplar trees with
an endophytic bacteria Burkholderia cepacia VM1468 that

contains the ability to code for toluene degradation. Follow-
ing inoculation, the plants grew well in the presence of
toluene and less toluene was released to the atmosphere by
ET , relative to control poplar trees also exposed to toluene
but not containing the bacterium. The gene that codes for
toluene also was observed to be horizontally transferred to
other endophytic bacteria. Such horizontal gene transfer can
allow a microbial community to adapt to changes in envi-
ronmental stresses. This area of the effect of endophytes on
plants and contaminant cleanup will undoubtedly provide a
wealth of research opportunities.

12.5 Summary

Organic compounds dissolved in groundwater enter the epi-
dermal layer of root hairs only if the compound is not first
absorbed by the soil or root itself. A useful parameter to
predict the extent of plant uptake of groundwater
contaminants is provided by the log transform of the parti-
tion coefficient K ow . In general, compounds that have log
K ow between 1 and 3.5 will cross the Casparian strip,
whereas compounds less than 1 will not enter as their solu-
bility is too high, and compounds greater than 3.5 have low
solubility and tend to partition onto soil or root surfaces.
Why is this information important to the phytoremediation
of contaminated groundwater? Such a fundamental property
of organic contaminant compounds provides a powerful
approach to understand the potential for phytoremediation of
groundwater contamination at a particular site based on the
primary contaminant released. Once in plants, the fate of a
particular organic compound will be governed by either diffu-
sion through the bark, volatilization through the leaves, or in
situ detoxification, where plants possess an arsenal of
approaches to deal with the threat of chemicals. In some
cases, these fundamental detoxification reactions can be
applied through phytoremediation—the end result

is a

decrease in groundwater contamination.