Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 300

trichloroacetic acid (TCAA) and dichloroacetic acid
(DCAA), formed by the P-450 pathway (Shang et al.
2003). Interestingly, the higher dosage of contaminant
exposed to a plant the higher the concentration of P-450 is
measured, suggesting that its production is induced.
Other oxidative reactions in plants to detoxify threats
involve the induction of plant peroxidases (POXs). These
oxidases are used by plants to catalyze the transformation of
many potentially harmful chemicals into less toxic forms.
Xenobiotics, for example, can be polymerized into the soil
humic fraction or root surface by POX and become essen-
tially nonbioavailable. Peroxidases can decrease H 2 O 2
concentrations in order to drive the oxidation of other
substrates. This typically is a reduction of the H 2 O 2 and the
oxidation of other substrates. Because these POXs are prev-
alent in most plants, their role in detoxification is more well
known than that of P-450. They are found in the cytosol of
the cell.
Many different classes of organics can be oxidized by
peroxidase. The putative enzyme peroxidase is a single
peptide chain, with one heme group. Bacteria in the rhizo-
sphere use a similar method to detoxify harmful substances
but use a dioxygenase to accomplish this goal. Hence, these
aerobic, heterotrophic bacteria can derive energy from this
reaction as the toxicant is mineralized to CO 2 and H 2 O. This
full detoxification by rhizospheric bacteria is more common,
however, when contaminant concentrations are low. When
concentrations are higher, some gets mineralized but the
majority is taken up into the plant and affected by Phase I
reactions (Kvesitadze et al. 2006). Other oxidative enzymes
released by microbes and fungi in the rhizosphere include
cellulose, lignase, and protease, among others (Walton et al.
1994).
Another important detoxification enzyme in both plants
and animals is glutathione-S-transferase (GST). Glutathione
is a tripeptide that reacts with oxidizing agents to form a
disulfide product. In this manner, GST acts sacrificially for
the protection of more important proteins such as DNA,
similar to how a zinc coating on various metal objects,
such as nails and screws, acts as a sacrificial metal to prevent
the underlying iron from oxidizing.
As can be inferred from the above comparison of Phase I
detoxification reactions plants and animals, plants process
xenobiotics using mechanisms that are fundamentally simi-
lar to how the mammalian liver detoxifies compounds.
Although plants lack true excretory organs, they can store
detoxification byproducts in vacuoles and in the lignin itself,
and thus separate them from the rest of the plant. But this
does not mean that plant exudates are not helpful in contam-
inant detoxification. Enzymes are released from plant
roots and form part of the exudates that are associated with
the rhizosphere. Compounds detected in the soil zone near
roots include dehalogenases and nitroreductases (Schnoor

et al. 1995). Peroxides have been found to polymerize
contaminants onto the root surface or soil organic matter in
the root zone. This action accelerates humification and
renders the contaminant less bioavailable for plant uptake.
From the perspective of the phytoremediation of
contaminated groundwater, plants have been shown to use
such enzymes to break down contaminants such as

chlorinated hydrocarbons. The C Cl bond of these

chlorinated compounds is attacked by monooxygenases,
glutathione-S-transferases, and anti-auxin cell receptor bind-
ing, and the

OH. Poplar trees were
shown by Noctor et al. (1998) to contain high concentrations
of glutathione. Komives et al. (2003) reported that, for
poplar trees exposed to increasing concentrations of
chlorinated herbicides, that increasing concentrations of glu-
tathione were detected in poplar leaf cuttings.

Cl is replaced with

12.4.1.2 Hydroxylation
Many xenobiotic compounds released to groundwater con-
tain an aromatic or multiple-ring structure. The process of
splitting these aromatic or heterocyclic rings and the
subsequent addition of an OH functional group is an oxi-

dative process called hydroxylation. Hydroxylation
reactions increase the contaminant reactivity in plant cells
by increasing the compound's polarity and, therefore, hydro-
philicity. Such “ring” cleavage occurs slowly in plants.
Unlike bacteria which can render such rings all the way to
CO 2 , further degradation by plants after cleavage and

OH
addition is limited. Instead of complete mineralization, these
cleaved, hydroxylated compounds are incorporated into
plant polymers as a bound residue. In many cases, because
of the slow kinetics of hydroxylation of aromatic or multi-
ple-ring compounds by plant cells, it is the rate-limiting step
in contaminant detoxification (Kvesitadze et al. 2006).
Various contaminant compounds that can enter ground-
water can undergo hydroxylation reactions after oxidation.
Aromatic hydrocarbons that contain an organic functional
group, such as the CH 3 on toluene, can be oxidized by

hydroxylation by OH functional group addition in the para
position. Organic compounds that contain a nitrogen, or N,

functional group undergo N-hydroxylation reactions, in

which the OH. Aliphatic

hydrocarbons also can have an -OH functional group
added, and proceed by the insertion of an oxygen atom
between the C

N is replaced by an

H bond catalyzed by P-450 enzymes.
Another oxidative reaction that undergoes hydroxylation is
the addition of oxygen to a C ¼ C carbon-carbon double

bond, called an epoxidation reaction.
Specific examples of hydroxylation reactions by plants
include those contaminants that are commonly detected in
groundwater. The degradation of the aromatic compound
benzene in gasoline is initiated by ring cleavage, and the
formation of a hydroxyl intermediate such as muconic or