the roots which allow larger surface area for the growth of microbes and provide
adequate supply of oxygen [50]. The inoculation of Lewia sp. which is a fungus
associated with Festuca arundinacea has shown the removal of polycyclic aromatic
hydrocarbon from spiked soil and accumulation of pyrene [51]. In this process, the
seeds are inoculated with the plant growth-promoting rhizobacteria (PGPR), which
target the roots to minimize the plant stress caused due to the presence of contaminants and enhance the plant biomass growth. Secondly, the exudates in the soil like
organic acids or sugars promote the growth and boost the metabolic activity in
rhizobacteria. The well-balanced positive interaction stimulates the degradation of
hydrocarbons. However, sometimes root exudates may also result in a repressive
effect on the biodegradation. For BTEX hydrocarbons, trees with deep rooting are
condign; grasses are usually practical for polycyclic aromatic hydrocarbon. The
higher rates of degradation of PAHs were detected at 3 millimeters from the root
zone [52]. Some points to be taken into consideration for the improvement and
longevity of the tree are by planting the tree before the growing season, making
sufficient hole diameter, and turning over the soil or mixed with fresh soil. Plantation
of mixed trees and grasses can also improve the efficacy of phytostimulation
technique [53].
Chemical fertilizers, e.g., atrazine, can be detoxified by the microbial consortia in
the rhizosphere leading to fewer detrimental effect on the plant. Caçador and Duarte
[54] used halophytes as phytoconverters for the phytoconversion of hexavalent
chromium, which is the toxic form of chromium to the less toxic trivalent form.
Lytle et al. [55] reported soluble hexavalent chromium reduction by water hyacinth
wherein hexavalent chromium is externally reduced by the lateral fine roots due to
oxalate exudation and the plants uptake less toxic trivalent chromium. Newman et al.
[56] have found that 2,4,6-trinitrotoluene (TNT) from contaminated soil can be
effectively degraded by a TNT-cometabolizing Pseudomonas sp. along with plant
species such as Bromus erectus Huds. The Pseudomonas strain cometabolized TNT
through aromatic nitroreduction and sufficiently reduced the phytotoxicity of TNT
and allowed plant growth. Stimulation of rhizosphere enhanced the growth of
bacterial populations possessing the genes specific for degradation of
2-nitrotoluene and 4-nitrotoluene [56].
During cometabolism biotransformation, numerous reactions which include
hydrolysis, reduction, oxidation, rearrangement, and conjugation [57] may occur
in the rhizosphere or soil by the action of enzymes from a consortium of bacteria,
which are phase I transformations. In plants, oxidative enzymes may act to form
hydroxylated metabolites of aromatic rings. These metabolites further conjugate to
sugars, amino acids, or glutathione, via glutathione S-transferase. These are phase II
transformations. The additional reactions [58], leading to further conjugations
followed by sequestration of the metabolites in organelles or incorporation into
plant tissues [59], also occur, which are phase III transformations. Thus,
microbial-mediated phytotransformation involves three phases [60].
52
S. Sophia and V. Shetty Kodialbail
adequate supply of oxygen [50]. The inoculation of Lewia sp. which is a fungus
associated with Festuca arundinacea has shown the removal of polycyclic aromatic
hydrocarbon from spiked soil and accumulation of pyrene [51]. In this process, the
seeds are inoculated with the plant growth-promoting rhizobacteria (PGPR), which
target the roots to minimize the plant stress caused due to the presence of contaminants and enhance the plant biomass growth. Secondly, the exudates in the soil like
organic acids or sugars promote the growth and boost the metabolic activity in
rhizobacteria. The well-balanced positive interaction stimulates the degradation of
hydrocarbons. However, sometimes root exudates may also result in a repressive
effect on the biodegradation. For BTEX hydrocarbons, trees with deep rooting are
condign; grasses are usually practical for polycyclic aromatic hydrocarbon. The
higher rates of degradation of PAHs were detected at 3 millimeters from the root
zone [52]. Some points to be taken into consideration for the improvement and
longevity of the tree are by planting the tree before the growing season, making
sufficient hole diameter, and turning over the soil or mixed with fresh soil. Plantation
of mixed trees and grasses can also improve the efficacy of phytostimulation
technique [53].
Chemical fertilizers, e.g., atrazine, can be detoxified by the microbial consortia in
the rhizosphere leading to fewer detrimental effect on the plant. Caçador and Duarte
[54] used halophytes as phytoconverters for the phytoconversion of hexavalent
chromium, which is the toxic form of chromium to the less toxic trivalent form.
Lytle et al. [55] reported soluble hexavalent chromium reduction by water hyacinth
wherein hexavalent chromium is externally reduced by the lateral fine roots due to
oxalate exudation and the plants uptake less toxic trivalent chromium. Newman et al.
[56] have found that 2,4,6-trinitrotoluene (TNT) from contaminated soil can be
effectively degraded by a TNT-cometabolizing Pseudomonas sp. along with plant
species such as Bromus erectus Huds. The Pseudomonas strain cometabolized TNT
through aromatic nitroreduction and sufficiently reduced the phytotoxicity of TNT
and allowed plant growth. Stimulation of rhizosphere enhanced the growth of
bacterial populations possessing the genes specific for degradation of
2-nitrotoluene and 4-nitrotoluene [56].
During cometabolism biotransformation, numerous reactions which include
hydrolysis, reduction, oxidation, rearrangement, and conjugation [57] may occur
in the rhizosphere or soil by the action of enzymes from a consortium of bacteria,
which are phase I transformations. In plants, oxidative enzymes may act to form
hydroxylated metabolites of aromatic rings. These metabolites further conjugate to
sugars, amino acids, or glutathione, via glutathione S-transferase. These are phase II
transformations. The additional reactions [58], leading to further conjugations
followed by sequestration of the metabolites in organelles or incorporation into
plant tissues [59], also occur, which are phase III transformations. Thus,
microbial-mediated phytotransformation involves three phases [60].
52
S. Sophia and V. Shetty Kodialbail