multiple pesticide resistance properties. When adaptation occurs through genetic
mutations, the pesticide-resistant organism may also tolerate other xenobiotic compounds that have mechanisms of action similar to those already exposed to pesticides; such resistance is called cross-resistance.
Genetic engineering of endophytic and rhizospheric bacteria for the degradation
of toxic compounds in the soil is considered to be one of the most promising new
technologies for the remediation of contaminated environmental sites (Divya and
Kumar 2011). To select the appropriate strain for genetic recombination and its
subsequent inoculation into the rhizosphere, three criteria have been recommended:
first, the strain should be stable after cloning, and the target gene should have high
expression; second, the species must be tolerant or insensitive to the contaminated/
toxic compound; and third, these strains may establish and live in specific plant
rhizosphere (Sindhu and Dadarwal 2000; Huang et al. 2004). In general, most
bacteria in the rhizosphere show only limited ability to reduce organic pollutants.
With the development of molecular biology, the genetically engineered rhizobacteria
with the contaminate-degenerating genes are constructed to enhance the
rhizoremediation (Glick 2010).
The microbial PCB-degradation system consists of two main metabolic stages:
(1) anaerobic reduction dechlorination, where PCBs are converted to low chlorinated
congeners; and (2) aerobic breakdown of the biphenyl structure in low-halogenated
congeners (less than five chlorines), resulting in chloro-HOPDA (2-hydroxy-6-oxo6-phenylhexa-2,4-dienoate), chlorobenzoic acid, ring opening, and complete mineralization (Passatore et al. 2014). The aerobic rhizobial degradation of PCBs is
usually carried out by the oxidative biphenyl pathway encoded by the bph genes,
which include the multicomponent dioxygenase (bphA, E, F, and G), dehydrogenase
(bphB), secondary dioxygenase (bphC), and a hydrolase (bphD) in other bacteria.
Genomic DNAs from Rhizobium and Bradyrhizobium have been found to be
strongly hybridized with the Comamonas testosteroni-derived bphABC gene
probe, suggesting the presence of a similar oxidative degradation system in rhizobia
(Damaj and Ahmad 1996; Ahmad et al. 1997). Molecular mechanisms involved in
the degradation of certain pollutants, such as trichloroethylene (TCE) and PCBs,
have also been studied.
Gong et al. (2016) reported the metabolic engineering of Pseudomonas putida
KT2440 for complete mineralization of methyl parathion. The strain was found
genetically stable, and its growth was not inhibited. Furthermore, engineering of the
strain showed a high degradation of methyl parathion (50 mg kg
À1 soil) in soil
samples. In another study, genetically engineered Pseudomonas putida X3 strain
was reported to utilize methyl parathion as the sole source of carbon for growth.
Engineered X3 strain hydrolyzed methyl parathion to p-nitrophenol. However, no
further degradation was observed, which may be due to the absence of p-nitrophenol
degrading genes in the X3 strain (Zhang et al. 2016).
In general, the combination of multiple OP degrading genes causes pesticides to
be converted into intermediate metabolites and eventually into small-molecule and
non-toxic substances (Barman et al. 2014; Acharya et al. 2015). OP-degrading genes
involved in the biodegradation and detoxification of OPs include opd, opdE, mpd
54
A. Sehrawat et al.
mutations, the pesticide-resistant organism may also tolerate other xenobiotic compounds that have mechanisms of action similar to those already exposed to pesticides; such resistance is called cross-resistance.
Genetic engineering of endophytic and rhizospheric bacteria for the degradation
of toxic compounds in the soil is considered to be one of the most promising new
technologies for the remediation of contaminated environmental sites (Divya and
Kumar 2011). To select the appropriate strain for genetic recombination and its
subsequent inoculation into the rhizosphere, three criteria have been recommended:
first, the strain should be stable after cloning, and the target gene should have high
expression; second, the species must be tolerant or insensitive to the contaminated/
toxic compound; and third, these strains may establish and live in specific plant
rhizosphere (Sindhu and Dadarwal 2000; Huang et al. 2004). In general, most
bacteria in the rhizosphere show only limited ability to reduce organic pollutants.
With the development of molecular biology, the genetically engineered rhizobacteria
with the contaminate-degenerating genes are constructed to enhance the
rhizoremediation (Glick 2010).
The microbial PCB-degradation system consists of two main metabolic stages:
(1) anaerobic reduction dechlorination, where PCBs are converted to low chlorinated
congeners; and (2) aerobic breakdown of the biphenyl structure in low-halogenated
congeners (less than five chlorines), resulting in chloro-HOPDA (2-hydroxy-6-oxo6-phenylhexa-2,4-dienoate), chlorobenzoic acid, ring opening, and complete mineralization (Passatore et al. 2014). The aerobic rhizobial degradation of PCBs is
usually carried out by the oxidative biphenyl pathway encoded by the bph genes,
which include the multicomponent dioxygenase (bphA, E, F, and G), dehydrogenase
(bphB), secondary dioxygenase (bphC), and a hydrolase (bphD) in other bacteria.
Genomic DNAs from Rhizobium and Bradyrhizobium have been found to be
strongly hybridized with the Comamonas testosteroni-derived bphABC gene
probe, suggesting the presence of a similar oxidative degradation system in rhizobia
(Damaj and Ahmad 1996; Ahmad et al. 1997). Molecular mechanisms involved in
the degradation of certain pollutants, such as trichloroethylene (TCE) and PCBs,
have also been studied.
Gong et al. (2016) reported the metabolic engineering of Pseudomonas putida
KT2440 for complete mineralization of methyl parathion. The strain was found
genetically stable, and its growth was not inhibited. Furthermore, engineering of the
strain showed a high degradation of methyl parathion (50 mg kg
À1 soil) in soil
samples. In another study, genetically engineered Pseudomonas putida X3 strain
was reported to utilize methyl parathion as the sole source of carbon for growth.
Engineered X3 strain hydrolyzed methyl parathion to p-nitrophenol. However, no
further degradation was observed, which may be due to the absence of p-nitrophenol
degrading genes in the X3 strain (Zhang et al. 2016).
In general, the combination of multiple OP degrading genes causes pesticides to
be converted into intermediate metabolites and eventually into small-molecule and
non-toxic substances (Barman et al. 2014; Acharya et al. 2015). OP-degrading genes
involved in the biodegradation and detoxification of OPs include opd, opdE, mpd
54
A. Sehrawat et al.