thereby supporting the hypothesis that apart from P450 cytochromes, peroxidases
are also involved in the first phase of detoxification.
The development of GM tobacco, which overexpressed glutathione-S-transferase
for the phytoremediation of chloroacetanilide herbicide (Karavangeli et al. 2005),
addresses the second phase in plant detoxification, namely the conjugation of the
activated compound. Similarly, the biodegradation of explosives by transgenic
plants expressing pentaerythritol tetranitrate reductase (French et al. 1999) is the
classic example of the exploitation of a bacterial gene for phytoremediation. In
addition, plants have been constructed that express bacterial enzymes capable of
TNT (trinitrotoluene) transformation and RDX degradation (hexahydro-1,3,5trinitro-1,3,5-triazine), an explosive nitroamine widely used in military and industrial applications (Bruce 2007).
The vital missing step in the efficient degradation of hydroxylated PCBs by plant
cells is the opening of the biphenyl ring by the bacterial enzyme encoded by bphC,
which is responsible for the cleavage of hydroxylated PCB derivatives, even those
formed by plants. Francova et al. (2003) reported the generation of tobacco plants
carrying the bphC gene. Subsequent testing of seeds for their ability to germinate in
high concentration of PCB showed significant germination. Besides this, improved
substrate specificity has been achieved by the expression of bacterial biphenylchlorophenyl dioxygenase gene in tobacco (Mohammadi et al. 2007).
2.10 Future Perspectives
Insect pests, pathogenic fungi, and weeds have always responded to the chemical
pesticides sprayed on them by developing resistance (Sindhu et al. 2010b, 2016).
Indeed, repeated use of the same agrochemicals is considered to be the main cause
behind the development of resistance (Miyata and Saito 1984; Heap 2014). On
the contrary, resistance has become the incentive for innovation on their fight against
the enemies of the crops (Jeschke 2016). For instance, widespread pest resistance to
the old organochlorine insecticides was the main reason that led to ban DDT and
cyclodienes. Their replacement with cholinesterase inhibitors was envisaged well
before the sublethal effects on birds of prey were noticed. The search for molecules
with different mode of actions in subsequent years was a necessity to confront
resistance mechanisms among insect pests. Despite this, pest resistance has developed within a few years of the introduction of the novel neonicotinoids and diamide
insecticides (Uchiyama and Ozawa 2014; Bass et al. 2015). Even more dramatic
change has been the development of resistance against glyphosate by many weeds
due to overuse of this herbicide in genetically modified crop varieties of cotton,
soybean, and maize (Shaner 2000; Beckie and Hall 2014; Dahiya et al. 2019b).
To address the resistance problem, the pesticide chemical industry is looking for
the production of novel chemicals to control the crop pests (Jeschke 2016). In recent
years, huge growth has been noticed in the marketing of neonicotinoids, phenylpyrazoles and diamide insecticides, strobilurin fungicides, and 4-HPPD herbicides
60
A. Sehrawat et al.
are also involved in the first phase of detoxification.
The development of GM tobacco, which overexpressed glutathione-S-transferase
for the phytoremediation of chloroacetanilide herbicide (Karavangeli et al. 2005),
addresses the second phase in plant detoxification, namely the conjugation of the
activated compound. Similarly, the biodegradation of explosives by transgenic
plants expressing pentaerythritol tetranitrate reductase (French et al. 1999) is the
classic example of the exploitation of a bacterial gene for phytoremediation. In
addition, plants have been constructed that express bacterial enzymes capable of
TNT (trinitrotoluene) transformation and RDX degradation (hexahydro-1,3,5trinitro-1,3,5-triazine), an explosive nitroamine widely used in military and industrial applications (Bruce 2007).
The vital missing step in the efficient degradation of hydroxylated PCBs by plant
cells is the opening of the biphenyl ring by the bacterial enzyme encoded by bphC,
which is responsible for the cleavage of hydroxylated PCB derivatives, even those
formed by plants. Francova et al. (2003) reported the generation of tobacco plants
carrying the bphC gene. Subsequent testing of seeds for their ability to germinate in
high concentration of PCB showed significant germination. Besides this, improved
substrate specificity has been achieved by the expression of bacterial biphenylchlorophenyl dioxygenase gene in tobacco (Mohammadi et al. 2007).
2.10 Future Perspectives
Insect pests, pathogenic fungi, and weeds have always responded to the chemical
pesticides sprayed on them by developing resistance (Sindhu et al. 2010b, 2016).
Indeed, repeated use of the same agrochemicals is considered to be the main cause
behind the development of resistance (Miyata and Saito 1984; Heap 2014). On
the contrary, resistance has become the incentive for innovation on their fight against
the enemies of the crops (Jeschke 2016). For instance, widespread pest resistance to
the old organochlorine insecticides was the main reason that led to ban DDT and
cyclodienes. Their replacement with cholinesterase inhibitors was envisaged well
before the sublethal effects on birds of prey were noticed. The search for molecules
with different mode of actions in subsequent years was a necessity to confront
resistance mechanisms among insect pests. Despite this, pest resistance has developed within a few years of the introduction of the novel neonicotinoids and diamide
insecticides (Uchiyama and Ozawa 2014; Bass et al. 2015). Even more dramatic
change has been the development of resistance against glyphosate by many weeds
due to overuse of this herbicide in genetically modified crop varieties of cotton,
soybean, and maize (Shaner 2000; Beckie and Hall 2014; Dahiya et al. 2019b).
To address the resistance problem, the pesticide chemical industry is looking for
the production of novel chemicals to control the crop pests (Jeschke 2016). In recent
years, huge growth has been noticed in the marketing of neonicotinoids, phenylpyrazoles and diamide insecticides, strobilurin fungicides, and 4-HPPD herbicides
60
A. Sehrawat et al.