2.9.4 Rapid Evolution Through Duplicated Genes
One gene copy can accumulate mutations and yield enzymes with altered properties,
while the other copy of the gene may continue its normal function. A number of
bacterial genes have been used to modify plants genetically and make them resistant
to specific herbicides. For example, the herbicide bromoxynil inhibits photosynthesis and uncouples oxidative phosphorylation. A gene originating from Klebsiella
pneumoniae subspp. ozaenae encoding a bromoxynil-modifying nitrilase was used
to generate bromoxynil-resistant transgenic plants (Stalker et al. 1988). Moreover, in
situ pesticide degradation rates can be manipulated by modifying the soil environment. For example, the plant rhizosphere can accelerate pesticide degradation
presumably through enhancement of microbial activity via the provision of carbon
in the form of root exudate or modification of O 2 concentrations. The organophosphorus insecticides diazinon and parathion were mineralized about twice as
fast in soil containing a bush bean (Phaseolus vulgaris) plant as in soil without a
plant (Hsu and Bartha 1979).
Focht and Reineke (2002) studied application of hybrid bacterium containing
sequences for complete degradation of polychlorinated biphenyls, Aroclor1221 in a
soil microcosm and found that both introduced bacterium and native microbes
remained unaffected. Although these studies demonstrated the potential of genetically engineered microorganisms in bioremediation of environmentally hazardous
compounds, there is also an example of GEM adversely affecting the indigenous
microbe during degradation of 2,4-D. Short et al. (1992) reported that genetically
engineered P. putida PP301 (pR0103) accumulated 2,4-dichlorophenol in arid soils
affecting an indigenous fungus.
2.9.5 Development of Transgenic Plants with Enhanced
Pesticide Degradation
To minimize the application of pesticides, transgenic plants have been developed,
which express the Bt (Bacillus thuringiensis) toxin. Such transgenic plants have
been released for cultivation in cotton, corn, brinjal, etc. Due to the cultivation of
these transgenic crops, lower amounts of pesticides are applied for control of
pathogens and insects. There is also possibility to develop transgenic plants with
enhanced ability to detoxify persistent organic compounds. To increase the natural
abilities of plants in the removal/detoxification of organic compounds, different
cytochromes have been introduced into plants, which are considered to be responsible for the first phase in plant detoxification. Doty et al. (2000) showed the
enhanced metabolism of halogenated hydrocarbons in transgenic plants containing
mammalian cytochrome P450 2EI. Similarly, overexpression of a basic peroxidase
in tomato (Wevar Oller et al. 2005) resulted in increased phenol phytoremediation,
2 Bioremediation of Pesticides: An Eco-Friendly Approach for Environment. . .
59
One gene copy can accumulate mutations and yield enzymes with altered properties,
while the other copy of the gene may continue its normal function. A number of
bacterial genes have been used to modify plants genetically and make them resistant
to specific herbicides. For example, the herbicide bromoxynil inhibits photosynthesis and uncouples oxidative phosphorylation. A gene originating from Klebsiella
pneumoniae subspp. ozaenae encoding a bromoxynil-modifying nitrilase was used
to generate bromoxynil-resistant transgenic plants (Stalker et al. 1988). Moreover, in
situ pesticide degradation rates can be manipulated by modifying the soil environment. For example, the plant rhizosphere can accelerate pesticide degradation
presumably through enhancement of microbial activity via the provision of carbon
in the form of root exudate or modification of O 2 concentrations. The organophosphorus insecticides diazinon and parathion were mineralized about twice as
fast in soil containing a bush bean (Phaseolus vulgaris) plant as in soil without a
plant (Hsu and Bartha 1979).
Focht and Reineke (2002) studied application of hybrid bacterium containing
sequences for complete degradation of polychlorinated biphenyls, Aroclor1221 in a
soil microcosm and found that both introduced bacterium and native microbes
remained unaffected. Although these studies demonstrated the potential of genetically engineered microorganisms in bioremediation of environmentally hazardous
compounds, there is also an example of GEM adversely affecting the indigenous
microbe during degradation of 2,4-D. Short et al. (1992) reported that genetically
engineered P. putida PP301 (pR0103) accumulated 2,4-dichlorophenol in arid soils
affecting an indigenous fungus.
2.9.5 Development of Transgenic Plants with Enhanced
Pesticide Degradation
To minimize the application of pesticides, transgenic plants have been developed,
which express the Bt (Bacillus thuringiensis) toxin. Such transgenic plants have
been released for cultivation in cotton, corn, brinjal, etc. Due to the cultivation of
these transgenic crops, lower amounts of pesticides are applied for control of
pathogens and insects. There is also possibility to develop transgenic plants with
enhanced ability to detoxify persistent organic compounds. To increase the natural
abilities of plants in the removal/detoxification of organic compounds, different
cytochromes have been introduced into plants, which are considered to be responsible for the first phase in plant detoxification. Doty et al. (2000) showed the
enhanced metabolism of halogenated hydrocarbons in transgenic plants containing
mammalian cytochrome P450 2EI. Similarly, overexpression of a basic peroxidase
in tomato (Wevar Oller et al. 2005) resulted in increased phenol phytoremediation,
2 Bioremediation of Pesticides: An Eco-Friendly Approach for Environment. . .
59