24.4.1.2 Genetically Modified Plants
Genetic modification can be used for either improving existing (accumulation of
heavy metals) or introducing new abilities (degradation of chlorinated hydrocarbons,
explosives, and pesticides). The natural ability of some plants to bioaccumulate
heavy metals is used in phytoremediation of contaminated sites.
Overproduction of metal transporters (metal bioaccumulation), overproduction of
the enzymes involved in glutathione synthesis and sulfur utilization (cell protection
against heavy metals), overproduction of phytochelatin synthase (metal
phytochelation), and enhanced phytovolatization of Hg and Se are the most common
approaches to using genetically modified organisms for enhanced phytoremediation
of heavy metals (Kotrba et al. 2009). For example, the cpSL transgenic Indian
mustard Brassica juncea accumulated twice as much Se in shoots, it had a
1.8 times higher leaf Se concentration, and was more metal-tolerant (showing a
faster growth) than the wild-type plants growing on selenium- and boroncontaminated saline sediments (Bañuelos et al. 2007). Transgenic Arabidopsis
thaliana plants overexpressing yeast protein YCF1 (detoxifying cadmium by
transporting it into vacuoles in yeast) showed a 2.2 times higher biomass yield and
1.5 times higher Cd and Pb accumulation in shoots as compared to wild-type plants
(Song et al. 2003).
Introduction of mammalian cytochrome P450 2E1 can drastically increase the
degradation of trichloroethylene, ethylene dibromide, carbon tetrachloride, chloroform, and vinyl chloride in the engineered plants (Gohel et al. 2006). The transgenic
plants showed a dramatic, up to 640-fold, enhancement of the metabolism of
trichloroethylene compared to null vector control plants. They also showed an
increased uptake and debromination of ethylene dibromide (Doty et al. 2000). A
field trial of trichloroethylene phytoremediation by transgenic poplars expressing
cytochrome P450 2E1 was conducted by Legault et al. (2017). The trichloroethylene
biodegradation was improved (but not as much as in the corresponding laboratoryscale study), via the evapotranspiration, i.e., evaporation facilitated by plant translocation, so the trichloroethylene content of transgenic leaves was reduced by 80%
and its diffusion from transgenic stems (measured as a loss of volatiles from the
stem) was reduced by 90% compared to the wild-type poplars.
Explosives and pesticides can also be effectively treated by phytoremediation
using genetically engineered plants. Expression of a bacterial nitroreductase gene in
Arabidopsis thaliana was used to enhance its tolerance to the uptake and degradation
of 2,4,6-trinitrotoluene (Kurumata et al. 2005). Genetically modified Arabidopsis
thaliana plants expressing mammalian cytochrome P450 enzyme CYP1A2 were
able to efficiently degrade the herbicide chlortoluron and showed an improved
herbicide resistance (Kebeish et al. 2014). Enhanced tolerance to two herbicides,
atrazine and metolachlor, as well as their uptake and degradation, were described by
Kawahigashi et al. (2006) for transgenic rice expressing human CYP1A1, CYP2B6,
and CYP2C19.
24 Modern Bioremediation Approaches: Use of Biosurfactants, Emulsifiers. . .
515
Genetic modification can be used for either improving existing (accumulation of
heavy metals) or introducing new abilities (degradation of chlorinated hydrocarbons,
explosives, and pesticides). The natural ability of some plants to bioaccumulate
heavy metals is used in phytoremediation of contaminated sites.
Overproduction of metal transporters (metal bioaccumulation), overproduction of
the enzymes involved in glutathione synthesis and sulfur utilization (cell protection
against heavy metals), overproduction of phytochelatin synthase (metal
phytochelation), and enhanced phytovolatization of Hg and Se are the most common
approaches to using genetically modified organisms for enhanced phytoremediation
of heavy metals (Kotrba et al. 2009). For example, the cpSL transgenic Indian
mustard Brassica juncea accumulated twice as much Se in shoots, it had a
1.8 times higher leaf Se concentration, and was more metal-tolerant (showing a
faster growth) than the wild-type plants growing on selenium- and boroncontaminated saline sediments (Bañuelos et al. 2007). Transgenic Arabidopsis
thaliana plants overexpressing yeast protein YCF1 (detoxifying cadmium by
transporting it into vacuoles in yeast) showed a 2.2 times higher biomass yield and
1.5 times higher Cd and Pb accumulation in shoots as compared to wild-type plants
(Song et al. 2003).
Introduction of mammalian cytochrome P450 2E1 can drastically increase the
degradation of trichloroethylene, ethylene dibromide, carbon tetrachloride, chloroform, and vinyl chloride in the engineered plants (Gohel et al. 2006). The transgenic
plants showed a dramatic, up to 640-fold, enhancement of the metabolism of
trichloroethylene compared to null vector control plants. They also showed an
increased uptake and debromination of ethylene dibromide (Doty et al. 2000). A
field trial of trichloroethylene phytoremediation by transgenic poplars expressing
cytochrome P450 2E1 was conducted by Legault et al. (2017). The trichloroethylene
biodegradation was improved (but not as much as in the corresponding laboratoryscale study), via the evapotranspiration, i.e., evaporation facilitated by plant translocation, so the trichloroethylene content of transgenic leaves was reduced by 80%
and its diffusion from transgenic stems (measured as a loss of volatiles from the
stem) was reduced by 90% compared to the wild-type poplars.
Explosives and pesticides can also be effectively treated by phytoremediation
using genetically engineered plants. Expression of a bacterial nitroreductase gene in
Arabidopsis thaliana was used to enhance its tolerance to the uptake and degradation
of 2,4,6-trinitrotoluene (Kurumata et al. 2005). Genetically modified Arabidopsis
thaliana plants expressing mammalian cytochrome P450 enzyme CYP1A2 were
able to efficiently degrade the herbicide chlortoluron and showed an improved
herbicide resistance (Kebeish et al. 2014). Enhanced tolerance to two herbicides,
atrazine and metolachlor, as well as their uptake and degradation, were described by
Kawahigashi et al. (2006) for transgenic rice expressing human CYP1A1, CYP2B6,
and CYP2C19.
24 Modern Bioremediation Approaches: Use of Biosurfactants, Emulsifiers. . .
515