growth-promoting rhizobacteria (PGPR) enhance plants’ growth and tolerance
towards varying environmental stresses by different action mechanisms (solubilizing
phosphates, producing phytohormones or fixing nitrogen) or by altering the plant
metabolism (increasing the absorption of water and minerals) that consequently
increases root development, enzymatic activities of the plant, support growth and
development of other microorganisms beneficial to plants and suppress plant
pathogens (Jacoby et al. 2017).
Genetic Alterations in Plants for Effective Remediation Many organic and
inorganic pollutants are recalcitrant to phytoremediation (Doty 2008). There are
many genes involved in metabolism, uptake, translocation and sequestration of
pollutants. Thus, the most feasible method for enhancing the effectiveness of
phytoremediation is to overexpress those genes in transgenic plants through genetic
engineering (Cherian and Oliveira 2005). Depending upon the strategy, transgenic
plants can be genetically modified for enhanced accumulation of contaminants in
different parts of the plant. There are several available reports on bioremediation of
pollutants through transgenic plants (Agnihotri and Seth 2019; Shah and Pathak
2019). Transgenic plants such as Brassica juncea, Arabidopsis thaliana and Thlaspi
caerulescens have been reported to effectively remove the heavy metals (Cd, Cr, Pb,
Zn and As) from soil (Agnihotri and Seth 2019). Genetically engineered plant
(Arabidopsis thaliana) with two expressed bacterial genes (one gene converts
arsenate into arsenite and the second binds to the arsenite) have been reported to
absorb arsenic efficiently from polluted soil and store in the vacuoles (Finnegan and
Chen 2012). Phytoremediation of organic pollutants is significantly improved with
transgenic plants (Hannink et al. 2002). For an instance, explosives such as 2,4,6trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and glycerol
trinitrate (GTN) are phytotoxic, and phytoremediation of such pollutants through
non-transgenic plants is relentlessly hindered (Van Aken 2009). But expression of
bacterial genes (nitroreductase, cytochrome P450 and pentaerythritol tetranitrate
reductase) specifically involved in degradation of TNT, RDX and GTN, respectively, enables the plants, viz. Nicotiana tabacum, Arabidopsis thaliana and Populus
sp., to be more tolerant of the pollutant in phytoremediation.
16.7 Future Prospects
A large area of land has been impacted by the organic and inorganic pollutants due to
anthropogenic activities which have become a major environmental issue and health
concern worldwide. The analysis for pollutants in soil would be useful in reassuring
the soil quality, food and health security of mankind. Several advanced technologies
are available to manage waste and contaminated sites, but there is growing interest in
in situ and ex situ remediation techniques using green plants, microbes and microbeassisted phytoremediation. Plants with high biomass production should be
accentuated and enhanced through genetic engineering for effective remediation of
pollutants from the environment. Similarly, genetic modification of microorganisms
16 Causes, Effects and Sustainable Approaches to Remediate Contaminated Soil
483
towards varying environmental stresses by different action mechanisms (solubilizing
phosphates, producing phytohormones or fixing nitrogen) or by altering the plant
metabolism (increasing the absorption of water and minerals) that consequently
increases root development, enzymatic activities of the plant, support growth and
development of other microorganisms beneficial to plants and suppress plant
pathogens (Jacoby et al. 2017).
Genetic Alterations in Plants for Effective Remediation Many organic and
inorganic pollutants are recalcitrant to phytoremediation (Doty 2008). There are
many genes involved in metabolism, uptake, translocation and sequestration of
pollutants. Thus, the most feasible method for enhancing the effectiveness of
phytoremediation is to overexpress those genes in transgenic plants through genetic
engineering (Cherian and Oliveira 2005). Depending upon the strategy, transgenic
plants can be genetically modified for enhanced accumulation of contaminants in
different parts of the plant. There are several available reports on bioremediation of
pollutants through transgenic plants (Agnihotri and Seth 2019; Shah and Pathak
2019). Transgenic plants such as Brassica juncea, Arabidopsis thaliana and Thlaspi
caerulescens have been reported to effectively remove the heavy metals (Cd, Cr, Pb,
Zn and As) from soil (Agnihotri and Seth 2019). Genetically engineered plant
(Arabidopsis thaliana) with two expressed bacterial genes (one gene converts
arsenate into arsenite and the second binds to the arsenite) have been reported to
absorb arsenic efficiently from polluted soil and store in the vacuoles (Finnegan and
Chen 2012). Phytoremediation of organic pollutants is significantly improved with
transgenic plants (Hannink et al. 2002). For an instance, explosives such as 2,4,6trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and glycerol
trinitrate (GTN) are phytotoxic, and phytoremediation of such pollutants through
non-transgenic plants is relentlessly hindered (Van Aken 2009). But expression of
bacterial genes (nitroreductase, cytochrome P450 and pentaerythritol tetranitrate
reductase) specifically involved in degradation of TNT, RDX and GTN, respectively, enables the plants, viz. Nicotiana tabacum, Arabidopsis thaliana and Populus
sp., to be more tolerant of the pollutant in phytoremediation.
16.7 Future Prospects
A large area of land has been impacted by the organic and inorganic pollutants due to
anthropogenic activities which have become a major environmental issue and health
concern worldwide. The analysis for pollutants in soil would be useful in reassuring
the soil quality, food and health security of mankind. Several advanced technologies
are available to manage waste and contaminated sites, but there is growing interest in
in situ and ex situ remediation techniques using green plants, microbes and microbeassisted phytoremediation. Plants with high biomass production should be
accentuated and enhanced through genetic engineering for effective remediation of
pollutants from the environment. Similarly, genetic modification of microorganisms
16 Causes, Effects and Sustainable Approaches to Remediate Contaminated Soil
483