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Transgenic plants for hytoremediation
Transgenic plants have also been produced for detoxification of soils contaminated with (i) metals like mercury (Hg), aluminium (Al), and arsenic (As) and (ii) explosive like trinitrotoluene (TNT). These transgenic plants can be grown in contaminated soils, to allow detoxification of these contaminated soils. Mercury detoxification.
Mercury toxicity is created due to the effluents released from chemical, paper, mining and defense industries, which use mercury in a variety of applications. Mercury toxicity led to two epidemics of human poisoning in Japan in the 1960s, so that methylmercury contamination of aquatic sediments is now regarded as one of the most serious environmental problems facing both developed and developing countries.
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According to a report published in February 2000 issue of Nature Biotechnology, transgenic Arabidopsis thaliana plants were produced using two bacterial genes for mercury detoxification pathway. The two genes included merA for mercury reductase and merB for organomercurials lyase. The gene merA was also artificially synthesized applying codon usage, so that such a synthesized gene merApe9 proved effective in releasing free mercury, which was not only less toxic (relative to Hg2+ ions), but was also volatile, so that it could volatilize into the atmosphere. The transgenic plants produced thus were also tolerant to gold (Au3+) contamination.
R – CH2 – Hg + H+  R- CH3 + Hg (II)
Hg(ii) + NADPH + H+  Hg(0) + NADP + H2
The transgenic plants carrying the above two genes could grow on 50-fold higher methylmercury concentration than wild type plants, and on up to 10-fold higher concentration than plants that carry the gene merB alone. The study was considered important, because methylmercury is highly toxic and is found in mercury polluted wetlands and coastal sediments worldwide.
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Aluminium detoxification Aluminium is another metal, which causes toxicity in the soil. Generally, lime is used to amend these AI-contaminated soils. However, transgenic plants have been produced, which can tolerate aluminium toxicity. The basic principle involved in the production of Al-tolerant plants is that citrate in the soil binds to aluminium, rendering it incapable of entering the roots. Keeping this in view, a gene for citrate synthase (CSb) was used for production of transgenic plants, which synthesized high level of citrate and secreted it into the soil through their roots, so that it will bind with available aluminium, rendering it incapable of entering the roots, thus making the transgenic plants aluminium tolerant. Such aluminium (Al3+) – tolerant transgenic plants carrying a gene for citrate synthase have been produced in tobacco, papaya, rice and corn, which exhibited tolerance to aluminium toxicity.
Arsenic detoxification. Arsenic is another metal, which contaminates thousands of sites world-over and adversely affects human health. It causes skin lesions and lung/kidney/liver cancers, and also damages nervous system. Arsenic contaminated sites are often not cleaned due to high cost involved. Production of arsenic tolerant and hyperaccumulating transgenic plants in Arabidopsis thaliana were reported in November, 2002 (Nature Biotechnology).
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Two bacterial genes (arsC, γ-ECS) were transferred, one for arsenate reductase (arsC) and the other for g- glutamylcysteine synthetase (γ-ECS), both under the influence of specific regulatory sequences. While ArsC converts arsenate (AsO43-) to arsenate
(ASO3-3) γ-ECS converts amino acids, glutamine and cysteine into g-glutamylcysteine, which in its turn is used for the synthesis of organic thiols (RS) including glutathione (GSH) and phytochelatins (PCs), to which arsenite (but not arsenate) binds. This led to a degree of tolerance against and hyperaccumulation of arsenic.
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ArsC and g-ECS catalysed reactions leading to the production of organic thiols, to which ASO33- binds.
TNT (an explosive) detoxification
Explosives that contaminate land and ground water, pose serious threat to the environment, since they do not breakdown in the natural environment.
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For instance, trinitrotoluene (TNT) is associated with anemia, liver damage and carcinogenicity. In the past, the only effective treatment of TNT-contaminated soils has been incineration, which produces unusable ash and causes air pollution. In December, 2001 (Nature Biotechnology), production of transgenic tobacco (Nicotiana tabacum) with enhanced TNT-detoxifying ability was reported. These transgenic plants were produced by transformation with the nitroreductases (NR) gene nfsl, isolated from Enterobacter cloacae, and tolerated TNT concentration upto the TNT-aqueous solubility limits (~0.5 mM), although the wild type controls exhibited phytotoxicity even at ~0.05mM concentration. If this gene is used for transformation of fast growing and deep rooted trees like poplar, this can provide a solution for clean-up of the TNT-contaminated sites.
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