In this article we will discuss about the examples of transgenic plants for phytoremediation.
1. Phytoremediation of Mercury:
There have been reports on the ability of certain plant species which have ability to accumulate mercury both from atmospheric and soil sources. However, plant species never ventured hyper accumulation of mercury, researcher use engineered plants by introducing bacterial specific genes for detoxyfying accumulated mercury within the transgenic plants.
Methyl mercury is an environmental toxicant cause’s neurological degeneration. Formation of methyl mercury is a bio-magnification process. Elemental mercury is initially released into the atmosphere, re-enter by precipitation and may be deposited in the sediments of lakes and oceans. Deposited mercury can undergo transformation to a methylated species by anaerobic bacteria.
Thus, methyl mercury produced by bacteria in contaminated soil is mercury. The amount of methyl mercury produced by anaerobic bacteria may be decreased by demethylation reaction and consequently enters volatilization of dimethyl mercury. To explore the potentials of plant to extract and detoxify this hazardous element, model plant Arabidopsis thaliana were modified genetically by introducing two bacterial genes, Mer A and Mer B to detoxify mercury.
The two bacterial genes Mer A and Mer B encodes for mercuric ion reductase and organomercurial lyase respectively. Transgenic plant expressing Mer B catalyses the release of Hg2+ from the organic compound. Subsequently, Mer A catalyses the reduction of Hg2+ to element Hg. Consequently elemental Hg is volatilized in the atmosphere.
Thus, transgenic plant, A. thaliana and tobacco expressing both bacterial origin Mer A and Mer B have the potential to transform methyl-mercury into elemental mercury, subsequently release them into atmosphere through a process of phytovolatilization. In an extended works, three modified Mer A proembryogenic masses, each having different amounts of altered coding sequences.
Each of these constructs was shown to confer Hg (II) resistance. Transgenic Populus deltoids overexpressing Mer Ag and Mer 18 gene when exposed to Hg (II) evolved 2 to 4 fold Hg (O) relative to wild plant. To improve the expression of Mer genes in plants, the bacterial Mer A DNA sequence was modified by reducing the GC content and replaced with plant regulatory elements.
Transgenic plants engineered for mercury detoxification and through them into the atmosphere by volatilization did not gained momentum as it was not acceptable under biosafety regulatory perspective. Efforts have been made to introduce only one bacterial gene, Mer B into the plant. The bacterial gene Mer B allows germination and survival of plants on a medium contaminated with organo-mercurial compound and methyl mercury.
Transgenic plant expressing Mer B gene are able to transform methyl-mercury into less toxic Hg2+. Expression of modified Mer B product, organomercurial lyase, catalyses the reaction, removing organic ligand and releasing Hg2 a less mobile mercury species. Thus, modified strategy can precludes the introduction of methyl, mercury into the food chain as well as retards Hg volatilization into the atmosphere.
Phytoremediation of Selenium:
Plants can be engineered to increase the efficiency of selenium (Se) accumulation, which is then view to increasing rate of Se volatilization. The selenium is a major environmental pollutant. The oxidised form of Se namely selenate or selenite are highly soluble and are easily removed by plants whereas inorganic forms such as selenide or elemental Se are less bioavailable.
The assimilation of sulfate and selenate is activated by ATP sulfurylase. Selenate is converted into adenosine phosphoselenate (ADP-Se) which is subsequently reduced to selenite. Over expression of genes for sulfate permease, a transporter for both sulfate and selenate, in Indian mustard had higher concentration of Se in their shoots compared to wild type plants.
Transgenic plants, which overexpressed ATP sulfurylase gene (APS), had 4 fold higher APS enzymatic activity and accumulated three times as much Se per plant than the wild type. Transgenic Indian mustard (Brassica juncea) has been genetically engineered by overexpressing five enzymes in the Se-assimilation pathway, of which overexpression of ATP sulfurylase (APS) and cystathionine β-lyase enhanced Se uptake and potentiate to metabolise Se to volatalisable form. A mouse Se-Cys lyase gene when transferred to plant showed enhanced shoot Se concentration upto 1.5 fold compared to wild type. The Arabidopsis the selenium hyper accumulation has been extensively studied in Astragalus species.
2. Phytoremediation of Arsenic:
Arsenic is an extremely toxic metal and its decontamination of polluted site is not environment friendly and therefore not recommended. Alternative possible exploration is to utilise transgenic plants for decontamination. A team of researcher at the University of Georgia, USA shows that genetically engineered plants can transport arsenic into plant and reduce to arsenite and sequester it in thiol peptide complexes. E. coli. arsC gene encoding arsenate reductase (arsC) which catalyzes the glutathione (GSH) coupled electro-chemical reduction of arsenate to the more toxic arsenite.
Transgenic strategy involves co-expression of two bacterial genes, arsC, and y-CECS, which encodes arsenate reductase and y-glutanyl cysteine synthetase respectively in Arabidopsis plants. Transgenic Arabidopsis also expressing SRSP/ARSC and ACT 2P/Y-ECS together showed enhanced tolerance towards arsenic than non-transformed plants.
Additional work suggested transgenic plant accumulate 4-17 fold greater fresh shoot weight and accumulated 2-3 fold more arsenic per gram of tissue than wild type (non-transformed) or plants expressing y-ECS or Ars C alone. Thus, transgenic technology led to the development of high biomass, fast growing arsenic hyper-accumulation.
3. High Tolerance to Cadmium and Lead:
Phytoremediation of metals by naturally grown plants have limited feasibility because of toxicity of the metals to the plants. In addition, extraction is restricted to the shallow contamination site due to depth of roots. Therefore, genetic engineering method has been devised by National Research Laboratory (NRL) for phytoremediation in Korea in developing transgenic plants potentiate to withstand hyper-accumulation of cadmium and lead. Transgenic extract of these heavy metals at much faster rate than traditional bioremediation of plants.
Before introducing novel genes into the plants for better phytoremediation, in certain experiments, yeast was chosen to conduct invitro experiments on heavy metal tolerance of yeast possess the YCF, or yeast cadmium factor 1 protein, YCF, is known to pump cadmium into vacuoles.
This protein is also known as vacuolar glutathionine S-conjugate transporter and belongs to the ATP-binding cassette super family. In vitro experiments proved that yeast is equipped with YCF1 protein confirmed tolerance to both lead and cadmium at the concentration 3 and 0.1 mol respectively. Arabidopsis thaliana was selected for the introduction of yeast based YCF gene.
Expression of sub cloned YCF1 gene in plants under the control of CaMV35S promoter, confirmed the presence of this protein in vacuolar as well as in the plasma membrane. Accumulation these protein in these region facilitate rapid uptake and sequestering of lead and cadmium in the transferred plants grown on medium supplemented with 0.75 mm lead or 70 µm cadmium.