This article throws light upon the three main areas of application of phytoremediation. The areas of application are: 1. Phyto-Extraction of Metals 2. Phytovolatilisation of Metals 3. Phytodegradation of Metals.
Application # 1. Phyto-Extraction of Metals:
A review of the phytoremediation literature reveals that, at present, there are two basic strategies of phyto-extraction being developed: chelate-assisted phyto-extraction, which we term induced phyto-extraction; and long-term continuous phyto-extraction.
Of the two processes, chelate-assisted phyto-extraction is the more developed and is presently being implemented commercially. Continuous phyto-extraction is also being studied by several groups for the removal of metals such as zinc, cadmium, and nickel and oxianionic metals such as selenium, arsenic, and chromium.
Metal Resistance Mechanisms:
Continuous phyto-extraction relies on the ability of plants to accumulate metals in their shoots, over extended periods. To achieve this, plants must possess efficient mechanisms for the detoxification of the accumulated metals. The recent observation that nickel resistance in Thlaspi goesingense is a primary determinant of nickel hyper-accumulation when plants are grown hydroponically supports this conclusion.
Therefore, the ability to manipulate metal tolerance in plants will be key to the development of efficient phytoremediation crops. As an elegant demonstration of this principle. Hg2+-resistant Arabidopsis thaliana overexpressing bacterial mercury reductase was recently shown to remove Hg2+ efficiently from solution.
Chelation:
Chelation of metal ions by specific high-affinity ligands reduces the solution concentration of free metal ions, thereby reducing their phytotoxicity. Two major classes of heavy metal chelating peptides are known to exist in plants- metallothioneins and phytochelatins. Metallothioneins are gene-encoded, low-molecular- weight, cysteine-rich polypeptides.
Plant metallothioneins are induced by Cu and have high affinity for this metal. Recent investigations of metallothionein (MT) expression levels in A. thaliana demonstrated that expression levels of MT2 mRNA strongly correlated with Cu resistance suggesting that metallothioneins are involved in Cu resistance.
Phytochelatins are low molecular weight, enzymatically synthesised cysteine-rich peptides known to bind cadmium and copper in plants. These peptides are essential for cadmium detoxification in A. thaliana. Although not strictly defined as chelation, precipitation of zinc as Zn- phytate has also been suggested as a zinc detoxification mechanism.
Compartmentalization:
Within cells, cadmium and phytochelatins accumulate in the vacuole and this accumulation appears to be driven by a Cd/H anti-port and an ATP-dependent PC-transporter. A similar system of cadmium detoxification also exists in the fission yeast, Schizosaccharomyces pombe. Mutants lacking the ability to accumulate Cd-PC complex in the vacuole yeast Cd-sensitive and have a defect in hmt I, a gene encoding an ATP- binding cassette-type transport protection.
The hmt I a gene product is responsible for transporting Cd- PC complex into the vauole, sulfide is added to the Cd-PC complex, forming a more stable high- molecular-weight Cd-PC-sulfide complex that may be essential for Cd resistance in the yeast.
Biotransformation:
The toxicity of such metals and metalloids as chromium, selenium, and arsenic can be reduced in plants by chemical reduction of the element and/or by its incorporation into organic compounds. Excess selenium is toxic to most plants because it is metabolised to selenocysteine and selenomethionine, which replace cysteine and methionine residues in proteins.
By funneling selenium into the non-protein amino acids methylselenocysteine and selenocystathionine, selenium accumulator species of Astragalus are able to reduce the amount of selenium incorporated into proteins, thereby tolerating elevated concentrations of selenium in shoots.
Recently the enzyme responsible for the methylation of selenocysteine in the selenium accumulator Astragalus bisculatushs been isolated and characterised, a first step in determining the molecular basis of selenium resistance in plants.
Cellular repair mechanisms:
A primary component of cellular resistance to elevated Cu concentrations appears to be enhanced plasma membrane resistance to, or repair of Cu-induced membrane damage. The intriguing observation that plant metallothioneins may be prenylated and targeted to the plasma membrane repair mechanisms in Cu resistance is also strongly supported by the recent observation that an acyl carrier protein (ACP) and AcylCoA binding protein (ACBP), two proteins known to be involved in lipid metabolism, are induced in Cu-exposed A. thaliana.
Application # 2. Phytovolatilisation of Metals:
Volatilisation of selenium from plant tissues may provide a mechanism of selenium detoxification. As early as 1894, Hofmeister proposed that selenium in animals is detoxified by releasing volatile dimethyl selenide from the lungs. He based this proposal on the fact that the odour of dimethyl tellurite was detected in the breath of dogs injected with sodium tellurite.
Using the same logic, it was suggested that the garlicky odour of plants that accumulate selenium may indicate the release of volatile selenium compounds. Lewis was the first to show that both selenium non-accumulator and accumulator species volatilize selenium.
Volatilisation of arsenic as dimethylarsenite has also been postulated as a resistance mechanism in marine algae. However, it is not known whether terrestrial plants also volatilize arsenic in significant quantities. Studies on arsenic uptake and distribution in higher plants indicate that arsenic predominantly accumulates in roots and that only small quantities are transported to shoots. However, plants may enhance the biotransformation of arsenic by rhizospheric bacteria, thus increasing rates of volatilisation.
Application # 3. Phytodegradation of Metals:
The use of plants to cleanse waters contaminated with organic and inorganic pollutants dates back hundreds of years and has been the basis for the present use of constructed wetlands in treating municipal and industrial waste streams.
The concept of using plants to remediate soils contaminated with organic pollutants is a more recent development based on observations that disappearance of organic chemicals is accelerated in vegetated soils compared with surrounding non-vegetated bulk soils.
Subsequent metabolic studies have established the ability of plants to take up and metabolize a range of environmentally problematic organic pollutants, including ammunition wastes (e.g., TNT and GTN), polychlorinated phenols [PBCs and trichloroethylene (TCE)].
Biotransformation and Compartmentalization:
Following uptake, organic compounds may have multiple fates. They may be trans-located to other plant tissues and subsequently volatilized, they may undergo partial or complete degradation or they may be transformed to less toxic compounds and found in plant tissues in non-available forms.
Biotransformation and sequestration of herbicides and pesticides, in particular, have been extensively investigated in plants. More recently, metabolism of non-agricultural xenobiotics such as TCE, TNT, and nitroglycerin (GTN) has been studied using axenic cell cultures and whole plants. 
