Heavy metal is defined as elements with metallic properties such as(Ductility, Conductivity, Stability as cations, Ligand specificity, etc.) and having atomic mass over 20 and specific gravity above 5. In biology, "heavy" refers to a series of metals and also metalloids that can be toxic to both plants and animals even at very low concentrations. The most common heavy metal contaminants are: Cd, Cr, Hg, Pb, As, Sn and Zn are not essential for normal growth as it does not have any physiological role in plant but such as Co, Cu, Fe, Mn, Mo, Ni and Zn, are essential elements required for normal growth and metabolism of plants. These elements can easily lead to poisoning when their concentration rises to supra-optimal values. Due to the vigorous industrialization, anthropogenic activity, modern agricultural practices and faulty waste disposal methods have increased the concentrations of elemental pollutants in the environment, which cause toxicity to all living organisms and enter the food web and causes serious threats to humans, animals and plants. Heavy metal phytotoxicity may result from alterations of numerous physiological processes caused at cellular/molecular level by inactivating enzymes, blocking functional groups of metabolically important molecules, displacing or substituting for essential elements and disrupting membrane integrity. This metal pollution has become one of the most serious environmental problems and in order to overcome these problems, scientific community is coming with new technologies such as phytoremediation which is a site remediation strategy, environmental friendly, aesthetically pleasing approach.

The term Phytoremediation ("photo" meaning plant, and the Latin suffix "remedium" meaning to clean or restore) also refers to a diverse collection of plant based technologies that use either naturally occurring or genetically engineered plants to extract, sequester, or detoxify pollutants, to clean contained environments. The phytoremediation is athe green technology. Mainly Brassica family species and related crop species that has been reported as potential candidates for phytoremediation for example Brassica juncea , B. oleracea (highest accumulation of Selenium), Thlaspi caerulescens, Thlaspi praecox, Arabidopsis thaliana etc among these Arabidopsis thaliana is a model plant for genetic, physiological and biochemical studies and the most important members of the Brassicaceae for phytoremediation.

Methods of phytoremediation:
a) Phytodegradation: Phytodegradation, also called "phytotransformation,"refers to the uptake of contaminants with the subsequent breakdown, mineralization, or metabolization by the plant itself through various internal enzymatic reactions and metabolic processes. Plants catalyze several internal reactions by producing enzymes with various activities and functions specifically; oxygenases have been identified in plants that are able to address hydrocarbons such as aliphatic and aromatic compounds.

b) Phytostabilation: Phytostabilization refers to the holding of contaminated soils and sediments in place by vegetation, and to immobilizing toxic contaminants in soils. It can occur through the sorption, precipitation, complexation, or metal valence reduction. It is useful for the treatment of lead (Pb) as well as arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu) and zinc(Zn).

c) Phytohydraulics: Phytohydraulics is the ability of vegetation to evapo transpire sources of surface water and ground water associated plant growth promoting rhizobacteria foster rhizoremediation of inorganic and organic pollutants groundwater.

d) Phytovolatilization: It involves the use of plants to take up contaminants from the soil, transforming them into volatile forms and transpiring them into the atmosphere. Some metals, like As, Hg and Se, may exist as gaseous species in the environment. Some naturally occurring or genetically modified plants, like Chara canescens (muskgrass), Brassica juncea (Indian mustard) and Arabidopsis thaliana, are reported to possess capability to absorb heavy metals and convert them to gaseous species within the plant and subsequently release them into the atmosphere. Phytovolatization has been used primarily for the removal of mercury wherein mercuric ion is transformed into the less toxic gaseous elemental mercury.

e) Phytohydraulics: Phytohydraulics is the ability of vegetation to evapotranspire sources of surface water and ground water associated plant growth promoting rhizobacteria foster rhizoremediation of inorganic and organic pollutants groundwater. One class of trees that has been widely studied in phytotechnologies is phreatophytes, which are deep-rooted, high-transpiring, water-loving trees that send their roots into regions of high moisture and that can survive in conditions of temporary saturation.Typical phreatophytes include species within the Salicaceae family such as cottonwoods, poplars, and willows.

f) Phytoextraction: Phytoextraction, a common process of phytoremediation, involves uptake of the contaminant by plant roots with subsequent accumulation in the aerial plant parts, 340 R. Jabeen et al.followed generally by harvest and then disposal of plant biomass. The metal accumulating plants are seeded or transplanted into the metal-contaminated soil and then cultivated with established agricultural practices. The roots of these plants absorb metal elements from the soil and translocate them to the aerial shoots, where they accumulate. After a sufficient plant growth and metal accumulation, the aerial plant parts are harvested and removed, thus ensuring a permanent removal of metals, such as Pb, Cd, Ni, Cu, Cr, and V, from the contaminated soils. However, it is applicable only those sites containing low to moderate levels of metal pollution, because plant growth does not sustain in heavily polluted sites .Brassica juncea is used to extract the Cr.

g) Rhizodegradation: It is the ability of plant roots to take up and sequester metal contaminants or excess nutrients from the aqueous growth substrates (waste-water streams, nutrient-recycling systems). It remediate metals like Pb, Cd, Ni, Cu, Cr, radionuclide (U, Cs & Sr). Indian mustard (Brassica juncea) and sunflower (Helianthus annuus) are most promising for metal removal from water such as Cd, Cr, Cu, Ni, Pb

KEY FEATURES FOR IDEAL PHYTOREMEDATING PLANT:
ïÆ'¼ Higher amount of biomass: Several Brassica species are able to produce significant amounts of biomass, which is of definite advantage in phytoremediation. for e.g. Armoracia rusticana, a significant proportion of this biomass is below ground.

ïÆ'¼ High metal accumulation and Tolerance: The plants having ability to hyperaccumulate the widest range of heavy metals. For example Thlaspi caerulescens and Macadamia neurophylla accumulate up to 51 g kg-1 dry wt of Zn and Mn, respectively

ïÆ'¼ Adaptability: Plants having more adaptability towards environmental condition and therefore, there is the potential to develop superior genotype for phytoremediation via through selection and breeding techniques.

ïÆ'¼ Potential under experimental condition: The plant should have potential for genetic manipulation, in vitro culture technique and also attractive candidate for introduction of gene e.g. Brassica juncea and Arabidopsis sp. has been appeared to be very tolerant under experimental condition.

ïÆ'¼ Rapid growth and easily harvested: Species which can be easily grown and harvested.

ïÆ'¼ Multiple Cropping: Plants in which multiple cropping can be achieved during the season and has limited seed and pollen.

ïÆ'¼ Other characters: Have ability to withstand difficult soil pH, salinity, soil structure; water content and produce a dense root system condition.

Factors which are affecting the uptake mechanisms of heavy metals.
I. The Plant Species: The success of the phytoextraction technique depends upon the identification of suitable plant species that hyperaccumulate.

II. Addition of Chelating Agent: The use of chelating agents in heavy-metal-contaminated soils could promote leaching of the contaminants into the soil.

III. Properties of Medium: the metal uptake is affected by (pH adjustment, addition of chelators, fertilizers) For example; the amount of lead absorbed by plants is affected by the pH, organic matter, and the phosphorus content of the soil.

IV. The Root Zone: The Root Zone is of special interest in phytoremediation. It can absorb contaminants and store or metabolize it inside the plant tissue.

V. Vegetative Uptake: Vegetative Uptake is affected by the environmental conditions The temperature affect growth substances and consequently root length.

Advantages of Phytoremediation:
1. Phytoremediation has potential to remediate both organic as well as inorganic compound.

2. It is considered as green technology as it is more eco-friendly and aesthetically pleasing.

3. Phytoremediation does not require expensive equipment or highly-specialized personnel, and it is relatively easy to implement.

4. The most important advantage of phytoremediation is its low cost compared to conventional clean-up technologies.

5. Phytoremediation can be used either as an in situ or ex situ application

6. In situ applications are frequently considered because minimizes disturbance of the soil and surrounding environment and reduce the spread of contamination via air and waterborne wastes.

7. Easy to implement and maintain.

Disadvantages of Phytoremediation:
1. Its lengthy process that is it takes several years or longer to clean up a hazardous waste site.

2. Affect the biodiversity due to the use of invasive and non-native species for phytoremediation.

3. Consumption of contaminated plant tissue is also of concern.

4. Harvested plant biomass produced from the process of phytoextraction may cause hazardous waste if not properly disposed.

5. Limits the plant growth due to the unfavourable climatic condition

Future prospects:
Phytoremediation is a green technology which is used to remediate the heavy metal with the help of plant. There are many natural hyperaccumlators can tolerate and accumulate high concentrations of toxic metals, they usually have small biomass, grow slowly and cannot be easily cultivated. Now, with the advancements of molecular biology use genetic engineering approaches such as manipulation of genes, phytochelation, microbial interaction, cell compartmentation, transgenic methods and regulation of the various genes to improve the metal accumulation capacity. Overexpression of proteins involved in intracellular metal sequestration (MTs, phytochelation synthase & vacuolar transporters) may significantly increase metal accumulation and subcellular storage. Identifying and cloning the gene or transfer specific genes for enhancement of specific character like high-biomass will be promising for hyperaccumlators and it would enchance other properties of plant for phytoremediation.

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