*Dept. of Biotechnology, Sri Padmavati Mahila Visva Vidyalayam, Tirupati, A.P., India**School of Engineering & Technology, Gurupur, Orissa, India
Key words : Phytoremediation, Heavy metals, Hyper accumulators, Transgenic plants.
ABSTRACT
Phytoremediation, the use of green plants to clean up polluted soil and water resources has receivedmuch attention in the last few years. Phytoremediation offers owners and managers of metal contami-nated sites is an innovative and cost effective option to address recalcitrant environmental contami-nants. Although not a new concept, phytoremediation is currently being re-examined as an environ-mentally friendly, cost-effective means of reducing metal contaminated soil. Genetic engineeringapproaches are currently being used to optimize the metabolic and physiological process that enableplants to phytoremediate, sites contaminated with heavy metals. Genetic manipulation of environ-mentally important plants can produce elite plant lines with enhanced remediation abilities. Recentresearch results include over expression of genes whose protein products are involved in metal up take,transport and sequestration, have opened up new possibilities in phytoremediation. This review articleprovides a critical review of the recent progress towards the development of transgenic plants withimproved phytoremediation capabilities and their potential use in environmental clean up.
INTRODUCTION
Environmental bio-technology is a new disciplinewhich integrates living materials, mainly plants, andvery small animals like earth worms, micro organ-isms to address the problems of environmental man-agement and sustainable developmentPhytoremediation is a word formed from the Greekprefix “Phyto” means plant and suffix “remedium”meaning to clean (or) restore (Cunningham et.al.,1996)The term actually refers to advise collection of plantbased technologies that use either naturally occurring(or) genetically engineered plants for cleaning con-taminated environments (Flathman and lanza, 1998),Phytoremediation consists of four different plantbased technologies, each having a different mecha-
* Address for correspondence: Usha Royalcheruvu Email : [email protected]
Jr. of Industrial Pollution Control 26 (1)(2010) pp 83-88
nism of action for the remediation of metal pollutedsoil, sediment or water. These include phytoextraction,where plants absorb metals from soil and translocatethem to harvestable shoots where they accumulate.Rhizofiltration involves the use of plant to clean vari-ous aquatic environments. Phytostabilization, whereplants are used to stabilize rather than clean contami-nated soil. Phytovolatalization, which involves theuse of plants to extract certain metals from soil andthen release them into the atmosphere through vola-tilization. Some plants tolerate and accumulate highconcentrations of metal in their tissues but not at thelevel required to be called as hyper accumulators.Researches in environmental biotechnology promiseto enhance the phytoremediation efficiency by aknown phytoremediator plant. A genetic combination
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of fast-growing, high biomass yielding, and high tol-erance and hyper accumulation of toxic metals inplant shoot is best suited for the purpose ofphytoremediation. It should also be easily cultivatedand harvested. The novel bioremediation systemcalled “Symbiotic Engineering” involves advantagesof both the bacterium rhizobium and the leguminousplants using many useful genes like ATPCS, MTL4,IRT1, may provide another valuable bioremediationtool. The Introduction of genes can be readily achievedfor many plant species using direct DNA methods ofgene transfer. Transformation of required gene for theproduction of transgenic plant using molecular tech-niques is a novel technology.
MATERIALS AND METHODS
Phytoremediation is a method which green plants usefor cleaning up contaminated hazardous wastes sites.Phytoremediation has applied Ex-situ and In-Situ,continually and induces to clean up contaminatedterrain of toxic metals. The following are the steps in-volved in the phytoremediation process
1. Identification of area.2. Chemical analysis of the soil before application of
the phytoremediation.3. Sowing the plant phytoaccumulators.4. Usage of agricultural and technical measures and
Inspection of vegetative development.5. Picking and drying the plants.6. Chemical analysis of the soil near the root after
finished phytoremediation.7. Chemical analysis of green leaves of plants.8. Determination of co-efficient (Concentration fac-
tors) of plants.
The material is dried in shadow and draft withoutsunlight presence. After sample preparation contentof heavy metal is determined by Atomic AbsorptionSpectroscopy. For enhancing the phytoremediationprocess the following steps are considered.- To enhance the speed and quantity of metal up-
takes by plants, some researchers are advocatingthe use of various chemicals like acidifying agents(Blaylocke and Huang, 2000), fertilizer salts (Lasatet al. 1997, 1998) and chelating materials (Blay locket al. 1997).
- Soil pH is a major factor influencing the availabil-ity of elements in the soil for plant uptake(Marschenev, 1995).
- Acidifying agents are also used to increase the avail-
ability of radio active elements in the soil for plantuptake.
Working process of phytoremediation
Plant roots take contaminants from the ground intothe plant body. The plant root zone is referred to asthe rhizosphere; this is where the action occurs. Thissoil supports large populations of diverse micro or-ganisms. This is due to chemicals exuded by plantsroots which provide carbon and energy for microbialgrowth. This combination of plants and micro organ-isms used to increase the biodegradation by com-pounds.
Proper plant selection
As a plant based technology, the success ofphytoextraction is inherently dependent upon properplant selection. Plants used for the phytoextractionmust be fast growing and have the ability to accumu-late large quantities of environmentally importantmetal contaminants in their shoot tissue (Blay lock etal. 1994) Researches initially envisioned using hyperaccumulators (Salt and Kramer, 2000) to clean metalpolluted sites. At present, there are nearly 400 knownhyper accumulators but majority are not appropriatefor phytoextraction, because of their slow growth andsmall size. Several researches are screened fast-grow-ing, high-biomass-accumulating plants, includingagronomic crops, for their ability to tolerate and accu-mulate metals in their shoots (Banuelus et al. 1997).
Types of Vegetation Used
Some of the plants used in phytoremediation are:- Alfalfa- Hybrid Poplar Trees- Blue-green Algae- Duck Weeds- Arrowroot- Sudan Grass- Rye Grass- Bermuda Grass- Alpine Bluegrass- Yellow or White Water Lilies- Sunflower- Vetiver grass- Poplar tree- Brake fern- Carrot- Periwinkle- Switch Grass- White reddish
PHYTOREMEDIATION – AN OVERVIEW REVIEW 85
Fig. 1 Various processes of phytoremediation
Some known hyper accumulator species of differentmetals.
Metal and Plant Species Concentration ofmetalaccuumu-lated (mg/kg)
Soil & water contaminated with toxic metals posemajor environmental and human health problems.According to EEA (European Environment Agency)estimation 1.4 million areas are contaminated(Puschenreither and Wenzel, 2003). Environmentalbiotechnology has given rise to an allied disciplinecalled “bio-engineering”. It is a ‘Green’ (or) ‘Soft” andalternative to the “hard” and costly Civil Engineeringworks for the environmental restoration and recon-struction.
In modern times, not only the biological organismsare used but their genetic materials (DNA) too. Mod-ern biotechnology has made wonders and has revo-lutionized and heralded a new era in the field of envi-ronmental management. With the new biotechnologi-cal tools (recombinant DNA technology, genetic engi-neering) working at genetic or molecular level scien-tists can change the genetic make up of organisms inwhich characteristics are not found naturally.Plant Enzymes implicated in Phytodegradation andphytotransformation of organic compounds
Enzymes Contaminated degraded /transformed into Less toxicforms
In the last few years, several commercial compa-nies practicing phytoremediation for environmentalclean up in USA and Europe. Important among themare phytotech (USA), phyto works (USA), earth care
Phytoremediation Values inworks carried out million dollars
Removal of heavy metal fromContaminated soils. 70-100Removal of heavy metal fromContaminated ground water 1-3Removal of heavy metal from
PHYTOREMEDIATION DESIGNS
Fig. 2 A typical process diagram of phytoremediation
Fig. 3 Ditoxification of xenobiotics in plant cell
PHYTOREMEDIATION – AN OVERVIEW REVIEW 87
Contaminated waste water 1-2Removal of heavy metal fromRadio nuclides 40-80Removal of organics froContaminated ground water 65-115Others 214-370 million
dollars
Role of Environmental biotechnology and genetic
Engineering in improving efficiency of
Phytoremediation.
Several genes that are involved in metal uptake, trans-location, sequestration and bioaccumulation has nowbeen identified. Transfer of these genes into candi-date plant will result in developing “Transgenicplants” with enhanced ability for metal uptake andaccumulation for the removal from environment. En-vironmental biotechnology is a tool to accumulate thephytoremediation process through over expression ofthose genes responsible for the sequestration of heavymetals and radio nuclide in plants or through “gene-transfer”. Any appropriate genes of foreign origin beentransferred in plants like Arabidopsis thaliana, Nic-otiana tobaccum, Brassica junceae, Brassicaolevaceae, Varbotrytis, Lycopersicon Esculentum etc.,to enhance the phytoremediation efficiency of theseplants.
Many hyper accumulator plants are rare, withsmall population occurring in remote places (or) haverestricted distribution. They often have slow growthrate and produce small biomass. Such hyper accumu-lator species may provide suitable genes involved inmetal uptake, translocation, and sequestration for en-hancing the phytoremediation. If genes from highlymetal tolerant and hyper accumulator plants are trans-ferred to high biomass yielding and growing culti-vars, this can do miracle.
Important Achievements
- Transfer of human MT-2 gene to tobacco (Nicoti-ana tobaccum) resulted in transgenic plant withenhanced cadmium (cd) tolerance and accumula-tion.
- Transfer of Pea MT gene in Arabidopsis thalianaresulted in enhanced copper (Cu) accumulationin the transgenic Arabidopsis Thaliana.
- Transfer of Yeast CUP 1 gene in Cauliflower (Bras-sica capitata) resulted in 16-fold higher accumula-tion of cd in transgenic Cauliflower ( Eaper et al.2006).
- Transfer MT gene in Nicotiana spp., Brassica Spp.,Arabidopsis thaliana resulted in highly tolerant to
Cd and other metals.- Somatic cell hybrid produced between Brassica
junceae (a high biomass yielding plant) andThalaspi caeruiescens (a known Zn and Ni hyperaccumulator) showed increased tolerance to Pband Ni, Zn and the total amount of leadphytoextracted was much greater because of highbiomass produced (Dushenkov et al. 200).
Advantages
i) The cost of the phytoremediation is lower than thatof traditional process both in-situ and ex-situ
ii) The plants can be easily monitored.iii) The possibility of the recovery and re-use of valu-
able metals.iv) It is the least harmful method because it uses natu-
rally occurring organisms and preserves the natu-ral state of the environment.
Limitations
i) With plant based systems of remediation, it is notpossible to completely prevent the leaching of con-taminants into the ground water.
ii) The survival of the plant is affected by the toxicityof the contaminated land and general conditionsof the soil.
iii) Possible bio accumulations of contaminants whichthen passed into the food-chain from primary levelconsumers up wards.
Future research direction
Further use of molecular biology expertise to greaterunderstanding of phytoremediation at the genetic andmolecular level research objectives in these areas.
i) Probing of the Bio-path ways involved in contami-nant degradation and sequestration.
ii) Identifying the specific genes involved inphytoremediation process.
iii) Investigating cell signaling path ways that affectthe genetic expression plant and microbial en-zymes.
iv) Studying the molecular ecology of root-microbialinteractions.
v) Analyzing and identifying root exudates.
Now it has become possible to:
- Create economically valuable and ecologicallyadapted crops called transgenic crops with desiredcharacters.
- Create hyper-accumulator plants which can toler-ate and bio accumulate high levels of toxic metals.
88 VASAVI ET AL.
- Grow thousands of replicas of economically im-portant plants in shorter time by tissue culture.
CONCLUSION
A phytoremediation is amenable to a variety of or-ganic and in organic compounds may be applied ei-ther in-situ or ex-situ. Phytoremediation is consideredto be an innovative technology and hope fully by in-creasing our knowledge and understanding of thisintricate clean up method, it will provide a cost effec-tive, environment friendly alternative to conventionalcleanup methods.
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