Phytoremediation is the use of green plants to modify/remove the pollutants from environment, thereby achieving eco-cleanup. Plants (woody or herbacious) which can accumulate desired levels of heavy metals concentration in their shoot known as metal hyperaccumulators and phenomenon is known as hyperaccumulation (i.e., the ability to accumulate at least 0.1% of the leaf dry weight in a heavy metal). Angiosperms are mainly take part as the hyperaccumulators. This includes Asteraceae, Brassicaceae, Euphorbiaceae, Fabaceae, Flacourtiaceae, Caryophyllaceae, Cyperaceae, Lamiacae, Poaceae and Violaceae. Amongst all Brassicaceae constitute a major part of hyperaccumulator group.

The Criteria for ideal plants used as hyperaccumulator for Phytoremediation should fulfil the following criteria:
i) Fast growth rate;
ii) High biomass;
iii) Ability to take up high concentration of heavy metals;
iv) Ability to tolerate high salinity and pH;
v) Easily harvestable and
vi) Must uptake and translocate metals to aerial parts efficiently.

The most extensively studied hyperaccumulator plant species include Thlaspi sp., Arabidopsis sp., and Sedum alfredii sp. The best known hyperaccumulator is the Thlaspi caerulescens. It can absorb zinc from the soil at a rate exceeding 40 Kg per hectare since it is a small and selfing diploid plant that can easily grow in vitro and also used as a model plant to demonstrate metal hyperaccumulation. Thlaspi sp. is also known to hyperaccumulate more than one metal i.e.

T. caerulescens Cd, Ni, Pb and Zn
T. goesingense Ni and Zn
T. ochroleucum Ni and Zn
T. rotundifolium Ni, Pb and Zn.

The strategies used in the development of a Phytoremediation plant include
i) Screening of hyperaccumulator candidate;
ii) Plant breeding, and
iii) Development of improved hyperaccumulators using genetic tools.

Recently, there are about 420 species belonging to about 45 plant families have been recorded as metals hyperaccumulators. While new hyperaccumulators continue to be discovered from field collections, only a few species have been tested in the laboratory to confirm their hyperaccumulating behaviors.

However, a major problem associated with most hyperaccumulator species is the low biomass and slow growth rate. Many researchers consider that the best way to transfer the appropriate characteristics of hyperaccumulators into high biomass plants via genetic engineering which substantially improves Phytoremediation efficiency of plants. To achieve this goal, it is necessary to understand that how these plants manage to tolerate and accumulate such high metals concentration.

Genetic engineering is a technology which is mainly focused on the understanding of genomics behind the ability of some plants to modify or remove pollutants. Till the date, no full-scale applications of transgenic, or genetically altered, plants for contaminated site remediation are known. A few laboratory and pilot studies have shown promising results in using transgenic plants for Phytoremediation.

The transgenic plants Arabidopsis thaliana L. and tobacco (Nicotiana tobacum) can transform methyl-mercury into elemental mercury before releasing it into the atmosphere. From a regulatory perspective, however, such mercury releases in the atmosphere are not environmentally acceptable; therefore, these genetically altered plants are not recommended for phytovolatilization of mercury. A transgenic yellow poplar (Liriodendron tulipifera) that is fast growing, pest resistant, and effective at absorbing mercury. This transformed ionic mercury to a much less toxic and less volatile metallic mercury.

Additional research is also focusing on:
i) Increasing plant tolerance to mercury and arsenic;
ii) Transforming the toxic elements to promote transport from roots to shoots;
iii) Transforming these toxic elements to promote storage in the aboveground plant parts;
iv) Enhancing the plants' ability to trap toxicants aboveground and
v) Enhancing transporters for uptake and storage.

Recent research results, including overexpression of genes whose protein products are involved in metal uptake, transport, and sequestration, or act as enzymes involved in the hazardous pollutants degradation, have opened up new possibilities in Phytoremediation. A better understanding of the mechanisms of rhizosphere interaction, uptake, transport and sequestration of metals in hyperaccumulator plants will lead to the designing of novel transgenic plants with improved remediation traits. It is also expected that recent advances in biotechnology will play a promising role in the development of new hyperaccumulators by transferring metal hyperaccumulating genes from low biomass wild species to the higher biomass producing cultivated species from times to come.

About Author / Additional Info:
Gaurav Saxena
Department of Environmental Microbiology
Babasaheb Bhimrao Ambedkar (Central) University
Lucknow (U.P) 206 025 India