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Significance of hyperaccumulation

The characteristic feature of heavy metal hyperaccumulation in plants has been effectively utilized as a successful tool in phytoremediation. Phytoremediation has emerged as an eco-friendly soil remediation technique that utilizes plant species. The naturally occurring heavy metal hyperaccumulator plants growing in metal enriched soils can accumulate 100-1000 folds higher level of metals, making them potential entities for phytoremediation (Navari-Izzo and Quatracci 2001, Kidd et al. 2009, Schwitzuebel et al. 2009). Thlaspi growing on Ni contaminated soils can accumulate about 3% of its diy matter and Thlaspi caerulescens can act as good hyperaccumulator of Cd and Zn, remediating as much as 60 kgZnlr1 and 80 kgCdlr1 from the soil (Robinson et al. 1998). Pteris vittata can accumulate as much as 22 gAskg'1 in its frond dry weight, with a bio concentration factor of 87, removing 26% of soil’s initial As (McGrath and Zhao 2003). Common buckwheat (Fagopymm esculentum) is quite effective in phytoremediation of Pb, which is largely immobile in soil and extraction rate is limited by solubility and diffusion to the root surface (Leitemnaier and Kupper 2013). Tamura et al. (2005) have shown that buckwheat can accumulate as much as 4.2 mgg'1 dxy weight of Pb in the shoots. Similarly, Phytolacca acinosa can act as a potential hyperaccumulator of Mn, when grown in Mn rich soils (Xue 2004).

However, hyperaccumulators have limited potential of phytoremediation because most of them are metal selective and can be used only in their natural habitats. The small biomass, shallow root systems and slow growth rates limit the speed of metal removal from the soils (Rascio and Navari- Izzo 2011). Hyperaccumulation can take decades to clean up the contaminated sites. Studies have shown that to decrease Zn concentration from 440 to 300 mgZnkg'1 in the soil, nine cropping of Thlaspi caerulescens would be required and 28 years of cultivation of this plant would be needed to remove 2100 mgZnkg'1 from a soil (McGarth et al. 1993, Kidd and Monterroso 2005).

Use of transgenic plants as hyperaccumulators

A biotechnological approach has been utilized to improve the potential of hyperaccumulator growth rate through selective breeding or by transfer of hyperaccumulator genes to high biomass species. Recently, various genetically modified plants have been raised by transferring genes from one plant to another. The transfer of phytochelatin synthetase (PCS) gene fr om Cyanodon dactylon to tobacco enhanced the amount of phytochelatin 3.88 fold and subsequently enhanced Cd accumulation by 3 folds (Li and Chen 2006). Similarly, in garlic {Allium sativum) (Zhang et al. 2005), overexpression of PCS favored the stress mitigation caused by heavy metal stress. Studies have shown that transgenic plants enhanced the metal transformation (from toxic to non-toxic form) 10 times more efficiently as compared to wild type (Kotrba et al. 2009). Bioengineered plants, tolerant to the presence of toxic levels of metals like Cd, Zn, Cr, Cu, Pb, As and Se, have been used for phytoremediation (Bexmet et al. 2003, Kawashima et al. 2004). Transgenic Brassica juncea showed enhanced uptake of Se and enhaixced Se tolerance than the wild species (Van Huysen et al. 2004). In an effort to overcome the shoxt biomass of hyperaccumulators, somatic hybrids have been generated beriveen Thlaspi caerulescens and Brassica napus which can be used as potential hyperaccumulators of Zn (Brewer et al. 1999) and Pb (Gleba et al. 1999).

Althoxxgh the biotechnological aspect to develop new- transgenic hyperaccumulators seems promising, it is not a nxxxch explored area. More experiments need to be performed under field conditions to bring out the actual potential of bioengineered hyperaccumulators.

Conclusions and future prospects

Human interferences with the biosphere have worsened the problem of heavy metal pollution. Heavy metal toxicity in soils has resulted in impeding growth and development of plants. Hyperaccumulator plants can act as potential tools for heavy metal remediation of the soil. An understanding of the mechanism of heavy metal uptake and their sequestration/detoxification will help us to investigate the possibilities of using them to remove metals from contaminated or natural metalliferous soils. The information obtained on the genes involved in transfer and sequestration of heavy metals in hyperaccumulators will open the opportunities to transfer these specific genes, through biotechnological processes, into plants with high biomass promising species. However, in spite of significant works done on this aspect in the recent years, the complexity and mechanism of hyperaccumulation still needs much exploration. Another important aspect is to search for and characterize other hyperaccumulator species, cultivate them and, using different agronomic management practices, enhance their plant growth and metal uptake by selective breeding and gene manipulation.

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