Metal hyperaccumulation in plants - Biodiversity prospecting for phytoremediation technology

Abstract

The importance of biodiversity (below and above ground) is increasingly considered for the cleanup of the metal contaminated and polluted ecosystems. This subject is emerging as a cutting edge area of research gaining commercial significance in the contemporary field of environmental biotechnology. Several microbes, including mycorrhizal and non-mycorrhizal fungi, agricultural and vegetable crops, ornamentals, and wild metal hyperaccumulating plants are being tested both in lab and field conditions for decontaminating the metalliferous substrates in the environment. As on todate about 400 plants that hyperaccumulate metals are reported. The families dominating these members are Asteraceae, Brassicaceae, Caryophyllaceae, Cyperaceae, Cunouniaceae, Fabaceae, Flacourtiaceae, Lamiaceae, Poaceae, Violaceae, and Euphobiaceae. Brassicaceae had the largest number of taxa viz. 11 genera and 87 species. Different genera of Brassicaceae are known to accumulate metals. Ni hyperaccumulation is reported in 7 genera and 72 species and Zn in 3 genera and 20 species. Thlaspi species are known to hyperaccumulate more than one metal i.e. T. caerulescence = Cd, Ni. Pb, and Zn; T. goesingense = Ni and Zn and T. ochroleucum = Ni and Zn and T. rotundifolium = Ni, Pb and Zn. Plants that hyperaccumulate metals have tremendous potential for application in remediation of metals in the environment. Significant progress in phytoremediation has been made with metals and radionuclides. This process involves rising of plants hydroponically and transplanting them into metal-polluted waters where plants absorb and concentrate the metals in their roots and shoots. As they become saturated with the metal contaminants, roots or whole plants are harvested for disposal. Most researchers believe that plants for phytoremediation should accumulate metals only in the roots. Several aquatic species have the ability to remove heavy metals from water, viz., water hyacinth (Eichhornia crassipes (Mart.) Solms); pennywort (Hydrocotyle umbellata L.) and duckweed (Lemna minor L.). The roots of Indian mustard are effective in the removal of Cd, Cr, Cu, Ni, Pb, and Zn and sunflower removes Pb, U, ¹³⁷Cs, and ⁹⁰Sr from hydroponic solutions. Aquatic plants in freshwater, marine and estuarine systems act as receptacle for several metals. Hyperaccumulators accumulate appreciable quantities of metal in their tissue regardless of the concentration of metal in the soil, as long as the metal in question is present. The phytoextraction process involves the use of plants to facilitate the removal of metal contaminants from a soil matrix. In practice, metal-accumulating plants are seeded or transplanted into metal-polluted soil and are cultivated using established agricultural practices. If metal availability in the soil is not adequate for sufficient plant uptake, chelates or acidifying agents would be applied to liberate them into the soil solution. Use of soil amendments such as synthetics (ammonium thiocyanate) and natural zeolites have yielded promising results. Synthetic cross-linked polyacrylates, hydrogels have protected plant roots from heavy metals toxicity and prevented the entry of toxic metals into roots. After sufficient plant growth and metal accumulation, the above-ground portions of the plant are harvested and removed, resulting the permanent removal of metals from the site. Soil metals should also be bioavailable, or subject to absorption by plant roots. Chemicals that are suggested for this purpose include various acidifying agents, fertilizer salts and chelating materials. The retention of metals to soil organic matter is also weaker at low pH, resulting in more available metal in the soil solution for root absorption. It is suggested that the phytoextraction process is enhanced when metal availability to plant roots is facilitated through the addition of acidifying agents to the soil. Chelates are used to enhance the phytoextraction of a number of metal contaminants including Cd, Cu, Ni, Pb, and Zn Researchers initially applied hyperaccumulators to clean metal polluted soils. Several researchers have screened fast-growing, high-biomass-accumulating plants, including agronomic crops, for their ability to tolerate and accumulate metals in their shoots. Genes responsible for metal hyperaccumulation in plant tissues have been identified and cloned. Glutathione and organic acids metabolism plays a key role in metal tolerance in plants. Glutathione is ubiquitous component cells from bacteria to plants and animals. In phytoremediation of metals in the environment, organic acids play a major role in metal tolerance. Organic acids acids form complexes with metals, a process of metal detoxification. Genetic strategies and transgenic plant and microbe production and field trials will fetch phytoremediaition field applications.The importance of biodiversity and biotechnology to remediate potentially toxic metals are discussed in this paper. Brassicaceae amenable to biotechnological improvement and phytoremediation hype are highlighted.