Induced Phytoextraction or Chelate Assisted Phytoextraction

Oligopeptide ligands, such as phytochelatins (PCs) and metallothioneins (MTs), are induced by the presence of or interact with heavy metals found in plant cells (Cobbett 2000). These peptides bind with the heavy metal, forming stable complexes, and thus neutralize them and minimize the toxicity of the metal ion[68]. Phytochelatins (PCs) are synthesized from glutathione, and have the structure Gly-(g-Glu-Cys)n, where n = 2-11. Around a hundred phytochelating ligands have been reported in plant species exposed to heavy metals (Rauser 1999). MTs are small, gene-encoded, Cys-rich polypeptides. PCs are functionally the same as MTs (Grill et al. 1987). Chelating agents such as ethylenediaminetetraacetic acid (EDTA) have been isolated from plants that are involved in the uptake of heavy metals and their detoxification. The addition of chelators to a Pb-contaminated soil (total soil Pb 2,500 mg kg-1) increased shoot Pb concentrations of Zea mays (corn) and Pisun sativum (pea) from less than 500 mg kg-1 to more than 10,000 mg kg-1. The synthetic chelator EDTA is used to enhance Ur uptake from the soil and also to enhance or facilitate Pb transport into the xylem, as well as to increase Pb translocation from roots to shoots. Various chelators can be ordered according to their ability to increase Pb desorption from the soil as follows: EDTA > hydroxyethylethylene-diaminetriacetic acid (HEDTA) > diethylenetriaminepentaacetic acid (DTPA) > eth-ylenediamine di(o-hydroxyphenylacetic acid) EDDHA (Huang et al. 1997). Vassil et al. (1998) reported that Brassica juncea exposed to Pb and EDTA in hydroponic solution was able to accumulate up to 55 mM kg-1 Pb in dry shoot tissue (1.1% w/w). This corresponds to a ~75-fold increase in the Pb concentration in its shoots compared to that found in the solution. Thus, EDTA (0.25 mM) was shown to stimulate this accumulation of Pb in shoots.

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