Phytoremediation, a Novel Method for Removal of Heavy Metals from Environment: Biochemical and Molecular Mechanisms

Abstract: Heavy metals are constantly released into the environment as a result of human, agricultural and industrial activities. Thus, there is an urgent need to develop a low-cost, effective, and sustainable method for their detoxification. Phytoremediation is the use of plants to remediate metal contaminated sites. Sesbania plants are being grown in the presence of different concentrations of lead (Pb). Metal concentration in shoots as well as in the roots increased with increasing concentrations in the growth medium. The X-ray absorption spectroscopic studies showed that metals were biotransformed in the Sesbania tissue. Scanning electron microscopy studies revealed that metals predominantly accumulated in the cortical and vascular (xylem) regions of root tissues. Transmission electron microscopy and X-ray microanalysis of tissue demonstrated the localization of metal nanoparticles in the plasma membrane, cell wall, and in the vacuole. To investigate the overall molecular response of Sesbania to Pb exposure, suppression subtractive hybridization was used to construct a cDNA library enriched in Pb induced mRNA transcripts. Screening the library by reverse Northern analysis revealed that 2025% of clones selected were differentially regulated in Pb treated plants. After differential screening, we isolated several differentially expressed cDNA clones, including a type 2 metallothionein (MT) gene which is involved in detoxification and homeostasis, and shown to be differentially regulated in Pb treated plants. The mRNA levels of MT increased substantially after Pb treatment indicating a potential role for it under Pb stress in Sesbania.