Phytoremediation of heavy metal-contaminated sites: Eco-environmental concerns, field studies, sustainability issues and future prospects

Environmental contamination due to heavy metals (HMs) is of serious ecotoxicological concern worldwide because of their increasing use at industries. Due to non-biodegradable and persistent nature, HMs cause serious soil/water pollution and severe health hazards in living beings upon exposure. HMs can be genotoxic, carcinogenic, mutagenic, and teratogenic in nature even at low concentration. They may also act as endocrine disruptors and induce developmental as well as neurological disorders and thus, their removal from our natural environment is crucial for the rehabilitation of contaminated sites. To cope with HM pollution, phytoremediation has emerged as a low-cost and eco-sustainable solution to conventional physico-chemical cleanup methods that require high capital investment and labor alter soil properties and disturb soil microflora. Phytoremediation is a green technology wherein plants and associated microbes are used to remediate HM-contaminated sites to safeguard the environment and protect public health. Hence, in view of the above, the present paper aims to examine the feasibility of phytoremediation as a sustainable remediation technology for the management of metals-contaminated sites. Therefore, this paper provides an in-depth review on both the conventional and novel phytoremediation approaches, evaluate their efficacy to remove toxic metals from our natural environment, explore current scientific progresses, field experiences and sustainability issues and revise world over trends in phytoremediation research for its wider recognition and public acceptance as a sustainable remediation technology for the management of contaminated sites in 21st century

Overexpression of AtATM3 in Brassica juncea confers enhanced heavy metal tolerance and accumulation

AtATM3, a member of the ATP-binding cassette transporter family, is localized at the mitochondrial membrane of Arabidopsis thaliana and is involved in the biogenesis of Fe-S clusters and iron homeostasis in plants. Through Agrobacterium-mediated genetic transformation, the AtATM3 gene driven by the cauliflower mosaic virus 35S promoter (CaMV35S) was introduced into Brassica juncea (Indian mustard), a plant species suitable for phytoremediation, with the aim of improving heavy metal tolerance and accumulation in plants. The presence of the AtATM3 gene in transgenic plants was confirmed by polymerase chain reaction (PCR). Reverse transcriptase-PCR analysis confirmed AtATM3 expression in transgenic plants, but the level of AtATM3 expression varied between lines. AtATM3 overexpression in B. juncea conferred enhanced tolerance to cadmium [Cd(II)] and lead [Pb(II)] stresses. Importantly, the shoot tissues of transgenic seedlings contained about 1.5- to 2.5-fold higher Cd(II) and Pb(II) levels than wild type (WT) seedlings, demonstrating significantly-increased accumulation of both Cd(II) and Pb(II) in transgenic plants. The enhanced capacity of heavy metal tolerance and accumulation by AtATM3 transgenic plants was attributed to higher BjGSHII (B. juncea glutathione synthetase II) and BjPCS1 (phytochelatin synthase 1) expression levels induced by AtATM3 overexpression. In addition, AtATM3 overexpression regulated the expression of several metal transporters in the transgenic B. juncea plants under heavy metal stress conditions. Therefore, AtATM3 transgenic plants are more tolerant of and can accumulate more heavy metals to enhance phytoremediation of contaminated soils.X112523sciescopu

An improved method for Agrobacterium-mediated genetic transformation from cotyledon explants of Brassica juncea

An efficient Agrobacterium-mediated genetic transformation method was established for Brassica juncea by investigating several factors responsible for successful gene transfer. Four-day-old cotyledon explants from in vitro grown seedlings were co-cultivated with Agrobacterium strain GV3101 harboring the binary vector EnPCAMBIA1302-YCF1, which contained the hygromycin phosphotransferase (HPT) gene as a selectable marker and the yeast cadmium factor 1 (YCF1) gene. Two days co-cultivation period on shoot induction medium (MS medium supplemented with 0.1 mg l(-1) alpha-naphthaleneacetic acid, 1.0 mg l(-1) 6-benzyladenine, and 2.0 mg l(-1) silver nitrate) containing 20 mg l(-1) acetosyringone and five days delaying exposure of explants to selective agent enhanced transformation efficiency significantly. A three-step selection strategy was developed to select hygromycin resistant shoots. Hygromycin-resistant shoots were subsequently rooted on root induction medium. Rooted plantlets were transferred to pot-soil, hardened, and grown in a greenhouse until maturity. Using the optimized transformation procedure, transformation efficiency reached at 16.2% in this study. Southern blot analysis was performed to confirm that transgenes (HPT and YCF1) were stably integrated into the plant genome. All transgenic plants showed single-copy of transgene integration in the host genome. Segregation analysis of T-1 progeny showed that the transgenes were stably integrated and transmitted to the progeny in a Mendelian fashion.X11912sciescopu

Overexpression of a yeast cadmium factor 1 (YCF1) enhances heavy metal tolerance and accumulation in Brassica juncea

A yeast cadmium factor 1 (YCF1) is a member of the ATP-binding cassette (ABC) transporter family associated with multi-drug resistance, and it is localized at the vacuolar membrane in Saccharomyces cerevisiae. To determine ability to increase heavy metal tolerance and accumulation, YCF1 was introduced into Brassica juncea plants by Agrobacterium-mediated genetic transformation. YCF1 gene presence in transgenic plants was demonstrated by polymerase chain reaction (PCR). Reverse transcriptase-PCR analysis confirmed YCF1 gene expression in the transgenic plants, but the degree of YCF1 expression varied among the lines. YCF1 overexpression in B. juncea conferred enhanced tolerance to cadmium (Cd[II]) and lead (Pb[II]) stress. Transgenic B. juncea seedlings showed 1.3- to 1.6-fold tolerance to Cd stress and 1.2- to 1.4-fold tolerance to Pb stress compared to wild type (WT) plants (per gram fresh weight). Most importantly, the shoot tissues of transgenic seedlings contained about 1.5- to 2-fold higher Cd(II) and Pb(II) levels than those of WT, demonstrating significantly increased accumulation of both Cd(II) and Pb(II) in transgenic plants.X112426sciescopu