Phytoremediation of Metals Using Transgenic Plants

An ideal plant for environmental cleanup can be envisioned as one with high biomass production, combined with superior capacity for pollutant tolerance, accumulation, and/or degradation, depending on the type of pollutant and the phytoremediation technology of choice. With the use of genetic engineering, it is feasible to manipulate a plant's capacity to tolerate, accumulate, and/or metabolize pollutants, and thus to create the ideal plant for environmental cleanup. In this review, we focus on the design and creation of transgenic plants for phytoremediation of metals. Plant properties important for metal phytoremediation are metal tolerance and accumulation, which are determined by metal uptake, root-shoot translocation, intracellular sequestration, chemical modification, and general stress resistance. If we know which molecular mechanisms are involved in these tolerance and accumulation processes, and which genes control these mechanisms, we can manipulate them to our advantage. This review aims to give a succinct overview of plant metal tolerance and accumulation mechanisms, and to identify possible strategies for genetic engineering of plants for metal phytoremediation. An overview is presented of what has been achieved so far regarding the manipulation of plant metal metabolism. In fact, both enhanced metal tolerance and accumulation have been achieved by overproducing metal chelating molecules (citrate, phytochelatins, metallothioneins, phytosiderophores, ferritin) or by the overexpression of metal transporter proteins. Mercury volatilization and tolerance was achieved by introduction of a bacterial pathway. The typical increase in metal accumulation as the result of these genetic engineering approaches is 2-to 3-fold more metal per plant, which could potentially enhance phytoremediation efficiency by the same factor. As for the applicability of these transgenics for environmental cleanup, results from lab and greenhouse studies look promising for several of these transgenics, but field studies will be the ultimate test to establish their phytoremediation potential, their competitiveness, and risks associated with their use.     

Phytoremediation: novel approaches to cleaning up polluted soils

Environmental pollution with metals and xenobiotics is a global problem, and the development of phytoremediation technologies for the plant-based clean-up of contaminated soils is therefore of significant interest. Phytoremediation technologies are currently available for only a small subset of pollution problems, such as arsenic. Arsenic removal employs naturally selected hyperaccumulator ferns, which accumulate very high concentrations of arsenic specifically in above-ground tissues. Elegant two-gene transgenic approaches have been designed for the development of mercury or arsenic phytoremediation technologies. In a plant that naturally hyperaccumulates zinc in leaves, approximately ten key metal homeostasis genes are expressed at very high levels. This outlines the extent of change in gene activities needed in the engineering of transgenic plants for soil clean-up. Further analysis and discovery of genes for phytoremediation will benefit from the recent development of segregating populations for a genetic analysis of naturally selected metal hyperaccumulation in plants, and from comprehensive ionomics data--multi-element concentration profiles from a large number of Arabidopsis mutants.     

Assessing the applicability of phytoremediation of soils with mixed organic and heavy metal contaminants

Soil pollution is a major environmental problem and many contaminated sites are tainted with a mixture of organic and heavy metal contaminants. Compared to other remedial strategies, phytoremediation is a low cost, environmentally-friendly, sustainable means of remediating the contamination. This review first provides an overview of phytoremediation studies where the soil is contaminated with just one type of pollutant (heavy metals or organics) and then critically evaluates the applicability of phytotechnologies for the remediation of contaminated sites where the soil is polluted by a mixture of organic and heavy metal contaminants. In most of the earlier research studies, mixed contamination was held to be detrimental to plant growth, yet there were instances where plant growth was more successful in soil with mixed contamination than in the soil with only individual contaminants. New effective phytoremediation strategies can be designed for remediation of co-contaminated sites using: (a) plants species especially adapted to grow in the contaminated site (hyperacumulators, local plants, transgenic plants); (b) endophytic bacteria to enhance the degradation in the rizhosphere; (c) soil amendments to increase the contaminants bioavailability [chelating agents and (bio)surfactants]; (d) soil fertilization to enhance the plant growth and microbial activity in the soil; and (e) coupling phytoremediation with other remediation technologies such as electrokinetic remediation or enhanced biodegradation in the rhizosphere.     

Phytoremediation of toxic trace elements in soil and water

Toxic heavy metals and metalloids, such as cadmium, lead, mercury, arsenic, and selenium, are constantly released into the environment. There is an urgent need to develop low-cost, effective, and sustainable methods for their removal or detoxification. Plant-based approaches, such as phytoremediation, are relatively inexpensive since they are performed in situ and are solar-driven. In this review, we discuss specific advances in plant-based approaches for the remediation of contaminated water and soil. Dilute concentrations of trace element contaminants can be removed from large volumes of wastewater by constructed wetlands. We discuss the potential of constructed wetlands for use in remediating agricultural drainage water and industrial effluent, as well as concerns over their potential ecotoxicity. In upland ecosystems, plants may be used to accumulate metals/metalloids in their harvestable biomass (phytoextraction). Plants can also convert and release certain metals/metalloids in a volatile form (phytovolatilization). We discuss how genetic engineering has been used to develop plants with enhanced efficiencies for phytoextraction and phytovolatilization. For example, metal-hyperaccumulating plants and microbes with unique abilities to tolerate, accumulate, and detoxify metals and metalloids represent an important reservoir of unique genes that could be transferred to fast-growing plant species for enhanced phytoremediation. There is also a need to develop new strategies to improve the acceptability of using genetically engineered plants for phytoremediation.

     

Phytoremediation of toxic trace elements in soil and water

Toxic heavy metals and metalloids, such as cadmium, lead, mercury, arsenic, and selenium, are constantly released into the environment. There is an urgent need to develop low-cost, effective, and sustainable methods for their removal or detoxification. Plant-based approaches, such as phytoremediation, are relatively inexpensive since they are performed in situ and are solar-driven. In this review, we discuss specific advances in plant-based approaches for the remediation of contaminated water and soil. Dilute concentrations of trace element contaminants can be removed from large volumes of wastewater by constructed wetlands. We discuss the potential of constructed wetlands for use in remediating agricultural drainage water and industrial effluent, as well as concerns over their potential ecotoxicity. In upland ecosystems, plants may be used to accumulate metals/metalloids in their harvestable biomass (phytoextraction). Plants can also convert and release certain metals/metalloids in a volatile form (phytovolatilization). We discuss how genetic engineering has been used to develop plants with enhanced efficiencies for phytoextraction and phytovolatilization. For example, metal-hyperaccumulating plants and microbes with unique abilities to tolerate, accumulate, and detoxify metals and metalloids represent an important reservoir of unique genes that could be transferred to fast-growing plant species for enhanced phytoremediation. There is also a need to develop new strategies to improve the acceptability of using genetically engineered plants for phytoremediation.     

Phytoremediation of toxic aromatic pollutants from soil

The enormous growth of industrialization, and the use of numerous aromatic compounds in dyestuffs, explosives, pesticides and pharmaceuticals has resulted in serious environmental pollution and has attracted considerable attention continuously over the last two decades. Many aromatic hydrocarbons, nitroaromatic compounds, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, diauxins and their derivatives are highly toxic, mutagenic and/or carcinogenic to natural microflora as well as to higher systems including humans. The increasing costs and limited efficiency of traditional physicochemical treatments of soil have spurred the development of new remediation technologies. Phytoremediation is emerging as an efficient treatment technology that uses plants to bioremediate pollutants from soil environments. Various modern tools and analytical devices have provided insight into the selection and optimization of remediation processes by various plant species. Sites heavily polluted with organic contaminants require hyperaccumulators, which could be developed by genetic engineering approaches. However, efficient hyperaccumulation by naturally occurring plants is also feasible and can be made practical by improving their nutritional and environmental requirements. Thus, phytoremediation of organics appears a very promising technology for the removal of contaminants from polluted soil. In this review, certain aspects of plant metabolism associated with phytoremediation of organic contaminants and their relevant phytoremediation efforts are discussed.IMTECH Communication No. 013/2002     

Phytoremediation of organic xenobiotics - Glutathione dependent detoxification in Phragmites plants from European treatment sites

Studies on the uptake of several organic xenobiotics and on their subsequent conjugation to biomolecules have been performed to elucidate the use of reed plants in phytoremediation of polluted water. Phragmites australis plants were able to accumulate organic xenobiotics in their rhizomes. The uptake was correlated to the logKOW and pKa of the xenobiotics and highest with compounds exhibiting logKOWs between 1 and 3. Detoxification of xenobiotics was demonstrated when the activity of glutathione S-transferase was determined in plants from various treatment sites. Enzyme activities were strongly dependent on the provenience of the plant and the history of the stand. Detoxification enzymes were also inducible. Naphthylic acetic acid (NAA), 2,4-dichlorophenol and BION were tested as potential inducers. BION was able to induce the GST activity 5-fold, albeit only for a short period of hours. The mechanism of induction and the flexibility of the detoxification system of certain ecotypes of reed toward stress or the pollution level will require further investigation.     

Rhizosphere processes and management in plant-assisted bioremediation (phytoremediation) of soils

Plant-assisted bioremediation or phytoremediation holds promise for in situ treatment of polluted soils. Enhancement of phytoremediation processes requires a sound understanding of the complex interactions in the rhizosphere. Evaluation of the current literature suggests that pollutant bioavailability in the rhizosphere of phytoremediation crops is decisive for designing phytoremediation technologies with improved, predictable remedial success. For phytoextraction, emphasis should be put on improved characterisation of the bioavailable metal pools and the kinetics of resupply from less available fractions to support decision making on the applicability of this technology to a given site. Limited pollutant bioavailability may be overcome by the design of plant–microbial consortia that are capable of mobilising metals/metalloids by modification of rhizosphere pH (e.g. by using Alnus sp. as co-cropping component) and ligand exudation, or enhancing bioavailability of organic pollutants by the release of biosurfactants. Apart from limited pollutant bioavailability, the lack of competitiveness of inoculated microbial strains (in particular degraders) in field conditions appears to be another major obstacle. Selecting/engineering of plant–microbial pairs where the competitiveness of the microbial partner is enhanced through a “nutritional bias” caused by exudates exclusively or primarily available to this partner (as known from the “opine concept”) may open new horizons for rhizodegradation of organically polluted soils. The complexity and heterogeneity of multiply polluted “real world” soils will require the design of integrated approaches of rhizosphere management, e.g. by combining co-cropping of phytoextraction and rhizodegradation crops, inoculation of microorganisms and soil management. An improved understanding of the rhizosphere will help to translate the results of simplified bench scale and pot experiments to the full complexity and heterogeneity of field applications.     

Phytoremediation—An Overview

The use of plants (directly or indirectly) to remediate contaminated soil or water is known as phytoremediation. This technology has emerged as a more cost effective, noninvasive, and publicly acceptable way to address the removal of environmental contaminants. Plants can be used to accumulate inorganic and organic contaminants, metabolize organic contaminants, and encourage microbial degradation of organic contaminants in the root zone. Widespread utilization of phytoremediation can be limited by the small habitat range or size of plants expressing remediation potential, and insufficient abilities of native plants to tolerate, detoxify, and accumulate contaminants. A better understanding and appreciation of the potential mechanisms for removing contaminants from the root zone and the interaction between plants, microorganisms, and contaminants will be useful in extending the application of phytoremediation to additional contaminated sites.     

Phytoremediation: an overview of metallic ion decontamination from soil

In recent years, phytoremediation has emerged as a promising ecoremediation technology, particularly for soil and water cleanup of large volumes of contaminated sites. The exploitation of plants to remediate soils contaminated with trace elements could provide a cheap and sustainable technology for bioremediation. Many modern tools and analytical devices have provided insight into the selection and optimization of the remediation process by plant species. This review describes certain factors for the phytoremediation of metal ion decontamination and various aspects of plant metabolism during metallic decontamination. Metal-hyperaccumulating plants, desirable for heavily polluted environments, can be developed by the introduction of novel traits into high biomass plants in a transgenic approach, which is a promising strategy for the development of effective phytoremediation technology. The genetic manipulation of a phytoremediator plant needs a number of optimization processes, including mobilization of trace elements/metal ions, their uptake into the root, stem and other viable parts of the plant and their detoxification and allocation within the plant. This upcoming science is expanding as technology continues to offer new, low-cost remediation options.     

Amendment Optimization to Enhance Lead Extractability from Contaminated Soils for Phytoremediation

Eight lead-contaminated soils and one background soil artificially contaminated with several lead compounds were examined to determine the factors that limit lead extractability and thus plant availability during phytoremediation, as lead must be in soluble form for plant uptake to occur. The effect of the chemical form of the lead as well as the association of the lead among the different soil chemical fractions on lead extractability was specifically addressed. Results indicate that all the added lead forms tested except PbCrO₄ were readily extracted and believed to be available for plant uptake, operationally defined as EDTA-extractable lead, as EDTA is the primary soil amendment for phytoremediation of lead-contaminated soils. Sequential extraction of the eight lead-contaminated soils that previously had been extracted with EDTA shows that the EDTA-extractable or plant available lead corresponds to mainly the exchangeable and carbonate fractions of each soil. Lead associated with oxide, organic, and residual fractions were less effectively targeted and solubilized by EDTA and therefore are not as readily available for plant uptake. Attempts to increase the available pool of soluble lead included the combination of EDTA with organic acids, reducing agents, and surfactants. Results from these studies indicate that high concentrations or extremely low pH conditions are required to enhance the plant available pool of lead by the organic acids and reducing agents. Surfactants, particularly caprylic acid in combination with 0.25 mM EDTA, were shown to be as effective as 0.50 mM EDTA alone. An amendment formulation combining less EDTA with surfactants is attractive for phytoremediation because of the biodegradability and cost concerns commonly associated with using larger amounts of EDTA.     

植物修复重金属污染及内生细菌效应

土壤和水体的重金属污染已严重危害人类生存环境与健康。由于受重金属污染的环境分布广泛,迫切需要开发经济的清除环境重金属的技术。植物修复是通过绿色植物降解或移除环境污染物,有望成为重金属污染环境的原位修复技术。植物内生菌是指定殖于健康植物的各种组织和器官内部的细菌,被感染的宿主植物不表现出外在病症,耐重金属的内生菌在多种超富集植物中存在。在植物修复过程中,野生型内生菌或基因工程内生菌的抗性系统能降低重金属植物毒性,促进其迁移金属。耐重金属内生菌还可以通过固氮、溶解矿物元素及产生类植物激素、铁载体和ACC脱氨酶等产物促进植物的生长。主要综述目前植物-内生菌相互作用及其潜在的促进植物修复重金属污染的研究进展。     

Conjugating Enzymes Involved in Xenobiotic Metabolism of Organic Xenobiotics in Plants

Phytoremediation of organic pollutants has become a topic of great interest in many countries due to the increasing number of recorded spill sites. When applying plant remediation techniques to unknown pollutant mixtures, information on the uptake rates as well as on the final fate of the compounds is generally lacking. A range of compounds are easily taken up by plants, whereas others may stay motionless and recalcitrant in the soil or sediment. Uptake is a necessary prerequisite for close contact between the pollutant and the detoxifying enzymes of plants that are localized in the cytosol of living cells. The presence and activity of these enzymes is crucial for a potential metabolization and further degradation of the chemicals under consideration. Conjugation to biomolecules is regarded as a beneficial detoxification reaction. The present review summarizes several prerequisites for pollutant uptake and discusses information on conjugating detoxification reactions. The final fate of compounds is critically discussed and perspectives for phytoremediation are given.     

Genetic Engineering of Plants to Enhance Selenium Phytoremediation

Referee: Dr. Dean A. Martens, USDAARS Southwest Watershed, Research Center, 200 E. Allen Road, Tucson, AZ 85719 Phytoremediation is the use of plants to remove, contain, or render harmless environmental pollutants. In recent years, much attention has been focused on the improvement of such technologies for this purpose. In this review, we introduce selenium phytoremediation and describe the attempts made to enhance it through genetic engineering. Initial efforts have taken advantage of the knowledge of the enzymatic pathways for selenium assimilation and volatilization, especially by overexpressing genes of rate-limiting enzymes in plants. Another possible approach is to introduce additional metabolic pathways from selenium hyperaccumulators or organisms other than plants that can help detoxify selenium compounds. In this way the capacity of plants to take up, accumulate, and volatilize compounds can be increased beyond that of any naturally occurring plant species. Here we report on the progress that has been made in overexpressing potentially important enzymes involved in the selenium/sulfur pathways and discuss possible future directions in the enhancement of phytoremediation through genetic engineering.     

Phytoremediation in Wetland Ecosystems: Progress,Problems, and Potential

Assessing the phytoremediation potential of wetlands is complex due to variable conditions of hydrology, soil/sediment types, plant species diversity, growing season, and water chemistry. Conclusions about long-term phytoremediation potential are further complicated by the process of ecological succession in wetlands. This review of wetlands phytoremediation addresses the role of wetland plants in reducing contaminant loads in water and sediments, including metals; volatile organic compounds (VOC), pesticides, and other organohalogens; TNT and other explosives; and petroleum hydrocarbons and additives. The review focuses on natural wetland conditions and does not attempt to review constructed wetland technologies. Physico-chemical properties of wetlands provide many positive attributes for remediating contaminants. The expansive rhizosphere of wetland herbaceous shrub and tree species provides an enriched culture zone for microbes involved in degradation. Redox conditions in most wetland soil/sediment zones enhance degradation pathways requiring reducing conditions. However, heterogeneity complicates generalizations within and between systems. Wetland phytoremediation studies have mainly involved laboratory microcosm and mesocosm technologies, with the exception of planted poplar communities. Fewer large-scale field studies have addressed remediation actions by natural wetland communities. Laboratory findings are encouraging with regards to phytoextraction and degradation by rhizosphere and plant tissue enzymes. However, the next phase in advancing the acceptance of phytoremediation as a regulatory alternative must demonstrate sustained contaminant removal by intact natural wetland ecosystems.     

Phytoremediation of chlorpyrifos by Populus and Salix

Chlorpyrifos is one of the commonly used organophosphorus insecticides that are implicated in serious environmental and human health problems. To evaluate plant potential for uptake of chlorpyrifos, several plant species of poplar (Populus sp.) and willow (Salix sp.) were investigated. Chlorpyrifos was taken up from nutrient solution by all seven plant species. Significant amounts of chlorpyrifos accumulated in plant tissues, and roots accumulated higher concentrations of chlorpyrifos than did shoots. Chlorpyrifos did not persist in the plant tissues, suggesting further metabolism of chlorpyrifos in plant tissue. To our knowledge, this work represents the first report for phytoremediation of chlorpyrifos using poplar and willow plants.     

Phytoremediation of Phosphorus-Enriched Soils

After numerous years of application of phosphorus (P) fertilizer and/or manures, many soils accumulate high levels of P, increasing the risk of P transfer via surface runoff. While current and future regulations may limit P application to soil, little effort is given to the reduction of soils already enriched with P. To evaluate the potential of phytoremediation for decreasing available P in soil, the P uptake of several crops and vegetables grown on a high P soil was studied. Plant shoot and root P contents, plant-available soil P, and potential P removal under optimum conditions were examined. None of the plants were “true” P hyperaccumulators, although all removed some P from the soil. Whole corn plants (for silage) and Indian mustard removed the highest amount of P; 114 and 108 kg ha^-1, respectively. If only the grain from corn and Indian mustard were harvested, significantly lower amounts of P were removed from the field. These results show that there is extensive variation in the uptake of P by various crop species. For soils highly enriched with P, it may be desirable to grow a plant species capable of removing above average concentrations of P and to harvest the whole plant because a large fraction of the plant P is contained within the leaves and stems.     

Potential of Inga sp. (Inga uruguensis Hook. and Arn.) in the Phytoremediation of Oily Compounds

Plant survival under conditions of low oxygen availability is an important aspect of the phytoremediation of oily compounds, as one of the problems associated with environmental contamination by such compounds is anaerobic stress. Since Inga spp. presents adaptation mechanisms to conditions of partial and total submersion, it would be useful in the phytoremediation of petroleum wastes. The aim of this work was to verify the efficiency of a phytoremediation system using¹ Inga uruguensis Hook. and Arn. to degrade organic compounds such as semisolid petroleum waste (SSPW) and used cooking oil (UCO). A greenhouse study was conducted with plants grown in pots containing 3.1 kg substrate (soil + organic waste) that contained 10.15% SSPW or 5.56% UCO. Plants were grown for 84 days, and the system was evaluated by measuring changes in the oil and grease content (OGC), plant development, and abundance of microorganisms in substrate. Treatment of the SSPW with Inga sp. yielded a 40.7% reduction in OGC, whereas there was no significant change in the OGC in unplanted systems. Conversely, there was no significant plant effect on the OGC in the UCO-treated systems. Furthermore, we found evidence that the plant effect is associated with microbial community changes. Considering the high dose of SSPW and the high rate of degradation within an experiment of short duration, it was concluded that Inga uruguensis Hook. and Arn. holds potential for the phytoremediation of recalcitrant oily residues in soil.     

Utilization of Multi-Tasking Non-Edible Plants for Phytoremediation and Bioenergy Source-A Review

Heavy metal contamination of land and freshwater resources is a serious concern worldwide. It adversely affects the health of animals, plants and humans. Therefore, remediation of toxic heavy metals must be highly considered. Unlike other techniques, phytoremediation is a holistic technology and can be used in large scale for soil remediation as it is costless, novel, environmentally-safe and solar-driven technology. Utilization of non-edible plants in phytoremediation is an ingenious technique as they are used to generate new bioenergy resources along with the remediation of contaminated soils. Some nonfood bioenergy crops such as Salix species, Miscanthus species, Populus species, Eucalyptus species, and Ricinus communis exhibit high capability to accumulate various metals and to grow in contaminated lands. However, there are still sustainable challenges facing coupling phytoremediation with bioenergy production from polluted lands. Therefore, there has long been a need for developing different strategies to resolve such challenges. In this article review, we will discuss the phytoremediation mechanism, the technique of phytoremediation coupling with bioenergy production, sustainable problems facing linking phytoremediation with energy production as well as possible strategies to enhance the efficiency of bioenergy plants for soil decontamination by improving their characteristics such as metal uptake, transport, accumulation, and tolerance.