Contribution of the arbuscular mycorrhizal symbiosis to heavy metal phytoremediation

High concentrations of heavy metals (HM) in the soil have detrimental effects on ecosystems and are a risk to human health as they can enter the food chain via agricultural products or contaminated drinking water. Phytoremediation, a sustainable and inexpensive technology based on the removal of pollutants from the environment by plants, is becoming an increasingly important objective in plant research. However, as phytoremediation is a slow process, improvement of efficiency and thus increased stabilization or removal of HMs from soils is an important goal. Arbuscular mycorrhizal (AM) fungi provide an attractive system to advance plant-based environmental clean-up. During symbiotic interaction the hyphal network functionally extends the root system of their hosts. Thus, plants in symbiosis with AM fungi have the potential to take up HM from an enlarged soil volume. In this review, we summarize current knowledge about the contribution of the AM symbiosis to phytoremediation of heavy metals.     

Recent Findings on the Phytoremediation of Soils Contaminated with Environmentally Toxic Heavy Metals and Metalloids Such as Zinc,Cadmium, Lead,and Arsenic

Due to their immutable nature, metals are a group of pollutants of much concern. As a result of human activities such as mining and smelting of metalliferous ores, electroplating, gas exhaust, energy and fuel production, fertilizer and pesticide application, etc., metal pollution has become one of the most serious environmental problems today. Phytoremediation, an emerging cost-effective, non-intrusive, and aesthetically pleasing technology, that uses the remarkable ability of plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues, appears very promising for the removal of pollutants from the environment. Within this field of phytoremediation, the utilization of plants to transport and concentrate metals from the soil into the harvestable parts of roots and above-ground shoots, i.e., phytoextraction, may be, at present, approaching commercialization. Improvement of the capacity of plants to tolerate and accumulate metals by genetic engineering should open up new possibilities for phytoremediation. The lack of understanding pertaining to metal uptake and translocation mechanisms, enhancement amendments, and external effects of phytoremediation is hindering its full scale application. Due to its great potential as a viable alternative to traditional contaminated land remediation methods, phytoremediation is currently an exciting area of active research.     

Reclamation of tannery polluted soil through phytoremediation

The huge volume of sludge emanating from the tannery effluent treatment plants poses a serious environmental problem. Phytoremediation is an emerging technology in which the plants are employed to reclamate the contaminated soil strewn with heavy metals (metalloids) and toxic compounds. This work focuses the impact of application of tannery sludge on biochemical properties of 6 months old tree saplings of Azadirachta indica A. Juss. (Neem), Melia azedarach Linn. (Wild Neem) and Leucaena leucocephala (Lam) de Wit (Subabool) raised over the tannery sludge in an attempt to use these plants for phytoremediation. The plants raised over the garden soil served as the control. The porosity and water holding capacity of the tannery sludge were higher. The plant growth supporting elements such as Ca, total N₂, NO₃ and Mg were higher in the sludge. The plants raised over the sludge were found to be dark green with increased morphometric parameters. Electrophoretic profile revealed amplification of a few polypeptides (100, 105, 49 and 55 KDa). The levels of biomolecules and the CO₂ absorption increased in 6 months old plants. There was a significant uptake and transport of chromium in all the three tree species suggesting that these plants could be employed in phytoremediation of soils contaminated with heavy metals.Key words: Azadirachta indica (Neem), CO₂ uptake, Leucaena leucocephala (Subabool), Melia azedarach (Wild Neem), metalloids, Phytoremediation, Stress biomolecules, Tannery sludge     

Endophyte-assisted phytoremediation: mechanisms and current application strategies for soil mixed pollutants

Abstract

Phytoremediation uses plants and associated microbes to remove pollutants from the environment and is considered a promising bioremediation method. Compared with well-described single contaminant treatments, the number of studies reporting phytoremediation of soil mixed pollutants has increased recently. Endophytes, including bacteria and fungi, exhibit beneficial traits for the promotion of plant growth, stress alleviation, and biodegradation. Moreover, endophytes either directly or indirectly assist host plants to survive high concentrations of organic and inorganic pollutants in the soil. Endophytic microorganisms can also regulate the plant metabolism in different ways, exhibiting a variety of physiological characteristics. This review summarizes the taxa and physiological properties of endophytic microorganisms that may participate in the detoxification of contaminant mixtures. Furthermore, potential biomolecules that may enhance endophyte mediated phytoremediation are discussed. The practical applications of pollutant-degrading endophytes and current strategies for applying this valuable bio-resource to soil phytoremediation are summarized.     

Perspectives of bacterial ACC deaminase in phytoremediation

Phytoremediation of contaminated soil and water environments is regulated and coordinated by the plant root system, yet root growth is often inhibited by pollutant-induced stress. Prolific root growth could maximize rates of hyperaccumulation of inorganic contaminants or rhizodegradation of organic pollutants, and thus accelerate phytoremediation. Accelerated ethylene production in response to stress induced by contaminants is known to inhibit root growth and is considered as a major limitation in improving phytoremediation efficiency. Recent work shows that bacterial 1-aminocyclopropane-1-carboxylate (ACC) deaminase regulates ethylene levels in plants by metabolizing its precursor ACC into alpha-ketobutyric acid and ammonia. Plants inoculated with ACC deaminase bacteria or transgenic plants that express bacterial ACC deaminase genes can regulate their ethylene levels and consequently contribute to a more extensive root system. Such proliferation of roots in contaminated soil can lead to enhanced uptake of heavy metals or rhizodegradation of xenobiotics.     

LEGUME-GRASS INTERCROPPING PHYTOREMEDIATION OF PHTHALIC ACID ESTERS IN SOIL NEAR AN ELECTRONIC WASTE RECYCLING SITE: A FIELD STUDY

A field experiment was conducted to study the phytoremediation of phthalic acid esters (PAEs) by legume (alfalfa, Medicago sativa L.)-grass (perennial ryegrass, Lolium perenne L. and tall fescue, Festuca arundinacea) intercropping in contaminated agricultural soil at one of the largest e-waste recycling sites in China. Two compounds, DEHP and DnBP, were present in the soil and in the shoots of the test plants at much higher concentrations than the other target PAEs studied. Over 80% of ‘total’ (i.e., all six) PAEs were removed from the soil across all treatments by the end of the experiment. Alfalfa in monoculture removed over 90% of PAEs and alfalfa in the intercrop of the three plant species contained the highest shoot concentration of total PAEs of about 4.7 mg kg^?1 DW (dry weight). Calculation of phytoextraction efficiency indicated that the most effective plant combinations in eliminating soil PAEs were the three-species intercrop (1.78%) and the alfalfa monocrop (1.41%). Phytoremediation with alfalfa was effective in both monoculture and intercropping. High bioconcentration factors (BCFs) indicated the occurrence of significant extraction of PAEs by plants from soil, suggesting that phytoremediation may have potential for the removal of PAEs from contaminated soils.     

Phase I xenobiotic metabolic systems in plants

Phytoremediation uses living higher plants for the removal and biochemical decomposition of environmental pollutants. In this paper Phase I metabolic pathways in the biotransformation reactions of organic pollutants in plants are reviewed. These reactions result in the introduction of functional groups in the xenobiotic molecule or the exposure of preexisting functional groups and lead to the formation of more polar, more water-soluble, chemically more reactive and sometimes biologically more active derivatives. Phase I type reactions are most important in the phytoremediation of hydrophobic, chemically stable organic pollutants, such as polycyclic aromatic hydrocarbons and (poly)chlorinated aliphatic and aromatic hydrocarbons. Although Phase I reactions involve a wide range of chemical transformations from hydrolysis to reduction, oxidative processes catalyzed by cytochrome P450 containing monooxygenases are the most important. Transgenic plants with tailored Phase I enzymatic activities may play major roles in the removal of environmentally stable organic pollutants from contaminated fields.     

From phytoremediation of soil contaminants to phytomanagement of ecosystem services in metal contaminated sites

Since the emergence of phytoremediation, much research has focused on its development for (i) the removal of metals from soil and/or (ii) the reduction of metal bioavailability, mobility, and ecotoxicity in soil. Here, we review the lights and shades of the two main strategies (i.e., phytoextraction and phytostabilization) currently used for the phytoremediation of metal contaminated soils, irrespective of the level of such contamination. Both strategies face limitations to become successful at commercial scale and, then, often generate skepticism regarding their usefulness. Recent innovative approaches and paradigms are gradually establishing these phytoremediation strategies as suitable options for the management of metal contaminated soils. The combination of these phytotechnologies with a sustainable and profitable site use (a strategy called phytomanagement) grants value to the many benefits that can be obtained during the phytoremediation of metal contaminated sites, such as, for instance, the restoration of important ecosystem services, e.g. nutrient cycling, carbon storage, water flow regulation, erosion control, water purification, fertility maintenance, etc.     

A Review on Mechanisms of Plant Tolerance and Role of Transgenic Plants in Environmental Clean-up

This article deals with the advances and implication of phytoremediation technologies with emphasis on remediation of toxic heavy metals from contaminated soil. Most of the conventional remedial technologies are expensive and inhibit the soil fertility and cause negative impacts on various ecosystem services. However, phytoremediation is a cost effective and ecofriendly approach, which does not adversely affect soil properties and ecosystem services. In recent years, major progresses have been made in understanding the physiological mechanisms of metal uptake and transport in hyper accumulators. However, the molecular mechanisms of metal uptake, translocation, accumulation and detoxification in plants and their further implication in transgenic development for efficient phytoremediation are not well understood. In view of above, present review article brings together existing bits of information to create a new direction for future research, critical gap in knowledge and a new perspective on remediation of toxic pollutants.     

根系分泌物及其在植物修复中的作用

 近年来环境污染日益严重,污染物在土壤植物中的行为引起了人们的高度关注。利用植物去除土壤水体等介质中污染物的植物修复是近10年来兴起的一项安全、廉价的技术,已成为污染生态学和环境生态学的研究热点,它通过植物吸收、根滤、稳定、挥发等方式清除环境中的重金属和有机污染物。国内外有关植物修复的研究报道和概述很多, 但对植物根系分泌物在植物修复中所起的作用及其机理少有述评。 本文从根系分泌物对土壤重金属和土壤有机污染物的去除作用出发,对根系分泌物的种类、数量及其在去除环境污染物中的作用机理和功能地位进行了总结,并借助研究事例对影响植物根系分泌的内外因子,如植物种类、营养胁迫、重金属胁迫、根际环境的理化性质、土壤微生物及其它环境因子进行了讨论。概言之,根系分泌物在修复污染土壤中的重金属途径是多种多样的,主要是通过调节根际pH值、与重金属形成螯合物、络合反应、沉淀、提高土壤微生物数量和活性来改变重金属在根际中的存在形态以及提高重金属的生物有效性,从而减轻它对环境的危害。在清除有机污染物时,根系分泌物中的酶可以对有机污染物进行直接降解,根系分泌物影响下的微生物也可以对有机污染物进行间接降解,且被认为是主要的降解途径。根系分泌物在植物修复过程中确实起着某些重要作用,今后应将这方面的研究重点放在某些特异性根系分泌物植物,尤其是某些重金属超富集植物资源的寻找、筛选上,通过室内实验和野外研究确定其根系分泌物对清除重金属和有机污染物的效率,证实超富集植物根系分泌物的特异性与污染物超富集的内在联系,找到污染土壤生态恢复和治理的有效方法并加以推广应用,如针对性地在被污染地大面积种植此类具特异性根分泌物植物,并辅以营林措施如修剪等,加快生物修复进程,提高修复效率。植物根系分泌物在植物修复过程中所具有的重要生态意义和可能应用前景,为污染生态学和化学生态学之间的联合研究开拓了全新的领域,今后将取得新的突破和重要进展。     

Suitability of aromatic plants for phytoremediation of heavy metal contaminated areas: a review

Abstract

This review briefly elucidates the research undertaken and benefits of using aromatic plants for remediation of heavy metal polluted sites. A sustainable approach to mitigate heavy metal contamination of environment is need of the hour. Phytoremediation has emerged to be one of the most preferable choices for combating the metal pollution problem. Aromatic plants can be used for remediation of contaminated sites as they are non-food crops thus minimizing the risk of food chain contamination. Most promising aromatic plants for phytoremediation of heavy metal contaminated sites have been identified from families – Poaceae, Lamiaceae, Asteraceae, and Geraniaceae. They act as potential phytostabilisers, hyper accumulators, bio-monitors, and facultative metallophytes. Being high value economic crops, monetary benefits can be obtained by growing them in tainted areas instead of food crops. It has been observed that heavy metal stress enhances the essential oil percentage of certain aromatic crops. Research conducted on some major aromatic plants in this context has been highlighted in the present review which suggests that aromatic plants hold a great potential for phytoremediation. It has been reported that essential oil from aromatic crops is not contaminated by heavy metals significantly. Thus, aromatic plants are emerging as an ideal candidate for phytoremediation.

Highlights

? Aromatic plants hold a great potential for phytoremediation of heavy metal contaminated sites.

? Being high value economic crops, monetary benefits can be obtained by growing them in contaminated areas instead of food crops.

? Research done on some major aromatic plants in this context has been highlighted in the present review.     

Phytoremediation—A Novel and Promising Approach for Environmental Clean-up

ABSTRACT

Phytoremediation is an eco friendly approach for remediation of contaminated soil and water using plants. Phytoremediation is comprised of two components, one by the root colonizing microbes and the other by plants themselves, which degrade the toxic compounds to further non-toxic metabolites. Various compounds, viz. organic compounds, xenobiotics, pesticides and heavy metals, are among the contaminants that can be effectively remediated by plants. Plant cell cultures, hairy roots and algae have been studied for their ability to degrade a number of contaminants. They exhibit various enzymatic activities for degradation of xenobiotics, viz. dehalogenation, denitrification leading to breakdown of complex compounds to simple and non-toxic products. Plants and algae also have the ability to hyper accumulate various heavy metals by the action of phytochelatins and metallothioneins forming complexes with heavy metals and translocate them into vacuoles. Molecular cloning and expression of heavy metal accumulator genes and xenobiotic degrading enzyme coding genes resulted in enhanced remediation rates, which will be helpful in making the process for large-scale application to remediate vast areas of contaminated soils. A few companies worldwide are also working on this aspect of bioremediation, mainly by transgenic plants to replace expensive physical or chemical remediation techniques. Selection and testing multiple hyperaccumulator plants, protein engineering of phytochelatin and membrane transporter genes and their expression would enhance the rate of phytoremediation, making this process a successful one for bioremediation of environmental contamination. Recent years have seen major investments in the R&D, which have also resulted in competition of filing patents by several companies for economic gains. The details of science & technology related to phytoremediation have been discussed with a focus on future trends and prospects of global relevance.     

Phytoremediation of toxic elemental and organic pollutants

Phytoremediation is the use of plants to extract, sequester, and/or detoxify pollutants. Phytoremediation is widely viewed as the ecologically responsible alternative to the environmentally destructive physical remediation methods currently practiced. Plants have many endogenous genetic, biochemical, and physiological properties that make them ideal agents for soil and water remediation. Significant progress has been made in recent years in developing native or genetically modified plants for the remediation of environmental contaminants. Because elements are immutable, phytoremediation strategies for radionuclide and heavy metal pollutants focus on hyperaccumulation above-ground. In contrast, organic pollutants can potentially be completely mineralized by plants.     

Phytoremediation in the Tropics—The Effect of Crude Oil on the Growth of Tropical Plants

Phytoremediation is a nondestructive, cost-effective in-situ technology to clean up contaminated soils. In the case of contamination with petroleum hydrocarbons, plants enhance microbial degradation of the contaminant in the rhizosphere. The potential of this technology for the tropics should be high due to prevailing climatic conditions favoring plant growth and stimulating microbial activity. Investigations of the potential of tropical plants for phytoremediation, however, are scarce. The present work studied two grasses and six legumes from the eastern savannah of Venezuela on their reaction to crude oil contamination in soil. Results shall help to identify plants with a potential for phytoremediation and subsequent studies. Seedling emergence and biomass production were determined for plants growing in soil contaminated with 0%, 3%, and 5% heavy crude oil. Contamination had, in general, a tendential but not significant negative influence on seedling emergence. Dry matter production was reduced by only a few percent to up to 85%. Furthermore, in some legumes inhibition of nodulation was observed. The grass Brachiaria brizantha and the legumes Centrosema brasilianum and Calopogonium mucunoides are promising for phytoremediation because in contaminated soil they combined high seedling emergence with least affected biomass production. Since they are cultivated forage/soil cover species also in other regions of the tropics, their potential for phytoremediation of petroleum contaminated soils extends beyond Venezuela.     

Phragmites australis — a helophytic grass — can establish successful partnership with phenol-degrading bacteria in a floating treatment wetland

Helophytic plants contribute significantly in phytoremediation of a variety of pollutants due to their physiological or biochemical mechanisms. Phenol, which is reported to have negative/deleterious effects on plant metabolism at concentrations higher than 500 mg/L, remains hard to be removed from the environmental compartments using conventional phytoremediation procedures. The present study aims to investigate the feasibility of using P. australis (a helophytic grass) in combination with three bacterial strains namely Acinetobacter lwofii ACRH76, Bacillus cereus LORH97, and Pseudomonas sp. LCRH90, in a floating treatment wetland (FTW) for the removal of phenol from contaminated water. The strains were screened based on their phenol degrading and plant growth promoting activities. We found that inoculated bacteria were able to colonize in the roots and shoots of P. australis, suggesting their potential role in the successful removal of phenol from the contaminated water. Pseudomonas sp. LCRH90 dominated the bacterial community structure followed by A. lowfii ACRH76 and B. cereus LORH97. The removal rate was significantly high when compared with the individual partners, i.e., plants and bacteria separately. The plant biomass, which was drastically reduced in the presence of phenol, recovered significantly with the inoculation of bacterial consortia. Likewise, highest reduction in chemical oxygen demand (COD), biochemical oxygen demand (BOD), and total organic carbon (TOC) is achieved when both plants and bacteria were employed. The study, therefore, suggests that P. australis in combination with efficient bacteria can be a suitable choice to FTWs for phenol-degradation in water.     

Linkage between bacterial and fungal rhizosphere communities in hydrocarbon-contaminated soils is related to plant phylogeny

Phytoremediation is an attractive alternative to excavating and chemically treating contaminated soils. Certain plants can directly bioremediate by sequestering and/or transforming pollutants, but plants may also enhance bioremediation by promoting contaminant-degrading microorganisms in soils. In this study, we used high-throughput sequencing of bacterial 16S rRNA genes and the fungal internal transcribed spacer (ITS) region to compare the community composition of 66 soil samples from the rhizosphere of planted willows (Salix spp.) and six unplanted control samples at the site of a former petrochemical plant. The Bray–Curtis distance between bacterial communities across willow cultivars was significantly correlated with the distance between fungal communities in uncontaminated and moderately contaminated soils but not in highly contaminated (HC) soils (>2000 mg kg⁻¹ hydrocarbons). The mean dissimilarity between fungal, but not bacterial, communities from the rhizosphere of different cultivars increased substantially in the HC blocks. This divergence was partly related to high fungal sensitivity to hydrocarbon contaminants, as demonstrated by reduced Shannon diversity, but also to a stronger influence of willows on fungal communities. Abundance of the fungal class Pezizomycetes in HC soils was directly related to willow phylogeny, with Pezizomycetes dominating the rhizosphere of a monophyletic cluster of cultivars, while remaining in low relative abundance in other soils. This has implications for plant selection in phytoremediation, as fungal associations may affect the health of introduced plants and the success of co-inoculated microbial strains. An integrated understanding of the relationships between fungi, bacteria and plants will enable the design of treatments that specifically promote effective bioremediating communities.     

Individual and combined effects of cadmium and polycyclic aromatic hydrocarbons on the phytoremediation potential of Xanthium sibiricum in co-contaminated soil

Soil contamination with heavy metals and organic pollutants continues to cause major ecological damage and human health problems. Phytoremediation offers a highly promising technology for the recovery of sites contaminated with mixed pollutants. In this study, we performed a greenhouse experiment to investigate the individual and combined effects of cadmium (Cd) and polycyclic aromatic hydrocarbon (PAH) contamination on the growth of Xanthium sibiricum, and also the ability of this species to accumulate and remove Cd and to reduce PAHs over a period of 75 days. Our results demonstrated that individual or combined contamination by Cd and PAHs showed no significant differences to the control treatment except in the high Cd treatment. The reduction of PAH concentration in the soil with the passage of time was similar in the presence or absence of plants. At higher levels of Cd, the removal of pyrene decreased in both planted and non-planted soils; however, this effect might be due to the higher Cd content. Soil dehydrogenase and polyphenol oxidase activities showed that soil contamination did not have a significant effect on the removal of PAHs. Overall, our results suggest that X. sibiricum might be a suitable species for use in the phytoremediation of contaminated soils.     

Plant Hairy Roots for Remediation of Aqueous Pollutants

Significant progress has been made in recent years in enhancing the ability of plants to tolerate, remove, and degrade pollutants. Plant root remediation of contaminated soils and groundwater shows great potential for future development due to its environmental compatibility and cost-effectiveness. Hairy roots are disease manifestations developed by plants that are wounded and infected by Agrobacterium rhizogenes. The application of transgenic hairy roots in phytoremediation has been suggested mainly because of their biochemical resemblance to the roots of the plant from which they are derived. The application of genetic engineering has greatly augmented removal rates of hazardous pollutants. In addition, the rhizospheric bacteria that live on or around plant hairy roots also lead to improved tolerance to normally phytotoxic chemicals and increased removal of pollutants. This paper provides a broad overview of the evidence supporting the suitability and prospects of hairy roots in phytoremediation of organic pollutants and heavy metals.     

Uptake and Bioaccumulation of Pentachlorophenol by Emergent Wetland Plant Phragmites australis (Common Reed) in Cadmium Co-contaminated Soil

Despite many studies on phytoremediation of soils contaminated with either heavy metals or organics, little information is available on the effectiveness of phytoremediation of co-occurring metal and organic pollutants especially by using wetland species. Phragmites australis is a common wetland plant and its potential for phytoremediation of cadmium pentachlorophenol (Cd-PCP) co-contaminated soil was investigated. A greenhouse study was executed to elucidate the effects of Cd (0, 10, and 20 mg kg^?1) without or with PCP (0, 50, and 250 mg kg^?1) on the growth of the wetland plant P. australis and its uptake, accumulation and removal of pollutant from soils. After 75 days, plant biomass was significantly influenced by interaction of Cd and PCP and the effect of Cd on plant growth being stronger than that of PCP. Coexistence of PCP at low level lessened Cd toxicity to plants, resulting in improved plant growth and increased Cd accumulation in plant tissues. The dissipation of PCP in soils was significantly influenced by interactions of Cd, PCP and plant presence or absence. As an evaluation of soil biological activities after remediation soil enzyme was measured.     

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