Implications of metal accumulation mechanisms to phytoremediation

Elucidating the phytoremediation potentials and ecophysiological mechanisms of indicator plants in the industrial polluted region

The integrity of natural ecosystems, particularly in the Global South, is increasingly compromised by industrial contaminants. Our study examines the growth of plant species adapted to ecosystems impacted by heavy metal pollution, specifically focusing on their phytoremediation capabilities and tolerance to contaminants. The potential of pollution-tolerant species was evaluated in the industrial subtropical wetland of Sialkot, Pakistan. Employing quantitative ecological methods, data on vegetation, phytosociological attributes, and soil properties were gathered from 450 plots across different pollution gradients. The study pinpointed 17 key indicator species tolerating high heavy metal pollution out of 182 surveyed, using a combination of Indicator Species Analysis (ISA) and the Importance Value Index (IVI). These species demonstrated diverse capacities to extract, stabilize, and accumulate heavy metals (Cr, Zn, Cu, As, Cd, Ni, Hg, and Pb) across varying pollution zones. Notably, Cannabis sativa demonstrated substantial phytoextraction of Zn and Cd, with concentrations reaching 1977.25 μg/g and 1362.78 μg/g, respectively. Arundo donax showed marked hyperaccumulation of Cd, peaking at 410.531 μg/g. Achyranthes aspera was remarkable for its extraction and accumulation of Ni and Cu, with concentrations of 242.412 μg/g and 77.2997 μg/g, respectively. Physiological changes, such as increased proline levels in Cannabis sativa and Achyranthes aspera reaching 39.041 μg/g and 27.523 μg/g under high metal concentrations, indicated adaptation to metal stress. Declines in chlorophyll and carotenoid levels were also observed as metal contamination increased, with up to 35% reductions in some species. These findings underscore the potential efficacy of selected plant species in phytoremediation and highlight the importance of physiological responses in their tolerance to metals, providing valuable information for targeted remediation strategies in polluted ecosystems and improving environmental management and sustainable practices.

Physiology and molecular basis of thallium toxicity and accumulation in Arabidopsis thaliana

Thallium (Tl) is a non-essential metal mobilized through industrial processes which can lead to it entering the environment and exerting toxic effects. Plants are fundamental components of all ecosystems. Therefore, understanding the impact of Tl on plant growth and development is of great importance for assessing the potential environmental risks of Tl. Here, the responses of Arabidopsis thaliana to Tl were elucidated using physiological, genetic, and transcriptome analyses. Thallium can be absorbed by plant roots and translocated to the aerial parts, accumulating at comparable concentrations throughout plant parts. Genetic evidence supported the regulation of Tl uptake and movement by different molecular compartments within plants. Thallium primarily caused growth inhibition, oxidative stress, leaf chlorosis, and the impairment of K homeostasis. The disturbance of redox balance toward oxidative stress was supported by significant differences in the expression of genes involved in oxidative stress and antioxidant defense under Tl exposure. Reduced GSH levels in cad2-1 mutant rendered plants highly sensitive to Tl, suggesting that GSH has a prominent role in alleviating Tl-triggered oxidative responses. Thallium down-regulation of the expression of LCHII-related genes is believed to be responsible for leaf chlorosis. These findings illuminate some of the mechanisms underlying Tl toxicity at the physiological and molecular levels in plants with an eye toward the future environment management of this heavy metal.

Biodegradable chelating agents for enhancing phytoremediation: Mechanisms, market feasibility, and future studies

Heavy metals in soil significantly threaten human health, and their remediation is essential. Among the various techniques used, phytoremediation is one of the safest, most innovative, and effective. In recent years, the use of biodegradable chelators to assist plants in improving their remediation efficiency has gained popularity. These biodegradable chelators aid in the transformation of metal ions or metalloids, thereby facilitating their mobilization and uptake by plants. Developed countries are increasingly adopting biodegradable chelators for phytoremediation, with a growing emphasis on green manufacturing and technological innovation in the chelating agent market. Therefore, it is crucial to gain a comprehensive understanding of the mechanisms and market prospects of biodegradable chelators for phytoremediation. This review focuses on elucidating the uptake, translocation, and detoxification mechanisms of chelators in plants. In this study, we focused on the effects of biodegradable chelators on the growth and environmental development of plants treated with phytoremediation agents. Finally, the potential risks associated with biodegradable chelator-assisted phytoremediation are presented in terms of their availability and application prospects in the market. This study provides a valuable reference for future research in this field.

Unveiling Innovative Approaches to Mitigate Metals/Metalloids Toxicity for Sustainable Agriculture

Due to anthropogenic activities, environmental pollution of heavy metals/metalloids (HMs) has increased and received growing attention in recent decades. Plants growing in HM-contaminated soils have slower growth and development, resulting in lower agricultural yield. Exposure to HMs leads to the generation of free radicals (oxidative stress), which alters plant morpho-physiological and biochemical pathways at the cellular and tissue levels. Plants have evolved complex defense mechanisms to avoid or tolerate the toxic effects of HMs, including HMs absorption and accumulation in cell organelles, immobilization by forming complexes with organic chelates, extraction via numerous transporters, ion channels, signaling cascades, and transcription elements, among others. Nonetheless, these internal defensive mechanisms are insufficient to overcome HMs toxicity. Therefore, unveiling HMs adaptation and tolerance mechanisms is necessary for sustainable agriculture. Recent breakthroughs in cutting-edge approaches such as phytohormone and gasotransmitters application, nanotechnology, omics, and genetic engineering tools have identified molecular regulators linked to HMs tolerance, which may be applied to generate HMs-tolerant future plants. This review summarizes numerous systems that plants have adapted to resist HMs toxicity, such as physiological, biochemical, and molecular responses. Diverse adaptation strategies have also been comprehensively presented to advance plant resilience to HMs toxicity that could enable sustainable agricultural production.

Root chemistry and microbe interactions contribute to metal(loid) tolerance of an aromatic plant - Vetiver grass

Aromatic plants, such as vetiver grass (Chrysopogon zizanioides), possess strong abilities to resist environmental stresses. However, whether such abilities stem from the interaction between specific chemical characteristics and the associated microbes in roots and rhizosphere remains unclear. We conducted pot experiments to analyze stress-tolerant parameters, organic compounds, and bacterial communities in roots and rhizosphere of vetiver under typical metal(loid) stress [cadmium (Cd), arsenic (As), or Cd + As] over time. The results showed that the vetiver displayed limited toxic symptoms in terms of oxidative stress-antioxidant balance and chlorophyll content. The root low-molecular-weight organic acids (LMWOAs), fatty acids, and sterols were highly sensitive to growth stage (increased from the 4-month to the 8-month stage), and less sensitive to metal(loid) stress. The sugar contents in the rhizosphere soils also notably increased over time. Such endo and rhizosphere chemical changes strongly correlated with and enriched the functional bacteria including Streptomyces, which can resist stress and promote plant growth. The compound-bacteria interaction highly depended on growth stage. Vetiver demonstrated a progressive adaptation to stresses through metabolite modulation and cellular defense reinforcement. Our study evidenced that vetiver shapes the interaction between organic compounds and bacterial community in the root-soil interface and provides notable stress-resistant functions.

Biochar applications for treating potentially toxic elements (PTEs) contaminated soils and water: a review

Environmental pollution with potentially toxic elements (PTEs) has become one of the critical and pressing issues worldwide. Although these pollutants occur naturally in the environment, their concentrations are continuously increasing, probably as a consequence of anthropic activities. They are very toxic even at very low concentrations and hence cause undesirable ecological impacts. Thus, the cleanup of polluted soils and water has become an obligation to ensure the safe handling of the available natural resources. Several remediation technologies can be followed to attain successful remediation, i.e., chemical, physical, and biological procedures; yet many of these techniques are expensive and/or may have negative impacts on the surroundings. Recycling agricultural wastes still represents the most promising economical, safe, and successful approach to achieving a healthy and sustainable environment. Briefly, biochar acts as an efficient biosorbent for many PTEs in soils and waters. Furthermore, biochar can considerably reduce concentrations of herbicides in solutions. This review article explains the main reasons for the increasing levels of potentially toxic elements in the environment and their negative impacts on the ecosystem. Moreover, it briefly describes the advantages and disadvantages of using conventional methods for soil and water remediation then clarifies the reasons for using biochar in the clean-up practice of polluted soils and waters, either solely or in combination with other methods such as phytoremediation and soil washing technologies to attain more efficient remediation protocols for the removal of some PTEs, e.g., Cr and As from soils and water.

The combined rhizoremediation by a triad: plant-microorganism-functional materials

The article describes new strategies for the remediation of soils contaminated with organic and inorganic pollutants. The aim of this study is to investigate the synergistic effects of combining plant-microorganism-functional materials for a more effective reduction of soil contamination with toxic chemicals. The innovative triad involves functional materials as a habitat for microorganisms, which helps to control the release of pollutants into the soil solution from the adsorbed form. This, in turn, reduces the toxic effect on microorganisms and plants. Microorganisms play a complex role, consisting of partial biodegradation of pollutants, stimulation of plant growth, and support for nutrient supply. Plants synthesize root exudates that facilitate microorganisms in biodegrading organic pollutants and stimulate their growth. The plant takes up pollutants through the root system, which can be further supported by endophytic microorganisms. The cooperation of the three players produces a synergistic effect that enhances the effectiveness of rhizodegradation supported by functional materials, which is more effective than using microorganisms, phytoremediation, or functional materials alone. The combination of physicochemical methods (functional materials) and microbiological methods (bacteria and fungi, rhizosphere, symbiotic and non-symbiotic) supported by plants (hyperaccumulators) is a promising approach for reducing chemicals from soil. Key examples of the synergistic effects of combining plant-microorganism-functional materials have been provided in this article.

Phytoremediation as a potential technique for vehicle hazardous pollutants around highways

With the synchronous development of highway construction and the urban economy, automobiles have entered thousands of households as essential means of transportation. This paper reviews the latest research progress in using phytoremediation technology to remediate the environmental pollution caused by automobile exhaust in recent years, including the prospects for stereoscopic forestry. Currently, most automobiles on the global market are internal combustion vehicles using fossil energy sources as the primary fuel, such as gasoline, diesel, and liquid or compressed natural gas. The composition of vehicle exhaust is relatively complex. When it enters the atmosphere, it is prone to a series of chemical reactions to generate various secondary pollutants, which are very harmful to human beings, plants, animals, and the eco-environment. Despite improving the automobile fuel quality and installing exhaust gas purification devices, helping to reduce air pollution, the treatment costs of these approaches are expensive and cannot achieve zero emissions of automobile exhaust pollutants. The purification of vehicle exhaust by plants is a crucial way to remediate the environmental pollution caused by automobile exhaust and improve the environment along the highway by utilizing the ecosystem's self-regulating ability. Therefore, it has become a global trend to use phytoremediation technology to restore the automobile exhaust pollution. Now, there is no scientific report or systematic review about how plants absorb vehicle pollutants. The screening and configuration of suitable plant species is the most crucial aspect of successful phytoremediation. The mechanisms of plant adsorption, metabolism, and detoxification are reviewed in this paper to address the problem of automobile exhaust pollution.

Applying fulvic acid for sediment metals remediation: Mechanism, factors, and prospect

Fulvic acid (FA) has been shown to play a decisive role in controlling the environmental geochemical behavior of metals. As a green and natural microbial metabolite, FA is widely used in environmental remediation because of its good adsorption complexation and redox ability. This paper introduces the reaction mechanism and properties of FA with metals, and reviews the progress of research on the remediation of metal pollutant by FA through physicochemical remediation and bioremediation. FA can control the biotoxicity and migration ability of some metals, such as Pb, Cr, Hg, Cd, and As, through adsorption complexation and redox reactions. The concentration, molecular weight, and source are the main factors that determine the remediation ability of FA. In addition, the ambient pH, temperature, metal ion concentrations, and competing components in sediment environments have significant effects on the extent and rate of a reaction between metals and FA during the remediation process. Finally, we summarize the challenges that this promising environmental remediation tool may face. The research directions of FA in the field of metals ecological remediation are also prospected. This review can provide new ideas and directions for the research of remediation of metals contaminants in sediments.

Phytoremediation technologies and their mechanism for removal of heavy metal from contaminated soil: An approach for a sustainable environment

The contamination of soils with heavy metals and its associated hazardous effects are a thrust area of today's research. Rapid industrialization, emissions from automobiles, agricultural inputs, improper disposal of waste, etc., are the major causes of soil contamination with heavy metals. These contaminants not only contaminate soil but also groundwater, reducing agricultural land and hence food quality. These contaminants enter the food chain and have a severe effect on human health. It is important to remove these contaminants from the soil. Various economic and ecological strategies are required to restore the soils contaminated with heavy metals. Phytoremediation is an emerging technology that is non-invasive, cost-effective, and aesthetically pleasing. Many metal-binding proteins (MBPs) of the plants are significantly involved in the phytoremediation of heavy metals; the MBPs include metallothioneins; phytochelatins; metalloenzymes; metal-activated enzymes; and many metal storage proteins, carrier proteins, and channel proteins. Plants are genetically modified to enhance their phytoremediation capacity. In Arabidopsis, the expression of the mercuric ion-binding protein in Bacillus megaterium improves the metal accumulation capacity. The phytoremediation efficiency of plants is also enhanced when assisted with microorganisms, biochar, and/or chemicals. Removing heavy metals from agricultural land without challenging food security is almost impossible. As a result, crop selections with the ability to sequester heavy metals and provide food security are in high demand. This paper summarizes the role of plant proteins and plant-microbe interaction in remediating soils contaminated with heavy metals. Biotechnological approaches or genetic engineering can also be used to tackle the problem of heavy metal contamination.

Potential application of enhanced phytoremediation for heavy metals treatment in Nepal

Heavy metals contamination in soil and water resources is a great threat to developing countries because of the lack of waste treatment facilities. A majority of wastewater treatment methods are known to be expensive and out of reach for municipalities and small pollution treatment enterprises. Phytotechnology is a promising, sustainable, environment-friendly, and cost-effective technique for domestic and industrial wastewater treatment in places where land is available. However, interest in conventional remediation methods and the lack of information on recent advances in a significant portion of the society in developing countries have restrained the applications of phytoremediation. This review discusses the concept of phytoremediation, mechanisms of heavy metals removal by plants, and the potential application of enhanced phytoremediation technologies in developing countries like Nepal. The authors also review the commercially viable hyperaccumulator species with their native distribution, heavy metals intake capacity, and their availability in Nepal. Those native plants can be utilized locally or introduced strategically in other parts/countries as well. Thus, for a flora-rich country like Nepal, this study holds great potential and presents enhanced phytoremediation as an effective and sustainable strategy for the future.

Insights on the bioremediation technologies for pesticide-contaminated soils

The fast pace of increasing human population has led to enhanced crop production, due to which a significant increase in the application of pesticides has been recorded worldwide. Following the enhancement in the utilization of pesticides, the degree of environmental pollution, particularly soil pollution, has increased. To address this challenge, different methods of controlling and eliminating such contaminants have been proposed. Various methods have been reported to eradicate or reduce the degree of contamination of pesticides in the soil. Several factors are crucial for soil contamination, including pH, temperature, the number, and type/nature of soil microorganisms. Among the accessible techniques, some of them respond better to contamination removal. One of these methods is bioremediation, and it is one of the ideal solutions for pollution reduction. In this innovative technique, microorganisms are utilized to decompose environmental pollutants or to curb pollution. This paper gives detailed insight into various strategies used for the reduction and removal of soil pollution.

Bioremoval capacity of Co+2 using Phormidium tenue and Chlorella vulgaris as biosorbents

Microalgae sorbents are microalgae that have the potential to passively bind heavy metals/contaminants to their cellular structures in a process called biosorption. This study investigates the use of two species of microalgae to remove the toxic heavy metal cobalt from aqueous solution. Two microalgae isolates, Phormidium tenue and Chlorella vulgaris, were collected from the Wadi Hanifah Stream in Riyadh, the Kingdom of Saudi Arabia. We determined the capacity of both isolates to bioremove Co⁺² ions and the optimum conditions under which this occurs. The two isolates were additionally characterized by microscopic and Fourier transform infrared spectroscopy (FTIR). In the current investigation, Phormidium tenue removed 94% of Co⁺² under ideal conditions of pH 6, contact duration (30 min), starting concentration (50 mgL^-1) and biosorbent dose (1gL^-1); while Chlorella vulgaris removed 87% of Co⁺² under the same parameters except pH 5.5 and contact duration (60 min). Fourier transform infrared spectroscopy (FTIR) confirms the binding of Co⁺² to the biomass, which comprises many of the functional groups. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed some alterations to the shape of algal cells and cellular components for both microalgae studied. In addition, equilibrium study by both Langmuir and Freundlich models was performed to detect the effect of certain equilibrium factors on the capacity of the biosorption mechanism. Finally, Phormidium tenue and Chlorella vulgaris were discovered to be promising microalgae for effective cobalt biosorption in aquatic conditions.

Ceratophyllum demersum-An accretion biotool for heavy metal remediation

Freshwater habitats are under serious threat due to the diverse pressures of development and restoration of these ecosystems is an important challenge in the present era. With a number of scientifically advanced methods available for restoration of these systems, phytoremediation finds its unique space as an ecologically sustainable technique. In this paper, a case study of Certophyllum demersum as a tool of heavy metal remediation in Dal lake, a natural freshwater system in Kashmir, India is presented. At all concentrations (2 ppm, 4 ppm, 6 ppm) the metal accumulation efficiency of C. demersum is of the order of Co²⁺ > Cd²⁺ > Mn²⁺ followed by other metals. The carbohydrate- protein plot reveals positive correlation (0.696) with the heavy metal uptake while the lipid-protein plot overall shows a weak correlation (0.296) and the carbohydrate-lipid plot shows an insignificant correlation (0.019). The results of the present study revealed attenuation of protein levels at low doses which lowered with increased heavy metal concentrations. Further, the overall lipid and carbohydrate content of the cultured C. demersum displayed a general decline with a rise in the concentration of heavy metals The overall study indicates the efficiency of C. demersum to adapt in polluted conditions and its potential to remove heavy metals for sustainable restoration of the degraded aquatic systems.

CsiLAC4 modulates boron flow in Arabidopsis and Citrus via high-boron-dependent lignification of cell walls

The mechanisms underlying plant tolerance to boron (B) excess are far from fully understood. Here we characterized the role of the miR397-CsiLAC4/CsiLAC17 (from Citrus sinensis) module in regulation of B flow. Live-cell imaging techniques were used in localization studies. A tobacco transient expression system tested modulations of CsiLAC4 and CsiLAC17 by miR397. Transgenic Arabidopsis were generated to analyze the biological functions of CsiLAC4 and CsiLAC17. CsiLAC4's role in xylem lignification was determined by mRNA hybridization and cytochemistry. In situ B distribution was analyzed by laser ablation inductively coupled plasma mass spectrometry. CsiLAC4 and CsiLAC17 are predominantly localized in the apoplast of tobacco epidermal cells. Overexpression of CsiLAC4 in Arabidopsis improves the plants' tolerance to boric acid excess by triggering high-B-dependent lignification of the vascular system's cell wall and reducing free B content in roots and shoots. In Citrus, CsiLAC4 is expressed explicitly in the xylem parenchyma and is modulated by B-responsive miR397. Upregulation of CsiLAC4 in Citrus results in lignification of the xylem cell walls, restricting B flow from xylem vessels to the phloem. CsiLAC4 contributes to plant tolerance to boric acid excess via high-B-dependent lignification of cell walls, which set up a 'physical barrier' preventing B flow.

A critical review on various remediation approaches for heavy metal contaminants removal from contaminated soils

Heavy metal pollution remains a global environmental challenge that poses a significant threat to human life. Various methods have been explored to eliminate heavy metal pollutants from the environment. However, most methods are constrained by high expenses, processing duration, geological problems, and political issues. The immobilization of metals, phytoextraction, and biological methods have proven practical in treating metal contaminants from the soil. This review focuses on the general status of heavy metal contamination of soils, including the excessive heavy metal concentrations in crops. The assessment of the recent advanced technologies and future challenges were reviewed. Molecular and genetic mechanisms that allow microbes and plants to collect and tolerate heavy metals were elaborated. Tremendous efforts to remediate contaminated soils have generated several challenges, including the need for remediation methodologies, degrees of soil contamination, site conditions, widespread adoptions and various possibilities occurring at different stages of remediation are discussed in detail.

Effect of the natural establishment of two plant species on microbial activity, on the composition of the fungal community, and on the mitigation of potentially toxic elements in an abandoned mine tailing

In Mexico, millions of tons of mining wastes are deposited in the open pit. Their content in potentially toxic elements (PTE) represents an environmental risk. In the tailings, pioneer plant communities are established, associated with a determined diversity of fungi; plants, and fungi are fundamental in the natural rehabilitation of mining wastes. The objective was to evaluate the impact of the natural establishment of two plant species on the microbial activity, on the composition of the fungal community, and on the mitigation of the effect of PTE in a contaminated mine tailing. In a tailing, we selected three sites: one non-vegetated; one vegetated by Reseda luteola, and one vegetated by Asphodelus fistulosus. In the substrates, we conducted a physical and chemical characterization; we evaluated the enzymatic activity, the mineralization of the carbon, and the concentration of PTE. We also determined the fungal diversity in the substrates and in the interior of the roots, and estimated the accumulation of carbon, nitrogen, phosphorus and PTE in plant tissues. The tailings had a high percentage of sand; the non-vegetated site presented the highest electric conductivity, and the plant cover reduced the concentration of PTE in the substrates. Plants increased the carbon content in tailings. The enzymatic activities of β-glucosidase and dehydrogenase, and the mineralization of carbon were highest at the site vegetated with A. fistulosus. Both plant species accumulated PTE in their tissues and exhibited potential in the phytoremediation of lead (Pb), cadmium (Cd), and copper (Cu). Fungal diversity was more elevated at the vegetated sites than in the bare substrate. Ascomycota prevailed in the substrates; the substrates and the plants shared some fungal taxa, but other taxa were specific. The plant coverage and the rhizosphere promoted the natural attenuation and a rehabilitation of the extreme conditions of the mining wastes, modulated by the plant species.

Response of Prosopis farcta to gradually increased soil copper and cadmium levels based on an integrated investigation

The impact of gradually increased soil levels of copper (Cu) and cadmium (Cd) on the medicinal plant, Prosopis farcta, irrigated with metal-enriched water was determined. Plants were treated with 2.54, 5.08, 10.16, and 20.32 µg mL^-1 for Cu²⁺ and 6.13, 12.26, 24.52, and 49 µg mL^-1 for Cd²⁺. The rate of phytoremediation was measured by bioconcentration factor (BCF) and the relative bioconcentration factor (RBCF). The movement of metal ions from roots to shoots was calculated as the Translocation Factor (TF). The exposure of plants to Cd or Cu decreased plant growth and increased Cd and Cu concentration in their shoots and roots. The weight of both shoots and roots decreased linearly with the increase of Cu and Cd contents in roots and shoots. Cd was more toxic than Cu as expected. The water content of shoots and roots decreased linearly as heavy metal levels increased. Prosopis farcta can take up Cu and Cd in both Cu- and Cd-contaminated soils but was more capable for transporting Cd from roots to shoots rather than Cu although more Cu is taken up by roots. Prosopis farcta is a natural accumulator of Cu and Cd and can be used in phytoremediation.CONCISE NOVEL ASPECTS OF THIS STUDYThis is the first report to show that the medicinal plant Prosopis farcta is an accumulator for Cu and Cd.This was determined by gradual addition of the metals to the soil via irrigation by heavy metal-polluted water which can provide an opportunity for the plant to develop a metal-resistance mechanism.Choosing suitable plant species for heavy metal accumulation is a critical step for successful phytoremediation of heavy metal pollutants.CORE IDEASProsopis farcta is of interest as a medicinal plant.P. farcta can take up Cu and Cd in both Cu- and Cd-contaminated soils.P. farcta transports more Cd from roots to shoots but more Cu is taken up.

Advances in Heavy Metal Bioremediation: An Overview

The pollution of toxic heavy metals is considered one of the most important environmental issues which has accelerated dramatically due to changing industrial activities. This review focuses on the most common methods, strategies, and biological approaches of heavy metal bioremediation. Also, it provides a general overview of the role of microorganisms in the bioremediation of heavy metals in polluted environments. Advanced methods of heavy metal remediation include physicochemical and biological methods; the latter can be further classified into in situ and ex situ bioremediation. The in situ process includes bioventing, biosparging, biostimulation, bioaugmentation, and phytoremediation. Ex situ bioremediation includes land farming, composting, biopiles, and bioreactors. Bioremediation uses naturally occurring microorganisms such as Pseudomonas, Sphingomonas, Rhodococcus, Alcaligenes, and Mycobacterium. Generally, bioremediation is of very less effort, less labor intensive, cheap, ecofriendly, sustainable, and relatively easy to implement. Most of the disadvantages of bioremediation relate to the slowness and time-consumption; furthermore, the products of biodegradation sometimes become more toxic than the original compound. The performance evaluation of bioremediation might be difficult as it has no acceptable endpoint. There is a need for further studies to develop bioremediation technologies in order to find more biological solutions for bioremediation of heavy metal contamination from different environmental systems.

Influence of sulfur amendments on heavy metals phytoextraction from agricultural contaminated soils: A meta-analysis

Heavy metal pollution is becoming recurrent and threatens biota biosafety in many agricultural fields. Diverse solutions explore the application of amendments to enable remediation. Sulfur represents a nonmetallic chemical element that actively affects heavy metals phytoextraction, and promotes and alternatively mitigates soil functions. In this study, we conduct a meta-analysis to synthesize the current knowledge on the influence of sulfur amendments on plants heavy metals uptake from contaminated soil media. Random-effects model was used to summarize effect sizes from 524 data points extracted from 30 peer reviewed studies. The phytoextraction of cadmium, chromium and nickel were 1.6-, 3.3-, and 12.6-fold, respectively, higher when sulfur amendment was applied; while copper uptake was 0.3-fold lower. Irrespective of the sulfur type, heavy metal extraction increased with the raising sulfur stress. Individual organs showed significant differences of heavy metal uptake between sulfur applied and non-sulfur treatments, and combined organs did not. The heavy metals uptake in leaves and roots were higher in sulfur applied than non-sulfur applied treatments, while those in grain, husk, and stalks were lower. The heavy metals phytoextraction (response ratio) followed the order roots > leaves > stalk > grain > husk. Moreover, heavy metals uptake was 2-fold higher in the sulfur applied than the non-sulfur treatments under ideal (5.5-8) and alkaline conditions (8-14), and 0.2-fold lower under acidic pH (1-5.5). Cadmium, manganese and nickel, and chromium were the most extracted under sulfur application by Vicia sp., Sorghum sp. and Brassica sp., respectively; while chromium, manganese, and iron were the most uptake without sulfur amendments by Oryza sp., Zea sp. and Sorghum sp., respectively. Our study highlights that the influence of sulfur on heavy metal phytoextraction depends on the single or combined effects of sulfur stress intensity, sulfur compounds, plant organ, plant type, and soil pH condition.

Heavy metals immobilization and improvement in maize (Zea mays L.) growth amended with biochar and compost

Soil with heavy metals contamination, mainly lead (Pb), cadmium (Cd), and chromium (Cr) is a progressively worldwide alarming environmental problem. Recently, biochar has been used as a soil amendment to remediate contaminated soils, but little work has been done to compare with other organic amendments like compost. We investigated biochar and compost's comparative effect on Pb, Cd, and Cr immobilization in soil, photosynthesis, and growth of maize plants. Ten kg soil was placed in pots and were spiked with Pb, Cd, and Cr at concentrations 20, 10, 20 mg kg^-1. The biochar and compost treatments included 0, 0.5, 1, 2, and 4% were separately applied to the soil. The crop from pots was harvested after 60 days. The results show that the highest reduction of AB-DTPA extractable Pb, Cd, and Cr in soil was 79%, 61% and 78% with 4% biochar, followed by 61%, 43% and 60% with 4% compost compared to the control, respectively. Similarly, the highest reduction in shoot Pb, Cd, and Cr concentration was 71%, 63% and 78%with 4% biochar, followed by 50%, 50% and 71% with 4% compost than the control, respectively. The maximum increase in shoot and dry root weight, total chlorophyll contents, and gas exchange characteristics were recorded with 4% biochar, followed by 4% compost than the control. The maximum increase in soil organic matter and total nitrogen (N) was recorded at 4% biochar application while available phosphorus and potassium in the soil at 4% compost application. It is concluded that both biochar and compost decreased heavy metals availability in the soil, reducing toxicity in the plant. However, biochar was most effective in reducing heavy metals content in soil and plant compared to compost. In the future, more low-cost, eco-friendly soil remediation methods should be developed for better soil health and plant productivity.

Study of Potentially Toxic Elements Uptake into Organs of Quercus spp. from Copper Deposits in Slovakia, Italy and Portugal

The article is focused on the application of Energy dispersive micro X-ray fluorescence spectroscopy as a specific method to determine the contents of potentially toxic elements and its spread in plant tissues. As a model species, Quercus spp. were selected. In order to compare the obtained results with previous research, four well-described abandoned Cu-deposits were selected for sampling: Ľubietová (Slovakia), Libiola and Caporciano (Italy), and São Domingos (Portugal). The results of micro X-ray fluorescence spectrometry confirm the irregular contamination of Quercus spp. by potentially toxic elements. The level of contamination is the highest predominantly in the root cortex, where is also the highest Ca contents (with exception of São Domingos). At Ľubietová and Caporciano, high Ni content was described in branches cortex, in branches mesoderm also Fe, Cu and Zn. At the same time, the inhibition influence of Ca was also confirmed regarding the input of these elements into plants.

Rhizosphere Microbial Communities and Heavy Metals

The rhizosphere is a microhabitat where there is an intense chemical dialogue between plants and microorganisms. The two coexist and develop synergistic actions, which can promote plants’ functions and productivity, but also their capacity to respond to stress conditions, including heavy metal (HM) contamination. If HMs are present in soils used for agriculture, there is a risk of metal uptake by edible plants with subsequent bioaccumulation in humans and animals and detrimental consequences for their health. Plant productivity can also be negatively affected. Many bacteria have defensive mechanisms for resisting heavy metals and, through various complex processes, can improve plant response to HM stress. Bacteria-plant synergic interactions in the rhizosphere, as a homeostatic ecosystem response to HM disturbance, are common in soil. However, this is hard to achieve in agroecosystems managed with traditional practices, because concentrating on maximizing crop yield does not make it possible to establish rhizosphere interactions. Improving knowledge of the complex interactions mediated by plant exudates and secondary metabolites can lead to nature-based solutions for plant health in HM contaminated soils. This paper reports the main ecotoxicological effects of HMs and the various compounds (including several secondary metabolites) produced by plant-microorganism holobionts for removing, immobilizing and containing toxic elements.

Humic + Fulvic acid mitigated Cd adverse effects on plant growth, physiology and biochemical properties of garden cress

Cadmium (Cd) is a toxic and very mobile heavy metal that can be adsorbed and uptaken by plants in large quantities without any visible sign. Therefore, stabilization of Cd before uptake is crucial to the conservation of biodiversity and food safety. Owing to the high number of carboxyl and phenolic hydroxyl groups in their structure, humic substances form strong bonds with heavy metals which makes them perfect stabilizing agents. The aim of this study was to determine the effects of humic and fulvic acid (HA + FA) levels (0, 3500, 5250, and 7000 mg/L) on alleviation of Cadmium (Cd) toxicity in garden cress (Lepidium sativum) contaminated with Cd (CdSO4.8H2O) (0, 100, and 200 Cd mg/kg) under greenhouse conditions. Our results showed that, Cd stress had a negative effect on the growth of garden cress, decreased leaf fresh, leaf dry, root fresh and root dry weights, leaf relative water content (LRWC), and mineral content except for Cd, and increased the membrane permeability (MP) and enzyme (CAT, SOD and POD) activity. However, the HA + FA applications decreased the adverse effects of the Cd pollution. At 200 mg/kg Cd pollution, HA + FA application at a concentration of 7000 mg/L increased the leaf fresh, leaf dry, root fresh, root dry weights, stem diameter, leaf area, chlorophyll reading value (CRV), MP, and LRWC values by 262%, 137%, 550%,133%, 92%, 104%, 34%, 537%, and 32% respectively, compared to the control. Although the highest H2O2, MDA, proline and sucrose values were obtained at 200 mg/L Cd pollution, HA + FA application at a concentration of 7000 mg/L successfully alleviated the deleterious effects of Cd stress by decreasing H2O2, MDA, proline, and sucrose values by 66%, 68%, 70%, and 56%, respectively at 200 mg/kg Cd pollution level. HA + FA application at a concentration of 7000 mg/L successfully mitigated the negative impacts of Cd pollution by enhanced N, P, K, Ca, Mg, Fe, Mn, Cu, Mn, Zn, and B by 75%, 23%, 84%, 87%, 40%, 85%, 143%, 1%, 65%, and 115%, respectively. In addition, HA + FA application at a concentration of 7000 mg/L successfully reduced Cd uptake by 95% and Cl uptake by 80%. Considering the plant growth parameters, the best results were determined when HA + FA concentration was 7000 mg/L. We have shown that, it is critical to apply a humic substance with high percentage of FA, which was 10% in this study, to mitigate the adverse effects of heavy metal stress on plant growth. In conclusion, the application of HA + FA may be suggested as an effective solution for reducing the Cd uptake of the plants by stabilizing Cd in soil and preventing translocation of Cd from the roots of plant to its shoot and leaves.

Integrative transcriptomic and metabolomic analyses provide insight into the long-term submergence response mechanisms of young Salix variegata stems

The mechanisms underlying long-term complete submergence tolerance in S. variegata involve enhanced oxidative stress responses, strengthened ethylene and ABA signaling, synthesis of raffinose family oligosaccharides, unsaturated fatty acids, and specific stress-related amino acids. Salix variegata Franch. is a riparian shrub species that can tolerate long-term complete submergence; however, the molecular mechanisms underlying this trait remain to be elucidated. In this study, we subjected S. variegata plants to complete submergence for 60 d and collected stems to perform transcriptomic and metabolomic analyses, as well as quantitative reverse transcription-polymerase chain reaction (qRT-PCR) assays. Results revealed that photosynthesis and the response to light stimulus were inhibited during submergence and recovered after desubmergence. Ethylene and abscisic acid (ABA) signaling could be important for the long-term submergence tolerance of S. variegata. Jasmonic acid (JA) signaling also participated in the response to submergence. Raffinose family oligosaccharides, highly unsaturated fatty acids, and specific stress-related amino acids accumulated in response to submergence, indicating that they may protect plants from submergence damage, as they do in response to other abiotic stressors. Several organic acids were produced in S. variegata plants after submergence, which may facilitate coping with the toxicity induced by submergence. After long-term submergence, cell wall reorganization and phenylpropanoid metabolic processes (the synthesis of specific phenolics and flavonoids) were activated, which may contribute to long-term S. variegata submergence tolerance; however, the detailed mechanisms require further investigation. Several transcription factors (TFs), such as MYB, continuously responded to submergence, indicating that they may play important roles in the responses and adaption to submergence. Genes related to oxidative stress tolerance were specifically expressed after desubmergence, potentially contributing to recovery of S. variegata plants within a short period of time.

Phytoremediation of potentially toxic elements in a polluted industrial soil using Poinsettia

Potentially toxic elements (PTEs) pollution has become a serious environmental threat, particularly in developing countries such as China. In response, there is a growing interest in phytoremediation studies to identify plant species as designated hyperaccumulators of PTEs in polluted soils. Poinsettia was selected as a candidate species for phytoremediation of six PTEs (Zn, Pb, Hg, Cr, As, Cu) in this study. A pot cultivation experiment (randomized incomplete block experimental design with 5 treatments and 4 blocks) was conducted using contaminated soils gathered from an industrial area in southcentral China. The bioaccumulation factor (BAF), translocation factor (TF), and bioconcentration factor were analyzed to determine the phytoremediation potential of poinsettia potted in different ratios of polluted soils. One-way ANOVA with post-hoc Tukey's test showed that poinsettia had significant uptake of Zn, Pb, Cu (BAF < 1 and TF < 1, p < 0.05) and Hg (BAF < 1 and TF > 1, p < 0.05). Poinsettias can therefore effectively accumulate Zn, Pb, and Cu in their lateral roots while extracting and transferring Hg into their leaves. Moreover, poinsettia exhibited tolerance towards As and Cr. Interestingly, it was also observed that PTEs can inhibit the height of potted poinsettia at a certain concentration.

Phytoremediation of Heavy Metals in Tropical Soils an Overview

The geomorphological characteristics of the materials inherent in tropical soils, in addition to the excessive use of fertilizers and pesticides, industrial waste and residues, and novel pollutants derived from emerging new technologies such as nanomaterials, affect the functionality and resilience of the soil-microorganism-plant ecosystem; impacting phytoremediation processes and increasing the risk of heavy metal transfer into the food chain. The aim of this review is to provide a general overview of phytoremediation in tropical soils, placing special emphasis on the factors that affect this process, such as nanoagrochemicals, and highlighting the value of biodiversity among plant species that have the potential to grow and develop in soils impacted by heavy metals, as a useful resource upon which to base further research.

Heavy-Metal Phytoremediation from Livestock Wastewater and Exploitation of Exhausted Biomass

Sustainable agriculture is aimed at long-term crop and livestock production with a minimal impact on the environment. However, agricultural practices from animal production can contribute to global pollution due to heavy metals from the feed additives that are used to ensure the nutritional requirements and also promote animal health and optimize production. The bioavailability of essential mineral sources is limited; thus, the metals are widely found in the manure. Via the manure, metallic ions can contaminate livestock wastewater, drastically reducing its potential recycling for irrigation. Phytoremediation, which is an efficient and cost-effective cleanup technique, could be implemented to reduce the wastewater pollution from livestock production, in order to maintain the water conservation. Plants use various strategies for the absorption and translocation of heavy metals, and they have been widely used to remediate livestock wastewater. In addition, the pollutants concentrated in the plants can be exhausted and used as heat to enhance plant growth and further concentrate the metals, making recycling a possible option. The biomass of the plants can also be used for biogas production in anaerobic fermentation. Combining phytoremediation and biorefinery processes would add value to both approaches and facilitate metal recovery. This review focuses on the concept of agro-ecology, specifically the excessive use of heavy metals in animal production, the various techniques and adaptations of the heavy-metal phytoremediation from livestock wastewater, and further applications of exhausted phytoremediated biomass.

Performance and efficiency services for the removal of hexavalent chromium from water by common macrophytes

Water contamination by hexavalent chromium(Cr) is an emerging issue. The removal of Cr(VI) using phytoremediation via different macrophytes was investigated in this study. To reduce Cr(VI) to the permissible level in irrigation water, the ability of four common macrophytes, viz. Pistia stratiotes (PS), Salvinia minima (SM), Ipomoea aquatica (IA) and Eichhornia crassipes (EC), to remove from 0.5 to 2.0 mg Cr(VI)/L was analyzed. The overall growth of PS was enhanced by 11 to 24%, SM by 36 to 53%, EC by 65 to 101% and IA by 4 to 13% by reducing Cr from 48 to 87% within 29 days of the experiment. In successive experiments, chromium uptake by SM surpassed ∼11.86-, ∼17.17- and ∼94-fold that of PS, EC and IA, respectively, after 15 days of growth in 0.35 to 1.75 mg Cr(VI)/L. The bioconcentration factor of SM surpassed that of PS, IA and EC by 0.64 to 1.73, 1.09 to 4.07 and 0.71 to 1.85 times, while PS exceeded IA and EC by1.71 to 2.35 and 1.07 to 1.11 times, respectively. SM was thus shown to offer efficient removal of Cr(VI), from a level ≅2.0 mg/L, while a suitable combination of SM and PS was efficient at ≤1.0 mg/L. Novelty It unravels the appropriate macrophytes in terms of biomass production and Cr- uptake pattern under natural condition for phytoremediation of aqueous Cr(VI) and in turn offer services to clean the environment.

Exogenous nitrogen enhances poplar resistance to leaf herbivory and pathogen infection after exposure to soil cadmium stress

Elemental defense hypothesis suggests that toxic metals accumulated in plant tissues could enhance plant defense against herbivores and pathogens. Since over-accumulation of metals in plant organs will pose negative effects on plant health, it is necessary to find a way to alleviate metal-induced toxicity in plants while keeping or even improving plant resistance. Exogenous nitrogen (N) application was reported to have such alleviation effect while stimulating metal accumulation in plant tissues. In this study, we examined whether soil N addition in three different doses to a poplar species under cadmium (Cd) stress can simultaneously improve plant growth and resistance to four herbivorous insects and a leaf pathogen. The results showed that N application to Cd-amended soil prominently enhanced plant growth and leaf Cd accumulation. While N addition in three doses all remarkably reduced herbivore growth than control plants, only the highest N dose exerted stronger inhibition than the sole Cd-treated plants. In the paired-choice experiment, plants supplied with the highest N dose showed an enhanced deterrent effect on herbivore preference than plants exposed to sole Cd. Furthermore, plant resistance to the leaf pathogen infection was strongly enhanced as the levels of N addition increased. Leaf sugar and three main defensive chemicals were not affected by N application implied that such enhanced effect of N on plant resistance was due to increased leaf Cd accumulation. Our results suggested that the application of exogenous N over a certain amount could enhance the resistance of Cd-treated plants to leaf herbivory and pathogen infection.

Comparative transcriptomic analysis reveals key genes and pathways in two different cadmium tolerance kenaf (Hibiscus cannabinus L.) cultivars

Soil cadmium (Cd) contamination has become a massive environmental problem. Kenaf is an industrial fiber crop with high tolerance to heavy metals and could be potentially used for soil phytoremediation. However, the molecular mechanism of Cd in kenaf tolerance remains largely unknown. In the present study, using two contrasting Cd sensitive kenaf (GH and YJ), the key factors accounting for differential Cd tolerance were investigated. GH has a stronger Cd transport and accumulation ability than YJ. In addition, physiological index investigation on malondialdehyde (MDA) contents and antioxidant enzyme (SOD, POD, and CAT) activities showed GH has a stronger detoxification capacity than YJ. Furthermore, the cell ultrastructure of GH is more stable than that of YJ under Cd stress. Transcriptome analysis revealed 2221 (689 up and 1532 down) and 3321 (2451 up and 870 down) genes were differentially expressed in GH and YJ, respectively. More DEGs (differentially expressed genes) were characterized as up-regulated in GH, indicating GH is inclined to activate gene expression to cope with cadmium stress. GO and KEGG analyses indicate that DEGs were assigned and enriched in different pathways. Plenty of critical Cd-induced DEGs such as SOD2, PODs, MT1, DTXs, NRT1, ABCs, CES, AP2/ERF, MYBs, NACs, and WRKYs were identified. The DEGs involved pathways, including antioxidant, heavy metal transport or detoxification, substance transport, plant hormone and calcium signals, ultrastructural component, and a wide range of transcription factors were suggested to play crucial roles in kenaf Cd tolerance, and accounting for the difference in Cd stress sensitivities.

Aluminium accumulation in excess and related anti-oxidation responses in C4 weed (Amaranthus viridis L.)

C₄ species, Amaranthus viridis L. exhibited a significant bioaccumulation of aluminium (Al) through the duration of 3- and 5-days exposure. As compared to control, Amaranthus appeared as excess-accumulator with maximum 5.85-fold bioaccumulation of Al in root. Cellular responses to Al tolerance initially scored tissue specific distribution of metal through cortical layers revealed by electron microscopy. The affected cells changed an oxidative status as read by histochemical stains, particularly, for hydrogen peroxide. Osmotic stress and its stability were scored by maximum proline and free amino acids accumulation with 1.53 and 1.59-fold increase over control. The accumulation of phenolics and flavonoids were over expressed in the ranges of 2.48-2.50-fold and 2.00-1.5-fold at 3- and 5-days respectively against control. Anti-oxidation to detoxify Al stress was facilitated by variants of peroxidases. For exclusion mechanism of metal, esterase activity significantly over expressed with maximum value of 1.80-fold at 5-days. The polymorphism of esterase exhibited few significant over produced bands, varied in numbers as detected by densitometric scanning. Moreover, plant extract was satisfactorily potential under in vitro anti-oxidation systems through assay of 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,2'-azino-bis-3-ethylbenzthiazoline-6-sulphonic acid (ABTS), ferric chelation activity etc. Therefore, weeds like Amaranthus would be a bioprospecting in role likely involved in phytoremediation of metal.

Implication of phytometabolites on metal tolerance of the pseudo-metallophyte -Rosmarinus officinalis- in a Mediterranean brownfield

This study highlights the trace metal and metalloid (TMM) accumulation in Rosmarinus officinalis L. and its chemical responses when exposed to high levels of contamination. R. officinalis individuals growing along a gradient of mixed TMM soil pollution, resulting from past industrial activities, were analysed. Several plant secondary metabolites, known to be involved in plant tolerance to TMM or as a plant health indicator, were investigated. The levels of thiol compounds and phytochelatin precursors (cysteine and glutathione) in the shoots were measured in the laboratory, while a portable non-destructive instrument was used to determine the level of phenolic compounds and chlorophylls directly on site. The level of Pb, As, Sb and Zn contaminations within the soil and plants was also determined. The results highlighted a decrease of TMM translocation with increases of soil contamination. The concentration of TMM in the shoots followed the Mitscherlich equation and reached a plateau at 0.41, 7.9, 0.37, 51.3 mg kg^-1 for As, Pb, Sb and Zn, respectively. In the shoots, the levels of thiols and phenols were correlated to concentrations of TMM. Glutathione seems to be the main thiol compounds involved in the tolerance to As, Pb and Sb. Phenols indices, using non-destructive measurements, may be considered as an easy way to establish a proxy to estimate the TMM contamination level of the R. officinalis shoots. The study highlights metabolic processes that contribute to the high potential of R. officinalis for phytostabilisation of TMM in contaminated areas in the Mediterranean.

Cell Wall Polysaccharide-Mediated Cadmium Tolerance Between Two Arabidopsis thaliana Ecotypes

Cadmium (Cd) is a toxic metal element and the mechanism(s) underlying Cd tolerance in plants are still unclear. Increasingly more studies have been conducted on Cd binding to plant cell walls (CW) but most of them have focused on Cd fixation by CW pectin, and few studies have examined Cd binding to cellulose and hemicellulose. Here we found that Cd binding to CW pectin, cellulose, and hemicellulose was significantly higher in Tor-1, a Cd tolerant A. thaliana ecotype, than in Ph2-23, a sensitive ecotype, as were the concentrations of pectin, cellulose, and hemicellulose. Transcriptome analysis revealed that the genes regulating CW pectin, cellulose, and hemicellulose polysaccharide concentrations in Tor-1 differed significantly from those in Ph2-23. The expressions of most genes such as pectin methyl esterase inhibitors (PMEIs), pectin lyases, xyloglucan endotransglucosylase/hydrolase, expansins (EXPAs), and cellulose hydrolase were higher in Ph2-23, while the expressions of cellulose synthase-like glycosyltransferase 3 (CSLG3) and pectin ethyl esterase 4 (PAE4) were higher in Tor-1. The candidate genes identified here seem to regulate CW Cd fixation by polysaccharides. In conclusion, an increase in pectin demethylation activity, the higher concentration of cellulose and hemicellulose, regulated by related genes, in Tor-1 than in Ph2-23 are likely involved in enhanced Cd CW retention and reduce Cd toxicity.

The combined effects of Cu and Pb on the sex-specific growth and physiology of the dioecious Populus yunnanensis

In recent years, pollution by heavy metals (HM) has become an increasingly serious problem in forest ecosystems, making their remediation a primary research focus in China. Poplars are ideal candidates for phytoremediation because of their great commercial value, ability to produce large biomass, and high capacity for HM uptake. The individual and combined effects of copper (Cu) and lead (Pb) on Populus yunnanensis growth and physiology were tested for both male and female potted plants in four treatment groups: control, Pb only (1,000 mg kg^-1 PbAc dry soil), Cu only (400 mg kg^-1 CuSO₄·5H₂O dry soil), and combined Pb and Cu. Each treatment group contained 25 male and 25 female individuals. The experimental duration was 3 months. Compared with the control plants, the Cu and Pb treatment groups experienced reduced leaf, stem, root, and total biomass for both sexes, but the impact on growth rate was more severe in females than in males. The cellular ultrastructure of leaves was extensively damaged in both male and female trees but was more severely damaged in females. Male trees demonstrated a stronger Cu absorption ability with a bioconcentration factor 2.30 times that of females. Significant changes in pigment content, membrane lipid peroxidation, and protein oxidation (carbonyl) also indicated that females were more sensitive than males to Cu- and Pb-induced stress. The higher Cu and Pb tolerance in males correlated with better H₂O₂ scavenging ability and proline accumulation. Nevertheless, the combined stress from both Cu and Pb yielded greater negative effect on the growth and physiology of P. yunnanensis for both sexes.

Testing the elemental defense hypothesis with a woody plant species: Cadmium accumulation protects Populus yunnanensis from leaf herbivory and pathogen infection

Elemental defense hypothesis states that metals accumulated in plant tissues may serve as defense against herbivores and pathogens. However, evidences collected so far are inconsistent and studies using woody plants as model species are still lacking. In this study we used a local woody plant species, Populus yunnanensis, to investigate whether cadmium (Cd) accumulation in leaves can protect plants from leaf herbivory and pathogen infection. Plants grown with or without Cd supplementation in the soil were subjected to herbivory by a specialist (Botyodes diniasalis) and a generalist (Spodoptera exigua), or to pathogen infection by a leaf pathogenic fungus (Pestalotiopsis microspora). Two additional tests with artificial media amended with a series of Cd concentrations were conducted for S. exigua and P. microspora to investigate the toxicity of Cd independently of other organic defenses present in P. yunnanensis leaves. The results showed that both herbivores strongly preferred control leaves over leaves containing high Cd. Feeding on leaves from Cd-treated plants significantly reduced the growth and survivals of both herbivores. Furthermore, plants grown on Cd-amended soil were more resistant to fungal infection. Growth of S. exigua and P. microspora on artificial media decreased with increasing Cd concentrations. In conclusion, we found that Cd accumulated in P. yunnanensis leaves could effectively reduce leaf herbivory and pathogen infection, which fully supported the Elemental defense hypothesis.

Phytoremediation: A Promising Approach for Revegetation of Heavy Metal-Polluted Land

Heavy metal accumulation in soil has been rapidly increased due to various natural processes and anthropogenic (industrial) activities. As heavy metals are non-biodegradable, they persist in the environment, have potential to enter the food chain through crop plants, and eventually may accumulate in the human body through biomagnification. Owing to their toxic nature, heavy metal contamination has posed a serious threat to human health and the ecosystem. Therefore, remediation of land contamination is of paramount importance. Phytoremediation is an eco-friendly approach that could be a successful mitigation measure to revegetate heavy metal-polluted soil in a cost-effective way. To improve the efficiency of phytoremediation, a better understanding of the mechanisms underlying heavy metal accumulation and tolerance in plant is indispensable. In this review, we describe the mechanisms of how heavy metals are taken up, translocated, and detoxified in plants. We focus on the strategies applied to improve the efficiency of phytostabilization and phytoextraction, including the application of genetic engineering, microbe-assisted and chelate-assisted approaches.

Heavy metal accumulation potential in pomegranate fruits and leaves grown in roadside orchards

This study was carried out to determine the possible heavy metal accumulation in fruits and leaves of Zivzik pomegranate (Punica granatum L.) grown in two different roadside orchards located in Pirinçli and Kapılı villages of Siirt province, Turkey. Leaf and fruit samples were collected from trees located at 0, 50, 100 m distances from the main roads. Plant samples were analyzed for cobalt (Co), nickel (Ni), cadmium (Cd), lead (Pb) and chromium (Cr) concentrations. The Co, Ni, Cd, Pb and Cr concentrations of fruit samples collected from Pirinçli village were ranged from 0.082 to 0.238 mg kg^-1, from 1.160 to 1.559 mg kg^-1, from 0.087 to 0.179 mg kg^-1, 0.326 to 0.449 mg kg^-1 and 0.606 to 1.054 mg kg^-1, respectively. The Co, Ni, Cd, Pb and Cr concentrations of fruit samples from Kapılı village were between 0.085 and 0.137 mg kg^-1, 1.042 and 1.123 mg kg^-1, 0.037 and 0.076 mg kg^-1, 0.277 and 0.520 mg kg^-1 and 0.762 and 0.932 mg kg^-1, respectively. Heavy metal concentrations of leaf samples from Pirinçli village varied from 0.191 to 0.227 mg Co kg^-1, 2.201 to 3.547 mg Ni kg^-1, 0.051 to 0.098 mg Cd kg^-1, 0.535 to 0.749 mg Pb kg^-1 and from 1.444 to 2.017 mg Cr kg^-1. Similarly, the heavy metal concentration of leaf samples from Kapılı villages were between 0.213 and 0.217 mg Co kg^-1, 2.160 and 2.511 mg Ni kg^-1, 0.058 and 0.114 mg Cd kg^-1, 0.579 and 0.676 mg Pb kg^-1 and 1.688 and 1.518 mg Cr kg^-1. The Co, Ni and Cr concentrations in fruit samples collected from 0, 50 and 100 meters to the main road in Pirinçli village were at statistically significant level, while only Ni concentration in leaf samples collected from 0, 50 and 100 meters to the main road was at significant level. In contrast, heavy metal concentrations in fruit and leaf samples collected from 0, 50 and 100 m to the main road in Kapılı village were not statistically significant level.

Differentiation of Nanoparticles Isolated from Distinct Plant Species Naturally Growing in a Heavy Metal Polluted Site

Leaves harvested from the plants of two different species (Dittrichia viscosa and Cichorium intybus) grown in their autogenous environment near a steel manufacturing company were characterized for naturally accumulated nanoparticles. These plant species are known to accumulate heavy metals. It was, however, unknown if these species would also accumulate these heavy metals in the form of nanoparticles. The isolated solid fractions were analyzed using dynamic light scattering, X-ray fluorescence, and transmission electron microscopy. These analyses revealed the presence of nanoparticles within the plants. The composition of nanoparticles found in each plant species is distinct: (i) for Dittrichia viscosa, the nanoparticle composition matched the heavy metal pollution anticipated from the surrounding industries; (ii) for Cichorium intybus, the nanoparticle composition was similar to the most abundant elements in the soil. The different behavior is a reflection of the phytoaccumulator characteristics of both species. This study provides the first evidence of sequestration of heavy metals in the form of nanoparticles by plants grown autogenously in polluted areas and will have implications in waste management of phytoremediation systems and in understanding the heavy metal life-cycle in the environment.

Remediation techniques for removal of heavy metals from the soil contaminated through different sources: a review

Heavy metal pollution is one of the serious problems and contaminates the environment by different means with the blow of industries in several countries. Different techniques like physical, chemical, and biological have been used for removal of heavy metal contaminants from the environment. Some of these have limitations such as cost, time consumption, logistical problems, and mechanical involvedness. Nowadays, in situ immobilization of metals, phytoremediation and biological techniques turned out to be best solution for elimination of metal(loid) s from the soil. Here, we reviewed the different remediation techniques for extraction of heavy metals from soil and especially highlighting in situ immobilization technique. The aim of remediation efforts at the contaminant site is to restrict the heavy metal to enter in the environment, food chain, and exposure to humans beings. The type of method used at a given site depends on the various factors like natural processes take place at the contaminated site, soil type, type of chemicals, and the depth of contaminated site.

Inhibitory effect of metals on animal and plant glutathione transferases

Glutathione transferases (GSTs) represent a widespread enzyme superfamily in eukaryotes and prokaryotes catalyzing different reactions with endogenous and xenobiotic substrates such as organic pollutants. The latter are often found together with metal contamination in the environment. Besides performing of essential functions, GSTs protect cells by conjugation of glutathione with various reactive electrophiles. The interference of toxic metals with this functionality of GSTs may have unpredictable toxicological consequences for the organisms. In this review results from the recent literature are summarized and discussed describing the ability of metals to inhibit intracellular detoxification processes in animals and plants.

Effects of Exogenous Organic Acids on Cd Tolerance Mechanism of Salix variegata Franch. Under Cd Stress

Chelate induction of organic acids has been recognized to enhance metal uptake and translocation in plants, but the underlying mechanism remains unclear. In this study, seedlings of Salix variegata were hydroponically exposed to the combinations of Cd (0 and 50 μM) and three exogenous organic acids (100 μM of citric, tartaric, or malic acid). Plant biomass, antioxidant enzymes, non-protein thiol compounds (NPT) content, and the expression of candidate genes associated with Cd accumulation and tolerance were determined. Results showed that Cd significantly inhibited plant biomass but stimulated the activity of antioxidant enzymes in the roots and leaves, while the lipid peroxidation increased as well. Respective addition of three organic acids greatly enhanced plant resistance to oxidative stress and reduced the lipid peroxidation induced by Cd, with the effect of malic acid showing greatest. The addition of organic acids also significantly increased the content of glutathione in the root, further improving the antioxidant capacity and potential of phytochelatin biosynthesis. Moreover, Cd induced the expression level of candidate genes in roots of S. variegata. The addition of three organic acids not only promoted the expression of candidate genes but also drastically increased Cd accumulation in S. variegata. In summary, application of citric, tartaric, or malic acid alleviated Cd-imposed toxicity through the boost of enzymatic and non-enzymatic antioxidants and candidate gene expression, while their effects on Cd tolerance and accumulation of S. variegata differed.

Oxidative stress mitigation and initiation of antioxidant and osmoprotectant responses mediated by ascorbic acid in Brassica juncea L. subjected to copper (II) stress

Copper (Cu) is an essential yet toxic metal, which holds the ability to induce production of reactive oxygen species (ROS) in living cells resulting in severe abiotic stress. Therefore, the aim of our current study was to investigate the effects of extrinsically added ascorbic acid (AA) on oxidative stress indicators and redox homoeostasis remediators in 7-day-old seedlings and 60-day-old plants of Brassica juncea L. (hyper-accumulator species) subjected to Cu (II) stress. Our findings showed that seed germination ballooned by 55.4% in Cu (II) stressed seedlings upon addition of 50 mg l^-1 AA. Copper content accelerated in stressed seedlings and plants; however, a negative interaction was seen upon addition of AA. Both seedlings and plants exposed to Cu (II) accumulated free radicals such as H₂O₂ and superoxide anion, however, the addition of AA in the growth media decreased H₂O₂ and superoxide anion generation indicating ROS detoxification. Confocal microscopy also revealed improved cell viability and reduced H₂O₂ content because of enhanced antioxidant activity upon addition of AA as a protective chelate. Antioxidants such as ascorbate, flavonoids and glutathione rose significantly in Cu (II) stressed seedlings and plants in the presence of AA. Protein content increased by 51.3% and 47.5% in seedlings and plants growing in a binary combination of 100 mg l^-1 Cu and AA (75 mg l^-1 and 25 mg l^-1), respectively. Sharp peaks for stress indicator amino acids such as cysteine and proline were seen in spectral analysis of B. juncea seedlings exposed to Cu (II). Protein thiols increased in plants grown in various binary doses Cu (II) and AA. This study provides sufficient evidence regarding the protective role of ascorbic acid (AA) against ROS and its suggested use as a soil amendment against Cu (II) toxicity in B. juncea.

Distribution and Redistribution of 109Cd and 65Zn in the Heavy Metal Hyperaccumulator Solanum nigrum L.: Influence of Cadmium and Zinc Concentrations in the Root Medium

Heavy metal redistribution is relevant for the quality of edible crops and the suitability of hyperaccumulators for bioremediation. Root-to-shoot transfer via the xylem and redistribution in the aerial parts via the phloem differ between various heavy metals. In general, cadmium is more slowly released to the shoot than zinc (e.g., in wheat, bean, and lupin). However, rapid cadmium transport to the shoot was detected in the hyperaccumulator Solanum nigrum L. This is a key aspect in this article and might be important for bioremediation. The radionuclides ¹⁰⁹Cd and ⁶⁵Zn were used to investigate the respective influence of elevated cadmium or zinc in the root medium on the dynamics of the two heavy metals in S. nigrum. Although transport via the xylem to the leaves was similar for ¹⁰⁹Cd and ⁶⁵Zn, the further redistribution from older leaves to younger leaves, flowers, and fruits via the phloem was far less efficient for ¹⁰⁹Cd than for ⁶⁵Zn. Furthermore, the redistribution of ¹⁰⁹Cd within the shoot was negatively influenced by increased cadmium (but not by increased zinc) concentrations in the nutrient medium. The redistribution of ⁶⁵Zn in the shoot was selectively decreased by increased zinc concentrations (but generally not by cadmium).

Role of nitric oxide in plant responses to heavy metal stress: exogenous application versus endogenous production

Anthropogenic activities, such as industrial processes, mining, and agriculture, lead to an increase in heavy metal concentrations in soil, water, and air. Given their stability in the environment, heavy metals are difficult to eliminate and can constitute a human health risk by entering the food chain through uptake by crop plants. An excess of heavy metals is toxic for plants, which have various mechanisms to prevent their accumulation. However, once metals enter the plant, oxidative damage sometimes occurs, which can lead to plant death. Initial production of nitric oxide (NO), which may play a role in plant perception, signalling, and stress acclimation, has been shown to protect against heavy metals. Very little is known about NO-dependent mechanisms downstream from signalling pathways in plant responses to heavy metal stress. In this review, using bioinformatic techniques, we analyse studies of the involvement of NO in plant responses to heavy metal stress, its possible role as a cytoprotective molecule, and its relationship with reactive oxygen species. Some conclusions are drawn and future research perspectives are outlined to further elucidate the signalling mechanisms underlying the role of NO in plant responses to heavy metal stress.

Concentration, Distribution, and Potential Aquatic Risk Assessment of Metals in Water from Chott Merouane (Ramsar Site), Algeria

In this study, 28 surface water samples were collected from eight different sites throughout the Chott Merouane. Samples were detected by atomic absorption spectrometry for Cd, Co, Cr, Cu, Fe, Ni, Mn, Pb, and Zn. The dissolved metal concentrations (mg/L) ranged from 0.05 to 0.90 mg/L for Cd, 0.13-6.45 mg/L for Co, bDL-2.05 mg/L for Cr, 0.03-0.27 mg/L for Cu, 0.34-7.41 mg/L for Fe, 01.6-4.54 mg/L for Ni, 0.15-1.19 mg/L for Mn, 0.23-5.88 mg/L for Pb, and 0.01-0.28 mg/L for Zn. Compared with U.S. EPA standards and other freshwaters worldwide, the most mean concentration of metals in surface water of this salt lake are higher than the guideline levels of aquatic life. This was further corroborated by results from the water quality indices that Chott Merouane is seriously polluted by metals. The values of the metal pollution index indicated that metal pollution level was Cd > Pb > Ni > Fe > Cr > Cu > Mn > Zn, and those metals belong to moderate or high pollution level. The Nemerow pollution index further indicated that Chott Merouane was suffering from serious metal contamination. Based on geostatistics analyses, generally distributions of these metal contents decreased in the order of the North Chott Merouane ≈ the Northwest Chott Merouane > the Eastern part of Chott Merouane > the South Chott Merouane. The quality of water has drastically deteriorated due to the mixed source of anthropogenic inputs. Therefore, necessary conservation and management measures should be taken to improve the water quality of this Ramsar wetland.

Ecological risk assessment of cerium for tropical agroecosystems

Cerium (Ce) is present in high technology materials and in mineral P fertilizers and the use and discharge of such resources may change the natural status of Ce in the soil environment. Brazilian soils in farming areas are significantly exposed to increased levels of unintentionally-added Ce through intensive input of phosphate fertilizers. The aims of this study were to evaluate the ecotoxicological risk to plants growing in tropical soils contaminated with Ce, as well as to create a database to support future legislation regulating the limits of this element in Brazilian and conceivably other tropical soils. Eight crop species (corn, sorghum, rice, wheat, soybeans, sunflower, radish, and beans) were exposed to a Ce concentration gradient in two typical tropical soils (Oxisol and Inceptsol), and an artificial soil. Our findings showed that among the endpoints measured, Ce phytotoxicity was more pronounced on shoot dry matter than on percent germination and germination speed index. Sensitivity of plants is species specific and our data showed that sunflower and radish exposed to Ce were the most sensitive crop species. Soil properties such as pH, cation exchange capacity, and organic carbon may have influenced the severity of Ce phytotoxicity. Because of that, the Oxisol contaminated with this element caused higher phytotoxicity than the other soils tested. Our risk assessment results (hazardous concentration, HC₅ = 281.6 mg Ce kg^-1) support the idea that unintentional Ce input through P fertilizers does not pose a risk to soils of Brazilian agroecosystems.

Improvement in fluoride remediation technology using GIS based mapping of fluoride contaminated groundwater and microbe assisted phytoremediation

Fluoride (F) in groundwater is a major issue of water pollution. Geo-statistical analysis of groundwater quality in Newai Tehsil, (India) has been done in order to identify the possible spatial distribution of water quality parameters and to assess the spatial dependence of water properties with the help of principal component analysis (PCA) structure. Two types of maps (spatial map and principal component map) of groundwater quality have been developed. A field experiment was conducted to investigate the effect of different Fluoride (F) concentration combined with Pseudomonas fluorescens (P.F) on Prosopis juliflora plant. The field design was used as completely randomized block design with three replicates. Study revealed that parameters were found to be positively and highly correlated with principal component. Low and high values (with their acceptable limit) have also been displayed over the each spatial map. Plants treated with P. fluorescens showed the highest F uptake in root, shoot and leaves tissues were 33.14, 19.41 and 15.15 mg kg^-1 after 120 days, respectively. Both total bioaccumulation factor (BF) and translocation factor (TF) were obtained above one i.e., 1.06 and 1.04, this confirmed the high accumulation and translocation of F in plant tissues. The F uptake efficiency of plant was enhanced to 67.7% and plant biomass was increased upto 57.03%. According to the available literature, this is the first spatial field study for the remediation of F polluted soil through P. fluorescens. The present study will be beneficial for researchers working towards further improvement of F phytoremediation technology. Also, GIS based study can be very useful for decision maker's exploration of groundwater to understand the potential of present research work on fluoride contamination.