Phytoremediation and hyperaccumulator plants

Phytoremediation mechanism and role of plant growth promoting rhizobacteria in weed plants for eco-restoration of hazardous industrial waste polluted site: a review

Plants have numerous strategies for phytoremediation depending upon the characteristic of pollutants. Plant growth promoting rhizobacteria (PGPR) are essential to the process of phytoremediation and play a key part in it. The mechanism of PGPR for phytoremediation is mediated by two methods; under the direct method there is phytohormone production, nitrogen fixation, nutrient mineral solubilization, and siderophore production while the indirect method includes quorum quenching, antibiosis, production of lytic enzyme, biofilm formation, and hydrogen cyanide production. Due to their economic and environmental viability, most researchers have recently concentrated on the potential of weed plants for phytoremediation. Although weed plants are considered unwanted and noxious, they have a high growth rate and adaptability which opens a high scope for its role in phytoremediation of contaminated site. The interaction of plant with rhizobacteria starts from root exudates containing various organic acids and peptides which act as nutrients essential for colonization and siderophore production by the rhizospheric bacteria. The rhizobacteria, while colonizing, tend to promote plant growth and health either directly by providing phytohormones and minerals or indirectly by suppressing growth of possible phytopathogens. Recently, several weed plants have been reported for phytoextraction of heavy metals (Ni, Pb, Zn, Hg, Cd, Cu, As, Fe, and Cr) contaminants from various agro-based industries. These potential native weed plants have high prospect of eco-restoration of polluted site with complex organo-metallic waste for sustainable development.

Floristic survey, trace element transfers between soil and vegetation and human health risk at an urban industrial wasteland

This study aimed to determine the trace element accumulation in the soil and plants in an industrial wasteland and to estimate the extent of transfer to humans to measure the effects on and risks to vegetation and human health and find bioindicator plants representative of the levels of the main contaminants. In areas with the highest extractable trace element levels, we observed decreases in plant biodiversity explained by the disappearance of several families, favouring the coverage of tolerant species, such as Urtica dioica and Hedera helix. Trace elements were also found in the leaves of several plants, especially in a dominant species that is poorly studied, Alliaria petiolata. Indeed, this species had the highest contents of Zn (1750 mg.kg-1 DW), Ni (13.1 mg.kg-1 DW), and Cd (18 mg.kg-1 DW) found at the site and is a potential Zn bioindicator since its leaf contents were also representative of the Zn extractable contents in soil (R² = 0.94). The hazard quotient and carcinogen risk revealed that most of the site had an identified or possible risk, mainly due to Pb and As. Native species, especially A. petiolata, could be used in phytoextraction to manage and limit these human and environmental risks.

How heavy metal stress affects the growth and development of pulse crops: insights into germination and physiological processes

The current work is an extensive review addressing the effects of heavy metals in major pulse crops such as Chickpea (Cicer arietinum L.), Pea (Pisum sativum L.), Pigeonpea (Cajanus cajan L.), Mung bean (Vigna radiata L.), Black gram (Vigna mungo L.) and Lentil (Lens culinaris Medik.). Pulses are important contributors to the global food supply in the world, due to their vast beneficial properties in providing protein, nutritional value and health benefits to the human population. Several studies have reported that heavy metals are injurious to plants causing inhibition in plant germination, a decrease in the root and shoot length, reduction in respiration rate and photosynthesis. Properly disposing of heavy metal wastes has become an increasingly difficult task to solve in developed countries. Heavy metals pose one of the substantial constraints to pulse crops growth and productivity even at low concentrations. This article attempts to present the morphological, biochemical and various physiological changes induced on the pulse crops grown under various heavy metal stress such as As, Cd, Cr, Cu, Pb, and Ni.

Individual and mutual effects of elevated carbon dioxide and temperature on salt and cadmium uptake and translocation by rice seedlings

Plant kingdoms are facing increasingly harsh environmental challenges marked by the coexposure of salinity and pollution in the pedosphere and elevated CO₂ and temperature in the atmosphere due to the rapid acceleration of industrialization and global climate change. In this study, we deployed a hydroponics-based experiment to explore the individual and mutual effects of different temperatures (low temperature, T1: 23°C; high temperature, T2: 27°C) and CO₂ concentrations (ambient CO₂: 360 ppm; medium CO₂: 450 ppm; high CO₂: 700 ppm) on the uptake and translocation of sodium chloride (NaCl, 0.0, 0.2, 0.6, and 1.1 g Na/L) and cadmium nitrate (Cd(NO₃)₂·4H₂O, 0.0, 0.2, 1.8, and 5.4 mg Cd/L) by rice seedlings. The results indicated that Cd and Na exposure significantly (P< 0.05) inhibited plant growth, but T2 and medium/high CO₂ alleviated the effects of Cd and Na on plant growth. Neither significant synergistic nor antagonistic effects of Cd and Na were observed, particularly not at T1 or high CO₂. At increasing temperatures, relative growth rates increased despite higher concentrations of Cd and Na in both rice roots and shoots. Similarly, higher CO₂ stimulated the growth rate but resulted in significantly lower concentrations of Na, while the Cd concentration was highest at medium CO₂. Coexposure experiments suggested that the concentration of Cd in roots slightly declined with additional Na and more at T2. Overall, our preliminary study suggested that global climate change may alter the distribution of mineral and toxic elements in rice plants as well as the tolerance of the plants.

Understanding the Effect of Irrigation with Chromium Loaded Tannery Effluent on Ocimum basilicum L. vis-a-vis Metal Uptake

In this study, we investigated the effect of irrigation with heavy metal loaded tannery effluent on Ocimum basilicum L., an important aromatic crop for utilization of tannery wastewater and remediation of chromium rich tannery affected soils. The soil was irrigated with three different dilutions of tannery wastewater viz. 50%, 75% and 100% of tannery effluents (T, UD and U) including control. The maximum herbal biomass (105.17 g plant^-1) was found in UD100. Although oil quality was not affected by application of tannery effluents, the oil yield was decreased significantly, as compared to control, in all treatments except in UD75. The chromium concentration in different plant parts was in order of root > shoot > leaf. This indicates O. basilicum is an effective aromatic crop for remediation of tannery affected sites without any effect on crop growth, oil yield per unit area and oil quality and increase in soil fertility.

Application of Cerium Dioxide Nanoparticles and Chromium-Resistant Bacteria Reduced Chromium Toxicity in Sunflower Plants

The continuous increase in the heavy metals concentration in the soil due to anthropogenic activities has become a global issue. The chromium, especially hexavalent chromium, is highly toxic for living organisms due to high mobility, solubility, and carcinogenic properties. Considering the beneficial role of nanoparticles and bacteria in alleviating the metal stress in plants, a study was carried out to evaluate the role of cerium dioxide (CeO₂) nanoparticles (NPs) and Staphylococcus aureus in alleviating the chromium toxicity in sunflower plants. Sunflower plants grown in chromium (Cr) contaminated soil (0, 25, and 50 mg kg^-1) were treated with CeO₂ nanoparticles (0, 25, and 50 mg L^-1) and S. aureus. The application of Cerium Dioxide Nanoparticles (CeO₂ NPs) significantly improved plant growth and biomass production, reduced oxidative stress, and enhanced the enzymatic activities in the sunflower plant grown under chromium stress. The application of S. aureus further enhanced the beneficial role of nanoparticles in alleviating metal-induced toxicity. The maximum improvement was noted in plants treated with both nanoparticles and S. aureus. The augmented application of CeO₂ NPs (50 mg l^-1) at Cr 50 mg kg^-1 increased the chl a contents from 1.2 to 2.0, chl b contents 0.5 to 0.8 and mg g^-1 FW, and decreased the leakage of the electrolyte from 121 to 104%. The findings proved that the application of CeO₂ nanoparticles and S. aureus could significantly ameliorate the metal-induced stress in sunflower plants. The findings from this study can provide new horizons for research in the application of nanoparticles in phytoremediation and bioremediation.

Environmental Survey of the Distribution and Metal Contents of Pteris vittata in Arsenic-Lead-Mercury-Contaminated Gold Mining Areas along the Bone River in Gorontalo Province, Indonesia

In this paper, we report ecological and environmental investigations on Pteris vittata in the As-Pb-Hg-polluted Bone River area, Gorontalo Province, Indonesia. The density distribution of P. vittata decreases from around the artisanal and small-scale gold mining (ASGM) site to the lower reaches of the Bone River, and it is rarely found near Gorontalo City. The maximum concentrations of As, Hg, and Pb recorded in the soil samples were 401, 36, and 159 mg kg^-1, respectively, with their maximum concentrations in P. vittata recorded as 17,700, 5.2, and 39 mg kg^-1, respectively. Around the ASGM sites, the concentrations of As, Pb, and Hg in P. vittata were highest in the study area. These data suggest that P. vittata, a hyperaccumulator of As, may be useful as a bioindicator for assessing environmental pollution by Pb and Hg.

Influence of low temperature on comparative arsenic accumulation and release by three Pteris hyperaccumulators

Arsenic (As) hyperaccumulator Pteris ferns are renowned for their capacity to accumulate As and have been used to remediate As-contaminated environmental. However, there is less information on how they perform under low temperature though it is important for practical phytoremediation. The purpose of this study was to identify the effect of temperature on As accumulation by three As hyperaccumulators, Pteris multifida, Pteris cretica and Pteris vittata. Ferns were cultured with 5 mg/L As addition under 25 °C to 5 °C for 15 days. The results showed that dropping of temperatures reduced As accumulation by P. vittata moderately but not P. multifida and P. cretica until 10 °C. At 5 °C, all ferns discontinued As accumulation, and the morphology showed necrosis in P. vittata, wherein P. multifida and P. cretica kept healthy. The As distribution showed that As was mainly accumulated in fronds, while P. multifida stored partial As in its root. Both translocation factor and As efflux showed that temperate zone ferns manage As more strictly as compared to P. vittata. Our findings demonstrated that temperature should be considered when applying Pteris ferns for As phytoremediation, and P. multifida could be the most suitable fern for treating As-contaminated water in temperate zone area.

Effects of Pseudomonas TCd-1 on rice (Oryza sativa) cadmium uptake, rhizosphere soils enzyme activities and cadmium bioavailability under cadmium contamination

Microbial remediation is a promising technique to reduce Cd accumulation in rice (Oryza sativa). In present study, a set of pot experiments were conducted to evaluate the effects of Cd-tolerate Pseudomonas TCd-1 inoculation on rice Cd uptake, soil enzyme activities and Cd bioavailability in the rhizosphere soils under Cd contaminated conditions. The results showed that at the ripening stage, with the inoculation of TCd-1, Cd contents in root, culm, leaf, hull and brown rice significantly reduced by 60.7%, 47.7%, 50.6%, 58.1% and 47.9%, respectively, and the cadmium bioconcentration factor (BCF) of rice lowered by 66.2% under 5 mg kg-1 Cd treatment. At the meantime, in the rhizosphere soils, pH increased by 0.05, the contents of exchangeable Cd (EX-Cd) and Fe-Mn oxides (OX-Cd) increased by 107.8% and 33.5%, whereas organic matter (OM-Cd) and residual (Res-Cd) decreased by 31.9% and 60.0%, respectively. The activity of acid phosphatase (ACP) increased by 28.3%, catalase (CAT), saccharase (SUC) activity decreased by 28.5% and 26.0%. Similarly, the Cd contents in root, culm, leaf, hull and brown rice reduced by 42.1%, 42.5%, 58.0%, 50.3%, and 68.8%, respectively, and the BCF lowered by 57.1%, under 10 mg kg-1 Cd treatment. Simultaneously, the soil pH increased by 0.06, the activities of CAT, SUC, urease (URE), ACP decreased by 26.4%, 34.6%, 63.8% and 15.3%, respectively. Furthermore, the correlation analysis showed that the inoculation of TCd-1 changed the correlation between rice Cd content and the biomass of roots, leaves, soil pH, CAT, PPO, URE activities, OM-Cd in rhizosphere soils. It suggested that Pseudomonas TCd-1 effectively reduced Cd uptake and Cd accumulation in rice was closely linked to the changes of soil pH, enzyme activities and Cd availability.

Mycorrhizal stress alleviation in Senecio bonariensis Hook & Arn growing in urban polluted soils

Loss of biodiversity and accumulation of contaminants in urban soils and water bodies cause serious issues in metropolitan areas. The Matanza-Riachuelo river basin (metropolitan area of Buenos Aires, Argentina) is one of the most environmentally degraded regions in the world. Senecio bonariensis Hook & Arn (Asteraceae) grows in the periodically flooded soils of this wetland. This plant concentrates potentially toxic trace elements (PTEs) in its tissues and establishes symbiosis with arbuscular mycorrhizal (AM) fungi that collaborate with PTE phytostabilization in soils. The objective of this work was to evaluate tolerance and stress alleviation of AM-colonized S. bonariensis when transplanting and exposing to highly polluted environmental conditions of the river basin. Plants were initially inoculated with different AM strains and maintained in greenhouse conditions. After 6 mo, they were transplanted to the field. These plants showed a more equal distribution between shoot and root biomass production in comparison to field spontaneous S. bonaerensis plants. Plants in earlier contact with native soil inoculum showed positive correlation with phosphorus content and a significant increase of vesicle frequency. Plants belatedly contacted with native inoculum in the field (control) showed a higher catalase level that was positively correlated with the total colonization frequency and chlorophyll content. The ability to establish symbiosis with Rhizophagus intraradices (strain GC3), commonly used in the formulation of biofertilizers, was also analyzed. Plants inoculated with GC3 at the beginning of the assay showed lower colonization and were less efficient in the field. The preservation of spontaneous native plants with ornamental value and bioaugmentation of their associated microbiome can contribute to the stabilization of contaminants in soils.

Efficiency of bacteria and bacterial assisted phytoremediation of heavy metals: An update

The aim of this review to address the plant-associated bacteria to enhance the phytoremediation efficiency of the heavy metals from polluted sites and it is also highlighted advances for the application in wastewater treatment. Plant-associated bacteria have potential to encourage the plant growth and resistance under stress conditions. Such bacteria could enhance plant growth by controlling growth hormone, nutrition security, producing siderophore, secondary metabolites, and improving the antioxidant enzymes system. This review also explores the concepts and applications of bacteria assisted phytoremediation, addressing aspects that affect phytoremediation and pathways for restoration. Significant review issues relating to production and application of bacteria for improvement of bioremediation were established and presented for possible future research. Bacteria assisted phytoremediation is cost-effective strategy and metal sequestration mechanism that hold high metal biosorption capacities. This also takes into consideration the current state of technology implementations and proposals for prospective clean-up studies.

Bioaugmented Phytoremediation of Metal-Contaminated Soils and Sediments by Hemp and Giant Reed

We assessed the effects of EDTA and selected plant growth-promoting rhizobacteria (PGPR) on the phytoremediation of soils and sediments historically contaminated by Cr, Ni, and Cu. A total of 42 bacterial strains resistant to these heavy metals (HMs) were isolated and screened for PGP traits and metal bioaccumulation, and two Enterobacter spp. strains were finally selected. Phytoremediation pot experiments of 2 months duration were carried out with hemp (Cannabis sativa L.) and giant reed (Arundo donax L.) grown on soils and sediments respectively, comparing in both cases the effects of bioaugmentation with a single PGPR and EDTA addition on plant and root growth, plant HM uptake, HM leaching, as well as the changes that occurred in soil microbial communities (structure, biomass, and activity). Good removal percentages on a dry mass basis of Cr (0.4%), Ni (0.6%), and Cu (0.9%) were observed in giant reed while negligible values (<100‰) in hemp. In giant reed, HMs accumulated differentially in plant (rhizomes > > roots > leaves > stems) with largest quantities in rhizomes (Cr 0.6, Ni 3.7, and Cu 2.2 g plant^–1). EDTA increased Ni and Cu translocation to aerial parts in both crops, despite that in sediments high HM concentrations in leachates were measured. PGPR did not impact fine root diameter distribution of both crops compared with control while EDTA negatively affected root diameter class length (DCL) distribution. Under HM contamination, giant reed roots become shorter (from 5.2 to 2.3 mm cm^–3) while hemp roots become shorter and thickened from 0.13 to 0.26 mm. A consistent indirect effect of HM levels on the soil microbiome (diversity and activity) mediated by plant response (root DCL distribution) was observed. Multivariate analysis of bacterial diversity and activity revealed not only significant effects of plant and soil type (rhizosphere vs. bulk) but also a clear and similar differentiation of communities between control, EDTA, and PGPR treatments. We propose root DCL distribution as a key plant trait to understand detrimental effect of HMs on microbial communities. Positive evidence of the soil-microbe-plant interactions occurring when bioaugmentation with PGPR is associated with deep-rooting perennial crops makes this combination preferable over the one with chelating agents. Such knowledge might help to yield better bioaugmented bioremediation results in contaminated sites.

Assessment the copper-induced changes in antioxidant defense mechanisms and copper phytoremediation potential of common mullein (Verbascum thapsus L.)

In this research, the Cu phytoremediation capacity of common mullein (Verbascum thapsus L.) was evaluated concerning plant growth, antioxidant enzymes, and photosynthetic activities. Plants were subjected to five Cu concentrations (0, 125, 250, 375, and 500 mg/L) under the hydroponic conditions for 2 weeks. The results showed that at 125 mg/L, root and shoot biomass and chlorophylls remained the same as that of the control and then declined with increasing concentrations of Cu, when compared with control. The carotenoid contents remained unchanged up to 250 mg/L compared with control and then dropped with raising Cu dose. The raising of antioxidant enzymes activity reflected the occurrence of stress due to Cu exposure as manifested by increased MDA and ion leakage level. However, increased antioxidant enzymes may be associated with the tolerance capacity of V. thapsus to protect the plant from oxidative damage. Except for the highest concentration (500 mg/L), Cu accumulation in the roots and shoots all increased significantly with increasing Cu concentration, and the Cu accumulation in shoots was greater than roots. The Cu accumulation reached its maximum level at 375 mg/L Cu concentration, with 492.8 and 447.3 mg/kg DW in shoots and roots, respectively, which is highly greater than the threshold value for a Cu (hyper)accumulator plant. The extraction coefficient (EC) close to 1, and translocation factor (TF) > 1 from 125 to 375 mg/L Cu, suggested that V. thapsus could be used as a viable plant species for Cu phytoextraction.

Phytoremediation: a sustainable environmental technology for heavy metals decontamination

Toxic metal contamination of soil is a major environmental hazard. Chemical methods for heavy metal's (HMs) decontamination such as heat treatment, electroremediation, soil replacement, precipitation and chemical leaching are generally very costly and not be applicable to agricultural lands. However, many strategies are being used to restore polluted environments. Among these, phytoremediation is a promising method based on the use of hyper-accumulator plant species that can tolerate high amounts of toxic HMs present in the environment/soil. Such a strategy uses green plants to remove, degrade, or detoxify toxic metals. Five types of phytoremediation technologies have often been employed for soil decontamination: phytostabilization, phytodegradation, rhizofiltration, phytoextraction and phytovolatilization. Traditional phytoremediation method presents some limitations regarding their applications at large scale, so the application of genetic engineering approaches such as transgenic transformation, nanoparticles addition and phytoremediation assisted with phytohormones, plant growth-promoting bacteria and AMF inoculation has been applied to ameliorate the efficacy of plants as candidates for HMs decontamination. In this review, aspects of HMs toxicity and their depollution procedures with focus on phytoremediation are discussed. Last, some recent innovative technologies for improving phytoremediation are highlighted.

Molecular mechanisms of plant adaptive responses to heavy metals stress

Heavy metals (HMs) are among the main environmental pollutants that can enter the soil, water bodies, and the atmosphere as a result of natural processes (weathering of rocks, volcanic activity), and also as a result of human activities (mining, metallurgical and chemical industries, transport, application of mineral fertilizers). Plants counteract the HMs stresses through morphological and physiological adaptations, which are imparted through well-coordinated molecular mechanisms. New approaches, which include transcriptomics, genomics, proteomics, and metabolomics analyses, have opened the paths to understand such complex networks. This review sheds light on molecular mechanisms included in plant adaptive and defense responses during metal stress. It is focused on the entry of HMs into plants, its transport and accumulation, effects on the main physiological processes, gene expressions included in plant adaptive and defense responses during HM stress. Analysis of new data allowed the authors to conclude that the most important mechanism of HM tolerance is extracellular and intracellular HM sequestration. Organic anions (malate, oxalate, etc.) provide extracellular sequestration of HM ions. Intracellular HM sequestration depends not only on a direct binding mechanism with different polymers (pectin, lignin, cellulose, hemicellulose, etc.) or organic anions but also on the action of cellular receptors and transmembrane transporters. We focused on the functioning chloroplasts, mitochondria, and the Golgi complex under HM stress. The currently known molecular mechanisms of plant tolerance to the toxic effects of HMs are analyzed.

The yield potential and growth responses of licorice (Glycyrrhiza glabra L.) to mycorrhization under Pb and Cd stress

The effects of mycorrhization (inoculation and non-inoculation) on growth and quality of two ecotypes (Baft and Ramjerd) of Glycyrrhiza glabra L. under heavy metals stress (0, 300 Pb + 20 Cd (H1) and 600 Pb + 40 Cd (H2) (mg kg^-1) was investigated. Higher concentration of heavy metals decreased shoot dry weight in Baft (7.05%) and Ramjerd (43.34%) than control. Root dry weight increased in mycorrhizal Baft (28.23%) and Ramjerd (31.84%) ecotypes under H2 than non-mycorrhizal plants. In mycorrhizal plants, root colonization percentage decreased 37.07% in H2 than control. Increasing heavy metals concentration led to increase of total antioxidant activity and total phenol content. Mycorrhizal Ramjerd showed the lowest shoot Pb concentration in both heavy metals concentrations and the highest root Pb concentration (107.25% higher than non-mycoorhizal one) in H2. For both ecotypes, the lowest shoot Cd concentration observed in mycorrhizal plants under H1 and mycorrhizal plants had more root Cd concentration (33.83 mg kg^-1dry matter) than non-mycorrhizal ones. In both concentrations of heavy metals, the lowest Pb (0.026) and Cd (0.153) translocation factor (TF) observed in mycorrhizal plants. Based on the results, licorice with TF< 1 is not a hyperaccumulator plant but stabilizes Cd and Pb in root. Novelty statement: Licorice is a well-known medicinal plant that its root and rhizome contains diverse applications in pharmaceutical and food industries. The main source of licorice supply is through harvesting from natural habitats of Iran (one of the first exporters of licorice in the world), which during the last years have been exposed to heavy metals contamination. Therefore, the growth response of the plant in polluted habitats and most importantly, the concentration of heavy metals especially in belowground parts of the plant need more consideration. Hence, this research was carried out with an objective to investigate growth and yield potential response of two ecotypes of licorice to mycorhization under heavy metal stress (Cd and Pb) and the mechanism of heavy metal management in above and belowground parts of licorice in order to achieve its potential for further sustainable phytoremediation programs and most importantly considering the heavy metal accumulation in rhizomes and roots in accordance with world standards for medicinal and edible consumption.

Assessment of phytostabilization potential of two Salix L. clones based on the effects of heavy metals on the root anatomical traits

Willow species (Salix L.) are a useful tool for assessing phytostabilization of the sites polluted by heavy metals. Phytostabilization potential of two willow genotypes (Salix alba L. clone '68/53/1' and Salix nigra Marshall clone '0408') has been evaluated in a 45-day hydroponic experiment, using stem cuttings (diameter 12 to 14 mm, length 20 cm) exposed to two concentrations (10^-4 M and 10^-5 M) of individually applied Cd, Ni, and Pb. Metals were diluted in 25% Hoagland's solution, in forms of CdCl₂·H₂O, NiSO₄·6H₂O, and Pb-EDTA. The control group of cuttings was grown in 25% Hoagland's solution without heavy metals. High Cd concentrations in willow roots, 8637 mg/kg (clone '68/53/1') and 6728 mg/kg of dry weight (clone '0408'), have indicated a high phytostabilization potential. However, detailed analyses of cross-sectional area of the root cortex and the central cylinder revealed that the excess concentration of Cd led to a significant reduction of measured anatomical root's traits of clone '68/53/1' in comparison with the control samples. Excessive concentration of Ni and Pb in nutrient solution increased the values of quantitatively measured root's traits of clone '0408', implying stimulatory effects of the applied concentrations. Concentration of 10^-4 M of each metal had more negative effects on the roots' anatomical traits, notably on parenchymal and exodermal cells and vessels. Deposits of metals were observed in root tissues. Clone '0408' demonstrated an increased tolerance to heavy metals, which could potentially make this clone useful in phytostabilization.

The dual benefit of a dominant mutation in Arabidopsis IRON DEFICIENCY TOLERANT1 for iron biofortification and heavy metal phytoremediation

One of the goals of biofortification is to generate iron-enriched crops to combat growth and developmental defects especially iron (Fe) deficiency anaemia. Fe-fortification of food is challenging because soluble Fe is unstable and insoluble Fe is nonbioavailable. Genetic engineering is an alternative approach for Fe-biofortification, but so far strategies to increase Fe content have only encompassed a few genes with limited success. In this study, we demonstrate that the ethyl methanesulfonate (EMS) mutant, iron deficiency tolerant1 (idt1), can accumulate 4-7 times higher amounts of Fe than the wild type in roots, shoots and seeds, and exhibits the metal tolerance and iron accumulation (Metina) phenotype in Arabidopsis. Fe-regulated protein stability and nuclear localisation of the upstream transcriptional regulator bHLH34 were uncovered. The C to T transition mutation resulting in substitution of alanine to valine at amino acid position 320 of bHLH34 (designated as IDT1_A320V ) in a conserved motif among mono- and dicots was found to be responsible for a dominant phenotype that possesses constitutive activation of the Fe regulatory pathway. Overexpression of IDT1_A320V in Arabidopsis and tobacco led to the Metina phenotype; a phenotype that has escalated specificity towards optimising Fe homeostasis and may be useful in Fe-biofortification. Knowledge of the high tolerance and accumulation of heavy metals of this mutant can aid the development of tools for phytoremediation of contaminants.

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.

Effect of EDTA and NTA on cadmium distribution and translocation in Pennisetum purpureum Schum cv. Mott

The primary objective of this research was to investigate the cadmium (Cd) distribution in Pennisetum purpurem (Napier grass) in the presence of 30 mg/L of Cd and different types and concentrations of chelating agents (ethylenediaminetetraacetic acid disodium dihydrate (EDTA), nitrilotriacetic acid (NTA), and EDTA-NTA mixtures). Plant samples were collected every 15 d during a 105-d experimental period. Accumulation of Cd in each part of the plant was determined using atomic absorption spectrometer (AAS), and the distribution of Cd was determined by laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) and synchrotron radiation micro X-ray fluorescence (SR-micro-XRF). The highest concentrations of Cd accumulation of 889 ± 53 mg kg^-1 in the underground part (roots) and 265 ± 26 mg kg^-1 in the aboveground part (stems and leaves) in the presence of 1:1 M ratio of Cd:EDTA after 30 d of exposure were observed. Plants grown in the presence of either NTA or EDTA-NTA mixtures showed significant lower Cd accumulation levels. The LA-ICP-MS analysis showed that Cd was primarily accumulated in the aboveground part (stems and leaves), especially in the xylem and intercalary meristem. In addition, translocation factor was very low. Thus, P. purpurem could be considered as a candidate plant for cadmium phytostabilization.

Root structural changes of two remediator plants as the first defective barrier against industrial pollution, and their hyperaccumulation ability

In the present day, plants are increasingly being utilized to safeguard the environment. In this study, we used Salsola crassa M. B. and Suaeda maritima L. Dumort for phytoremediation of water contaminated with heavy metals and simultaneous examination of the effect of industrial pollution on their root structures. After irrigation of a treatment group with wastewater and a control group with fresh water for 3 months, we fixed the root parts in the FAA fixator for developmental study, and measured the concentrations of Co, Ni, Zn, As, Cu, and Pb in the roots, shoots, soil, and irrigating water. The plants irrigated with wastewater showed significant accumulation of heavy metals in the roots and some translocation of heavy metals from the roots to the shoots. We also performed an experiment with two 0.3 m³ pools to more closely study the feasibility of these plants for filtering water of contaminants, including mineral compounds, and altering its chemical characteristics. In our anatomical studies, the cells of the treatment roots showed irregularities and abnormal appearances in all tissue layers. The diameter and area of the xylem and the size of the cortical parenchyma have increased in the treatment plants of both species, confirmed by Stereolite software. Phytoremediation studies indicated that S. crassa accumulated As, Cu, Zn, Pb, Co, and Ni, and S. maritima accumulated As, Co, Zn, and Cu. S. crassa accumulated more heavy metals in its roots, whereas S. maritima accumulated more in its shoots. The biological oxygen demand and chemical oxygen demand were also significantly reduced in the wastewater passed through pools with S. crassa. Our results indicate that both genera are hyperaccumulators of heavy metals and therefore hold promise for industrial wastewater treatment, especially the absorption of As.

Lead uptake by the symbiotic Daucus carota L.-Glomus intraradices system and its effect on the morphology of extra- and intraradical fungal microstructures

This work examines the strategies adopted by an arbuscular mycorrhizal symbiotic system to ameliorate environmental Pb stress by examining the concentrations of P, Fe, and Pb in the fungal microstructures and the host's root. In vitro cultures of Ri-T DNA-transformed carrot (Daucus carota L.) roots were inoculated with Glomus intraradices and treated with Pb(NO₃)₂ solution and the extraradical spores and mycelia (S/M) and the root with the vesicles, mycelia, and root cells were subsequently analyzed by polarized energy dispersive x-ray fluorescence (PEDXRF) spectrometry. Upon Pb treatment, within the root, the percentages of mycorrhizal colonization, the vesicles, and mycelia increased as well as the areas of the vesicles and the (extraradical) spores, although the number of spores and arbuscules decreased. The S/M and the mycorrhizal root showed enhanced concentrations of Pb, Fe, and P. These were particularly marked for Fe in the Pb-treated cultures. This indicates a synergistic relationship between the arbuscular mycorrhizal fungus and the host that confers a higher Pb tolerance to the latter by the induction of higher Fe absorption in the host. The intraradical vesicle, mycelia, and arbuscule numbers are interpreted as a "tactic to divert" the intraradical Pb traffic away from the root cells to the higher affinity cell walls of the arbuscular mycorrhizal fungi (AMF) microstructures in the apoplast. The results of this work show that the symbiosis between the AMF G. intraradices and the host plant D. carota distinctly improves the latter's Pb tolerance, and imply that the appropriate metal tolerant host-AMF combinations could be employed in process designs for the phytoremediation of Pb.

Differential effects of plant root systems on nickel, copper and silver bioavailability in contaminated soil

A pot experiment was conducted to investigate the effect of diverse plant species (four herbaceous and four woody species) on the labile pool of six metals (Ag, Cu, Pb, Zn, Ni and Se) present in their rhizosphere. After three months of cultivation, concentrations of trace elements (TE) in above and below-ground biomass of each species were determined. The labile and presumably bioavailable fraction of these TE in the rhizosphere as well as key soil parameters (e.g. pH, electrical conductivity (EC), percent of organic matter and dissolved organic carbon (DOC)) were also measured and compared as a function of plant species. The concentration of TE in plant tissues differed among species. In general and as expected, concentrations were higher in root tissues of tested plants than in above-ground tissues. While the labile pool of several TE in the rhizosphere, notably Ag, Ni, and Cu, was significantly and differently affected by the presence of the plants, pH, EC and percentage of organic matter remained unchanged. In contrast, DOC was higher in the rhizosphere of all tested plants than in soil of the control pots without plants. In addition, there was a positive relationship between Ni and Cu availability concentrations, and DOC levels. This suggests that root systems of different species can have a distinct influence on soil DOC and consequently modify the labile pools of Ni and Cu in the rhizosphere. These findings have important implications for plant selection in phytoremediation projects.

A tabulated review on distribution of heavy metals in various plants

Plants are a rich source of elements, and knowledge of their elemental composition determines their use for various purposes, especially for food and medicine. Therefore, it is necessary to create a database of the elemental composition of plants. The present review focuses on the concentration of various heavy metals as reported by various workers from time to time by using different sophisticated techniques. Cluster analysis was applied on the basis of mean values of heavy metals in plants. Co, Cu, and Cr have similar proximities. Cluster analysis was also applied to different families on the basis of their heavy metal contents. Elaeagnaceae, Adoxaceae, Thymelaeaceae, Cupressaceae, and Acoraceae had close proximities with each other. First three components of principal component analysis explained 95.7 % of the total variance. Factor analysis explained four underlying factors for heavy metal analysis. Factor 1 explained for 26.5 % of the total variance and had maximum loadings on Co, Cu, and Cr. Of the total variance, 21.7 % was explained by factor 2 and had maximum loadings on Zn and Cd. Factor 3 accounted for 19.2 % of the total variance and had maximum loadings on Ni and Pb. Mn had maximum loading on factor 4. The mean values of heavy metals as listed in this paper are Cu (18.7 μg/g dw), Mn (99.67 μg/g dw), Cr (22.9 μg/g dw), Co (19.7 μg/g dw), As (1.25 μg/g dw), Hg (0.17 μg/g dw), Zn (94.0 μg/g dw), Pb (6.93 μg/g dw), Cd (26.9 μg/g dw), Ni (19.9 μg/g dw), and Sb (0.25 μg/g dw).

Phytoaccumulation of Heavy Metals in Natural Vegetation at the Municipal Wastewater Site in Abbottabad, Pakistan

Heavy metal accumulation in crops and soils from wastewater irrigation poses a significant threat to the human health. A study was carried out to investigate the removal potential of heavy metals (HM) by native plant species, namely Cannabis sativa L., Chenopodium album L., Datura stramonium L., Sonchus asper L., Amaranthus viridus L., Oenothera rosea (LHer), Xanthium stramonium L., Polygonum macalosa L., Nasturtium officinale L. and Conyza canadensis L. growing at the municipal wastewater site in Abbottabad city, Pakistan. The HM concentrations varied among plants depending on the species. Metal concentrations across species varied in the order iron (Fe) > zinc (Zn) > chromium (Cr) > nickel (Ni) > cadmium (Cd). Majority of the species accumulated more HM in roots than shoots. Among species, the concentrations (both in roots and shoots) were in the order C. sativa > C. album > X. stramonium > C. canadensis > A. viridus > N. officinale > P. macalosa > D. stramonium > S. asper > O. rosea. No species was identified as a hyperaccumulator. All species exhibited a translocation factor (TF) less than 1. Species like C. sativa, C. album and X. stramonium gave higher (> 1) biological concentration factor (BCF) and biological accumulation coefficient (BAC) especially for Fe, Cr and Cd than other species. Higher accumulation of heavy metals in these plant species signifies the general application of these species for phytostabilization and phytoextraction of HM from polluted soils.

Heavy Metal Uptakes by Myriophyllum verticillatum from Two Environmental Matrices: The Water and the Sediment

To determine the preferred elements of the benthic plant Myriophyllum verticillatum, changes in the element concentrations in the plant were investigated in laboratory condition. The reactor was fed with synthetically contaminated water consisting of 2×10(-6) M of the heavy metals Fe, Cr, Zn, Ni, and Cu for 1060 hours. The elements that were preferentially taken up by the tested plant body were evaluated with respect to translocation factor, bio-concentration factor, and the amounts of partial elements and relative uptakes. Both the changing physical properties of the aqueous solution in the reactor during the experiment and the growth of the plant were tested using a two-sample t-test. The Zn and Cu levels in the combination of the leaves and stems were found to be significantly higher than the levels in the roots at the end of the trial. Based on the partial amount of each element, the affinity of the plant for different elements was found to follow the order of Ca>Fe>Mn. Scanning electron microscope (SEM) analyses of the plant bodies indicated that these elements were located both inside the organs and on the surface of the tissues alone or with microorganisms such as diatoms.

Phytoremediation of Mono-, Di-, and Triethylene Glycol by Echinodorus cordifolius L. Griseb

Mono-, di-, and triethylene glycol are chemicals used in various industrial (polyester products, plasticizers, printing, etc.) and domestic settings. The toxicity of these compounds is relatively low, but they do pose risks to the environment. Phytoremediation of the three glycols by Echinodorus cordifolius L. Griseb. were studied. The glycols were degraded in the leaves and roots, but leaves were the main source of degradation. The results of this study indicate that the plant can degrade triethylene glycol to diethylene glycol, diethylene glycol to 1,4-dioxan-2-one, or even further to monoethylene glycol. Moreover, 2-methoxy-4-vinylphenol, 1,2-cyclopentanedione, 1,4:3,6-dianhydro-.alpha.-d-glucopyranose, 2-propenamide, and 2,5-anhydro-1,6-dideoxyhexo-3,4-diulose were produced by this plant in response to the glycols.

Sulfur-selenium-molybdenum interactions distinguish selenium hyperaccumulator Stanleya pinnata from non-hyperaccumulator Brassica juncea (Brassicaceae)

Long-term sulfate, selenate and molybdate accumulation and translocation were investigated in two ecotypes of Stanleya pinnata and non-hyperaccumulator Brassica juncea under different levels of applied sulfate and selenate. Morphological differences were observed between the ecotypes of S. pinnata, but few differences in selenium (Se) and sulfur (S) accumulation were measured. Se-to-S ratios were nearly identical between the ecotypes under all treatments. When compared with B. juncea, several unique trends were observed in the hyperaccumulators. While both S. pinnata ecotypes showed no significant effect on Se content of young leaves when the supplied sulfate in the growth medium was increased tenfold (from 0.5 to 5 mM), the Se levels in B. juncea decreased 4- to 12-fold with increased sulfate in the growth medium. Furthermore, S. pinnata’s S levels decreased slightly with high levels of supplied Se, suggesting competitive inhibition of uptake, while B. juncea showed higher S levels with increasing Se, possibly due to up-regulation of sulfate transporters. Both ecotypes of S. pinnata showed much larger Se concentrations in young leaves, while B. juncea showed slightly higher levels of Se in older leaves relative to young. Molybdenum (Mo) levels significantly decreased in S. pinnata with increasing sulfate and selenate in the medium; B. juncea did not show the same trends. These findings support the hypothesis that S. pinnata contains a modified sulfate transporter with a higher specificity for selenate.

Phytoremediation of wastewater with Limnocharis flava, Thalia geniculata and Typha latifolia in constructed wetlands

Phytoremediation is thought to be the most sustainable wastewater treatment option for developing countries. However, its application is often limited by unavailability of suitable candidate species. In the present study, the potentials of Limnocharis flava, Thalia geniculata and Typha latifolia for remediation of heavy metal contaminated wastewater with a constructed wetland system were evaluated. The wetland consisted of three treatment lines each planted with sufficient and equal number of a species. Duplicate plant and water samples were collected bi-monthly and analyzed for Fe, Cu, Zn, Pb, and Hg using the atomic absorption spectrophotometer over a six month period. Bioaccumulation rates generally increased over time and varied among plants for these metals, with Fe (456-1549 mg kg1 roots; 20-183 mg kg(-1) shoot) being the most sequestered and Pb (1.2-7.6 mg kg(-1) roots; 1.55-3.95 mg kg(-1) shoot) the least. Translocation factors differed among the species but generally remained stable over time. L flava showed potential for hyperaccumulating Hg. Removal efficiencies varied for the studied metals (approximately 20-77 %) and were generally related to metal uptake by the plants. These results demonstrate the suitability of the species for phytoremediation, and the usefulness of the technique as an option for improving irrigation water quality in Ghana.

Photocatalytic reduction of Cr(VI) from agricultural soil column leachates using zinc oxide under UV light irradiation

The photocatalytic reduction of Cr(VI) from agricultural soil leachates irrigated with Cr(VI)-containing waste hydroponic solution was evaluated in this work. For this purpose, zinc oxide was used as a catalyst under UV irradiation (lambda = 365 nm). The reduction of Cr(VI) was preliminarily evaluated on synthetic solutions with a concentration of 15 mg L(-1) to optimize the catalyst loading and the solution pH and to determine the effect of organic matter. Greater removal of Cr(VI) was observed at pH 7, and the optimum catalyst loading was found to be 2 g L(-1), which achieved an 84% Cr(VI) reduction in 6 h. The influence of dissolved organic matter on the reduction of Cr(VI) was evaluated through the addition of different concentrations of humic acid (HA) to the chromium solution. The removal of Cr(VI) was continuously enhanced as the HA concentration gradually increased from 0 to 14 mg L(-1). The percentage of hexavalent chromium reduction from soil leachates was in the range of 13-99%, and the rate constant was significantly enhanced by the presence of organic compounds in the soil pore water. Thus, a marked synergistic effect between the photocatalytic reduction of Cr(VI) and the organic matter in soil (e.g. humic substances) was observed in real samples and was similar to that observed in the Cr(VI) synthetic solution that contained HA.

Hydroponical estimation of interactions among selected heavy metals accumulated by Salix viminalis in phytoremediation process

Determination of interactions between selected heavy metals (Cd, Co, Cr, Cu, Ni, Pb and Zn) in their phytoremediation by one-year-old cuttings of Salix viminalis 'Cannabina' was the purpose of this work. The achieved results indicate that Salix cuttings may successfully be used in phytoremediation of polluted soil and/or sewage not only with one metal at high concentrations but also in different combinations with the other metals. Under controlled conditions (the hydroponic experiment) new interactions were found as well as known data concerning interactions between-presented in the matrix-heavy metals, depending on whether their concentration and composition were confirmed. The results showed that the ratio of metal concentration can change the interaction intensity. The achieved results enable one to indirectly estimate the accumulation efficiency of dominating metals as well as accompanying ones at lower concentrations.

Cadmium tolerance and accumulation characteristics of Bidens pilosa L. as a potential Cd-hyperaccumulator

Recently, researchers are becoming interested in using hyperaccumulators for decontamination of heavy metal polluted soils, whereas few species that hyperaccumulate cadmium (Cd) has been identified in the plant kingdom. In this study, the physiological mechanisms at the seedling stage and growth responses and Cd uptake and accumulation at flowering and mature stages of Bidens pilosa L. under Cd treatments were investigated. At the seedling stage, when soil Cd was lower than 16mgkg(-1), the plant did not show obvious symptom of phytoxicity, and the alterations of chlorophyll (CHL), superoxide dismutase (SOD), peroxidase (POD), malondialdehyde (MDA), and soluble protein (SP) did not have significant differences when compared with the control. At the flowering and mature stages, under low Cd treatments (</=16mgkg(-1)), the application of Cd could facilitate plant growth, resulting in 3.9-11.0% and 5.9-13.8%, respectively, increase in shoots dry biomass compared with the control. The Cd concentrations in stems, leaves and shoots exceeded 100mgkg(-1) when soil Cd was at 8mgkg(-1), and they were positively correlated with Cd concentration in soils, the bioaccumulation factor (BF) and translocation factor (TF) values were all greater than 1.0. Thus, it is clear that B. pilosa has the basic characteristics of a Cd-hyperaccumulator. All the results elementarily indicated that B. pilosa is a potential Cd-hyperaccumulating plant.

Light quanta modulated characteristics of Ni uptake by Brassica juncea seedlings: the interdependence of plant metal concentration and biomass

The relationships between the concentration of metal in the growth medium, Cs, the concentration of metal absorbed by the plant, Cp, and the total biomass achieved, M, all of which are factors relevant to the efficiency of metal uptake and tolerance by the plant, have been investigated via the physiological response of Brassica juncea seedlings to Ni stress. The factorial growth experiments treated the Ni concentration in agar medium and the diurnal light quanta as independently variable parameters. Observations included the evidence of light enhancement of Ni toxicity in the root, as well as at the whole-plant level. The shoot mass index possibly is an indicator of the amount of shoot metal sequestration in B. juncea, as are the logarithmic variation of Cp with Cs and the power-law dependence of M on Cp. The sum total of these observations indicates that, for the Ni accumulating plant B. juncea, the overall metabolic allocation to either growth or metal tolerance of the plant is important. Neither a rapid biomass increase nor a high metal absorbed concentration favored the removal of high metal mass from the medium. Rather, the plants with a moderate rate of biomass growth and a moderate absorbed metal concentration demonstrated the ability to remove the maximum mass of metal from the medium. The implication of these results as related to the extant model of phyoextraction efficiency is discussed.