Mn accumulation and tolerance in Celosia argentea Linn.: a new Mn-hyperaccumulating plant species

Constructed wetlands and hyperaccumulators for the removal of heavy metal and metalloids: A review

Water pollutions of heavy metal and metalloids (HMMs), typically including As, Cd, Cu, Cr, Mn, Ni, Pb, and Zn, are becoming a severe environmental problem to be controlled. Constructed wetlands (CWs) have been intensively investigated and applied for the removal of HMMs. By analyzing a mass of data from the existing literatures, this review found that the HMM removals in CWs varied from 12.35 % to 91.01 %, depending upon the HMM species and CW conditions. Nonetheless, 88.50 % of the influent HMMs were eventually immobilized in the CW sediments, while the common wetland plants are inefficient (i.e., accounting for 4.64 %) to uptake and accumulate the HMMs. It was also found that the concentrations of certain HMMs in the CW sediments have already exceeded up to 100 % of various environmental standards, indicating the urgency of introducing HMM hyperaccumulators in the systems. Through comparison, both the aboveground and belowground HMM accumulating capacities of reported hyperaccumulators were higher by magnitudes than common wetland plants. Following this, the efficacies and mechanisms of candidate hyperaccumulators were provided for the various scenarios of HMM control in CWs. Further, the selection principals, culture methods, and harvest strategies of hyperaccumulator in CWs were discussed. Finally, several perspectives were suggested for the future research. Overall, this review provided guiding information for the utilization of hyperaccumulators in CWs, which can improve the efficiency and sustainability of HMM removal in the CW systems.

Morphological Enrichment and Environmental Factors Correlation of Heavy Metals in Dominant Plants in Typical Manganese Ore Areas in Guizhou, China

Bioavailable heavy metal and their efficient phytoremediation in mining areas have major implications for environmental and human health. In this study, we investigated 12 dominant plants in a typical Mn ore area of Zunyi, Guizhou Province, China, to determine the heavy metal contents, morphologies, and environmental factors affecting Mn, Cd, Pb, Cu, Zn, and Cr in the plant parts and rhizosphere soil. The bioavailabilities and degrees of metals were evaluated using the ratios of the secondary to primary phase distributions and potential ecological risk indices. Principal component analysis, cluster analysis, positive matrix factorisation modelling, and redundancy analysis were used to trace the origins and correlations among the metals. The results indicate that the bioavailabilities were the highest for Mn and Cd in the study area, and all of the target heavy metals had bioavailabilities above the moderate ecological harm level. Statistical modelling indicates that there are four main pollution sources: mining, smelting, processing operations, and atmospheric deposition. The dominant plants had high heavy metal enrichments, bioconcentration factors, and translocation factors for Mn, Cu, Cr, Cd, and Zn. The redundancy analysis indicates that soil total N, total P, and pH affect metal absorption and distributions in Compositae and non-Compositae plants in low-N, low-P, and slightly alkaline mining environments. This study provides a feasible basis for the screening of heavy metal enrichment plants and the improvement of remediation technology in manganese ore area under the extreme environment of poor nutrition.

Physiological and transcriptomic response reveals new insight into manganese tolerance of Celosia argentea Linn

Celosia argentea is a manganese (Mn) hyperaccumulator with high ornamental value and strong stress resistance. It is important to understand the molecular mechanism of tolerance to heavy metals of hyperaccumulators to improve the efficiency of phytoremediation. In this study, the effects of different Mn concentrations (0, 0.8, 3, and 10 mM) on physiological characteristics and molecular changes were determined. Low concentrations of Mn increased the growth of C. argentea, while high concentrations of Mn suppressed its growth, A concentration up to 3 mM did not affect the growth of C. argentea, and the highest transfer factor (TF) was 6.16. Oxidative damage of different Mn level treatments in C. argentea was verified through relative water content, electrolyte leakage, MDA content, H2O2 content and superoxide contents. With an increase in Mn concentration, the contents of chlorophyll a, chlorophyll b, and carotenoids decreased. Our results indicated that low-concentration manganese treatment can reduce the reactive oxygen burst and MDA, soluble sugar and proline, making C. argentea have strong abiotic stress tolerance. The molecular mechanism of C. argentea after 10 mM Mn treatment was analysed through transcriptome analysis, and differentially expressed genes (DEGs) in these pathways were further verified by qRTPCR. Plantpathogen interactions, plant hormone signal transduction, the MAPK signalling pathway and the phenylpropanoid biosynthesis pathway were important in the response to Mn stress, and the heavy metal-associated isoprenylated plant protein, metal transporter Nramp, and zinc transporter play key roles in the strong ability of C. argentea to tolerate heavy metals. These results suggest that C. argentea exhibits strong manganese tolerance and provide new insight into the molecular mechanisms of plant responses to heavy metal stress.

Effect of hesperidin on growth, photosynthesis, antioxidant systems and uptake of cadmium, copper, chromium and zinc by Celosia argentea plants

Rapid industrialization and extensive agricultural practices are the major causes of soil heavy metal contamination, which needs urgent attention to safeguard the soils from contamination. However, the phytotoxic effects of excessive metals in plants are the primary obstacle to efficient phytoextraction. The present study evaluated the effects of hesperidin (HSP) on metals (Cu, Cd, Cr, Zn) phytoextraction by hyperaccumulator (Celosia argentea L.) plants. For this purpose, HSP, a flavonoid compound with strong antioxidant potential to assist metal phytoextraction was used under metal stress in plants. Celosia argentea plants suffered significant (P ≤ 0.001) oxidative damage due to the colossal accumulation of metals (Cu, Cd, Cr, Zn). However, HSP supplementation notably (P ≤ 0.001) abated ROS generation (O2•‒, •OH, H2O2), lipoxygenase activity, methylglyoxal production, and relative membrane permeability that clearly indicated HSP-mediated decline in oxidative injury in plants. Exogenous HSP improved (P ≤ 0.001) the production of non-protein thiol, phytochelatins, osmolytes, and antioxidant compounds. Further, HSP enhanced (P ≤ 0.001) H2S and NO endogenous production, which might have improved the GSH: GSSG ratio. Consequently, HSP-treated C. argentea plants had higher biomass alongside elevated metal accumulation mirrored as profound modifications in translocation factor (TF), bioaccumulation coefficient (BAC), and bioconcentration factor (BCF). In this context, HSP significantly enhanced TF of Cr (P ≤ 0.001), Cd (P ≤ 0.001), and Zn (P ≤ 0.01), while BAC of Cr (P ≤ 0.001), Cd (P ≤ 0.001), and Zn (P ≤ 0.001). Further, BCF was significant (P ≤ 0.05) only in plants grown under Cr-spiked soil. Overall, HSP has the potential for phytoremediation of metals by C. argentea, which might be a suitable strategy for metal-polluted soils.

Integrated transcriptome and metabolome analysis reveals the mechanism of tolerance to manganese and cadmium toxicity in the Mn/Cd hyperaccumulator Celosia argentea Linn

Understanding the molecular mechanism of tolerance to heavy metals in hyperaccumulators is important for improving the efficiency of phytoremediation and is interesting for evolutionary studies on plant adaption to abiotic stress. Celosia argentea Linn. was recently discovered to hyperaccumulate both manganese (Mn) and cadmium (Cd). However, the molecular mechanisms underlying Mn and Cd detoxification in C. argentea are poorly understood. Laboratory studies were conducted using C. argentea seedlings exposed to 360 μM Mn and 8.9 μM Cd hydroponic solutions. Plant leaves were analyzed using transcriptional and metabolomic techniques. A total of 3960 differentially expressed genes (DEGs) in plants were identified under Cd stress, among which 17 were associated with metal transport, and 10 belonged to the ATP transporter families. Exposures to Mn or Cd led to the differential expression of three metal transport genes (HMA3, ABCC15, and ATPase 4). In addition, 33 and 77 differentially expressed metabolites (DEMs) were identified under Mn and Cd stresses, respectively. Metabolic pathway analysis showed that the ABC transporter pathway was the most affected in Mn/Cd exposed seedlings. Conjoint transcriptome and metabolome analysis showed that the glutathione (GSH) metabolic pathway was over-represented in the KEGG pathway of both DEGs and DEMs. Our results confirm that the ABC transporter and GSH metabolic pathways play important roles in Mn and Cd detoxification. These findings provide new insight into the molecular mechanisms of tolerance to Mn and Cd toxicity in plants.

Metal tolerance in plants: Molecular and physicochemical interface determines the "not so heavy effect" of heavy metals

An increase in technological interventions and ruthless urbanization in the name of development has deteriorated our environment over time and caused the buildup of heavy metals (HMs) in the soil and water resources. These heavy metals are gaining increased access into our food chain through the plant and/or animal-based products, to adversely impact human health. The issue of how to restrict the entry of HMs or modulate their response in event of their ingress into the plant system is worrisome. The current knowledge on the interactive-regulatory role and contribution of different physical, biophysical, biochemical, physiological, and molecular factors that determine the heavy metal availability-uptake-partitioning dynamics in the soil-plant-environment needs to be updated. The present review critically analyses the interactive overlaps between different adaptation and tolerance strategies that may be causally related to their cellular localization, conjugation and homeostasis, a relative affinity for the transporters, rhizosphere modifications, activation of efflux pumps and vacuolar sequestration that singly or collectively determine a plant's response to HM stress. Recently postulated role of gaseous pollutants such as SO₂ and other secondary metabolites in heavy metal tolerance, which may be regulated at the whole plant and/or tissue/cell is discussed to delineate and work towards a "not so heavy" response of plants to heavy metals present in the contaminated soils.

Phylogenetic analysis of hyperaccumulator plant species for heavy metals and polycyclic aromatic hydrocarbons

Increasing concentration of heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs) in the soil may impose a serious threat to living organisms due to their toxicity and the ability to accumulate in plant tissues. The present review focuses on the phylogenetic relationships, sources, biotransformation and accumulation potential of hyperaccumulators for the priority HMs and PAHs. This review provides an opportunity to reveal the role of hyperaccumulators in removal of HMs and PAHs from soils, to understand the relationships between pollutants and their influence on the environment and to find potential plant species for soil remediation. The phylogenetic analysis results showed that the hyperaccumulators of some chemicals (Co, Cu, Mn, Ni, Zn, Cd) are clustered on the evolutionary tree and that the ability to hyperaccumulate different pollutants can be correlated either positively (Cd-Zn, Pb-Zn, Co-Cu, Cd-Pb) or negatively (Cu-PAHs, Co-Cd, Co-PAHs, Ni-PAHs, Cu-Ni, Mn-PAHs). Further research needs to be extended on the focus of commercializing the techniques including the native hyperaccumulators to remediate the highly contaminated soils.

Applications of Nanomaterials for Heavy Metal Removal from Water and Soil: A Review

Heavy metals are toxic and non-biodegradable environmental contaminants that seriously threaten human health. The remediation of heavy metal-contaminated water and soil is an urgent issue from both environmental and biological points of view. Recently, nanomaterials with excellent adsorption capacities, great chemical reactivity, active atomicity, and environmentally friendly performance have attracted widespread interest as potential adsorbents for heavy metal removal. This review first introduces the application of nanomaterials for removing heavy metal ions from the environment. Then, the environmental factors affecting the adsorption of nanomaterials, their toxicity, and environmental risks are discussed. Finally, the challenges and opportunities of applying nanomaterials in environmental remediation are discussed, which can provide perspectives for future in-depth studies and applications.

Myriophyllum elatinoides: A potential candidate for the phytoremediation of water with low level boron contamination

Phytoremediation is considered to be a cost-effective strategy for removing boron (B) from polluted water. In this study, Myriophyllum elatinoides, a widespread submerged or floating macrophyte, was found to survive in 40 mg B/L. Time-dependent kinetics show that the shoot exhibits a much longer period of B uptake and a much higher maximal tissue B concentration than the root. High values of the bioconcentration factor (BCF) and translocation factor (TF) indicate that M. elatinoides is a potential hyperaccumulator of B. Transmission electron micrographs show that excess B damages the cells of M. elatinoides, and the major target organelles are the chloroplast (leaf), mitochondria (stem and root), and nucleolus (root). Energy dispersive spectroscopy (EDS) shows that B is mainly deposited in the cytoplasm and on the surface of the chloroplast of the leaf cell. In the stem and root cells, B is mainly deposited on the mitochondrial membrane and in the vacuoles, respectively. This study indicates that the mechanisms of B toxicity, tolerance, and accumulation in M. elatinoides are involved in the cellular localization of B. Future work should focus on the evaluation of the physiological and genetic mechanisms involved in B tolerance and accumulation in M. elatinoides under different conditions.

Cadmium hyperaccumulation as an inexpensive metal armor against disease in Crofton weed

Invasive plants readily invade metal-contaminated areas. The hyperaccumulation of toxic heavy metals is not an uncommon feature among plant species. Although several hypotheses were proposed to explain this phenomenon, it is currently unclear how hyperaccumulation may benefit plants. The invasive Crofton weed (Ageratina adenophora) is a known hyperaccumulator of chromium and lead. We previously found that the species can also hyperaccumulate cadmium. The role of phytoaccumulation in defense to pathogen attack is unclear. We inoculated A. adenophora plants with a common generalist pathogen (Rhizoctonia solani) to test its resistance under cadmium treatment. We found evidence that cadmium hyperaccumulation reduced pathogen infection in A. adenophora. Our findings indicate elemental defense is highly cost efficient for hyperaccumulators inhabiting metal-contaminated sites, where plants were only modestly affected by cadmium. The reduction in pathogen damage conferred by cadmium was relatively high, particularly under lower cadmium levels. However, the benefits at higher levels may be capped. Elemental defense may be a key mechanism for plant invasion into polluted sites, especially in regions with widespread industrial activity. Our study highlights the importance of testing different metal concentrations when testing plant resistance and the importance of considering enemy attack when selecting plants for phytoremediation.

Retention of Contaminants Elements from Tailings from Lead Mine Washing Plants in Ceramics for Bricks

Mining activity is essential for the social welfare of the population. However, this activity produces a series of mining waste. These mining wastes, if not properly treated, can produce significant environmental pollution. This study develops the incorporation of tailings from washing plants in ceramic materials for bricks in order to retain the contaminating elements in the ceramic matrix. To this end, firstly, a physical and chemical characterisation of the mining waste is carried out and different groups of samples are conformed with clay and mining waste. These conformed samples with mining waste are evaluated through different physical and mechanical tests typical in the ceramic industry, studying the variation of properties by the incorporation of the waste. In turn, the leachates from the groups of conformed samples are analyzed, confirming the retention of the contaminating elements of the mining waste in the ceramic matrix. The results of these tests showed that ceramics can be made for bricks with up to 90% mining waste, obtaining physical and mechanical properties acceptable regarding the regulations and retaining the contaminating elements in the ceramic matrix, as confirmed by the leachate tests.

Manganese accumulation and plant physiology behavior of Camellia oleifera in response to different levels of potassium fertilization

The effects of potassium (K) fertilization (KCl, analytically pure; 0, 60, 200, and 400 mg kg^-1) on the growth and Mn accumulation of Camellia oleifera in two types of Mn-contaminated soils were investigated. The potential mechanisms underlying the impacts of K fertilization were explored. C. oleifera accumulated high amounts of Mn in both soil conditions. The addition of K fertilizer decreased the soil pH and promoted Mn accumulation in C. oleifera. However, the plant biomass decreased significantly under the high level of K fertilization (400 mg kg^-1), and the oxidative stress was stimulated under Mn contamination. But an appropriate concentration of K fertilizer (200 mg kg^-1) was necessary for the formation of photosynthesis pigments, nonenzymatic antioxidants and antioxidant enzymes, metabolic processes, and nutrient uptake. Furthermore, when plants supplemented with a low level of K fertilization (200 mg kg^-1), the catalase activity in C. oleifera leaves was enhanced to alleviate oxidative stress and protect the plant from Mn contamination. Our study demonstrated that 200 mg kg^-1 of K fertilizer has the potential to further enhance the efficiency of Mn phytoremediation by C. oleifera.

Subcellular distribution and chemical forms involved in manganese accumulation and detoxification for Xanthium strumarium L

Xanthium strumarium L. is a candidate species for manganese (Mn)-phyto-remediation. To reveal the mechanism of this species adaptive to Mn stress, the growth, Mn subcellular distribution, chemical forms, as well as micro-structure and ultra-structure responses of the mining ecotype (ME) of X. strumarium to Mn stress were studied with the non-mining ecotype (NME) as the reference by a hydroponic experiment. The results showed the ME demonstrated a higher tolerance to Mn stress with a superior growth and a higher tolerance index (TI) when compared with the NME. The concentrations of Mn in leaves, stems, and roots of the ME were 1.1-1.8, 1.2-1.9, and 1.3-1.9 times higher than those in the corresponding organs of the NME, respectively. The micro-structure and ultra-structure showed abnormal alterations, such as shrunken ducts and sieve canals, round-shaped chloroplasts, increased starch and osmiophilic granules, as well as expanded and non-compact granum thylakoids in the NME, compared to the ME. More than 83% of Mn was localized in cell wall and soluble fraction, while the Mn concentration in all fractions had a direct linear relationship with Mn treatment in the ME. The proportions of pectates and protein integrated-Mn, phosphate-Mn, and oxalate-Mn forms were dominant in leaves and stems of the ME, whereas, in the NME the relative proportions of inorganic Mn and water-soluble Mn forms in the roots was higher than the other forms. Altogether, the combination of preferential distribution of Mn in the cell wall and soluble fraction, and storage of Mn in low toxicity forms, such as phosphate-Mn, pectates and protein-bound Mn, and oxalate-Mn, might be responsible for alleviating Mn toxicity in the ME.

Manganese accumulation and plant physiology behavior of Camellia oleifera in response to different levels of nitrogen fertilization

Manganese (Mn) pollution in soil, especially around the mining areas, is a severe problem in China. Seeking for effective remediation methods for Mn-contaminated soil is therefore urgent and necessary. Camellia oleifera (C. oleifera) is one of the world's four major woody oil plants, which is widely cultivated in subtropical acidic soils for oil production and has become an important economic and ecological resource in Guangxi Province. Nitrogen (N) is one of the most common limiting factors for plant growth and development in soils. We carried out this study to evaluate the effects of different N fertilization levels (0, 100, 300 and 500 mg kg^-1) on the morphological and physiological characteristics of C. oleifera in two soils with different Mn-contamination degrees. The results indicate that N fertilization affected the plant growth and the content of photosynthetic pigments, while C. oleifera accumulated great amounts of Mn in both soils. However, the plant biomass reduced significantly at the high-level N fertilization (≥300 mg kg^-1), and the oxidative stress was stimulated under Mn contamination. As a comparison, the plant biomass remained unaffected at the low-level N fertilization (100 mg kg^-1), and the ascorbate peroxidase (APX) activity in C. oleifera leaves were enhanced to alleviate the oxidative stress and therefore protecting the plant from Mn contamination. Meanwhile, plants supplemented with a low-level of N fertilizer (100 mg kg^-1) had appropriate antioxidant enzyme and nonenzymatic antioxidant activities, which indicates that this was favorable growth conditions for C. oleifera. Thus, the recommended N fertilization level for maintaining plant biomass and increasing Mn accumulation in plant is 100 mg kg^-1 N; at which level the efficiency of Mn phytoremediation by C. oleifera can be further enhanced.

Integrative study of subcellular distribution, chemical forms, and physiological responses for understanding manganese tolerance in the herb Macleaya cordata (papaveraceae)

Macleaya cordata is a perennial herb, a candidate phytoremediation plant with high biomass and manganese (Mn) tolerance. To study the mechanism underlying its Mn adaptability, Mn²⁺ at various concentrations (0, 1000, 5000, 10000, 15000, and 20000 μM) were applied to M. cordata to investigate the subcellular distribution and chemical forms of Mn, as well as the resulting physiological and biochemical changes by pot culture experiment under natural light in a greenhouse. According to our results, Mn level in M. cordata increased with exogenous Mn concentrations; and Mn contents in different tissues exhibited a leaf > root > stem pattern. Meanwhile, biomass and the level of photosynthetic pigments increased at lower Mn concentrations but declined as Mn concentration further ascended. Subcellular distribution analysis revealed that Mn was sequestered in cell wall and vacuole; in addition, it was incorporated into pectates and protein, phosphates, and oxalates. These findings revealed a possible effective strategy for M. cordata to reduce Mn mobility and toxicity. Moreover, a continuous boost in the level of malondialdehyde was observed with Mn gradient; whereas contents of soluble proteins and proline, and the activities of superoxide dismutase and peroxidase were initially increased and then dropped. Altogether, these results indicated that most Mn was trapped in the cell wall and soluble fractions in low toxicity forms such as pectates and protein, phosphates, and oxalates. These strategies, that is functioning cooperatively with the well-coordinated antioxidant defense systems and the non-enzymatic metabolites, confer strong resistance to Mn in M. cordata.

Phytoextraction of cadmium-contaminated soils: comparison of plant species and low molecular weight organic acids

To select suitable plants for phytoextraction of Cd-contaminated soils, we evaluated the phytoextraction potential of five local Cd-accumulators: Amaranthus hypochondriacus L., Solanum nigrum L., Phytolacca acinosa Roxb., Celosia argentea L., and Sedum spectabile Boreau. The plants were grown in three naturally contaminated soils with different total Cd levels (1.57, 3.89, and 22.4 mg kg^-1). Throughout the experimental period, no plants showed any visible symptoms of metal toxicity. The Cd uptake of C. argentea was the greatest in the S-YS soil (105 μg plant^-1) and among the greatest in the S-HC soil and S-TJ soil. Besides, C. argentea exhibited the highest bioconcentration factor (12.3) in three soils. To improve the phytoextraction efficiency of C. argentea, we applied four low molecular weight organic acids (LMWOAs): tartaric acid, malic acid, oxalic acid, and citric acid. Malic acid was more effective in enhancing Cd uptake by C. argentea than the other LMWOAs. Therefore, C. argentea may be a potential choice in actual remediation projects. Moreover, application of malic acid is an effective way to increase the phytoextraction efficiency of C. argentea.

Manganese tolerance and accumulation characteristics of a woody accumulator Camellia oleifera

This study intended to help illustrate the Mn accumulation ability of Camellia oleifera and provide it as a novel species for possible use in Mn-contaminated sites. Field surveys have been carried out on Mn accumulation in C. oleifera growing near Mn mining area in Hezhou. Pot growth experiments in soil and sand culture were conducted to investigate Mn tolerance, accumulation, and translocation patterns in C. oleifera. C. oleifera grew well and showed no symptoms of Mn toxicity at a Mn treatment level below 1026 mg kg^-1 in soil culture and 15.0 mmol L^-1 in sand culture. Mn concentrations in leaves and stems reached a maximum of 9612.8 ± 83.5 and 6134.8 ± 94.0 mg kg^-1, respectively, in soil culture and 28,465.8 ± 1276.7 and 15,398.4 ± 1148.6 mg kg^-1, respectively, in sand culture. Meanwhile, most of the Mn taken from the substrates was transported to the aboveground tissues in soil and sand culture, e.g., over 92.07% of the total Mn taken up by C. oleifera was translocated to shoots in the 10.0 mmol L^-1 treatment. Our findings confirmed that C. oleifera exhibited extraordinary Mn accumulation and toleration abilities, and C. oleifera was a suitable species for phytoremediation of Mn-contaminated sites in Guangxi Province.

The effects of EDTA on plant growth and manganese (Mn) accumulation in Polygonum pubescens Blume cultured in unexplored soil, mining soil and tailing soil from the Pingle Mn mine, China

The effects of water-extractable Mn concentration, bioaccumulation factor (BAF), translocation factor (TF), and Mn uptake by Polygonum pubescens Blume cultured in the unexplored soil, mining soil and tailing soil from the Pingle Mn mine in China were quantified in a pot experiment to determine the effects of EDTA exposure on the success of phytoremediation. The results showed that EDTA significantly (P < 0.05) increased the water-extractable Mn concentration, and soils with different amounts of artificial disturbances had different responses to EDTA exposure. Low and medium EDTA concentrations might have positive effect on plant growth of P. pubescens cultured in the unexplored soil, as indicated by comparable increases in biomass, plant height and photosynthetic pigment content, but opposite results were found with high EDTA concentrations exposure. EDTA exposure had a negative effect on the growth of P. pubescens cultured in the mining soil and tailing soil. In general, the concentration of Mn in different tissues significantly (P < 0.05) increased as the EDTA concentration increased in each soil. The efficacy of Mn remediation by P. pubescens was enhanced in all three soils, with all EDTA treatments.

Nitrogen fertilizers promote plant growth and assist in manganese (Mn) accumulation by Polygonum pubescens Blume cultured in Mn tailings soil

This study examined how different nitrogen (N) forms and application levels promote plant growth and assist in manganese (Mn) remediation of Polygonum pubescens Blume (P. pubescens) cultured in soil with a high Mn level. The effects of ammonium chloride (a) and urea (u), at three application levels (10, 20, and 30 mg L^-1 N) and control (no N addition, CK) on the growth, Mn accumulation, and enzymatic anti-oxidative defenses of P. pubescens were examined. In general, both ammonium-N and urea-N promoted the plant mass and height of P. pubescens. The total Mn amount of roots, stems, and leaves in N treatments were higher (p < 0.05) than that of CK. The ammonium-N treatments showed greater plant biomass and Mn accumulation compared to the urea-N ones. In general, the accumulations of Mn, Cr, Zn, and Cu were significantly lower (p < 0.05) in the N fertilizer treatment than those in the control; while the accumulations of Pb were higher (p < 0.05) in P. pubescens across all N fertilizer treatments than those in the control. The N addition decreased the contents of O₂^- and H₂O₂ in the leaves of P. pubescens, while increasing the activities of enzymatic anti-oxidative defenses.

A new conceptual framework for plant responses to soil metals based on metal transporter kinetic parameters

Based on a review of the literature, we have developed a functional conceptual framework of plant metal uptake in relation to plant available metal concentration in the soil. This framework applies to all plant parts and plant available metal levels in soils, and was validated using independent datasets from field surveys and the literature. This is the first framework based on metal transporter kinetic parameters and combining Michaelis-Menten (hyperbolic) kinetics facilitated by the High Affinity Transport System (HATS) for soil concentrations below the transition concentration between transport systems, and linear metal uptake facilitated by the Low Affinity Transport System (LATS) for higher soil available metal concentrations. We propose a new terminology for metal tolerant plants, i.e. metal tolerators, based on this framework. Depending on the plant available metal levels in the soil, tolerator responses to metals can be described best by either V_max and K_m for soil concentrations below the transition concentration between metal transport systems (HATS), or by the slope for greater soil concentrations (LATS). This conceptual framework may be a useful tool for selecting suitable metal tolerators for specific phytoremediation purposes, and may be also applied to non-metal elements or ions.

Mn concentration and mycorrhizal colonization in understory native species grown at areas of manganese mine tailings disposal

Revegetation of areas with mine tailings should consider not only the best technique but also the choice of species adapted to this condition. This study aimed to analyze the occurrence of plant species, their Mn concentrations, and mycorrhizal colonization after implementation of two revegetation techniques (replacement of topsoil with natural seed bank and planting of seedlings) in an area of disposal of tailings from Mn processing compared to a native forest area as well as to a place that was not revegetated. Plant samples (at least three individuals/species) were collected from the understory at revegetated locations and forest. The established plant species and their Mn concentrations, mycorrhizal colonization, and forms of Mn in the soil were analyzed. The use of topsoil led to greater plant diversity. The high concentrations of Mn in the substrates did not affect the occurrence of vegetation in the understory and mycorrhizal colonization. The plant species established in the revegetated areas differed in relation to Mn concentration (471-27,842 mg kg^-1 in leaves), Mn translocation factor (0.2-125.3) and mycorrhizal colonization rates (1-35%). Four potential Mn hyperaccumulators species were identified: Aparisthmium cordatum, Clidemia hirta, Socratea exorrhiza, and Vismia latifolia.

Hyperaccumulator Plants from China: A Synthesis of the Current State of Knowledge

Hyperaccumulator plants are the material basis for phytoextraction research and for practical applications in decontaminating polluted soils and industrial wastes. China's high biodiversity and substantial mineral resources make it a global hotspot for hyperaccumulator plant species. Intensive screening efforts over the past 20 years by researchers working in China have led to the discovery of many different hyperaccumulators for a range of elements. In this review, we present the state of knowledge on all currently reported hyperaccumulator species from China, including Cardamine hupingshanensis (selenium, Se), Dicranopteris dichotoma (rare earth elements, REEs), Elsholtzia splendens (copper, Cu), Phytolacca americana (manganese, Mn), Pteris vittata (arsenic, As), Sedum alfredii, and Sedum plumbizincicola (cadmium/zinc, Cd/Zn). This review covers aspects of the ecophysiology and molecular biology of tolerance and hyperaccumulation for each element. The major scientific advances resulting from the study of hyperaccumulator plants in China are summarized and synthesized.

Screening ornamental plants to identify potential Cd hyperaccumulators for bioremediation

To identify possible cadmium (Cd) accumulators or hyperaccumulators among ornamental plants, a pot experiment involving increasing Cd concentration (0, 5, 15, 30, 60, and 100 mg kg^-1) was conducted among seven species. The principal objective was to screen for ornamental plants with an exceptional ability to accumulate and translocate Cd ions as well as sufficient biomass for harvesting. Regarding shoot biomass, root biomass, plant height and tolerance index (TI), Malva rotundifolia showed high tolerance to Cd and Malva crispa, Sida rhombifolia, Celosia argentea and Celosia cristata medium tolerance; Althaea rosea and Abutilon theophrasti were more sensitive to Cd than the other plants. A hormetic response was induced by Cd in M. crispa, C. argentea, C. cristata and M. rotundifolia. Based on its capacity for Cd accumulation, bioaccumulation coefficients (BCFs) and translocation factors (TFs), M. rotundifolia was selected from candidate plants after 60 days of exposure to Cd-contaminated soil and found to have accumulated more than 200 mg kg^-1 Cd in its roots and 900 mg kg^-1 in its shoots. Moreover, M. rotundifolia BCFs and TFs were higher than 1.0, with the former ranging from 1.41 to 3.31 and the latter from 1.03 to 7.37. Taken together, these results indicate that M. rotundifolia can be classified as a model hyperaccumulator.

Simultaneous hyperaccumulation of cadmium and manganese in Celosia argentea Linn

To indentify Mn/Cd co-hyperaccumulatoion in Celosia argentea Linn., 2 pot experiments were conducted using Cd/Mn-amended and real contaminated soils, respectively. The interaction between Cd and Mn with regard to their accumulation in the plants was also assessed. The results indicated that C. argentea can simultaneously hyperaccumulate Cd and Mn. The maximum Cd and Mn concentrations in leaves were 276 and 29,000 mg/kg, respectively. Mn application significantly enhanced the biomass production and Cd accumulation in shoots (p < 0.05). However, Cd addition did not reduce Mn accumulation in the plants. The interactions between Cd and Mn in C. argentea differ from what was previously found in hydroponic experiments. This species grew healthy in soils taken from a Cd/Mn-contaminated site, indicating a high tolerance to Cd and Mn. The transfer and bioaccumulation factors of Cd and Mn were greater than 1, which showed that C. argentea had potential for Cd and Mn phytoextraction. Besides its potential practical benefits, C. argentea is an important resource to study the mechanisms of Cd/Mn hyperaccumulation and tolerance in plants.

Phytoremediation of Mn-contaminated paddy soil by two hyperaccumulators (Phytolacca americana and Polygonum hydropiper) aided with citric acid

The purpose of this study was to investigate the phytoremediation potential of two hyperaccumulator plants, Phytolacca americana L. and Polygonum hydropiper L., on manganese-contaminated paddy soils. The biomass growth, Mn concentrations in plant tissues, and potential Mn removal efficiency from soils of these two plants were studied with citric acid, and the mechanisms of citric acid on these two plants were analyzed by examining the root activity, the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in leaves, as well as the concentrations of O₂·^- and H₂O₂ in leaves. The results showed that the biomass of these two plants were both promoted under low level of citric acid (3 mmol kg^-1). The concentration of Mn in the plants and the amount of Mn removed from the soil by the plants through harvesting were enhanced at low and intermediate (10 mmol kg^-1) citric acid application levels. The results also showed that root activity was enhanced at the low citric acid level and significantly inhibited under the intermediate and high levels (15 mmol kg^-1), which indicates the facilitative function of the low level of citric acid and the inhibitive function of the high level of citric acid application on plant biomass growth. Under the low and intermediate levels of citric acid application, O₂·^- in the plant leaves increased sharply, and the SOD, POD, and CAT activities also increased sharply, which made the level of H₂O₂ very similar to that of the control, ensuring the health of the plants. At the high level of citric acid application, however, the O₂·^- continued to rise sharply, while the activity of the three antioxidant enzymes declined sharply, causing the concentration of hydrogen peroxide to be much higher than that in the control, thus endangering the plants. The present study shows the potential of P. hydropiper for use in the phytoremediation of soil contaminated with a relatively low level of manganese.

Divergent biology of facultative heavy metal plants

Among heavy metal plants (the metallophytes), facultative species can live both in soils contaminated by an excess of heavy metals and in non-affected sites. In contrast, obligate metallophytes are restricted to polluted areas. Metallophytes offer a fascinating biology, due to the fact that species have developed different strategies to cope with the adverse conditions of heavy metal soils. The literature distinguishes between hyperaccumulating, accumulating, tolerant and excluding metallophytes, but the borderline between these categories is blurred. Due to the fact that heavy metal soils are dry, nutrient limited and are not uniform but have a patchy distribution in many instances, drought-tolerant or low nutrient demanding species are often regarded as metallophytes in the literature. In only a few cases, the concentrations of heavy metals in soils are so toxic that only a few specifically adapted plants, the genuine metallophytes, can cope with these adverse soil conditions. Current molecular biological studies focus on the genetically amenable and hyperaccumulating Arabidopsis halleri and Noccaea (Thlaspi) caerulescens of the Brassicaceae. Armeria maritima ssp. halleri utilizes glands for the excretion of heavy metals and is, therefore, a heavy metal excluder. The two endemic zinc violets of Western Europe, Viola lutea ssp. calaminaria of the Aachen-Liège area and Viola lutea ssp. westfalica of the Pb-Cu-ditch of Blankenrode, Eastern Westphalia, as well as Viola tricolor ecotypes of Eastern Europe, keep their cells free of excess heavy metals by arbuscular mycorrhizal fungi which bind heavy metals. The Caryophyllaceae, Silene vulgaris f. humilis and Minuartia verna, apparently discard leaves when overloaded with heavy metals. All Central European metallophytes have close relatives that grow in areas outside of heavy metal soils, mainly in the Alps, and have, therefore, been considered as relicts of the glacial epoch in the past. However, the current literature favours the idea that hyperaccumulation of heavy metals serves plants as deterrent against attack by feeding animals (termed elemental defense hypothesis). The capability to hyperaccumulate heavy metals in A. halleri and N. caerulescens is achieved by duplications and alterations of the cis-regulatory properties of genes coding for heavy metal transporting/excreting proteins. Several metallophytes have developed ecotypes with a varying content of such heavy metal transporters as an adaption to the specific toxicity of a heavy metal site.

Boron stress response and accumulation potential of the extremely tolerant species Puccinellia frigida

Phytoremediation is a promising technology to tackle boron toxicity, which restricts agricultural activities in many arid and semi-arid areas. Puccinellia frigida is a perennial grass that was reported to hyperaccumulate boron in extremely boron-contaminated sites. To further investigate its potential for phytoremediation, we determined its response to boron stress under controlled conditions (hydroponic culture). Also, as a first step towards understanding the mechanisms underlying its extreme tolerance, we evaluated the presence and expression of genes related with boron tolerance. We found that P. frigida grew normally even at highly toxic boron concentrations in the medium (500mg/L), and within its tissues (>5000mg/kg DW). We postulate that the strategies conferring this extreme tolerance involve both restricting boron accumulation and an internal tolerance mechanism; this is consistent with the identification of putative genes involved in both mechanisms, including the expression of a possible boron efflux transporter. We also found that P. frigida hyperaccumulated boron over a wide range of boron concentrations. We propose that P. frigida could be used for boron phytoremediation strategies in places with different soil characteristics and boron concentrations. Further studies should pave the way for the development of clean and low-cost solutions to boron toxicity problems.

A newly found manganese hyperaccumulator--Polygonum lapathifolium Linn

In the present work, both field investigation and laboratory experiment were carried out to testify whether Polygonum lapathifolium L. is a potential manganese (Mn) hyperaccumulator. Results from field investigation showed that P. lapathifolium had great tolerance and accumulation to Mn. Mn concentrations in leaves were the highest, varied from 6889.2 mg kg-1 dry weight (DW) to 18841.7 mg kg(-1) DW with the average of 12180.6 mg kg(-1). The values of translocation factor (the concentrations of Mn in leaf to that in root) ranged from 5.72 to 9.53. Results from laboratory experiment illuminated that P. lapathifolium could grow well and show no toxic symptoms even under high Mn stress (16 mmol L(-1)). Although the changes of antioxidant enzymes activities were triggered under Mn stress, the alterations of pigments were not significant (P > 0.05) as compared with control. Total plant biomass and plant height increased with increasing Mn supply. Mn concentrations in leaves and stems were constantly greater than those in roots, the ratio of concentrations in leaves to that in roots were 2.58-6.72 and the corresponding values in stems to that in roots were 1.45-3.18. The results showed that P. lapathifolium is a Mn-hyperaccumulator.

A Newly Identified Passive Hyperaccumulator Eucalyptus grandis × E. urophylla under Manganese Stress

Manganese (Mn) is an essential micronutrient needed for plant growth and development, but can be toxic to plants in excess amounts. However, some plant species have detoxification mechanisms that allow them to accumulate Mn to levels that are normally toxic, a phenomenon known as hyperaccumulation. These species are excellent candidates for developing a cost-effective remediation strategy for Mn-polluted soils. In this study, we identified a new passive Mn-hyperaccumulator Eucalyptus grandis × E. urophylla during a field survey in southern China in July 2010. This hybrid can accumulate as much as 13,549 mg/kg DW Mn in its leaves. Our results from Scanning Electron Microscope (SEM) X-ray microanalysis indicate that Mn is distributed in the entire leaf and stem cross-section, especially in photosynthetic palisade, spongy mesophyll tissue, and stem xylem vessels. Results from size-exclusion chromatography coupled with ICP-MS (Inductively coupled plasma mass spectrometry) lead us to speculate that Mn associates with relatively high molecular weight proteins and low molecular weight organic acids, including tartaric acid, to avoid Mn toxicity. Our results provide experimental evidence that both proteins and organic acids play important roles in Mn detoxification in Eucalyptus grandis × E. urophylla. The key characteristics of Eucalyptus grandis × E. urophylla are an increased Mn translocation facilitated by transpiration through the xylem to the leaves and further distribution throughout the leaf tissues. Moreover, the Mn-speciation profile obtained for the first time in different cellular organelles of Eucalyptus grandis × E. urophylla suggested that different organelles have differential accumulating abilities and unique mechanisms for Mn-detoxification.