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Aims: The plant available phosphorus (P) is deemed as the main soil factor affecting the accumulation of rare earth elements (REEs) in the hyperaccumulator plant Phytolacca americana, but the underlying mechanisms are largely unknown. Methods: Soil P amendment and exogenous organic acids addition experiments were employed to determinate the P-induced soil chemical properties changes and subsequent REE accumulation variations in P. americana. Manipulations of P and REE concentration were also conducted under hydroponic conditions, and the translocation of P and REE from roots to shoots was analyzed. Results: P-deficiency induced a decrease of pH (0.8 unit) and secretion of organic acids in the rhizosphere, thus contributing to the mobilization of P and REEs, and the increased REE accumulation in P. americana eventually. However, an elevated soil P supply reduced REE accumulation by 90% in shoots. P spray did not affect the accumulation of yttrium (Y) in P. americana under 1–10 μmol L−1 Y treatment, but limited Y translocation from roots to shoots under 100 μmol L−1 Y treatment. Moreover, compared to 1 μmol L−1 Y treatment, under 100 μmol L−1 Y the root P concentration was nearly tripled and more P was allocated to the apoplast. The linear regression slope between Y and P concentrations in the roots of hydroponic P. americana was 3.4, being close to the Y/P ratio of YPO4 precipitates (2.9). Conclusions: Our study demonstrates that P mediated REE mobilization in the rhizosphere and apoplast precipitation within the endosphere can regulate the REE accumulation in P. americana. [ABSTRACT FROM AUTHOR]

Aims: Dicranopteris linearis is a rare earth element (REE), aluminium (Al) and silicon (Si) hyperaccumulator plant which occurs in southern China. To date, there have been no studies on the variation in elemental accumulation among populations of Dicranopteris linearis occurring on REE-enriched and non-REE enriched soils. Methods: In total 43 Dicranopteris linearis individuals and the corresponding rhizosphere soils from 7 populations in southern China were sampled and the plant material and soil composition were chemically analysed for REE and other elements. Results: Dicranopteris linearis populations from REE-enriched soils and non-REE enriched soils both have the ability to accumulate high concentrations of REE (> 1000 mg kg− 1), Al (> 1000 mg kg− 1) and Si (> 10 000 mg kg− 1). The ratios of light REEs (LREEs) to heavy REEs (HREEs) in Dicranopteris linearis pinnae are consistently > 1, regardless of their ratios in the corresponding soils. Concentrations of REE in Dicranopteris linearis pinnae vary significantly among populations and have positive correlations with total and extractable soil REE concentrations. The plant Al and Si concentrations in Dicranopteris linearis populations are uncoupled from variations in soil Al and Si concentrations. The plant REE concentrations have no obvious relationships with the prevailing Al concentrations, but are positively correlated with plant Si and Mn, and negatively correlated with soil extractable P concentrations. Conclusions: The hyperaccumulation of REEs (LREE > HREE), Al and Si in non-REE enriched soils is a species-wide trait. The accumulation of REEs is highly variable and associated with soil REE concentrations, nutrients status, and other soil properties. [ABSTRACT FROM AUTHOR]

Highlights • Zinc could severely reduced Ni uptake in the hyperaccumulator Noccaea caerulescens. • Zinc or Fe deficiency could stimulate Ni absorption. • Cobalt, the non-essential element for plants, could severely suppress Ni uptake. • The expression levels of ZIP10 and IRT1 could be stimulated by Ni exposure. Abstract Nickel hyperaccumulators are a group of particular plant species which possess high efficient Ni uptake transport systems in their roots, the mechanisms of which however are far from being understood. Study on the interactions between nickel and other trace metals during root absorption process can provide insights into the Ni uptake patterns. Therefore, this work used model hyperaccumulator species Noccaea caerulescens to comprehensively assess the interaction between Ni and other trace metals (Zn, Fe, Mn, Cu and Co) during the root uptake process. Plants were grown in nutrient solutions containing different concentrations of Ni, Zn, Fe and Co, and the metal concentrations in roots and shoots were determined. Furthermore, the gene expression levels of Zn and Fe transporters were determined by quantitative real-time reverse transcriptase PCR. Results showed that Ni uptake could be severely suppressed by the presence of Zn or Co, while greatly stimulated by Zn or Fe deficiency. Iron uptake could be also stimulated by Ni exposure. In addition, the expression levels of the Zn transporter ZIP10 and the Fe transporter IRT1 were increased after Ni exposure. It could be concluded that the great competition between Ni and the non-essential element Co indicates that Ni uptake is mainly controlled by non-specific transporters. Nickel may be mainly taken up via Zn transporters, while Fe transporters also play a role for Ni absorption. [ABSTRACT FROM AUTHOR]

Background: Hyperaccumulator plants are unusual plants that accumulate particular metals or metalloids, such as nickel, zinc, cadmium and arsenic, in their living tissues to concentrations that are hundreds to thousands of times greater than what is normal for most plants. The hyperaccumulation phenomenon is rare (exhibited by less than 0.2% of all angiosperms), with most of the ~500 hyperaccumulator species known globally for nickel.Scope: This review highlights the contemporary understanding of nickel hyperaccumulation processes, which include root uptake and sequestration, xylem loading and transport, leaf compartmentation and phloem translocation processes.Conclusions: Hyperaccumulator plants have evolved highly efficient physiological mechanisms for taking up nickel in their roots followed by rapid translocation and sequestration into the aerial shoots. The uptake of nickel is mainly involved with low affinity transport systems, presumably from the ZIP family. The presence of high concentrations of histidine prevents nickel sequestration in roots. Nickel is efficiently loaded into the xylem, where it mainly presents as Ni2+. The leaf is the main storage organ, which sequestrates nickel in non-active sites, e.g. vacuoles and apoplast. Recent studies show that phloem translocates high levels of nickel, which has a strong impact on nickel accumulation in young growing tissues. [ABSTRACT FROM AUTHOR]

On the basis of our previous field survey, we postulate that the pattern and degree of zinc (Zn) isotope fractionation in the Zn hyperaccumulator Noccaea caerulescens (J. & C. Presl) F. K. Mey may reflect a relationship between Zn bioavailability and plant uptake strategies. Here, we investigated Zn isotope discrimination during Zn uptake and translocation in N. caerulescens and in a nonaccumulator Thlaspi arvense L. with a contrasting Zn accumulation ability in response to low (Zn-L) and high (Zn-H) Zn supplies. The average isotope fractionations of the N. caerulescens plant as a whole, relative to solution (Δ66Znplant-solution), were -0.06 and -0.12‰ at Zn-L-C and Zn-H-C, respectively, indicative of the predominance of a high-affinity (e.g., ZIP transporter proteins) transport across the root cell membrane. For T. arvense, plants were more enriched in light isotopes under Zn-H-A (Δ66Znplant-solution = -0.26‰) than under Zn-L-A and N. caerulescens plants, implying that a low-affinity (e.g., ion channel) transport might begin to function in the nonaccumulating plants when external Zn supply increases. Within the root tissues of both species, the apoplast fractions retained up to 30% of Zn mass under Zn-H. Moreover, the highest δ66Zn (0.75‰-0.86‰) was found in tightly bound apoplastic Zn, pointing to the strong sequestration in roots (e.g., binding to high-affinity ligands/precipitation with phosphate) when plants suffer from high Zn stress. During translocation, the magnitude of isotope fractionation was significantly greater at Zn-H (Δ66Znroot-shoot = 0.79‰) than at Zn-L, indicating that fractionation mechanisms associated with root-shoot translocation might be identical to the two plant species. Hence, we clearly demonstrated that Zn isotope fractionation could provide insight into the internal sequestration mechanisms of roots when plants respond to low and high Zn supplies. [ABSTRACT FROM AUTHOR]

Aims: This study evaluated the effect of phloem translocation on Ni accumulation in the hyperaccumulator Noccaea caerulescens. Methods: The first experiment assessed the metal and organic compound concentrations in phloem sap. Leaves were cut from plants grown in nutrient solutions added with 100 μM Ni or Zn, and then used for phloem sap extraction by means of the EDTA-stimulated exudation method. In the second experiment, Ni was applied to old leaves as a foliar spray to assess bidirectional movement of Ni in phloem. Results: In the first experiment, enrichment of Ni or Zn was found in phloem exudates, indicating high Ni and Zn phloem loading and translocation capacity in N. caerulescens. Amino acids, e.g. nicotianamine and histidine, were present at low concentrations in exudates, which are insufficient for Ni and Zn chelation. On the contrary, concentrations of organic acids, especially malate, were high in all treatments, which may be involved in Ni and Zn chelation in phloem. The second experiment showed that 89 % of Ni exported from old leaves was translocated upward to young leaves, whereas only 11 % moved downward to roots. Conclusions: Phloem sap of N. caerulescens is enriched in Ni and malate, the majority of which moves upward to young tissues. Phloem translocation may play an important role for Ni accumulation in young leaves of N. caerulescens. [ABSTRACT FROM AUTHOR]

Stable Zn isotope signatures offer a potential tool for tracing Zn uptake and transfer mechanisms within plant-soil systems. Zinc isotopic compositions were determined in the Zn hyperaccumulator Noccaea caerulescens collected at a Zn-contaminated site (Viviez), a serpentine site (Vosges), and a noncontaminated site (Sainte Eulalie) in France. Meanwhile, a Zn-tolerant plant (Silene vulgaris) was also collected at Viviez for comparison. While δ66Zn was substantially differentiated among N. caerulescens from the three localities, they all exhibited an enrichment in heavy Zn isotopes of 0.40-0.72‰ from soil to root, followed by a depletion in heavy Zn from root to shoot (-0.10 to -0.50‰). The enrichment of heavy Zn in roots is ascribed to the transport systems responsible for Zn absorption into root symplast and root-to-shoot translocation, while the depletion in heavy Zn in shoots is likely to be mediated by a diffusive process and an efficient translocation driven by energy-required transporters (e.g., NcHMA4). The mass balance yielded a bulk Zn isotopic composition between plant and soil (Δ66Znplant-soil) of -0.01‰ to 0.63‰ in N. caerulescens, indicative of high- and/or low-affinity transport systems operating in the three ecotypes. In S. vulgaris, however, there was no significant isotope fractionation between whole plant and rhizosphere soil and between root and shoot, suggesting that this species appears to have a particular Zn homeostasis. We confirm that quantifying stable Zn isotopes is useful for understanding Zn accumulation mechanisms in plants. [ABSTRACT FROM AUTHOR]

In this paper, the effects of indole-3-acetic acid (IAA) and/or ethylenediaminetetraacetic acid (EDTA) on lead uptake by a Zn/Cd hyperaccumulator Picris divaricata were studied. P. divaricata responded to Pb by better root system and increased biomass in presence of phytohormone IAA, which was able to reduce the inhibiting effects of Pb on transpiration without reducing the uptake of Pb The application of 100 μM IAA increased plant transpiration rate by about 20% and Pb concentration in leaves by about 37.3% as compared to treatment exposed to Pb alone. The enhanced phytoextraction efficiency could be attributed to the mechanisms played by IAA through alleviating Pb toxicity, creating better root system and plant biomass, promoting a higher transpiration rate as well as regulating the level of nutrient elements. On the contrary, inefficiency of phytoextraction was found with EDTA or the combination of IAA and EDTA probably because most Pb was in the form of Pb-EDTA complex which blocked the uptake by P. divaricata. The present study demonstrated that IAA was able to enhance the phytoextraction of Pb by Zn/Cd hyperaccumulator P. divaricata, providing a feasible method for the phytoremediation of polymetallic contaminated soils. [ABSTRACT FROM AUTHOR]

• Rare earth elements can be recovered from an hyperaccumulating fern. • REEs can be precipitated as oxalate after direct extraction from the biomass. • H 2 SO 4 is used to extract REEs and to obtain a highly concentrated final solid. • Major REEs behave in the same way throughout the process. Dicranopteris linearis is a fern that grows naturally on the tailings of former rare earth element (REE) mines in southern China. It accumulates REEs in its aerial parts at concentrations that classify this plant as hyperaccumulator. The main objective of this research is to design a process to recover REEs directly from the dry biomass. This process involves three steps: (i) extraction of REEs from the dry plant by leaching with a solution containing acid and/or complexing agents, (ii) precipitation of REEs with oxalate and (iii) calcination to prepare REE oxide. A methodology based on speciation calculation made it possible to determine operating conditions for precipitation by accounting for the influence of the species present according to the extraction method. Simulations were carried out using Chess software, allowing the calculation of the composition of a multicomponent solution at thermodynamic equilibrium. These operating conditions were then validated experimentally. REE extraction from the dry plant was significantly improved under acid conditions (with H 2 SO 4) or with a strong complexing agent (ethylenediaminetetraacetic acid). After optimization of the precipitation conditions, REE precipitation yields were above 90 %. After calcination, the final solid (REE oxide) contained ca 69 % of the REEs present in the initial biomass. The use of sulphuric acid as an extracting agent resulted in the recovery of a solid composed of 52 wt % of REEs (based on metal content). Thus, this study proposes a new recovery route for these strategic elements with a REE concentration factor from soil to final solid of 2500. [ABSTRACT FROM AUTHOR]

Abstract: To better understand the photosynthesis under stress, the effect of cadmium on carbon assimilation and chloroplast ultrastructure of a newly found Zn/Cd hyperaccumulator Picris divaricata in China was investigated in solution culture. The shoot and root Cd concentrations increased with increase in Cd supply, reaching maxima of 1109 and 5604mgkg−1 dry weight at 75μM Cd, respectively. As Cd supply to P. divaricata increased, the shoot and root dry weight, leaf water content (except 75μM Cd), concentrations of chlorophyll a and b, chlorophyll a/b ratio and the concentration of carotenoids were not depressed at high Cd. However, the stomatal conductance, transpiration rate, net photosynthetic rate and intercellular CO2 concentration were significantly affected when the Cd concentration reached 10, 10, 25 and 75μM, respectively. Meanwhile, carbonic anhydrase (CA; EC 4.2.1.1) activity and Rubisco (EC 4.1.1.39) content reached maxima in the presence of 50 and 5μM Cd, respectively. In addition, CA activity correlated positively with shoot Cd in plants treated with Cd at a range of 0–50μM. Moreover, the activities of NADP+-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13), Rubisco and fructose-1, 6-bisphosphatase (EC 3.1.3.11) were not significantly suppressed by increased Cd supply. Although the mesophyll cell size was reduced, chloroplast ultrastructure remained intact at the highest Cd treatment. Our finding revealed that P. divaricata chloroplast and the enzymes of carbon assimilation tolerate high levels of Cd, demonstrating its potential in possible application in phytoremediation. [Copyright &y& Elsevier]

A hydroponic experiment was carried out to study the effect of cadmium (Cd) on growth, Cd accumulation, lipid peroxidation, reactive oxygen species (ROS) content and antioxidative enzymes in leaves and roots of Arabis paniculata F., a new Cd hyperaccumuator found in China. The results showed that 22–89μM Cd in solution enhanced the growth of A. paniculata after three weeks, with 21–27% biomass increase compared to the control. Cd concentrations in shoots and roots increased with increasing Cd supply levels, and reached a maximum of 1662 and 8670mgkg−1 Cd dry weight at 178μM Cd treatment, respectively. In roots, 22–89μM Cd reduced the content of malondialdehyde (MDA), superoxide () and H2O2 as well as the activities of superoxide dismutase (SOD), guaiacol peroxidase (GPX), ascorbate peroxidase (APX) and glutathione reductase (GR). In leaves, the contents of MDA, and H2O2 remained unaffected by 22–89μM Cd, while 178μM Cd treatment significantly increased the MDA content, 69.5% higher than that of the control; generally, the activities of SOD, catalase (CAT), GPX and APX showed an increasing pattern with increasing Cd supply levels. Our present work concluded that A. paniculata has a great capability of Cd tolerance and accumulation. Moderate Cd treatment (22–89μM Cd) alleviated the oxidative stress in roots, while higher level of Cd addition (178μM) could cause an increasing generation of ROS, which was effectively scavenged by the antioxidative system. [Copyright &y& Elsevier]

Dicranopteris linearis is the best-known hyperaccumulator species of rare earth elements (REEs) and silicon (Si), capable of dealing with toxic level of REEs. Hence, this study aimed to clarify how D. linearis leaves cope with excessive REE stress, and whether Si plays a role in REE detoxification. The results show that lanthanum (La – as a representative of the REEs) stress led to decreased biomass and an increase of metabolism related to leaf cell wall synthesis and modification. However, the La stress-induced responses, especially the increase of pectin-related gene expression level, pectin polysaccharides concentration, and methylesterase activity, could be mitigated by Si supply. Approximately 70% of the Si in D. linearis leaves interacted with the cell walls to form organosilicon Si-O-C linkages. The Si-modified cell walls contained more hydroxyl groups, leading to a more efficient REE retention compared to the Si-free ones. Moreover, this [Si-cell wall] matrix increased the pectin-La accumulation capacity by 64%, with no effect on hemicellulose-La and cellulose-La accumulation capacity. These results suggest that [Si-pectin] matrix fixation is key in REE detoxification in D. linearis , laying the foundation for the development of phytotechnological applications (e.g. , REE phytomining) using this species in REE-contaminated sites. [Display omitted] • How hyperaccumulator plants detoxify excessive REE stress remain unclear. • Si supply can alleviate the REE toxicity in a hyperaccumulator fern. • Si mitigates the increase of pectin synthesis and modification induced by REE stress. • Most Si in the leaves form Si-O-C linkage with the cell wall component pectin. • [Si-pectin] matrix enhances the REE accumulation and retention capacity. [ABSTRACT FROM AUTHOR]

Abstract: Mechanisms of cadmium (Cd)-induced inhibition of photosynthesis in the Zn/Cd hyperaccumulator Picris divaricata were investigated using photosynthesis limitation analysis. P. divaricata seedlings were grown in nutrient solution containing 0, 5, 10, 25, 50, or 75 μM Cd for 2 weeks. Total limitations to photosynthesis (T L) increased from 0% at 5 μM Cd to 68.8% at 75 μM Cd. CO2 diffusional limitation (D L) made the largest contribution to T L, accounting for 93–98% of T L in the three highest Cd treatments, compared to just 2–7% of T L attributable to biochemical limitation (B L). Microscopic imaging revealed significantly decreased stomatal density and mesophyll thickness in the three highest Cd treatments. Chlorophyll fluorescence parameters related to photosynthetic biochemistry (F v/F m, NPQ, Φ PSII, and q P) were not significantly decreased by increased Cd supply. Our results suggest that increased D L in leaves is the main cause of Cd-induced inhibition of photosynthesis in P. divaricata, possibly due to suppressed function of mesophyll and stomata. Analysis of chlorophyll fluorescence showed that Cd supply had little effect on photochemistry parameters, suggesting that the PSII reaction centers are not a main target of Cd inhibition of photosynthesis in P. divaricata. [Copyright &y& Elsevier]

Abstract: Potentilla griffithii Hook is a newly found hyperaccumulator plant capable of high tolerance and accumulation of Zn and Cd. We investigated the interactive effects between Cd and Zn on accumulation and vacuolar sequestration in P. griffithii. Stimulatory effect of growth was noted at 0.2mM Cd and 1.25 and 2.5mM Zn tested. Accumulation of Zn and Cd in roots, petioles and leaves were increased significantly with addition of these metals individually. However, the Zn supplement decreased root Cd accumulation but increased the concentration of Cd in petioles and leaves. The results from sub-cellular distribution showed that up to 94% and 70% of the total Zn and Cd in the leaves were present in the protoplasts, and more than 90% Cd and Zn in the protoplasts were localized in the vacuoles. Nearly, 88% and 85% of total Cd and Zn were extracted in the cell sap of the leaves suggesting that most of the Cd and Zn in the leaves were available in soluble form. The present results indicate that Zn supplement significantly enhanced the petiole accumulation of Cd and further vacuolar sequestration plays an important role in tolerance, detoxification and hyperaccumulation of these metals in P. griffithii. [Copyright &y& Elsevier]

Abstract: This experiment investigated the Zn accumulation and tolerance, and responses of non-protein thiols in Zn hyperaccumulator Arabis paniculata under different Zn stress (0, 100, 250, 500, 1000 and 2000μM Zn). No obvious phytotoxicity or biomass reduction was observed in the plants during a 3-w of growth. Compared to the control, addition of 100–500μM Zn increased plant biomass by 18–38%. The high Zn concentrations in the roots (9025–12,892mgkg−1) and the shoots (777–6030mgkg−1) did not impair the membrane stability or increase the lipid peroxidation except for the roots treated with 2000μM Zn. Substantial amounts of Zn accumulated in the plant were water-soluble, with 58–73% in the shoots and 30–53% in the roots. Low Zn translocation from the shoots to the roots was observed in all treatments. This phenomenon was consistent with the XRD analysis, which showed the presence of ZnO and ZnS in the ashed-roots. No phytochelatin was detected in the shoots and only PC4 was eluted in the roots, suggesting their limited role in Zn tolerance in A. paniculata. While glutathione did not correspond to Zn loading in plants, cysteine had significant positive correlation with Zn concentration in plants. Our data suggested that A. paniculata exhibited high Zn tolerance and cysteine may be involved in its Zn tolerance. [ABSTRACT FROM AUTHOR]

Abstract: The Zn/Cd hyperaccumulator Arabis paniculata is able to tolerate high level of Zn and Cd. To clarify the molecular basis of Zn and Cd tolerance, proteomic approaches were applied to identify proteins involved in Zn and Cd stress response in A. paniculata. Plants were exposed to both low and high Zn or Cd levels for 10 d. Proteins of leaves in each treatment were separated by 2-DE (two-dimensional electrophoresis). Nineteen differentially-expressed proteins upon Zn treatments and 18 proteins upon Cd treatments were observed. Seventeen out of 19 of Zn-responsive proteins and 16 out of 18 of Cd-responsive proteins were identified using MALDI-TOF/TOF-MS (matrix-assisted laser desorption/ionization time of flight mass spectrometry). The most of identified proteins were known to function in energy metabolism, xenobiotic/antioxidant defense, cellular metabolism, protein metabolism, suggesting the responses of A. paniculata to Zn and Cd share similar pathway to certain extend. However, the different metal defense was also revealed between Zn and Cd treatment in A. paniculata. These results indicated that A. paniculata against to Zn stress mainly by enhancement of energy metabolism including auxin biosynthesis and protein metabolism to maintain plant growth and correct misfolded proteins. In the case of Cd, plants adopted antioxidative/xenobiotic defense and cellular metabolism to keep cellular redox homeostasis and metal-transportation under Cd stress. [Copyright &y& Elsevier]

Abstract: In this study, zinc (Zn) and cadmium (Cd) tolerance, accumulation and distribution was conducted in Potentilla griffithii H., which has been identified as a new Zn hyperaccumulator found in China. Plants were grown hydroponically with different levels of Zn2+ (20, 40, 80 and 160mgL−1) and Cd2+ (5, 10, 20 and 40mgL−1) for 60 days. All plants grew healthy and attained more biomass than the control, except 40mgL−1 Cd treatment. Zn or Cd concentration in plants increased steadily with the increasing addition of Zn or Cd in solution. The maximum metal concentrations in roots, petioles and leaves were 14,060, 19,600 and 11,400mgkg−1 Zn dry weight (DW) at 160mgL−1 Zn treatment, and 9098, 3077 and 852mgkg−1 Cd DW at 40mgL−1 Cd treatment, respectively. These results suggest that P. griffithii has a high ability to tolerate and accumulate Cd and Zn, and it can be considered not only as Zn but also as a potential cadmium hyperaccumulator. Light microscope (LM) with histochemical method, scanning electron microscope combined with energy dispersive spectrometry (SEM-EDS) and transmission electron microscope (TEM) were used to determine the distribution of Zn and Cd in P. griffithii at tissue and cellular levels. In roots, SEM-EDS confirmed that the highest Zn concentration was found in xylem parenchyma cells and epidermal cells, while for Cd, a gradient was observed with the highest Cd concentration in rhizodermal and cortex cells, followed by central cylinder. LM results showed that Zn and Cd distributed mainly along the walls of epidermis, cortex, endodermis and some xylem parenchyma. In leaves, Zn and Cd shared the similar distribution pattern, and both were mostly accumulated in epidermis and bundle sheath. However, in leaves of 40mgL−1 Cd treatment, which caused the phytotoxicity, Cd was also found in the mesophyll cells. The major storage site for Zn and Cd in leaves of P. griffithii was vacuoles, to a lesser extent cell wall or cytosol. The present study demonstrates that the predominant sequestration of Zn and Cd in cell walls of roots and in vacuoles of epidermis and bundle sheath of leaves may play a major role in strong tolerance and hyperaccumulation of Zn and Cd in P. griffithii. [Copyright &y& Elsevier]