Differential metal-specific tolerance and accumulation patterns among Thlaspi caerulescens populations originating from different soil types

Root Zn sequestration transporter heavy metal ATPase 3 from Odontarrhena chalcidica enhance Cd tolerance and accumulation in Arabidopsis thaliana

The Ni hyperaccumulator Odontarrhena chalcidica (formerly Alyssum murale), exhibits a significant capacity to accumulate Zn in the roots. However, the molecular mechanisms underlying the variation in Ni and Zn accumulation are poorly understood. Here, we isolated a homolog of heavy metal ATPase 3 from O. chalcidica (OcHMA3) and characterized its functions using heterologous systems. Phylogenetic analysis revealed that OcHMA3 protein shares 87.6 % identity with AtHMA3, with similar metal binding sites to other HMA3 proteins. Heterologous expression of OcHMA3 in yeast increased sensitivity to Cd, Ni and Zn, suggesting it functions as a broad-specificity transporter. Further investigation showed OcHMA3 is constitutively expressed in the roots and localized to the tonoplast. Overexpression of OcHMA3 in A. thaliana shoots increased its roots Zn concentrations by 41.9 % - 74.1 %. However, overexpression of OcHMA3 in roots enhanced its tolerance to Cd and increased roots Cd concentrations by 50.9 % - 90.6 %. Our findings indicated OcHMA3 is responsible for Zn sequestration in root vacuoles, likely leading to Zn retention in roots and subsequent Ni hyperaccumulation in shoots. This study elucidates the molecular mechanism of Ni and Zn accumulation in O. chalcidica, and identifies OcHMA3 as a potential gene for developing Zn-rich plants and for phytoextraction in Cd-contaminated soils.

Species-Specific and Pollution-Induced Changes in Gene Expression and Metabolome of Closely Related Noccaea Species Under Natural Conditions

Hyperaccumulators within the Noccaea genus possess many promising genetic and metabolic adaptations that could be potentially exploited to support phytoremediation efforts and/or crop improvement and biofortification. Although hyperaccumulation is very common in this genus, individual species display specific traits as they can accumulate different elements (e.g., zinc, cadmium, and/or nickel). Moreover, there appears to be some populational variability with natural selection increasing the metal tolerance in metallicolous populations. Therefore, employing robust methods, such as integrated analysis of the transcriptome and metabolome, is crucial for uncovering pivotal candidate genes and pathways orchestrating the response to metal stress in Noccaea hyperaccumulators. Our study highlights several species-specific traits linked to the detoxification of metals and metal-induced oxidative stress in hyperaccumulating N. praecox when compared to a closely related model species, N. caerulescens, when grown in the field. Transcriptome analysis revealed distinct differences between the three studied natural Noccaea populations. Notably, we observed several pathways frequently connected to metal stress, i.e., glutathione metabolism, phenylpropanoid biosynthesis, and flavonoid biosynthesis, which were enriched. These differences were observed despite the relative evolutionary closeness of studied species, which emphasizes the importance of further expanding our knowledge on hyperaccumulators if we want to exploit their mechanisms for phytoremediation efforts or food quality improvements.

Physiological dynamics as indicators of plant response to manganese binary effect

Introduction

Heavy metals negatively affect plant physiology. However, plants can reduce their toxicity through physiological responses. Broussonetia papyrifera is a suitable candidate tree for carrying out the phytoremediation of manganese (Mn)-contaminated soil.

Methods

Considering that Mn stress typically exerts a binary effect on plants, to reveal the dynamic characteristics of the physiological indexes of B. papyrifera to Mn stress, we conducted pot experiments with six different Mn concentrations (0, 0.25, 0.5, 1, 2, and 5 mmol/L) for 60 days. In addition to the chlorophyll content, malondialdehyde (MDA), proline (PRO), soluble sugar, superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), the absorption and transfer characteristics of Mn, and root structure were also measured.

Results

Phytoremedial potential parameters such as the bioconcentration factor (BCF) and translocation factor (TF) displayed an increasing trend with the increase of Mn concentration. At lower Mn concentrations (<0.5 mmol/L), the TF value was <1 but crossed 1 when the Mn concentration exceeded 100 mmol/L. The Mn distribution in various tissues was in the following order: leaf > stem > root. The root structure analysis revealed that low-level concentrations of Mn (1 mmol/L) promoted root development. Mn concentration and stress duration had significant effects on all measured physiological indexes, and except soluble sugar, Mn concentration and stress time displayed a significant interaction on the physiological indexes.

Discussion

Our study demonstrates that the physiological indexes of B. papyrifera display dynamic characteristics under Mn stress. Thus, during the monitoring process of Mn stress, it appears to be necessary to appropriately select sampling parts according to Mn concentration.

DNA methylation is enhanced during Cd hyperaccumulation in Noccaea caerulescens ecotype Ganges

In this study, we assess the DNA damage occurring in response to cadmium (Cd) in the Cd hyperaccumulator Noccaea caerulescens Ganges (GA) vs the non-accumulator and close-relative species Arabidopsis thaliana. At this purpose, the alkaline comet assay was utilized to evaluate the Cd-induced variations in nucleoids and the methy-sens comet assay, and semiquantitative real-time (qRT)-PCR were also performed to associate nucleus variations to possible DNA modifications. Cadmium induced high DNA damages in nuclei of A. thaliana while only a small increase in DNA migration was observed in N. caerulescens GA. In addition, in N. caerulescens GA, CpG DNA methylation increase upon Cd when compared to control condition, along with an increase in the expression of MET1 gene, coding for the DNA-methyltransferase. N. caerulescens GA does not show any oxidative stress under Cd treatment, while A. thaliana Cd-treated plants showed an upregulation of transcripts of the respiratory burst oxidase, accumulation of reactive oxygen species, and enhanced superoxide dismutase activity. These data suggest that epigenetic modifications occur in the N. caerulescens GA exposed to Cd to preserve genome integrity, contributing to Cd tolerance.

Construction and analysis of a Noccaea caerulescens TILLING population

BACKGROUND

Metals such as Zn or Cd are toxic to plant and humans when they are exposed in high quantities through contaminated soil or food. Noccaea caerulescens, an extraordinary Zn/Cd/Ni hyperaccumulating species, is used as a model plant for metal hyperaccumulation and phytoremediation studies. Current reverse genetic techniques to generate mutants based on transgenesis is cumbersome due to the low transformation efficiency of this species. We aimed to establish a mutant library for functional genomics by a non-transgenic approach, to identify mutants with an altered mineral profiling, and to screen for mutations in bZIP19, a regulator of Zn homeostasis in N. caerulescens.

RESULTS

To generate the N. caerulescens mutant library, 3000 and 5000 seeds from two sister plants of a single-seed recurrent inbred descendant of the southern French accession Saint-Félix-de-Pallières (SF) were mutagenized respectively by 0.3 or 0.4% ethyl methane sulfonate (EMS). Two subpopulations of 5000 and 7000 M2 plants were obtained after 0.3 or 0.4% EMS treatment. The 0.4% EMS treatment population had a higher mutant frequency and was used for TILLING. A High Resolution Melting curve analysis (HRM) mutation screening platform was optimized and successfully applied to detect mutations for NcbZIP19, encoding a transcription factor controlling Zn homeostasis. Of four identified point mutations in NcbZIP19, two caused non-synonymous substitutions, however, these two mutations did not alter the ionome profile compared to the wild type. Forward screening of the 0.4% EMS treatment population by mineral concentration analysis (ionomics) in leaf material of each M2 plant revealed putative mutants affected in the concentration of one or more of the 20 trace elements tested. Several of the low-Zn mutants identified in the ionomic screen did not give progeny, illustrating the importance of Zn for the species. The mutant frequency of the population was evaluated based on an average of 2.3 knockout mutants per tested monogenic locus.

CONCLUSIONS

The 0.4% EMS treatment population is effectively mutagenized suitable for forward mutant screens and TILLING. Difficulties in seed production in low Zn mutants, obtained by both forward and reverse genetic approach, hampered further analysis of the nature of the low Zn phenotypes.

Multimodal synchrotron X-ray fluorescence imaging reveals elemental distribution in seeds and seedlings of the Zn-Cd-Ni hyperaccumulator Noccaea caerulescens

The molecular biology and genetics of the Ni-Cd-Zn hyperaccumulator Noccaea caerulescens has been extensively studied, but no information is yet available on Ni and Zn redistribution and mobilization during seed germination. Due to the different physiological functions of these elements, and their associated transporter pathways, we expected differential tissue distribution and different modes of translocation of Ni and Zn during germination. This study used synchrotron X-ray fluorescence tomography techniques as well as planar elemental X-ray imaging to elucidate elemental (re)distribution at various stages of the germination process in contrasting accessions of N. caerulescens. The results show that Ni and Zn are both located primarily in the cotyledons of the emerging seedlings and Ni is highest in the ultramafic accessions (up to 0.15 wt%), whereas Zn is highest in the calamine accession (up to 600 μg g-1). The distribution of Ni and Zn in seeds was very similar, and neither element was translocated during germination. The Fe maps were especially useful to obtain spatial reference within the seeds, as it clearly marked the vasculature. This study shows how a multimodal combination of synchrotron techniques can be used to obtain powerful insights about the metal distribution in physically intact seeds and seedlings.

Metal accumulation by Alyssum serpyllifolium subsp. malacitanum Rivas Goday (Brassicaceae) across different petrographic entities in South-Iberian ultramafic massifs: plant-soil relationships and prospects for phytomining

To date, studies of hyperaccumulation in plant tissues on ultramafic rocks have not considered the great diversity of petrographic entities in the world's outcrops. One of them is the one that we studied in Spain with more than eight petrographic entities and different soils. Our hypothesis is that the different chemical compositions of the soils in ultramafic rocks significantly affect the hyperaccumulation of metals by specialized plants, which may have consequences for phytomining. For this purpose, individuals, populations, and different soils have been tested and the results have been subjected to the corresponding statistical tests. The obtained knowledge reflects the different behavior of the studied plant not only for the Ni: the obtained results for Sr and for Ba revealed interesting results for the hyperaccumulation in Alyssum of both metals.

Low-molecular-weight ligands in plants: role in metal homeostasis and hyperaccumulation

Mineral nutrition is one of the key factors determining plant productivity. In plants, metal homeostasis is achieved through the functioning of a complex system governing metal uptake, translocation, distribution, and sequestration, leading to the maintenance of a regulated delivery of micronutrients to metal-requiring processes as well as detoxification of excess or non-essential metals. Low-molecular-weight ligands, such as nicotianamine, histidine, phytochelatins, phytosiderophores, and organic acids, play an important role in metal transport and detoxification in plants. Nicotianamine and histidine are also involved in metal hyperaccumulation, which determines the ability of some plant species to accumulate a large amount of metals in their shoots. In this review we extensively summarize and discuss the current knowledge of the main pathways for the biosynthesis of these ligands, their involvement in metal uptake, radial and long-distance transport, as well as metal influx, isolation and sequestration in plant tissues and cell compartments. It is analyzed how diverse endogenous ligand levels in plants can determine their different tolerance to metal toxic effects. This review focuses on recent advances in understanding the physiological role of these compounds in metal homeostasis, which is an essential task of modern ionomics and plant physiology. It is of key importance in studying the influence of metal deficiency or excess on various physiological processes, which is a prerequisite to the improvement of micronutrient uptake efficiency and crop productivity and to the development of a variety of applications in phytoremediation, phytomining, biofortification, and nutritional crop safety.

Overexpression of ZNT1 and NRAMP4 from the Ni Hyperaccumulator Noccaea caerulescens Population Monte Prinzera in Arabidopsis thaliana Perturbs Fe, Mn, and Ni Accumulation

Metalliferous soils are characterized by a high content of metal compounds that can hamper plant growth. The pseudometallophyte Noccaea caerulescens is able to grow on metalliferous substrates by implementing both tolerance and accumulation of usually toxic metal ions. Expression of particular transmembrane transporter proteins (e.g., members of the ZIP and NRAMP families) leads to metal tolerance and accumulation, and its comparison between hyperaccumulator N. caerulescens with non-accumulator relatives Arabidopsis thaliana and Thlaspi arvense has deepened our knowledge on mechanisms adopted by plants to survive in metalliferous soils. In this work, two transporters, ZNT1 and NRAMP4, expressed in a serpentinic population of N. caerulescens identified on the Monte Prinzera (Italy) are considered, and their expression has been induced in yeast and in A. thaliana. In the latter, single transgenic lines were crossed to test the effect of the combined over-expression of the two transporters. An enhanced iron and manganese translocation towards the shoot was induced by overexpression of NcZNT1. The combined overexpression of NcZNT1 and NcNRAMP4 did perturb the metal accumulation in plants.

Differences in mineral accumulation and gene expression profiles between two metal hyperaccumulators, Noccaea japonica and Noccaea caerulescens ecotype Ganges, under excess nickel condition

Here we compare mineral accumulation and global gene expression patterns between two metal hyperaccumulator plants - Noccaea japonica, originating from Ni-rich serpentine soils, and Noccaea caerulescens (ecotype Ganges), originating from Zn/Pb-mine soils - under excess Ni conditions. Significant differences in the accumulation of K, P, Mg, B, and Mo were explained by the expression levels of specific transporters for each mineral. We previously showed that total Ni accumulation in the whole plant is higher in N. caerulescens than in N. japonica. Here we found a similar tendency for Fe under excess Ni; however, the expression of iron-regulated transporter 1 (IRT1), which encodes the primary Fe uptake transporter and causes excess Ni uptake in Arabidopsis thaliana, was higher in N. japonica. NjIRT1 has a point mutation at Asp100, which is essential for Fe transport, and so might lack its Fe and possibly Ni transport function. Noccaea japonica might have lost its IRT1 function, which would prevent excess Ni uptake via IRT1 in Ni-rich soils, and come to rely on other transporters.

Isotopic signatures reveal zinc cycling in the natural habitat of hyperaccumulator Dichapetalum gelonioides subspecies from Malaysian Borneo

BACKGROUND

Some subspecies of Dichapetalum gelonioides are the only tropical woody zinc (Zn)-hyperaccumulator plants described so far and the first Zn hyperaccumulators identified to occur exclusively on non-Zn enriched 'normal' soils. The aim of this study was to investigate Zn cycling in the parent rock-soil-plant interface in the native habitats of hyperaccumulating Dichapetalum gelonioides subspecies (subsp. pilosum and subsp. sumatranum). We measured the Zn isotope ratios (δ66Zn) of Dichapetalum plant material, and associated soil and parent rock materials collected from Sabah (Malaysian Borneo).

RESULTS

We found enrichment in heavy Zn isotopes in the topsoil (δ66Zn 0.13 ‰) relative to deep soil (δ66Zn -0.15 ‰) and bedrock (δ66Zn -0.90 ‰). This finding suggests that both weathering and organic matter influenced the Zn isotope pattern in the soil-plant system, with leaf litter cycling contributing significantly to enriched heavier Zn in topsoil. Within the plant, the roots were enriched in heavy Zn isotopes (δ66Zn ~ 0.60 ‰) compared to mature leaves (δ66Zn ~ 0.30 ‰), which suggests highly expressed membrane transporters in these Dichapetalum subspecies preferentially transporting lighter Zn isotopes during root-to-shoot translocation. The shoots, mature leaves and phloem tissues were enriched in heavy Zn isotopes (δ66Zn 0.34-0.70 ‰) relative to young leaves (δ66Zn 0.25 ‰). Thisindicates that phloem sources are enriched in heavy Zn isotopes relative to phloem sinks, likely because of apoplastic retention and compartmentalization in the Dichapetalum subspecies.

CONCLUSIONS

The findings of this study reveal Zn cycling in the rock-soil-plant continuum within the natural habitat of Zn hyperaccumulating subspecies of Dichapetalum gelonioides from Malaysian Borneo. This study broadens our understanding of the role of a tropical woody Zn hyperaccumulator plant in local Zn cycling, and highlights the important role of leaf litter recycling in the topsoil Zn budget. Within the plant, phloem plays key role in Zn accumulation and redistribution during growth and development. This study provides an improved understanding of the fate and behaviour of Zn in hyperaccumulator soil-plant systems, and these insights may be applied in the biofortification of crops with Zn.

Natural Fungal Endophytes From Noccaea caerulescens Mediate Neutral to Positive Effects on Plant Biomass, Mineral Nutrition and Zn Phytoextraction

Phytoextraction using hyperaccumulating plants is a method for the remediation of soils contaminated with trace elements (TEs). As a strategy for improvement, the concept of fungal-assisted phytoextraction has emerged in the last decade. However, the role played by fungal endophytes of hyperaccumulating plants in phytoextraction is poorly studied. Here, fungal endophytes isolated from calamine or non-metalliferous populations of the Cd/Zn hyperaccumulator Noccaea caerulescens were tested for their growth promotion abilities affecting the host plant. Plants were inoculated with seven different isolates and grown for 2 months in trace element (TE)-contaminated soil. The outcomes of the interactions between N. caerulescens and its native strains ranged from neutral to beneficial. Among the strains, Alternaria thlaspis and Metapochonia rubescens, respectively, isolated from the roots of a non-metallicolous and a calamine population of N. caerulescens, respectively, exhibited the most promising abilities to enhance the Zn phytoextraction potential of N. caerulescens related to a significant increase of the plant biomass. These strains significantly increased the root elemental composition, particularly in the case of K, P, and S, suggesting an improvement of the plant nutrition. Results obtained in this study provide new insights into the relevance of microbial-assisted phytoextraction approaches in the case of hyperaccumulating plants.

Imaging Zn and Ni distributions in leaves of different ages of the hyperaccumulator Noccaea caerulescens by synchrotron-based X-ray fluorescence

Mapping of leaves of hyperaccumulators can provide insights into the mechanisms these species utilize to accumulate high metal concentrations. We used synchrotron-based X-ray fluorescence (SXRF) to perform Zn and Ni imaging in leaves of different ages of Noccaea caerulescens. A mature leaf of the related non-hyperaccumulator Thlaspi arvense was also imaged. The concentrations of Zn, Ni, Co, and Cr in N. caerulescens grown on an ultramafic soil were 9-, 10-, 12-, and 3-fold higher than T. arvense. N. caerulescens showed an exceptional ability to accumulate Zn from the soil, posing a bioconcentration factor of 6.7. T. arvense had Zn and Ni distributed uniformly in the leaf blade with doubling fluorescence counts in the tip and margins, suggesting a strategy to excrete metals and avoid toxicity. On the other hand, N. caerulescens displayed distinctly different Zn and Ni accumulation patterns, regardless of the age or metal concentration in the leaves. Zinc was mainly distributed in the cells surrounding the central and secondary veins. Nickel accumulated in the margins and tips of the leaf blade. Given the time required to image large leaves in synchrotron facilities, small leaves can be used to represent the leaf distribution of Zn and Ni in N. caerulescens.

Rapid Hormetic Responses of Photosystem II Photochemistry of Clary Sage to Cadmium Exposure

Five-day exposure of clary sage (Salvia sclarea L.) to 100 μM cadmium (Cd) in hydroponics was sufficient to increase Cd concentrations significantly in roots and aboveground parts and affect negatively whole plant levels of calcium (Ca) and magnesium (Mg), since Cd competes for Ca channels, while reduced Mg concentrations are associated with increased Cd tolerance. Total zinc (Zn), copper (Cu), and iron (Fe) uptake increased but their translocation to the aboveground parts decreased. Despite the substantial levels of Cd in leaves, without any observed defects on chloroplast ultrastructure, an enhanced photosystem II (PSII) efficiency was observed, with a higher fraction of absorbed light energy to be directed to photochemistry (ΦPSΙΙ). The concomitant increase in the photoprotective mechanism of non-photochemical quenching of photosynthesis (NPQ) resulted in an important decrease in the dissipated non-regulated energy (ΦNO), modifying the homeostasis of reactive oxygen species (ROS), through a decreased singlet oxygen (1O2) formation. A basal ROS level was detected in control plant leaves for optimal growth, while a low increased level of ROS under 5 days Cd exposure seemed to be beneficial for triggering defense responses, and a high level of ROS out of the boundaries (8 days Cd exposure), was harmful to plants. Thus, when clary sage was exposed to Cd for a short period, tolerance mechanisms were triggered. However, exposure to a combination of Cd and high light or to Cd alone (8 days) resulted in an inhibition of PSII functionality, indicating Cd toxicity. Thus, the rapid activation of PSII functionality at short time exposure and the inhibition at longer duration suggests a hormetic response and describes these effects in terms of "adaptive response" and "toxicity", respectively.

The Evolutionary Genomics of Serpentine Adaptation

Serpentine barrens are among the most challenging settings for plant life. Representing a perfect storm of hazards, serpentines consist of broadly skewed elemental profiles, including abundant toxic metals and low nutrient contents on drought-prone, patchily distributed substrates. Accordingly, plants that can tolerate the challenges of serpentine have fascinated biologists for decades, yielding important insights into adaptation to novel ecologies through physiological change. Here we highlight recent progress from studies which demonstrate the power of serpentine as a model for the genomics of adaptation. Given the moderate - but still tractable - complexity presented by the mix of hazards on serpentine, these venues are well-suited for the experimental inquiry of adaptation both in natural and manipulated conditions. Moreover, the island-like distribution of serpentines across landscapes provides abundant natural replicates, offering power to evolutionary genomic inference. Exciting recent insights into the genomic basis of serpentine adaptation point to a partly shared basis that involves sampling from common allele pools available from retained ancestral polymorphism or via gene flow. However, a lack of integrated studies deconstructing complex adaptations and linking candidate alleles with fitness consequences leaves room for much deeper exploration. Thus, we still seek the crucial direct link between the phenotypic effect of candidate alleles and their measured adaptive value - a prize that is exceedingly rare to achieve in any study of adaptation. We expect that closing this gap is not far off using the promising model systems described here.

Evolution of the metal hyperaccumulation and hypertolerance traits

To succeed in life, living organisms have to adapt to the environmental issues to which they are subjected. Some plants, defined as hyperaccumulators, have adapted to metalliferous environments, acquiring the ability to tolerate and accommodate high amounts of toxic metal into their shoot, without showing symptoms of toxicity. The determinants for these traits and their mode of action have long been the subject of research, whose attention lately moved to the evolution of the hypertolerance and hyperaccumulation traits. Genetic evidence indicates that the evolution of both traits includes significant evolutionary events that result in species-wide tolerant and accumulating backgrounds. Different edaphic environments are responsible for subsequent refinement, by local adaptive processes, leading to specific strategies and various degrees of hypertolerance and hyperaccumulation, which characterize metallicolous from non-metallicolous ecotypes belonging to the same genetic unit. In this review, we overview the most updated concepts regarding the evolution of hyperaccumulation and hypertolerance, highlighting also the ecological context concerning the plant populations displaying this fascinating phenomenon.

FLC and SVP Are Key Regulators of Flowering Time in the Biennial/Perennial Species Noccaea caerulescens

The appropriate timing of flowering is crucial for plant reproductive success. Studies of the molecular mechanism of flower induction in the model plant Arabidopsis thaliana showed long days and vernalization as major environmental promotive factors. Noccaea caerulescens has an obligate vernalization requirement that has not been studied at the molecular genetics level. Here, we characterize the vernalization requirement and response of four geographically diverse biennial/perennial N. caerulescens accessions: Ganges (GA), Lellingen (LE), La Calamine (LC), and St. Felix de Pallières (SF). Differences in vernalization responsiveness among accessions suggest that natural variation for this trait exists within N. caerulescens. Mutants which fully abolish the vernalization requirement were identified and were shown to contain mutations in the FLOWERING LOCUS C (NcFLC) and SHORT VEGETATIVE PHASE (NcSVP) genes, two key floral repressors in this species. At high temperatures, the non-vernalization requiring flc-1 mutant reverts from flowering to vegetative growth, which is accompanied with a reduced expression of LFY and AP1. This suggested there is "crosstalk" between vernalization and ambient temperature, which might be a strategy to cope with fluctuations in temperature or adopt a more perennial flowering attitude and thus facilitate a flexible evolutionary response to the changing environment across the species range.

Nickel tolerance, translocation and accumulation in a Cd/Zn co-hyperaccumulator plant Sedum alfredii

Multi-elements hyperaccumulators are of high scientific interest to be applied in remediation of mix-contaminated soils. Sedum alfredii is a well-known Cd/Zn co-hyperaccumulator with high Pb and Cu tolerance. This study investigated the ability of the hyperaccumulating ecotype (HE) S. alfredii to tolerate and accumulate Ni. Differed from the non-hyperaccumulating ecotype (NHE), HE plants grew healthy after 50 μM Ni exposure for 4 weeks. The HE plants translocated up to 40 % Ni to the shoots under high Ni stress and accumulated >3000 and 200 mg kg^-1 Ni in roots and shoots, respectively. Micro-XRF image showed that Ni was highly restricted within the HE stem and leaf vascular bundles, especially the xylem tissues. The HE roots were of high Ni tolerance, showing much less pronounced Ni-induced phytotoxicity as compared with the NHEs. Ni-induced O₂- was observed in the apoplastic part of HE root cells, but both Ni and the induced O₂- were highly accumulated in the sensitive zone (root cap, meristem, and cylinder) of NHE roots. These results suggest that although low Ni mobility out of vascular tissues limits the metal accumulation in stems and leaves, HE S. alfredii is highly tolerant towards Ni stress by metal homeostasis in root cells.

Contrasting growth responses to aluminium addition among populations of the aluminium accumulator Melastoma malabathricum

Aluminium (Al) hyper-accumulation is a common trait expressed by tropical woody plants growing on acidic soils. Studies on Al accumulators have suggested that Al addition may enhance plant growth rates, but the functional significance of this trait and the mechanistic basis of the growth response are uncertain. This study aimed to test the hypothesis that differential growth responses to Al among populations of an Al accumulator species are associated with variation in biomass allocation and nutrient uptake. We conducted two experiments to test differential responses to the presence of Al in the growth medium for seedlings of the Al accumulator shrub Melastoma malabathricum collected from 18 populations across Peninsular Malaysia. Total dry mass and relative growth rate of dry mass were significantly greater for seedlings that had received Al in the growth medium than for control plants that did not receive Al, but growth declined in response to 5.0 mM Al addition. The increase in growth rate in response to Al addition was greater for a fast-growing than a slow-growing population. The increase in growth rate in response to Al addition occurred despite a reduction in dry mass allocation to leaves, at the expense of higher allocation to roots and stems, for plants grown with Al. Foliar concentrations of P, K, Mg and Ca increased in response to Al addition and the first axis of a PCA summarizing foliar nutrient concentrations among populations was correlated positively with seedling relative growth rates. Some populations of the Al hyper-accumulator M. malabathricum express a physiological response to Al addition which leads to a stimulation of growth up to an optimum value of Al in the growth medium, beyond which growth declines. This was associated with enhanced nutrient concentrations in leaves, which suggests that Al accumulation functions to optimize elemental stoichiometry and growth rate.

Spatially-resolved localization and chemical speciation of nickel and zinc in Noccaea tymphaea and Bornmuellera emarginata

Hyperaccumulator plants present the ideal model system for studying the physiological regulation of the essential (and potentially toxic) transition elements nickel and zinc. This study used synchrotron X-ray Fluorescence Microscopy (XFM) elemental imaging and spatially resolved X-ray Absorption Spectroscopy (XAS) to elucidate elemental localization and chemical speciation of nickel and zinc in the hyperaccumulators Noccaea tymphaea and Bornmuellera emarginata (synonym Leptoplax emarginata). The results show that in the leaves of N. tymphaea nickel and zinc have contrasting localization, and whereas nickel is present in vacuoles of epidermal cells, zinc occurs mainly in the mesophyll cells. In the seeds Ni and Zn are similarly localized and strongly enriched in the cotyledons in N. tymphaea. Nickel is strongly enriched in the tip of the radicle of B. emarginata. Noccaea tymphaea has an Fe-rich provascular strand network in the cotyledons of the seed. The chemical speciation of Ni in the seeds of N. tymphaea is unequivocally associated with carboxylic acids, whereas Zn is present as the phytate complex. The spatially resolved spectroscopy did not reveal any spatial variation in chemical speciation of Ni and Zn within the N. tymphaea seed. The dissimilar ecophysiological behaviour of Ni and Zn in N. tymphaea and B. emarginata raises questions about the evolution of hyperaccumulation in these species.

Re-investigation of cadmium accumulation in Mirabilis jalapa L.: evidences from field and laboratory

Mirabilis jalapa L. was identified as a cadmium (Cd) hyperaccumulator, but data were mainly from laboratory conditions. The main aim of the present study was to confirm whether M. jalapa is a Cd hyperaccumulator by field survey and laboratory experiment. The field survey was conducted at 3 sites and 66 samples were collected, and the results showed that although M. jalapa did not exhibit any visible damage when growing on soil containing 139 mg Cd kg^-1, a low concentration of Cd (11.85 ± 3.45 mg kg^-1) in its leaves was observed. Although the translocation factor (TF) was up to 3.24 ± 0.42, the bioconcentration factor (BCF) was only 0.13 ± 0.07. The Cd accumulation in leaves of Lanping (LP, contaminated site) and Kunming (KM, clean site) populations reached 93.88 and 81.76 mg kg^-1 when artificially spiked soil Cd was 175 mg kg^-1, respectively. The BCFs of LP and KM populations were 0.55 and 0.48, and the TFs of the two populations were 3.98 and 4.15, respectively. Under hydroponic condition, the Cd concentration in young leaves of LP and KM populations was 78.5 ± 0.8 and 46.3 ± 1.2 mg kg^-1 at 5 mg L^-1 Cd treatment, respectively. Furthermore, a significantly positive correlation between tissue Cd concentration and total Cd, CaCl₂-extractable Cd, and TCLP-Cd (toxicity characteristic leaching procedure) in soil was established. Therefore, M. jalapa had constitutional characteristics for Cd tolerance and accumulation, but it was not a Cd hyperaccumulator.

Biomolecular approaches to understanding metal tolerance and hyperaccumulation in plants

Trace metal elements are essential for plant growth but become toxic at high concentrations, while some non-essential elements, such as Cd and As, show toxicity even in traces.

Elevated Expression of Vacuolar Nickel Transporter Gene IREG2 Is Associated With Reduced Root-to-Shoot Nickel Translocation in Noccaea japonica

A number of metal hyperaccumulator plants, including nickel (Ni) hyperaccumulators, have been identified in the genus Noccaea. The ability to accumulate Ni in shoots varies widely among species and ecotypes in this genus; however, little is known about the molecular mechanisms underlying this intra- and inter-specific variation. Here, in hydroponic culture, we compared Ni accumulation patterns between Noccaea japonica, which originated in Ni-enriched serpentine soils in Mt. Yubari (Hokkaido, Japan), and Noccaea caerulescens ecotype Ganges, which originated in zinc/lead-mine soils in Southern France. Both Noccaea species showed extremely high Ni tolerance compared with that of the non-accumulator Arabidopsis thaliana. But, following treatment with 200 μM Ni, N. caerulescens showed leaf chlorosis, whereas N. japonica did not show any stress symptoms. Shoot Ni concentration was higher in N. caerulescens than in N. japonica; this difference was due to higher efficiency of root-to-shoot Ni translocation in N. caerulescens than N. japonica. It is known that the vacuole Ni transporter IREG2 suppresses Ni translocation from roots to shoots by sequestering Ni in the root vacuoles. The expression level of the IREG2 gene in the roots of N. japonica was 10-fold that in the roots of N. caerulescens. Moreover, the copy number of IREG2 per genome was higher in N. japonica than in N. caerulescens, suggesting that IREG2 expression is elevated by gene multiplication in N. japonica. The heterologous expression of IREG2 of N. japonica and N. caerulescens in yeast and A. thaliana confirmed that both IREG2 genes encode functional vacuole Ni transporters. Taking these results together, we hypothesize that the elevation of IREG2 expression by gene multiplication causes the lower root-to-shoot Ni translocation in N. japonica.

Evaluation of heavy metal-induced responses in Silene vulgaris ecotypes

Silene vulgaris is a pseudometallophyte that spontaneously occurs in various ecological niches. Therefore, three ecotypes of this species representing calamine (CAL), serpentine (SER), and non-metallicolous (NM) populations were investigated in this study. Owing to the presence of Pb or Ni ions in natural habitats from metallicolous populations originated, we used these metals as model stressors to determine the survival strategy of tested ecotypes and analyze metal distribution at various levels of organism organization. We focused on growth tolerance, non-enzymatic antioxidants, and photosynthetic apparatus efficiency as well as anatomical and ultrastructural changes occurred in contrasting ecotypes exposed in vitro to excess amounts of Pb2+ and Ni2+. Although Ni application contributed to shoot culture death, the study revealed that the mechanisms of Pb detoxification differed between ecotypes. The unspecific reaction of both metallicolous specimens relied on the formation of effective mechanical barrier against toxic ion penetration, while the Pb appearance in the protoplasts led to the activation of ecotype-specific intracellular defense mechanisms. Hence, the response of CAL and SER ecotypes was almost unchanged under Pb treatment, whereas the reaction of NM one resulted in growth disturbances and physiological alternations. Moreover, both metallicolous ecotypes exhibited increase generation of reactive oxygen species (ROS) in leaves, even before the harmful ions got into these parts of plants. It may implicate the potential role of ROS in CAL and SER adaptation to heavy metals and, for the first time, indicate on integral function of ROS as signaling molecules in metal-tolerant species.

Gene expression and biochemical response of giant reed under Ni and Cu stress

Giant reed (Arundo donax) has proved to be effective in detoxification, accumulation and tolerance of toxic metals. The present study explored the stress response of giant reed against Cu and Ni stress. The effect of metal stress was studied on dry weight, chlorophyll pigments antioxidant enzymes production and selected genes expression. The accumulation of heavy metals increased in a concentration-dependent manner and depicted toxicity symptoms in leaves beyond 75 mg/L of Cu or Ni. Oxidative stress was evident in giant reed under highest exposure of Ni and Cu which increased antioxidants activities (SOD, POD and CAT). It was observed that metal transport and detoxification were possible due to the expression of glutathione reductase, Natural Resistance-Associated Macrophage Protein (NRAMP) and Yellow Stripe-Like (YSL) genes. These insights into the genetic basis of a successful remediating plant species will be useful in understanding heavy metals tolerance in giant reed.

Spatiotemporal heterogeneity of photosystem II function during acclimation to zinc exposure and mineral nutrition changes in the hyperaccumulator Noccaea caerulescens

We investigated changes in mineral nutrient uptake and translocation and photosystem II (PSII) functionality, in the hyperaccumulator Noccaea caerulescens after exposure to 800 μM Zn in hydroponic culture. Exposure to Zn inhibited the uptake of K, Mn, Cu, Ca, and Mg, while the uptake of Fe and Zn enhanced. Yet, Ca and Mg aboveground tissue concentrations remain unchanged while Cu increased significantly. In the present study, we provide new data on the mechanism of N. caerulescens acclimation to Zn exposure by elucidating the process of photosynthetic acclimation. A spatial heterogeneity in PSII functionality in N. caerulescens leaves exposed to Zn for 3 days was detected, while a threshold time of 4 days was needed for the activation of Zn detoxification mechanism(s) to decrease Zn toxicity and for the stomatal closure to decrease Zn supply at the severely affected leaf area. After 10-day exposure to Zn, the allocation of absorbed light energy in PSII under low light did not differ compared to control ones, while under high light, the quantum yield of non-regulated energy loss in PSII (Φ_NO) was lower than the control, due to an efficient photoprotective mechanism. The chlorophyll fluorescence images of non-photochemical quenching (NPQ) and photochemical quenching (q_p) clearly showed spatial and temporal heterogeneity in N. caerulescens exposure to Zn and provided further information on the particular leaf area that was most sensitive to heavy metal stress. We propose the use of chlorophyll fluorescence imaging, and in particular the redox state of the plastoquinone (PQ) pool that was found to display the highest spatiotemporal heterogeneity, as a sensitive bio-indicator to measure the environmental pressure by heavy metals on plants.

Variability of trace element distribution in Noccaea spp., Arabidopsis spp., and Thlaspi arvense leaves: the role of plant species and element accumulation ability

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was applied for the determination of Cd and Zn distributions within the leaves of Cd- and Zn-hyperaccumulating plants, Noccaea caerulescens, N. praecox, and Arabidopsis halleri, in contrast to nonaccumulator species, Thlaspi arvense and A. thaliana. The elemental mapping of the selected leaf area was accomplished via line scans with a 110-μm-diameter laser beam at a 37-μm s^-1 scan speed and repetition rate of 10 Hz. The lines were spaced 180 μm apart and ablated at an energy density of 2 J cm^-2. The elemental imaging clearly confirmed that Cd was predominantly distributed within the parenchyma of the T. arvense, whereas in the Noccaea spp. and A. halleri, the highest intensity Cd signal was observed in the veins of the leaves. For Zn, higher intensities were observed in the veins for all the plant species except for A. thaliana. Close relationships between Zn and Ca were identified for the Noccaea spp. leaves. These relationships were not confirmed for A. halleri. Significant correlations were also proved between the Cd and Zn distribution in A. halleri, but not for the Noccaea spp. For both T. arvense and A. thaliana, no relevant significant relationship for the interpretation of the results was observed. Thus, the LA-ICP-MS imaging is proved as a relevant technique for the description and understanding of the elements in hyperaccumulating or highly accumulating plant species, although its sensitivity for the natural element contents in nonaccumulator plant species is still insufficient.

Variation in defence strategies in the metal hyperaccumulator plant Noccaea caerulescens is indicative of synergies and trade-offs between forms of defence

In the metal hyperaccumulator plant Noccaea caerulescens, zinc may provide a defence against pathogens. However, zinc accumulation is a variable trait in this species. We hypothesize that this variability affects the outcome of interactions between metal accumulation and the various constitutive and inducible defences that N. caerulescens shares with non-accumulator plants. We compare zinc concentrations, glucosinolate concentrations and inducible stress responses, including reactive oxygen species (ROS) and cell death, in four N. caerulescens populations, and relate these to the growth of the plant pathogen Pseudomonas syringae, its zinc tolerance mutants and Pseudomonas pathogens isolated from a natural population of N. caerulescens. The populations display strikingly different combinations of defences. Where defences are successful, pathogens are limited primarily by metals, cell death or organic defences; there is evidence of population-dependent trade-offs or synergies between these. In addition, we find evidence that Pseudomonas pathogens have the capacity to overcome any of these defences, indicating that the arms race continues. These data indicate that defensive enhancement, joint effects and trade-offs between different forms of defence are all plausible explanations for the variation we observe between populations, with factors including metal availability and metal-tolerant pathogen load probably shaping the response of each population to infection.

Transcriptional effects of cadmium on iron homeostasis differ in calamine accessions of Noccaea caerulescens

Calamine accessions of the zinc/cadmium/nickel hyperaccumulator, Noccaea caerulescens, exhibit striking variation in foliar cadmium accumulation in nature. The Ganges accession (GA) from Southern France displays foliar cadmium hyperaccumulation (>1000 μg g^-1 DW), whereas the accession La Calamine (LC) from Belgium, with similar local soil metal composition, does not (<100 μg g^-1 DW). All calamine accessions are cadmium hypertolerant. To find out the differences between LC and GA in their basic adaptation mechanisms, we bypassed the cadmium excluding phenotype of LC by exposing the plants to 50 μm cadmium in hydroponics, achieving equal cadmium accumulation in the shoots. The iron content increased in the roots of both accessions. GA exhibited significant decreases in manganese and zinc contents in the roots and shoots, approaching those in LC. Altogether 702 genes responded differently to cadmium exposure between the accessions, 157 and 545 in the roots and shoots, respectively. Cadmium-exposed LC showed a stress response and had decreased levels of a wide range of photosynthesis-related transcripts. GA showed less changes, mainly exhibiting an iron deficiency-like response. This included increased expression of genes encoding five iron deficiency-regulated bHLH transcription factors, ferric reduction oxidase FRO2, iron transporters IRT1 and OPT3, and nicotianamine synthase NAS1, and decreased expression of genes encoding ferritins and NEET (a NEET family iron-sulfur protein), which is possibly involved in iron transfer, distribution and/or management. The function of the IRT1 gene in the accessions was compared. We conclude that the major difference between the two accessions is in the way they cope with iron under cadmium exposure.

Chlorophyll Fluorescence Imaging Analysis for Elucidating the Mechanism of Photosystem II Acclimation to Cadmium Exposure in the Hyperaccumulating Plant Noccaea caerulescens

We provide new data on the mechanism of Noccaea caerulescens acclimation to Cd exposure by elucidating the process of photosystem II (PSII) acclimation by chlorophyll fluorescence imaging analysis. Seeds from the metallophyte N. caerulescens were grown in hydroponic culture for 12 weeks before exposure to 40 and 120 μM Cd for 3 and 4 days. At the beginning of exposure to 40 μM Cd, we observed a spatial leaf heterogeneity of decreased PSII photochemistry, that later recovered completely. This acclimation was achieved possibly through the reduced plastoquinone (PQ) pool signaling. Exposure to 120 μM Cd under the growth light did not affect PSII photochemistry, while under high light due to a photoprotective mechanism (regulated heat dissipation for protection) that down-regulated PSII quantum yield, the quantum yield of non-regulated energy loss in PSII (ΦNO) decreased even more than control values. Thus, N. caerulescens plants exposed to 120 μM Cd for 4 days exhibited lower reactive oxygen species (ROS) production as singlet oxygen (¹O₂). The response of N. caerulescens to Cd exposure fits the 'Threshold for Tolerance Model', with a lag time of 4 d and a threshold concentration of 40 μM Cd required for the induction of the acclimation mechanism.

Can zinc pollution promote adaptive evolution in plants? Insights from a one-generation selection experiment

Human activities generate environmental stresses that can lead plant populations to become extinct. Population survival would require the evolution of adaptive responses that increase tolerance to these stresses. Thus, in pseudometallophyte species that have colonized anthropogenic metalliferous habitats, the evolution of increased metal tolerance is expected in metallicolous populations. However, the mechanisms by which metal tolerance evolves remain unclear. In this study, parent populations were created from non-metallicolous families of Noccaea caerulescens. They were cultivated for one generation in mesocosms and under various levels of zinc (Zn) contamination to assess whether Zn in soil represents a selective pressure. Individual plant fitness estimates were used to create descendant populations, which were cultivated in controlled conditions with moderate Zn contamination to test for adaptive evolution in functional traits. The number of families showing high fitness estimates in mesocosms was progressively reduced with increasing Zn levels in soil, suggesting increasing selection for metal tolerance. In the next generation, adaptive evolution was suggested for some physiological and ecological traits in descendants of the most exposed populations, together with a significant decrease of Zn hyperaccumulation. Our results confirm experimentally that Zn alone can be a significant evolutionary pressure promoting adaptive divergence among populations.

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.

Micro-edaphic factors affect intra-specific variations in trace element profiles of Noccaea praecox on ultramafic soils

The aim of this study was to compare trace element profiles of Noccaea praecox (Wulfen) F. K. Mey. growing on ultramafic soils in different habitat types and to observe differences in uptake and translocation of trace elements. Physico-chemical characteristics of the soil and concentrations of P₂O₅, K₂O, Ca, Fe, Mn, Zn, Cu, Ni, Cr, Pb, Cd, and Co in plant samples were presented. Biological concentration, accumulation, and translocation factors were calculated to estimate accumulation potential of different N. praecox accessions. All of the studied accessions were Ni hyperaccumulators (with shoot concentrations up to 14,593 mg kg^-1), but with notable differences in accumulation and translocation rates. Significant differences in accumulation and translocation patterns of trace elements were observed among accessions from habitats characterized as serpentine steppes on dry, shallow soils in contrast to the accessions from habitats with higher soil moisture, and higher content of organic matter.

Comparative RNA-seq analysis of nickel hyperaccumulating and non-accumulating populations of Senecio coronatus (Asteraceae)

Most metal hyperaccumulating plants accumulate nickel, yet the molecular basis of Ni hyperaccumulation is not well understood. We chose Senecio coronatus to investigate this phenomenon as this species displays marked variation in shoot Ni content across ultramafic outcrops in the Barberton Greenstone Belt (South Africa), thus allowing an intraspecific comparative approach to be employed. No correlation between soil and shoot Ni contents was observed, suggesting that this variation has a genetic rather than environmental basis. This was confirmed by our observation that the accumulation phenotype of plants from two hyperaccumulator and two non-accumulator populations was maintained when the plants were grown on a soil mix from these four sites for 12 months. We analysed the genetic variation among 12 serpentine populations of S. coronatus, and used RNA-seq for de novo transcriptome assembly and analysis of gene expression in hyperaccumulator versus non-accumulator populations. Genetic analysis revealed the presence of hyperaccumulators in two well supported evolutionary lineages, indicating that Ni hyperaccumulation may have evolved more than once in this species. RNA-Seq analysis indicated that putative homologues of transporters associated with root iron uptake in plants are expressed at elevated levels in roots and shoots of hyperaccumulating populations of S. coronatus from both evolutionary lineages. We hypothesise that Ni hyperaccumulation in S. coronatus may have evolved through recruitment of these transporters, which play a role in the iron-deficiency response in other plant species.

Evolutionary dynamics of quantitative variation in an adaptive trait at the regional scale: The case of zinc hyperaccumulation in Arabidopsis halleri

Metal hyperaccumulation in plants is an ecological trait whose biological significance remains debated, in particular because the selective pressures that govern its evolutionary dynamics are complex. One of the possible causes of quantitative variation in hyperaccumulation may be local adaptation to metalliferous soils. Here, we explored the population genetic structure of Arabidopsis halleri at fourteen metalliferous and nonmetalliferous sampling sites in southern Poland. The results were integrated with a quantitative assessment of variation in zinc hyperaccumulation to trace local adaptation. We identified a clear hierarchical structure with two distinct genetic groups at the upper level of clustering. Interestingly, these groups corresponded to different geographic subregions, rather than to ecological types (i.e., metallicolous vs. nonmetallicolous). Also, approximate Bayesian computation analyses suggested that the current distribution of A. halleri in southern Poland could be relictual as a result of habitat fragmentation caused by climatic shifts during the Holocene, rather than due to recent colonization of industrially polluted sites. In addition, we find evidence that some nonmetallicolous lowland populations may have actually derived from metallicolous populations. Meanwhile, the distribution of quantitative variation in zinc hyperaccumulation did separate metallicolous and nonmetallicolous accessions, indicating more recent adaptive evolution and diversifying selection between metalliferous and nonmetalliferous habitats. This suggests that zinc hyperaccumulation evolves both ways-towards higher levels at nonmetalliferous sites and lower levels at metalliferous sites. Our results open a new perspective on possible evolutionary relationships between A. halleri edaphic types that may inspire future genetic studies of quantitative variation in metal hyperaccumulation.

Simultaneous hyperaccumulation of nickel and cobalt in the tree Glochidion cf. sericeum (Phyllanthaceae): elemental distribution and chemical speciation

Hyperaccumulation is generally highly specific for a single element, for example nickel (Ni). The recently-discovered hyperaccumulator Glochidion cf. sericeum (Phyllanthaceae) from Malaysia is unusual in that it simultaneously accumulates nickel and cobalt (Co) with up to 1500 μg g-1 foliar of both elements. We set out to determine whether distribution and associated ligands for Ni and Co complexation differ in this species. We postulated that Co hyperaccumulation coincides with Ni hyperaccumulation operating on similar physiological pathways. However, the ostensibly lower tolerance for Co at the cellular level results in the exudation of Co on the leaf surface in the form of lesions. The formation of such lesions is akin to phytotoxicity responses described for manganese (Mn). Hence, in contrast to Ni, which is stored principally inside the foliar epidermal cells, the accumulation response to Co consists of an extracellular mechanism. The chemical speciation of Ni and Co, in terms of the coordinating ligands involved and principal oxidation state, is similar and associated with carboxylic acids (citrate for Ni and tartrate or malate for Co) and the hydrated metal ion. Some oxidation to Co3+, presumably on the surface of leaves after exudation, was observed.

Contrasting nickel and zinc hyperaccumulation in subspecies of Dichapetalum gelonioides from Southeast Asia

Hyperaccumulator plants have the unique ability to concentrate specific elements in their shoot in concentrations that can be thousands of times greater than in normal plants. Whereas all known zinc hyperaccumulator plants are facultative hyperaccumulators with only populations on metalliferous soils hyperaccumulating zinc (except for Arabidopsis halleri and Noccaea species that hyperaccumulate zinc irrespective of the substrate), the present study discovered that Dichapetalum gelonioides is the only (zinc) hyperaccumulator known to occur exclusively on 'normal' soils, while hyperaccumulating zinc. We recorded remarkable foliar zinc concentrations (10 730 µg g-1, dry weight) in Dichapetalum gelonioides subsp. sumatranum growing on 'normal' soils with total soil zinc concentrations of only 20 µg g-1. The discovery of zinc hyperaccumulation in this tropical woody plant, especially the extreme zinc concentrations in phloem and phloem-fed tissues (reaching up to 8465 µg g-1), has possible implications for advancing zinc biofortification in Southeast Asia. Furthermore, we report exceptionally high foliar nickel concentrations in D. subsp. tuberculatum (30 260 µg g-1) and >10 wt% nickel in the ash, which can be exploited for agromining. The unusual nickel and zinc accumulation behaviour suggest that Dichapetalum-species may be an attractive model to study hyperaccumulation and hypertolerance of these elements in tropical hyperaccumulator plants.

Comparison of three types of oil crop rotation systems for effective use and remediation of heavy metal contaminated agricultural soil

Selecting suitable plants tolerant to heavy metals and producing products of economic value may be a key factor in promoting the practical application of phytoremediation polluted soils. The aim of this study is to further understand the utilization and remediation of seriously contaminated agricultural soil. In a one-year field experiment, we grew oilseed rape over the winter and then subsequently sunflowers, peanuts and sesame after the first harvest. This three rotation system produced high yields of dry biomass; the oilseed rape-sunflower, oilseed rape-peanut and oilseed rape-sesame rotation allowed us to extract 458.6, 285.7, and 134.5 g ha^-1 of cadmium, and 1264.7, 1006.1, and 831.1 g ha^-1 of lead from soil, respectively. The oilseed rape-sunflower rotation showed the highest phytoextraction efficiency (1.98%) for cadmium. Lead and cadmium in oils are consistent with standards after extraction with n-hexane. Following successive extractions with potassium tartrate, concentrations of lead and cadmium in oilseed rape and peanut seed meals were lower than levels currently permissible for feeds. Thus, this rotation system could be useful for local farmers as it would enable the generation of income during otherwise sparse phytoremediation periods.

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.

Genome-wide identification and expression profile analysis of CCH gene family in Populus

Copper plays key roles in plant physiological activities. To maintain copper cellular homeostasis, copper chaperones have important functions in binding and transporting copper to target proteins. Detailed characterization and function analysis of a copper chaperone, CCH, is presently limited to Arabidopsis. This study reports the identification of 21 genes encoding putative CCH proteins in Populus trichocarpa. Besides sharing the conserved metal-binding motif MXCXXC and forming a βαββαβ secondary structure at the N-terminal, all the PtCCHs possessed the plant-exclusive extended C-terminal. Based on their gene structure, conserved motifs, and phylogenetic analysis, the PtCCHs were divided into three subgroups. Our analysis indicated that whole-genome duplication and tandem duplication events likely contributed to expansion of the CCH gene family in Populus. Tissue-specific data from PlantGenIE revealed that PtCCH genes had broad expression patterns in different tissues. Quantitative real-time RT-PCR (qRT-PCR) analysis revealed that PnCCH genes of P. simonii × P. nigra also had different tissue-specific expression traits, as well as different inducible-expression patterns in response to copper stresses (excessive and deficiency). In summary, our study of CCH genes in the Populus genome provides a comprehensive analysis of this gene family, and lays an important foundation for further investigation of their roles in copper homeostasis of poplar.

Roots alterations in presence of phenanthrene may limit co-remediation implementation with Noccaea caerulescens

Co-phytoremediation of both trace elements and polycyclic aromatic hydrocarbons (PAH) is an emerging technique to treat multi-contaminated soils. In this study, root morphological and structural features of the heavy metal hyperaccumulator Noccaea caerulescens, exposed to a model PAH phenanthrene (PHE) in combination with cadmium (Cd), were observed. In vitro cultivated seedlings were exposed to 2 mM of PHE and/or 5 μM of Cd for 1 week. Co-phytoremediation effectiveness appeared restricted because of a serious inhibition (about 40%) of root and shoot biomass production in presence of PHE, while Cd had no significant adverse effect on these parameters. The most striking effects of PHE on roots were a decreased average root diameter, the inhibition of cell and root hair elongation and the promotion of lateral root formation. Moreover, endodermal cells with suberin lamellae appeared closer to the root apex when exposed to PHE compared to control and Cd treatments, possibly due to modified lateral root formation. The stage with well-developed suberin lamellae was not influenced by PHE whereas peri-endodermal layer development was impaired in PHE-treated plants. Many of these symptoms were similar to a water-deficit response. These morphological and structural root modifications in response to PHE exposition might in turn limit Cd phytoextraction by N. caerulescens in co-contaminated soils.

Salt and heavy metal tolerance and expression levels of candidate tolerance genes among four extremophile Cochlearia species with contrasting habitat preferences

To test the concept of a general "mineral stress tolerance", we compared four extremophile Cochlearia species for salt (NaCl), zinc (Zn) and cadmium (Cd) tolerance and accumulation, and for expression of candidate tolerance genes for salt and Zn tolerance. Salt tolerance decreased in the order C. anglica>C. x hollandica>C. danica>C. pyrenaica, corresponding with the average salinity levels in the species' natural environments. The glycophytic metallophyte, C. pyrenaica, showed a relatively high level of salt tolerance, compared to other glycophytic Brassicaceae. Salt tolerance was positively correlated with HKT1 expression and the K⁺ concentration in roots under salt exposure, but uncorrelated with the Na⁺ concentrations in roots and shoots. All the species accumulated Na⁺ primarily in their leaves, and exhibited a high NHX1 expression in leaves, in comparison with other glycophytic Brassicaceae, suggesting that salt tolerance in Cochlearia is based on an efficient vacuolar sequestration of Na⁺ in leaves. The metallicolous C. pyrenaica population was hypertolerant to Zn, but not to Cd, in comparison with the other Cochlearia species. All the Cochlearia species accumulated Zn and Cd primarily in roots, and showed high levels of Cd and Zn tolerance, with unusually low rates of metal accumulation, in comparison with non-metallophytes, or non-metallicolous metallophyte populations, of species belonging to other genera or families. Although Cochlearia, as a genus, shows relatively high levels of tolerance to both salt and heavy metals, this is most probably not due to a common 'mineral stress tolerance' mechanism.

Perturbations and 3R in carbon management

Perturbations in various carbon pools like biological, geological, oceanic, and missing carbon sink affect its global data, which are generally neglected or ignored in routine calculations. These natural and anthropogenic events need to be considered before projecting a sustainable carbon management plan. These plans have both general and experimental aspects. General plans should focus on (a) minimizing emission; (b) maximizing environmentally sound reuse, reduce, and recycling; (c) effective treatment; and (d) converting carbon into valuable products with atom economy. Experimental carbon management plans involving various biological and chemical techniques with limitation in terms of research level and economic feasibility. Chemical options have benefits of higher productivity and wider product range, but it suffers from its higher-energy requirements and environmental unfriendliness. In contrast to this, biological options are more selective and less energy intensive, but their productivity is very low. Hence, there is a requirement of hybrid process where the benefits of both the options, i.e., biological and chemical, can be reaped. In view of above, the proposed review targets to highlight the various perturbations in the global carbon cycle and their effects; study the currently practiced options of carbon management, specifically in light of 3R principle; and propose various new hybrid methods by compatible combinations of chemical and biological processes to develop better and safer carbon management. These methods are hypothetical so they may require further research and validations but may provide a comprehensive base for developing such management methods.

De novo transcriptome assemblies of four accessions of the metal hyperaccumulator plant Noccaea caerulescens

Noccaea caerulescens of the Brassicaceae family has become the key model plant among the metal hyperaccumulator plants. Populations/accessions of N. caerulescens from geographic locations with different soil metal concentrations differ in their ability to hyperaccumulate and hypertolerate metals. Comparison of transcriptomes in several accessions provides candidates for detailed exploration of the mechanisms of metal accumulation and tolerance and local adaptation. This can have implications in the development of plants for phytoremediation and improved mineral nutrition. Transcriptomes from root and shoot tissues of four N. caerulescens accessions with contrasting Zn, Cd and Ni hyperaccumulation and tolerance traits were sequenced with Illumina Hiseq2000. Transcriptomes were assembled using the Trinity de novo assembler and were annotated and the protein sequences predicted. The comparison against the BUSCO plant early release dataset indicated high-quality assemblies. The predicted protein sequences have been clustered into ortholog groups with closely related species. The data serve as important reference sequences in whole transcriptome studies, in analyses of genetic differences between the accessions and other species, and for primer design.

Cadmium-zinc accumulation and photosystem II responses of Noccaea caerulescens to Cd and Zn exposure

A population of the metallophyte Noccaea (Thlaspi) caerulescens originating from a Zn-enriched area at Røros Copper Mine (Norway) was studied. N. caerulescens tolerance to accumulate Cd and Zn was evaluated in hydroponic experiments by chlorophyll fluorescence imaging analysis. In the field-collected N. caerulescens mother plants, Zn shoot concentrations were above Zn hyperaccumulation threshold while, in hydroponic experiments under 40-μM Cd exposure, shoot Cd concentrations were clearly above Cd hyperaccumulation threshold. Cadmium ions and, to a less extent, Zn were mainly retained in the roots. Exposure to Cd enhanced Zn translocation to the shoot, while decreased significant total Ca²⁺ uptake, suggesting that Cd uptake occurs through Ca²⁺ transporters. Nevertheless, it increased Ca²⁺ translocation to the leaf, possibly for photoprotection of photosystem II (PSII). Exposure to 800 μM Zn or 40 μM Cd resulted in increased Fe³⁺ uptake suggesting that in N. caerulescens, Cd uptake does not take place through the pathway of Fe³⁺ uptake and that conditions that lead to Cd and Zn accumulation in plants may also favor Fe accumulation. Despite the significant high toxicity levels of Zn and Cd in leaves, under Zn and Cd exposure, respectively, the allocation of absorbed light energy at PSII did not differ compared to controls. The results showed that N. caerulescens keep Cd and Zn concentrations in the mesophyll cells in non-toxic forms for PSII and that the increased Ca and Fe accumulation in leaves alleviates the toxicity effects. Chlorophyll fluorescence imaging revealed that PSII of N. caerulescens resisted better the phytotoxic effects of 20 times higher Zn than Cd exposure concentration. Overall, it is concluded that the use of chlorophyll fluorescence imaging constitutes a promising basis for investigating heavy metal tolerance of plants.

Demographic history of the trace metal hyperaccumulator Noccaea caerulescens (J. Presl and C. Presl) F. K. Mey. in Western Europe

Noccaea caerulescens (Brassicaceae) is a major pseudometallophyte model for the investigation of the genetics and evolution of metal hyperaccumulation in plants. We studied the population genetics and demographic history of this species to advance the understanding of among-population differences in metal hyperaccumulation and tolerance abilities. Sampling of seven to 30 plants was carried out in 62 sites in Western Europe. Genotyping was carried out using a combination of new chloroplast and nuclear neutral markers. A strong genetic structure was detected, allowing the definition of three genetic subunits. Subunits showed a good geographic coherence. Accordingly, distant metallicolous populations generally belonged to distinct subunits. Approximate Bayesian computation analysis of demographic scenarios among subunits further supported a primary isolation of populations from the southern Massif Central prior to last glacial maximum, whereas northern populations may have derived during postglacial recolonization events. Estimated divergence times among subunits were rather recent in comparison with the species history, but certainly before the establishment of anthropogenic metalliferous sites. Our results suggest that the large-scale genetic structure of N. caerulescens populations pre-existed to the local adaptation to metalliferous sites. The population structure of quantitative variation for metal-related adaptive traits must have established independently in isolated gene pools. However, features of the most divergent genetic unit (e.g. extreme levels of Cd accumulation observed in previous studies) question the putative relationships between adaptive evolution of metal-related traits and subunits isolation. Finally, admixture signals among distant metallicolous populations suggest a putative role of human activities in facilitating long-distance genetic exchanges.

Terminalia arjuna bark extract alleviates nickel toxicity by suppressing its uptake and modulating antioxidative defence in rice seedlings

Terminalia arjuna (Ta) bark contains various natural antioxidants and has been used to protect animal cells against oxidative stress. In the present study, we have examined alleviating effects of Ta bark aqueous extract against Ni toxicity in rice (Oryza sativa L.). When rice seedlings were raised for 8 days in hydroponics in Yoshida nutrient medium containing 200 μM NiSO₄, a decline in height, reduced biomass, increased Ni uptake, loss of root plasma membrane integrity, increase in the level of O₂˙^-, H₂O₂ and ˙OH, increased lipid peroxidation, decline in photosynthetic pigments, increase in the level of antioxidative enzymes superoxide dismutase, catalase and glutathione peroxidase and alterations in their isoenzyme profile patterns were observed. Transmission electron microscopy (TEM) showed damage to chloroplasts marked by disorganised enlarged starch granules and disrupted thylakoids under Ni toxicity. Exogenously adding Ta bark extract (3.2 mg ml^-1) to the growth medium considerably alleviated Ni toxicity in the seedlings by reducing Ni uptake, suppressing generation of reactive oxygen species, reducing lipid peroxidation, restoring level of photosynthesis pigments and ultrastructure of chloroplasts, and restoring levels of antioxidative enzymes. Results suggest that Ta bark extract considerably alleviates Ni toxicity in rice seedlings by preventing Ni uptake and reducing oxidative stress in the seedlings.

Geography Plays a More Important Role than Soil Composition on Structuring Genetic Variation of Pseudometallophyte Commelina communis

Pseudometallophytes are excellent models to study microevolution and local adaptation to soil pollution, as they can grow both on metalliferous and contrasting non-metalliferous soils. Although, there has been accumulating evidence for the effects of edaphic conditions and geographical isolation on the genetic structure of pesudometallophytes, it is still a difficult problem in evolutionary biology to assess their relative importance. In this study, we investigated the spatial patterns of genetic variability, population differentiation and genetic groups in pseudometallophyte Commelina communis with 12 microsatellite loci. Eight metallicolous and six non-metallicolous populations of C. communis were sampled from cupriferous sites and surrounding non-contaminated areas in China. Neither significant reduction in genetic diversity nor apparent founder and bottleneck effects were observed in metallicolous populations of C. communis. Based on Bayesian and Neighbor-Joining clustering analyses and a principal coordinates analysis, all sampled populations were found to be mainly separated into three genetic groups, corresponding well to their geographical locations rather than edaphic origins. Moreover, a significant and strong correlation between population genetic divergence and geographical distance were detected by Mantel test (r = 0.33; P < 0.05) and multiple matrix regression with randomization (MMRR; βD = 0.57, P < 0.01). However, the effect of copper concentration on genetic patterns of C. communis was not significant (MMRR; βE = -0.17, P = 0.12). Our study clearly demonstrated that the extreme edaphic conditions in metalliferous areas had limited effects on the genetic variability in C. communis. Geographic distance played a more important role in affecting the genetic structure of C. communis than soil composition did. In C. communis, the geographically disjunctive populations on metalliferous soils had multiple origins and evolved independently from nearby non-metallicolous populations.

Effect of Heavy Metals Pollution on Soil Microbial Diversity and Bermudagrass Genetic Variation

Heavy metal pollution is a serious global environmental problem as it adversely affects plant growth and genetic variation. It also alters the composition and activity of soil microbial communities. The objectives of this study were to determine the soil microbial diversity, bermudagrass genetic variation in Cd contaminated or uncontaminated soils from Hunan province of China, and to evaluate Cd-tolerance of bermudagrass at different soils. The Biolog method, hydroponic experiments and simple sequence repeat markers were used to assess the functional diversity of microorganisms, Cd-tolerance and the genetic diversity of bermudagrass, respectively. Four of the sampling sites were heavily contaminated with heavy metals. The total bioactivity, richness, and microbial diversity decreased with increasing concentration of heavy metal. The hydroponic experiment revealed that bermudagrass populations collected from polluted sites have evolved, encompassing the feature of a higher resistance to Cd toxicity. Higher genetic diversity was observed to be more in contaminated populations than in uncontaminated populations. Heavy metal pollution can result in adverse effects on plant growth, soil microbial diversity and activity, and apparently has a stronger impact on the genetic structure. The results of this study provide new insights and a background to produce a genetic description of populations in a species that is suitable for use in phytoremediation practices.