Transcriptional regulation of metal transport genes and mineral nutrition during acclimatization to cadmium and zinc in the Cd/Zn hyperaccumulator, Thlaspi caerulescens (Ganges population)

NtZIP5A/B is involved in the regulation of Zn/Cu/Fe/Mn/Cd homeostasis in tobacco

Plants grow in soils with varying concentrations of microelements, often in the presence of toxic metals e.g. Cd. To cope, they developed molecular mechanisms to regulate metal cross-homeostasis. Understanding underlying complex relationships is key to improving crop productivity. Recent research suggests that the Zn and Cd uptake protein NtZIP5A/B [Zinc-regulated, Iron-regulated transporter-like Proteins (ZIPs)] from tobacco (Nicotiana tabacum L. v. Xanthi) is involved in the regulation of a cross-talk between the two metals. Here, we support this conclusion by showing that RNAi-mediated silencing of NtZIP5A/B resulted in a reduction of Zn accumulation and that this effect was significantly enhanced by the presence of Cd. Our data also point to involvement of NtZIP5B in regulating a cross-talk between Cu, Fe, and Mn. Using yeast growth assays, Cu (but not Fe or Mn) was identified as a substrate for NtZIP5B. Furthermore, GUS-based analysis showed that the tissue-specific activity of the NtZIP5B promoter was different in each of the Zn-/Cu-/Fe-/Mn deficiencies applied with/without Cd. The results indicate that NtZIP5B is involved in maintaining multi-metal homeostasis under conditions of Zn, Cu, Fe, and Mn deficiency, and also in the presence of Cd. It was concluded that the protein regulates the delivery of Zn and Cu specifically to targeted different root cells depending on the Zn/Cu/Fe/Mn status. Importantly, in the presence of Cd, the activity of the NtZIP5B promoter is lost in meristematic cells and increased in mature root cortex cells, which can be considered a manifestation of a defense mechanism against its toxic effects.

Uptake and translocation of cadmium and trace metals in common rice varieties at different growth stages

Rice (Oryza sativa) is an important nutritional grain for the majority of Asian countries, but it is also a major source of cadmium (Cd) accumulation. A pot experiment was carried out to investigate the Cd uptake and translocation of high Cd (IR-50) and low Cd (White Ponni) rice cultivars in Cd-contaminated soils. The findings revealed that Cd impacts on rice development and growth differed depending on rice cultivars. Soil Cd levels in the seedling stage exceeded the critical levels (3-6 mg kg-1) only 5.0 mg kg-1 Cd treatment for the IR-50 (7.47 mg kg-1). At higher Cd treatments (1.0 and 5.0 mg kg-1), morphometric characteristics and yield of grains showed a declining and increasing trend in both rice varieties, respectively. The accumulation of Cd was higher in soil and roots during seedling and tillering stages, whereas in booting and maturity stages increased in stems and leaves in IR-50 and WP rice varieties. Cd levels in rice grains above the maximum allowable limit (0.4 mg kg-1) only in IR-50 (0.51 mg kg-1) rice cultivar at maturity stage. The EF of Cd were classified as minor enrichment to 'moderate enrichment' in both rice cultivars. TF values exhibited > 1 in booting and maturity stages in both rice cultivars at higher Cd treatments. The study concluded that the IR-50 rice variety exhibited increased Cd intake and transported to various parts of rice plants, particularly grains. The findings indicate that WP rice cultivar is more resistant to Cd toxicity, reducing health hazards for persons who preferred the staple food rice.

Copper-dependent control of uptake, translocation and accumulation of cadmium in hyperaccumlator Sedum alfredii

Cadmium (Cd) is detrimental to plant growth and threatens human health. Here, we investigated the potential for remediation of Cd-contaminated soil with high copper (Cu) background using Cd hyperaccumulator ecotype (HE) Sedum alfredii. We assessed effects of Cu on Cd accumulation, compartmentation and translocation in HE S. alfredii, and compared with those in a related non-accumulator ecotype (NHE). We found that Cu supply significantly induced Cd accumulation in roots and shoots of long-term soil-cultivated HE S. alfredii. A large fraction of root Cd was accumulated in the organelles, but a small fraction was stored in the cell wall. Importantly, Cu addition reduced Cd accumulation in the cell wall and the organelles in root cells. Furthermore, leaf cell capacity to sequestrate Cd in the organelles was greatly improved upon Cu exposure. We also found that genes involving metal transport and cell wall remodeling were distinctly regulated to mediate Cd accumulation in HE S. alfredii. These findings indicate that Cu-dependent decrease of root cell-wall-bound Cd, and stimulation of efflux/influx of organelle Cd transport in root and leaf cells plays a role in the dramatic Cd hyperaccumulation expressed in naturally survived HE S. alfredii.

Symplasmic and transmembrane zinc transport is modulated by cadmium in the Cd/Zn hyperaccumulator Sedum alfredii

This study investigated the influence of Cd (25 µM) on Zn accumulation in a hyperaccumulating (HE) and a non-hyperaccumulating (NHE) ecotype of Sedum alfredii Hance at short-term supply of replete (Zn5, 5 µM) and excess (Zn400, 400 µM) Zn. Cd inhibited Zn accumulation in both ecotypes, especially under Zn400, in organs with active metal sequestration, i.e. roots of NHE and shoots of HE. Direct biochemical Cd/Zn competition at the metal-protein interaction and changes in transporter gene expression contributed to the observed accumulation patterns in the roots. Specifically, in HE, Cd stimulated SaZIP4 and SaPCR2 under Zn5, but downregulated SaIRT1 and SaZIP4 under Zn400. However, Cd downregulated related transporter genes, except for SaNRAMP1, in NHE, irrespective of Zn. Cadmium stimulated casparian strip (CSs) development in NHE, as part of the defense response, while it had a subtle effect on the (CS) in HE. Moreover, Cd delayed the initiation of the suberin lamellae (SL) in HE, but stimulated SL deposition in NHE under both Zn5 or Zn400. Changes in suberization were mainly ascribed to suberin-biosynthesis-related genes and hormonal signaling. Altogether, Cd regulated Zn accumulation mainly via symplasmic and transmembrane transport in HE, while Cd inhibited both symplasmic and apoplasmic Zn transport in NHE.

Cadmium and Zn hyperaccumulation provide efficient constitutive defense against Turnip yellow mosaic virus infection in Noccaea caerulescens

To understand the role of Zn and Cd in anti-viral defence, Zn/Cd hyperaccumulator Noccaea caerulescens plants grown with deficient (0.3 µM), replete (10 µM) and excess (100 µM) Zn2+ and Cd (10 µM Zn2+ + 1 µM Cd2+) were infected with Turnip yellow mosaic virus (TYMV). Gas exchange and chlorophyll fluorescence kinetics analyses demonstrated direct TYMV effects on photosynthetic light reactions but N. caerulescens was more resistant against TYMV than the previously studied non-hyperaccumulator N. ochroleucum. Virus abundance and photosynthesis inhibition were the lowest in the high Zn and Cd treatments. RNAseq analysis of 10 µM Zn2+ plants revealed TYMV-induced upregulation of Ca transporters, chloroplastic ZTP29 and defence genes, but none of those that are known to be strongly involved in hyperaccumulation. Synchrotron µ-XRF tomography, however, showed that Zn hyperaccumulation remained strongest in vacuoles of epidermal storage cells regardless of infection. This was in contrast to N. ochroleucum, where apoplastic Zn drastically increased in response to TYMV. These results suggest that the antiviral response of N. caerulescens is less induced by the onset of this biotic stress, but it is rather a permanent resistant state of the plant. Real-time qPCR revealed upregulation of ferritin in Zn10 infected plants, suggesting Fe deprivation as a virus defence strategy under suboptimal Zn supply.

Significance and genetic control of membrane transporters to improve phytoremediation and biofortification processes

Humans frequently consume plant-based foods in their daily life. Contamination of agricultural soils by heavy metals (HMs) is a major food and nutritional security issue. The crop plants grown in HM-contaminated agricultural soil may accumulate more HMs in their edible part, further transferring into the food chain. Consumption of HM-rich crops can cause severe health issues in humans. On the other hand, the low content of the essential HM in the edible part of the crop also causes health problems. Therefore, researchers must try to reduce the non-essential HM in the edible part of the crop plants and improve the essential HMs. Phytoremediation and biofortification are the two strategies for resolving this problem. The genetic component helps to improve the efficiency of phytoremediation and biofortification processes in plants. They help eliminate HMs from soil and improve essential HM content in crop plants. The membrane transporter genes (genetic components) are critical in these two strategies. Therefore, engineering membrane transporter genes may help reduce the non-essential HM content in the edible part of crop plants. Targeted gene editing by genome editing tools like CRISPR could help plants achieve efficient phytoremediation and biofortification. This article covers gene editing's scope, application, and implication to improve the phytoremediation and biofortification processes in non-crop and crop plants.

Overexpression of Sedum SpHMA2, SpHMA3 and SpNramp6 in Brassica napus increases multiple heavy metals accumulation for phytoextraction

Phytoextraction is an environmentally friendly phytoremediation technology that can reduce the total amount of heavy metals (HMs) in the soil. Hyperaccumulators or hyperaccumulating transgenic plants with biomass are important biomaterials for phytoextraction. In this study, we show that three different HM transporters from the hyperaccumulator Sedum pumbizincicola, SpHMA2, SpHMA3, and SpNramp6, possess Cd transport. These three transporters are located at the plasma membrane, tonoplast, and plasma membrane, respectively. Their transcripts could be strongly stimulated by multiple HMs treatments. To create potential biomaterials for phytoextraction, we overexpressed the three single genes and two combining genes, SpHMA2&SpHMA3 and SpHMA2&SpNramp6, in rapes having high biomass and environmental adaptability, and found that the aerial parts of the SpHMA2-OE3 and SpHMA2&SpNramp6-OE4 lines accumulated more Cd from single Cd-contaminated soil because SpNramp6 transports Cd from root cells to the xylem and SpHMA2 from the stems to the leaves. However, the accumulation of each HM in the aerial parts of all selected transgenic rapes was strengthened in multiple HMs-contaminated soils, probably due to the synergistic transport. The HMs residuals in the soil after the transgenic plant phytoremediation were also greatly reduced. These results provide effective solutions for phytoextraction in both Cd and multiple HMs-contaminated soils.

Phytoremediation as an Effective Remedy for Removing Trace Elements from Ecosystems

The pollution of soil by trace elements is a global problem. Conventional methods of soil remediation are often inapplicable, so it is necessary to search intensively for innovative and environment-friendly techniques for cleaning up ecosystems, such as phytoremediation. Basic research methods, their strengths and weaknesses, and the effects of microorganisms on metallophytes and plant endophytes resistant to trace elements (TEs) were summarised and described in this manuscript. Prospectively, bio-combined phytoremediation with microorganisms appears to be an ideal, economically viable and environmentally sound solution. The novelty of the work is the description of the potential of "green roofs" to contribute to the capture and accumulation of many metal-bearing and suspended dust and other toxic compounds resulting from anthropopressure. Attention was drawn to the great potential of using phytoremediation on less contaminated soils located along traffic routes and urban parks and green spaces. It also focused on the supportive treatments for phytoremediation using genetic engineering, sorbents, phytohormones, microbiota, microalgae or nanoparticles and highlighted the important role of energy crops in phytoremediation. Perceptions of phytoremediation on different continents are also presented, and new international perspectives are presented. Further development of phytoremediation requires much more funding and increased interdisciplinary research in this direction.

Protein Analysis of A. halleri and N. caerulescens Hyperaccumulators When Exposed to Nano and Ionic Forms of Cd and Zn

Hyperaccumulator plant species growing on metal-rich soils can accumulate high quantity of metals and metalloids in aerial tissues, and several proteomic studies on the molecular mechanisms at the basis of metals resistance and hyperaccumulation have been published. Hyperaccumulator are also at the basis of the phytoremediation strategy to remove metals more efficiently from polluted soils or water. Arabidopsis halleri and Noccea caerulescens are both hyperaccumulators of metals and nano-metals. In this study, the change in some proteins in A. halleri and N. caerulescens was assessed after the growth in soil with cadmium and zinc, provided as sulphate salts (CdSO₄ and ZnSO₄) or sulfide quantum dots (CdS QDs and ZnS QDs). The protein extracts obtained from plants after 30 days of growth were analyzed by 2D-gel electrophoresis (2D SDS-PAGE) and identified by MALDI-TOF/TOF mass spectrometry. A bioinformatics analysis was carried out on quantitative protein differences between control and treated plants. In total, 43 proteins resulted in being significatively modulated in A. halleri, while 61 resulted in being modulated in N. caerulescens. Although these two plants are hyperaccumulator of both metals and nano-metals, at protein levels the mechanisms involved do not proceed in the same way, but at the end bring a similar physiological result.

Rice Plants (Oryza sativa L.) under Cd Stress in Fe Deficiency Conditions

Due to the environment pollution by cadmium (Cd) near industrial metallurgic factories and the widespread use of phosphorus fertilizers, the problem of toxic Cd effect on plants is well discussed by many authors, but the phytotoxicity of Cd under iron (Fe) deficiency stress has not been sufficiently studied. The aim of the work was to study comprehensively the effect of Cd under Fe deficiency conditions on physiological, biochemical, and anatomical parameters of rice varieties, to identify varietal differences in plant response to the effect of double stress. Relative resistance and sensitivity to the joint effect of Cd and Fe deficiency stress rice varieties have been identified. Double stress decreased a linear growth and biomass accumulation of roots and shoots (by 36-50% and 33-46% and 32-56% and 32-48%, accordingly), content of photosynthetic pigments (Chla, Chlb, and carotenoids by 36-51%, 32-47%, and 64-78%, accordingly), and relative water content (by 18-26%). Proline content increased by 28-103% in all rice varieties, but to a lesser extent in sensitive varieties. The thickness of the lower and upper epidermis and the diameter of vascular bundles of leaves decreased by 18-50%, 46-60%, and 13-48%, accordingly. The thickness of the root endodermis and exodermis and diameter of the central cylinder mainly decreased. The thickness of the exodermis increased slightly by 7%, and the diameter of the central cylinder remained at the control level in resistant Madina variety while in sensitive Chapsari variety, these indicators decreased significantly by 50 and 45%, accordingly. Thus, the aggravation of adverse effect of Cd under Fe deficiency conditions and the varietal specificity of plants' response to double stress were shown. It creates the need for further study of these rice varieties using Fe to identify mechanisms for reducing the toxic effect of Cd on plants as well as the study of Fe and Cd transporter genes at the molecular level.

Soil amendments affect the potential of Gomphrena claussenii for phytoremediation of a Zn- and Cd-contaminated soil

This study assessed the impact of inorganic and organic amendments upon zinc (Zn) and cadmium (Cd) availabilities in leachates collected from a Cd- and Zn-contaminated soil, while also evaluating the beneficial use of the tested amendments for decreasing metal availability, hence improving the phytoremediation potential of Gomphrena claussenii Moq. Plants were grown for 60 days in a Zn-smelting-affected soil containing 45,000 and 621 mg kg^-1 of Zn and Cd, respectively (pseudo-total concentrations), after application of the following amendments: limestone, calcium silicate, sewage sludge, triple superphosphate, and red mud. Zinc and Cd availabilities in the soil decreased following the addition of limestone, calcium silicate, and red mud. These amendments were effective in reducing metal mobility and availability, positively affecting plant growth. Plants grown in the soil amended with limestone and calcium silicate accumulated Zn mainly in the roots, while Cd was translocated to plant shoots, with smaller amounts being detected in the roots. Reductions of Zn and Cd concentrations in the leachate were found by adding red mud, with this decrease for Zn being less pronounced compared to what was verified after the application of limestone and calcium silicate. Moreover, the use of red mud resulted in a higher Zn:Cd ratio in the leachate, which favored a greater absorption and transport of Zn from root to shoot. In conclusion, the tested soil amendments reduced the availability of excessive concentrations of Cd and Zn in naturally contaminated soil, which resulted in improved growth and survival of Zn- and Cd-tolerant G. claussenii plants, with the application of limestone, calcium silicate, and red mud - i.e., alkaline amendments - standing out as the best combinations with G. Claussenii when designing a strategy to achieve optimal phytoremediation.

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.

Uptake of hematite nanoparticles in maize and their role in cell cycle dynamics, PCNA expression and mitigation of cadmium stress

Cadmium toxicity is considered a major threat to several crops worldwide. Hematite nanoparticles (NPs), due to their small size and large specific surface area, could be applied as an adsorbent for toxic heavy metals in soil. Also, they serve as an efficient nano-fertilizer, promoting Fe availability and biomass production in plants, thus enabling Cd²⁺ -induced stress tolerance. The phytotoxicity of five different concentrations of hematite NPs, ranging from 500 to 8,000 mg·kg^-1 , and Cd²⁺ concentrations (110 or 130 mg·kg^-1 Cd²⁺ ) alone or combined with 500 mg·kg^-1 NPs was evaluated in maize. The changes in fresh weight, element analysis, cell cycle regulation, DNA banding patterns and proliferating cell nuclear antigen (PCNA) expression were used as biomarkers. The results revealed that increased fresh weight and fewest polymorphic DNA bands were detectable after treatment with 500 mg·kg^-1 NPs. However, at 8,000 mg·kg^-1 NPs, PCNA expression increased significantly, which resulted in cell cycle arrest at the G1/S checkpoint in roots. Significant reductions in fresh weight, altered nutrient profiles and cell cycle perturbations are considered symptoms of Cd²⁺ toxicity in maize. Conversely, amending 500 mg·kg^-1 NPs with 130 mg·kg^-1 Cd²⁺ increased fresh weight, Fe concentration and genomic template stability, while reducing Cd²⁺ uptake and PCNA1 expression. Overall, 8,000 mg·kg^-1 hematite NPs interfered with the cellular homeostatic balance of maize, resulting in a cascade of genotoxic events, leading to growth inhibition. Although 500 mg·kg^-1 hematite NPs alleviated Cd²⁺ -induced DNA damage to a certain extent, their impact on cell cycle progression requires further verification.

Research on the Mechanisms of Plant Enrichment and Detoxification of Cadmium

The heavy metal cadmium (Cd), as one of the major environmentally toxic pollutants, has serious impacts on the growth, development, and physiological functions of plants and animals, leading to deterioration of environmental quality and threats to human health. Research on how plants absorb and transport Cd, as well as its enrichment and detoxification mechanisms, is of great significance to the development of phytoremediation technologies for ecological and environmental management. This article summarises the research progress on the enrichment of heavy metal cadmium in plants in recent years, including the uptake, transport, and accumulation of Cd in plants. The role of plant roots, compartmentalisation, chelation, antioxidation, stress, and osmotic adjustment in the process of plant Cd enrichment are discussed. Finally, problems are proposed to provide a more comprehensive theoretical basis for the further application of phytoremediation technology in the field of heavy metal pollution.

Curium(III) and europium(III) as luminescence probes for plant cell (Brassica napus) interactions with potentially toxic metals

We have investigated the interaction of the actinide Cm(III) and its lanthanide homologue Eu(III) with cells of Brassica napus in suspension. This study combines biochemical techniques (plant cell response) with spectroscopic experiments to determine the chemical speciation of hazardous metals in contact with the plant cells. Experiments conducted over a period of 7 d showed that B. napus cells were able to bioassociate both potentially toxic metals in significant amounts up to 0.58 µmol Eu/g_{fresh cells} and 1.82 µmol Cm/g_{fresh cells} at 30 µM Eu(III) and 0.68 µM Cm(III), respectively. For Cm(III), a biosorption process could be identified as soon as 5 h post-exposure with 73 ± 4% of the Cm(III) bioassociated. Luminescence spectroscopy results based on UV and site-selective excitation confirmed the existence of three Cm(III)/Eu(III) [M(III)] species in both the supernatants and cells. The findings detailed herein support that M(III) coordinates to two kinds of carboxyl groups and phosphate groups.

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

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

Earthworm and arbuscular mycorrhiza interactions: Strategies to motivate antioxidant responses and improve soil functionality

Earthworms and arbuscular mycorrhizal fungi (AMF) act synergistically in the rhizosphere and may increase host plant tolerance to Cd. However, mechanisms by which earthworm-AMF-plant partnerships counteract Cd phytotoxicity are unknown. Thus, we evaluated individual and interactive effects of these soil organisms on photosynthesis, antioxidant capacity, and essential nutrient uptake by Solanum nigrum, as well as on soil quality following Cd exposure (0-120 mg kg^-1). Decreases in biomass and photosynthetic activity, as well as nutrient imbalances were observed in Cd-stressed plants; however, the addition of AMF and earthworms reversed these effects. Cd exposure increased superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities, whereas inoculation with Rhizophagus intraradices decreased those. Soil enzymatic activity decreased by 15-60% with increasing Cd concentrations. However, Cd-mediated toxicity was partially reversed by soil organisms. Earthworms and AMF ameliorated soil quality based on soil enzyme activity. At 120 mg kg^-1 Cd, the urease, catalase, and acid phosphatase activities were 1.6-, 1.4-, and 1.2-fold higher, respectively, in soils co-incubated with earthworms and AMF than in uninoculated soil. Cd inhibited shoot Fe and Ca phytoaccumulation, whereas AMF and earthworms normalized the status of essential elements in plants. Cd detoxification by earthworm-AMF-S. nigrum symbiosis was manifested by increases in plant biomass accumulation (22-117%), chlorophyll content (17-63%), antioxidant levels (SOD 10-18%, POD 9-25%, total polyphenols 17-22%, flavonoids 15-29%, and glutathione 7-61%). It also ameliorated the photosynthetic capacity, and macro- and micronutrient statuses of plants; markedly reduced the levels of malondialdehyde (20-27%), superoxide anion (29-36%), and hydrogen peroxide (19-30%); and upregulated the transcription level of FeSOD. Thus, the combined action of earthworms and AMF feasibly enhances metal tolerance of hyperaccumulating plants and improves the quality of polluted soil.

Molecular Mechanisms and Biochemical Pathways for Micronutrient Acquisition and Storage in Legumes to Support Biofortification for Nutritional Security

The world faces a grave situation of nutrient deficiency as a consequence of increased uptake of calorie-rich food that threaten nutritional security. More than half the world's population is affected by different forms of malnutrition. Unhealthy diets associated with poor nutrition carry a significant risk of developing non-communicable diseases, leading to a high mortality rate. Although considerable efforts have been made in agriculture to increase nutrient content in cereals, the successes are insufficient. The number of people affected by different forms of malnutrition has not decreased much in the recent past. While legumes are an integral part of the food system and widely grown in sub-Saharan Africa and South Asia, only limited efforts have been made to increase their nutrient content in these regions. Genetic variation for a majority of nutritional traits that ensure nutritional security in adverse conditions exists in the germplasm pool of legume crops. This diversity can be utilized by selective breeding for increased nutrients in seeds. The targeted identification of precise factors related to nutritional traits and their utilization in a breeding program can help mitigate malnutrition. The principal objective of this review is to present the molecular mechanisms of nutrient acquisition, transport and metabolism to support a biofortification strategy in legume crops to contribute to addressing malnutrition.

Plant growth-promoting Bacillus sp. strain SDA-4 confers Cd tolerance by physio-biochemical improvements, better nutrient acquisition and diminished Cd uptake in Spinacia oleracea L

Cadmium (Cd) is highly toxic metal for plant metabolic processes even in low concentration due to its longer half-life and non-biodegradable nature. The current study was designed to assess the bioremediation potential of a Cd-tolerant phytobeneficial bacterial strain Bacillus sp. SDA-4, isolated, characterized and identified from Chakera wastewater reservoir, Faisalabad, Pakistan, together with spinach (as a test plant) under different Cd regimes. Spinach plants were grown with and without Bacillus sp. SDA-4 inoculation in pots filled with 0, 5 or 10 mg kg^-1 CdCl₂-spiked soil. Without Bacillus sp. SDA-4 inoculation, spinach plants exhibited reduction in biomass accumulation, antioxidative enzymes and nutrient retention. However, plants inoculated with Bacillus sp. SDA-4 revealed significantly augmented growth, biomass accumulation and efficiency of antioxidative machinery with concomitant reduction in proline and MDA contents under Cd stress. Furthermore, application of Bacillus sp. SDA-4 assisted the Cd-stressed plants to sustain optimal levels of essential nutrients (N, P, K, Ca and Mg). It was inferred that the characterized Cd-tolerant PGPR strain, Bacillus sp. SDA-4 has a potential to reduce Cd uptake and lipid peroxidation which in turn maintained the optimum balance of nutrients and augmented the growth of Cd-stressed spinach. Analysis of bioconcentration factor (BCF) and translocation factor (TF) revealed that Bacillus sp. SDA-4 inoculation with spinach sequestered Cd in rhizospheric zone. Research outcomes are important for understanding morpho-physio-biochemical attributes of spinach-Bacillus sp. SDA-4 synergy which might provide efficient strategies to decrease Cd retention in edible plants and/or bioremediation of Cd polluted soil colloids.

Different strategies of Cd tolerance and accumulation in Arabidopsis halleri and Arabidopsis arenosa

Pseudometallophytes are commonly used to study the evolution of metal tolerance and accumulation traits in plants. Within the Arabidopsis genus, the adaptation of Arabidopsis halleri to metalliferous soils has been widely studied, which is not the case for the closely related species Arabidopsis arenosa. We performed an in-depth physiological comparison between the A. halleri and A. arenosa populations from the same polluted site, together with the geographically close non-metallicolous (NM) populations of both species. The ionomes, growth, photosynthetic parameters and pigment content were characterized in the plants that were growing on their native site and in a hydroponic culture under Cd treatments. In situ, the metallicolous (M) populations of both species hyperaccumulated Cd and Zn. The NM population of A. halleri hyperaccumulated Cd and Zn while the NM A. arenosa did not. In the hydroponic experiments, the NM populations of both species accumulated more Cd in their shoots than the M populations. Our research suggests that the two Arabidopsis species evolved different strategies of adaptation to extreme metallic environments that involve fine regulation of metal homeostasis, adjustment of the photosynthetic apparatus and accumulation of flavonols and anthocyanins.

Dynamic interactions between soil cadmium and zinc affect cadmium phytoavailability to rice and wheat: Regional investigation and risk modeling

Characterizing the interactions between Cd and Zn with respect to the soil soluble Cd and crop Cd uptake allows the development of risk-based approaches to the performance of grain crops. By means of a three-year survey of 358 rice fields and 206 wheat fields across China, this study investigated the effect of Cd-Zn interactions on the phytoavailability of Cd to rice and wheat. The interactive nature between the Cd:Zn ratio and pH of soil affected crop Cd uptake, and the resulting grain Cd intake risk, were examined by the Free-Ion Activity-based model and probability analysis. In highly acidic rice soils (pH < 5.9), soil Zn had no effect on rice Cd uptake, whereas, under near-neutral conditions (pH > 5.9), a site-specific influence of soil Zn on grain Cd concentration was found. Soil Zn could inhibit Cd uptake and translocation by the plant in soil-wheat system when the soil Cd:Zn ratio decreased to 0.0083 and lower. Rice grain poses a significant health risk to local consumers due to its high Cd accumulation and its low Zn accumulation. In order to reduce the health risks from dietary Cd to local consumers, approximately 63.9% of the rice fields and 30.5% of the wheat fields require strategies ameliorating soil acidity in rice soils and increasing Zn concentrations in wheat soils.

Direct inhibition of photosynthesis by Cd dominates over inhibition caused by micronutrient deficiency in the Cd/Zn hyperaccumulator Arabidopsis halleri

This work reveals, by imaging in vivo measurements in the Cd/Zn hyperaccumulator Arabidopsis halleri, in how far Cd stress affects macronutrient (Ca, K) and micronutrient (Fe, Zn) distribution in the leaves. We directly correlate these changes with biophysics of the photosynthetic light reactions. Plants were grown for 2 months at 10 μM Zn (=control), and supplemented with 10, 15, 50 or 75 μM Cd. Direct imaging of OJIP transients revealed that bundle sheath cells were more sensitive to Cd toxicity than mesophyll cells further from the vein. Progressive inhibition of photosystem (PS) II reaction centres and decrease in quantum yield of electron transport between Q_A and Q_B and further to PSI acceptors was observed. This was correlated with the decreased dynamics of Q_A re-oxidation and lower operating efficiency of PSII. Analysis by a benchtop micro X-ray fluorescence device showed that Cd mostly accumulated in the veins, and restricted Fe and Zn distribution from the veins, especially in the 75 μM Cd, while K concentration increased in the whole leaf. Calcium distribution was apparently not affected by Cd, but Cd excess inhibited trichome formation and thereby diminished total Ca concentration in the leaves. The results point to differential tissue sensitivity to Cd, evident by heterogeneous inhibition of photosynthesis. Part of this may be a result of selective disturbances in the leaf nutrient homeostasis. The better photosynthetic performance away from the veins compared to the bundle sheath cells, however, indicates that direct inhibition of photosynthesis by Cd dominates over inhibition caused by micronutrient deficiency.

Plant cell (Brassica napus) response to europium(III) and uranium(VI) exposure

Experiments conducted over a period of 6 weeks using Brassica napus callus cells grown in vitro under Eu(III) or U(VI) stress showed that B. napus cells were able to bioassociate both potentially toxic metals (PTM), 628 nmol Eu/gfresh cells and 995 nmol U/gfresh cells. Most of the Eu(III) and U(VI) was found to be enriched in the cell wall fraction. Under high metal stress (200 μM), cells responded with reduced cell viability and growth. Subsequent speciation analyses using both metals as luminescence probes confirmed that B. napus callus cells provided multiple-binding environments for Eu(III) and U(VI). Moreover, two different inner-sphere Eu3+ species could be distinguished. For U(VI), a dominant binding by organic and/or inorganic phosphate groups of the plant biomass can be concluded.

Boron inhibits cadmium uptake in wheat (Triticum aestivum) by regulating gene expression

Various nutrients (Mg, Zn, Fe, Mn, Si, etc.) can supress cadmium (Cd) uptake and alleviate Cd toxicity, but the mechanisms are not the same. In this study, the molecular mechanism governing the effects of boron (B) on uptake of Cd in hydroponically grown wheat was characterized. As compared to control (0 μM Cd), B concentration per plant decreased by 22% and 29% under 5 μM Cd and 50 μM Cd treatment respectively. In addition, B application decreased Cd concentration and accumulation in whole wheat. Correlation analysis of different elements show that there was a highly negative correlation between concentrations of B and Cd (r = -0.854 with significant correlation) in wheat. Additionally, 16,543 differentially expressed genes (DEGs) (7666 up- and 8877 down-regulated) were detected between 0 and 5 μM Cd treatments in wheat roots by transcriptome sequencing. Gene ontology functional category and Kyoto encyclopedia of genes and genomes pathway analyses indicated that the DEGs were involved in biological process, cellular component, and molecular function. Five highly homologous genes to Cd transporters were identified; these genes were involved in metal ion binding, transmembrane ion transport, and protein transport. According to the qRT-PCR results, expression of all these genes was down-regulated in the 462 μM of B treatment compared with the 46.2 μM of B treatment regardless of the Cd treatments (0.5 or 5 μM Cd). These results suggest that B is an inhibitor of Cd uptake, and the down-regulation of five highly homologous genes could be associated with decreased uptake of Cd after B application.

Analysis of trace metal distribution in plants with lab-based microscopic X-ray fluorescence imaging

BACKGROUND

Many metals are essential for plants and humans. Knowledge of metal distribution in plant tissues in vivo contributes to the understanding of physiological mechanisms of metal uptake, accumulation and sequestration. For those studies, X-rays are a non-destructive tool, especially suited to study metals in plants.

RESULTS

We present microfluorescence imaging of trace elements in living plants using a customized benchtop X-ray fluorescence machine. The system was optimized by additional detector shielding to minimize stray counts, and by a custom-made measuring chamber to ensure sample integrity. Protocols of data recording and analysis were optimised to minimise artefacts. We show that Zn distribution maps of whole leaves in high resolution are easily attainable in the hyperaccumulator Noccaea caerulescens. The sensitivity of the method was further shown by analysis of micro- (Cu, Ni, Fe, Zn) and macronutrients (Ca, K) in non-hyperaccumulating crop plants (soybean roots and pepper leaves), which could be obtained in high resolution for scan areas of several millimetres. This allows to study trace metal distribution in shoots and roots with a wide overview of the object, and thus avoids making conclusions based on singular features of tiny spots. The custom-made measuring chamber with continuous humidity and air supply coupled to devices for imaging chlorophyll fluorescence kinetic measurements enabled direct correlation of element distribution with photosynthesis. Leaf samples remained vital even after 20 h of X-ray measurements. Subtle changes in some of photosynthetic parameters in response to the X-ray radiation are discussed.

CONCLUSIONS

We show that using an optimized benchtop machine, with protocols for measurement and quantification tailored for plant analyses, trace metal distribution can be investigated in a reliable manner in intact, living plant leaves and roots. Zinc distribution maps showed higher accumulation in the tips and the veins of young leaves compared to the mesophyll tissue, while in the older leaves the distribution was more homogeneous.

Effects and interactions of cadmium and zinc on root morphology and metal translocation in two populations of Hylotelephium spectabile (Boreau) H. Ohba, a potential Cd-accumulating species

The interactions between Cd and Zn in their effects on plants are inconsistent and difficult to predict. A hydroponic experiment was conducted to investigate the effects of Cd and Zn and their interactions on root morphology and metal translocation in two populations of Hylotelephium spectabile (Boreau) H. Ohba (Crassulaceae, HB1 and HB2). Both populations showed relative tolerance to high levels of Cd and Zn, except that the leaf biomass of HB1 significantly decreased by 44.6% with 5-mg/L Cd plus 10-mg/L Zn. Root growth was inhibited in both populations by addition of 20-mg/L Zn under Cd stress, while 10-mg/L Zn showed little impact on the root growth inhibition of HB2. Roots with diameter 0.1-0.4 mm contributed most of the total root length (RL) and root surface area (RSA) of H. spectabile. In both populations, these root parameters showed greater suppression with the combined stress of Cd plus Zn than under Cd or Zn single stress, except by adding 10-mg/L Zn under Cd stress. Moreover, HB2 maintained relatively higher RL and RSA than HB1 under the different treatments, which implied that HB2 might possess a more effective mechanism than HB1 for coping in response to Cd and Zn stress. The addition of Zn not only affected the absorption of Cd but also significantly affected the distribution of Cd in different tissues of H. spectabile. A low level of Zn led to increased Cd in the stem of HB2, but an increase in Cd in the leaf and root of HB1. Addition of 10-mg/L Zn led to a significant increase by 188% and 170% in Cd accumulation in aboveground part of HB2 under 2- and 5-mg/L Cd stress, whereas the addition of Zn had little effect on Cd accumulation in HB1. Thus, strong positive interactions of Cd and Zn occurred in HB2, which showed great potential for application in phytoremediation of soil contaminated with both Cd and Zn, warranting further investigation under field condition.

Zinc Hyperaccumulation in Plants: A Review

Zinc is an essential microelement involved in many aspects of plant growth and development. Abnormal zinc amounts, mostly due to human activities, can be toxic to flora, fauna, and humans. In plants, excess zinc causes morphological, biochemical, and physiological disorders. Some plants have the ability to resist and even accumulate zinc in their tissues. To date, 28 plant species have been described as zinc hyperaccumulators. These plants display several morphological, physiological, and biochemical adaptations resulting from the activation of molecular Zn hyperaccumulation mechanisms. These adaptations can be varied between species and within populations. In this review, we describe the physiological and biochemical as well as molecular mechanisms involved in zinc hyperaccumulation in plants.

Physiological stress responses, mineral element uptake and phytoremediation potential of Morus alba L. in cadmium-contaminated soil

Fast growing woody plants are proposed for potential application for phytoremediation of contaminated soil. In this study, the plant growth, physiological responses, mineral element uptake, and phytoremediation potential of the woody plant Morus alba L. were studied in different levels of Cd-contaminated soil through dynamic sampling (30, 60, 120, and 180 d). The results indicated that M. alba L. had strong physiological coordination, tolerance and detoxification capacity in response to Cd in contaminated soil. Compared with the control, the photosynthetic pigment content in M. alba L. leaves was significantly suppressed during initial cultivation (30-60 d) and the malonaldehyde (MDA) content and electrolyte leakage (EL) were increased from 30 to 120 d of cultivation. Furthermore, the uptake of Cu, Mn, and Zn in plant tissues was imbalanced throughout cultivation (30-180 d) under 55 mg·kg^-1 Cd stress. However, the chlorophyll a, chlorophyll b, carotenoid, soluble protein, and soluble sugar contents and the peroxidase (POD) and ascorbate peroxidase (APX) activities in plant leaves, as well as the uptake of macronutrients (K, Ca, and Mg) in plant stems and leaves were maintained at normal levels. Furthermore, the catalase (CAT) activities in plant leaves and the Ca and Mg contents in plant roots were significantly (p < 0.05) enhanced in response to Cd stress after 180 d of cultivation. Furthermore, the biomass of M. alba L. was significantly increased with cultivation time in Cd-contaminated soil. Therefore, normal photosynthesis, antioxidant protection, and macronutrient regulation contribute to M. alba L. with high tolerance to Cd. Moreover, the uptake and total extraction amount of Cd in aboveground M. alba L. were significantly (p < 0.05) increased with both the plant growth period and soil Cd level, and the maximum amount of Cd reached up to 340.5 μg·plant^-1. Thus, M. alba L. can be regarded as a potential candidate for phytoremediation in Cd-contaminated sites.

Zn/Cd status-dependent accumulation of Zn and Cd in root parts in tobacco is accompanied by specific expression of ZIP genes

BACKGROUND

Root-to-shoot translocation of zinc (Zn) and cadmium (Cd) depends on the concentrations of both metals in the medium. A previous study on tobacco (Nicotiana tabacum) pointed to the contribution of NtZIP1, NtZIP2, NtZIP4 and NtIRT1-like in the regulation of this phenomenon. To learn more, Zn and Cd accumulation, root/shoot distribution and the expression of ZIP genes were investigated in the apical, middle and basal root parts.

RESULTS

We show that Zn/Cd status-dependent root-shoot distribution of both metals was related to distinct metal accumulation in root parts. At low Zn and Cd in the medium, the apical part contained the highest metal level; at higher concentrations, the middle and basal parts were the major sink for excess metal. The above were accompanied by root part-specific expression pattern modifications of ZIPs (NtZIP1-like, NtZIP2, NtZIP4A/B, NtZIP5A/B, NtZIP5-like, NtZIP8, NtZIP11, NtIRT1, and NtIRT1-like) that fell into four categories with respect to the root part. Furthermore, for lower Zn/Cd concentrations changes were noted for NtZIP5A/B and NtZIP5-like only, but at higher Zn and Cd levels for NtZIP1-like, NtZIP5-like, NtZIP8, NtZIP11, NtIRT1, and NtIRT1-like. NtZIP1, here renamed to NtZIP5B, was cloned and characterized. We found that it was a zinc deficiency-inducible transporter involved in zinc and cadmium uptake from the soil solution primarily by the middle root part.

CONCLUSIONS

We conclude that regulation of the longitudinal distribution of Zn and Cd is highly specific, and that the apical, middle and basal root parts play distinct roles in Zn/Cd status-dependent control of metal translocation efficiency to shoots, including the stimulation of Zn translocation to shoots in the presence of Cd. These results provide new insight into the root part-specific unique role of NtZIP5B and other ZIP genes in the longitudinal distribution of zinc and cadmium and their contribution to the regulation of root-to-shoot translocation.

Interaction Between Zn Deficiency, Toxicity and Turnip Yellow Mosaic Virus Infection in Noccaea ochroleucum

Zinc is essential for the functioning of numerous proteins in plants. To investigate how Zn homeostasis interacts with virus infection, Zn-tolerant Noccaea ochroleucum plants exposed to deficient (Zn'0'), optimal (Zn10), and excess Zn (Zn100) concentrations, as well as Cd amendment, were infected with Turnip yellow mosaic virus (TYMV). Imaging analysis of fluorescence kinetics from the μs (OJIP) to the minutes (Kautsky effect, quenching analysis) time domain revealed strong patchiness of systemic virus-induced photosystem II (PSII) inhibition. That was more pronounced in Zn-deficient plants, while Zn excess acted synergistically with TYMV, in both cases resulting in reduced PSII reaction centers. Infected Cd-treated plants, already severely stressed, showed inhibited non-photochemical quenching and PSII activity. Quantitative in situ hybridization at the cellular level showed increased gene expression of ZNT5 and downregulation of HMA4 in infected Zn-deficient leaves. In Zn10 and Zn100 infected leaves, vacuolar sequestration of Zn increased by activation of HMA3 (mesophyll) and MTP1 (epidermis). This correlated with Zn accumulation in the mesophyll and formation of biomineralization dots in the cell wall (Zn100) visible by micro X-ray fluorescence tomography. The study reveals the importance of adequate Zn supply and distribution in the maintenance of photosynthesis under TYMV infection, achieved by tissue-targeted activation of metal transporter gene expression.

Changes in Proteome and Protein Phosphorylation Reveal the Protective Roles of Exogenous Nitrogen in Alleviating Cadmium Toxicity in Poplar Plants

Phytoremediation soil polluted by cadmium has drawn worldwide attention. However, how to improve the efficiency of plant remediation of cadmium contaminated soil remains unknown. Previous studies showed that nitrogen (N) significantly enhances cadmium uptake and accumulation in poplar plants. In order to explore the important role of nitrogen in plants’ responses to cadmium stress, this study investigates the poplar proteome and phosphoproteome difference between Cd stress and Cd + N treatment. In total, 6573 proteins were identified, and 5838 of them were quantified. With a fold-change threshold of > 1.3, and a p-value < 0.05, 375 and 108 proteins were up- and down-regulated by Cd stress when compared to the control, respectively. Compared to the Cd stress group, 42 and 89 proteins were up- and down-regulated by Cd + N treatment, respectively. Moreover, 522 and 127 proteins were up- and down-regulated by Cd + N treatment compared to the CK group. In addition, 1471 phosphosites in 721 proteins were identified. Based on a fold-change threshold of > 1.2, and a p-value < 0.05, the Cd stress up-regulated eight proteins containing eight phosphosites, and down-regulated 58 proteins containing 69 phosphosites, whereas N + Cd treatment up-regulated 86 proteins containing 95 phosphosites, and down-regulated 17 proteins containing 17 phosphosites, when compared to Cd stress alone. N + Cd treatment up-regulated 60 proteins containing 74 phosphosites and down-regulated 37 proteins containing 42 phosphosites, when compared to the control. Several putative responses to stress proteins, as well as transcriptional and translational regulation factors, were up-regulated by the addition of exogenous nitrogen following Cd stress. Especially, heat shock protein 70 (HSP70), 14-3-3 protein, peroxidase (POD), zinc finger protein (ZFP), ABC transporter protein, eukaryotic translation initiation factor (elF) and splicing factor 3 B subunit 1-like (SF3BI) were up-regulated by Cd + N treatment at both the proteome and the phosphoproteome levels. Combing the proteomic data and phosphoproteomics data, the mechanism by which exogenous nitrogen can alleviate cadmium toxicity in poplar plants was explained at the molecular level. The results of this study will establish the solid molecular foundation of the phytoremediation method to improve cadmium-contaminated soil.

Hyperaccumulation of Cu, Zn, Ni, and Cd in Azolla species inducing expression of methallothionein and phytochelatin synthase genes

Azolla is a floating aquatic fern, having amazing capacity for concentrating toxic heavy metals. Metallothioneins (MTs) and phytochelatins (PCs) are well-defined heavy metal-binding ligands in plants. Bioaccumulation potential of different Azolla species varies according to their heavy metal ions. Therefore, the accumulation of Ni, Zn, Cu, and Cd was studied in A. pinnata, A. filiculoides, and a sample taken from Anzali wetland. Moreover, the expression of metallothionein and phytochelatin synthase encoding genes was examined at different metal concentrations. The highest level of Cu and Cd absorption was detected in A. pinnata, while the maximum amount of Ni and Zn absorption was observed in A. filiculoides and the sample taken from Anzali, respectively. The MT2 and PCS1 gene expression patterns were significantly induced by the heavy metal treatments, confirming their roles in phytoremediation potential of Azolla. However, as the results concerning heavy metal accumulation and gene expression vary in different species, only specific species of Azolla can be used for special purposes. It can be concluded that the Azolla is a good candidate for phytoremediation purposes, and the formation of phytochelatin-heavy metal complexes and their sequestration in vacuole are the main processes influencing susceptibility of Azolla to heavy metals.

Molecular dissection of cadmium-responsive transcriptome profile in a low-cadmium-accumulating cultivar of Brassica parachinensis

Brassica parachinensis L., a daily consumed leaf vegetable, is a high-Cd accumulator that substantially threatens human health. Screening and breeding Cd pollution-safe cultivars (Cd-PSCs) of crops is a low-cost strategy to restrict human Cd intake from contaminated soils via the food chain. However, little is known about the molecular mechanisms underlying the low-Cd-accumulating traits of B. parachinensis Cd-PSCs. In the current study, we analyzed the transcriptomes of the Cd-treated (5 μM) roots and shoots of a low-Cd-accumulating cultivar (SJ19) and a high-Cd-accumulating cultivar (CX4) of B. parachinensis to reveal the molecular mechanisms in response to Cd stress. Compared to CX4, many pathways involved in carbohydrate and amino acid metabolisms were exclusively up-regulated in SJ19 roots upon exposure to low Cd concentrations, which may produce more energy and metabolites for Cd detoxification. Antioxidant enzymes in the peroxisome were up-regulated in both SJ19 and CX4 roots in response to Cd, while glutathione biosynthesis was only activated in SJ19 roots. In SJ19 shoots, pathways of photosynthesis and cell growth were activated to mitigate Cd-induced damages. Furthermore, Cd transport genes, such as MTP1, HMA3 and CAX family genes, were highly induced by Cd stress in SJ19 roots in accordance with the high Cd concentration in roots, while genes involved in root-to-shoot Cd translocation such as FRD3 and CESA3 were suppressed, which may contribute to the low Cd concertation in edible part of SJ19. Our study provides a genetic basis for further Cd-PSCs screening and breeding.

Interaction between Cd and Zn on Metal Accumulation, Translocation and Mineral Nutrition in Tall Fescue (Festuca arundinacea)

Tall fescue (Festuca arundinacea), an accumulator that is able to accumulate and excrete cadmium (Cd), has attracted much attention for its possible use in phytoremediation of heavy metal contaminated soils. In the present study, the interaction between Cd and Zn, and their uptake, translocation and accumulation under external Cd and Zn treatment in tall fescue were investigated. The concentrations of K, Ca, Mg in xylem sap under Cd and Zn treatment were measured to determine the level of mineral nutrients and their relationship with Cd alleviation. The result showed that Cd and Zn antagonized each other in the roots, while Cd antagonized Zn and Zn synergized Cd in the shoots of tall fescue. Compared with Cd only treatment, the concentrations of Ca, Mg and K in xylem sap increased after the addition of Zn, and they increased the most in the guttation. This result indicated that the addition of Zn facilitates the level of mineral elements to alleviate Cd toxicity, which might be used to improve the phytoremediation efficiency of Cd contaminated soils by tall fescue.

Characteristics of cadmium accumulation and isotope fractionation in higher plants

Cadmium (Cd) pollution of the soil is an important global environmental issue owing to its great toxicity. The study of metal isotope fractionation is a novel technique that could be used to identify and quantify metal uptake and transport mechanisms in plant. In this study, cadmium tolerant Ricinus communis and hyperaccumulator Solanum nigrum have been cultured in different Cd concentration nutrient solutions. The Cd isotope values, metal elements concentrations in the organs (root, stem and leaf) in the two plant species have been measured during the growth periods (10d, 15d, 20d, 25d, and 30d). The results indicate that the organs of S. nigrum could be enriched with lighter Cd isotopes compared with R. communis. In addition, the Cd isotope fractionation become smaller when the plants were subjected to high Cd toxicity, which indicates that Cd isotope fractionation reflected the extent of Cd toxicity to plants. This study advances our current view of Cd translocation machination in plants.

Site-specific regulation of transcriptional responses to cadmium stress in the hyperaccumulator, Sedum alfredii: based on stem parenchymal and vascular cells

We compared the transcriptomes of parenchymal and vascular cells of Sedum alfredii stem under Cd stress to reveal gene regulatory networks underlying Cd hyperaccumulation. Cadmium (Cd) hyperaccumulation in plants is a complex biological process controlled by gene regulatory networks. Efficient transport through vascular systems and storage by parenchymal cells are vital for Cd hyperaccumulation in the Cd hyperaccumulator Sedum alfredii, but the genes involved are poorly understood. We investigated the spatial gene expression profiles of transport and storage sites in S. alfredii stem using laser-capture microdissection coupled with RNA sequencing. Gene expression patterns in response to Cd were distinct in vascular and parenchymal cells, indicating functional divisions that corresponded to Cd transportation and storage, respectively. In vascular cells, plasma membrane-related terms enriched a large number of differentially-expressed genes (DEGs) for foundational roles in Cd transportation. Parenchymal cells contained considerable DEGs specifically concentrated on vacuole-related terms associated with Cd sequestration and detoxification. In both cell types, DEGs were classified into different metabolic pathways in a similar way, indicating the role of Cd in activating a systemic stress signalling network where ATP-binding cassette transporters and Ca²⁺ signal pathways were probably involved. This study identified site-specific regulation of transcriptional responses to Cd stress in S. alfredii and analysed a collection of genes that possibly function in Cd transportation and detoxification, thus providing systemic information and direction for further investigation of Cd hyperaccumulation molecular mechanisms.

Uptake, translocation, and accumulation of Cd and its interaction with mineral nutrients (Fe, Zn, Ni, Ca, Mg) in upland rice

An analysis of the Cd and mineral nutrients accumulation of upland rice was performed in an experimental field with hard-ridged plots containing three soils with exogenous Cd addition at rates of 0, 0.25, 0.5, 1, 2, 4, 8, 16 mg kg^-1 five years prior to commence this experiment. Aims of this investigation were to study uptake, translocation, and accumulation of Cd by an upland rice (Huyou2) and effects of Cd addition on the accumulation of Fe, Zn, Ni, Ca, and Mg. The results demonstrate the mean Cd content in the plant parts, from highest to lowest, were as follows: root, stem, leaf, and brown rice. The Cd content in the brown rice of the upland rice was below the limit of Cd in rice (0.2 mg kg^-1) from China (GB 2762-2017) when the amount of Cd added was ≤ 1 mg kg^-1. This observation can be attributed to lower TF_soil-grain of Cd in upland rice. Significant differences were observed between Cd concentrations present in brown rice from the three soils which can be mainly attributed to the differences in DTPA-extractable soil Cd because of different soil pH. Addition of high concentrations of Cd to soil was found to reduce Fe, Zn, Mg, and increased Ni uptake by the roots and their accumulation in brown rice. Altogether, results of this study suggest that it may be possible to cultivate upland rice in slightly Cd-polluted soils and Cd toxicity and accumulation in upland rice can be minimized by optimizing the macro and micronutrient composition of the soil.

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.

Effect of water cadmium concentration and water level on the growth performance of Salix triandroides cuttings

The growth performance of Salix triandroides cuttings at three water cadmium (Cd) concentrations (0, 20, and 40 mg L^-1) and three water levels (- 40 cm, water level 40 cm below the soil surface; 0 cm, water level even with the soil surface; and 40 cm, water level 40 cm above soil surface) was investigated to evaluate its potential in phytoextraction strategies. Compared to cuttings in the - 40 or 0 cm water levels, cuttings in the 40 cm water level showed significantly lower biomass, height, and adventitious root length and significantly fewer leaves and adventitious roots. However, these growth and morphological parameters were not different among the three water Cd concentrations. Water level decreased stomatal conduction and transpiration rate but showed no significant effects on chlorophyll concentration or photosynthetic rate. Chlorophyll concentration and stomatal conductance were higher at 40 mg L^-1 Cd treatment than at 0 or 20 mg L^-1 Cd treatment; yet, photosynthetic rate and transpiration rate were not different. Cd concentration in the leaves and stems increased as the water level increased, but the highest Cd concentration in the roots occurred in the 0 cm water level. As water Cd concentration increased, Cd concentration in the leaves, stems, and roots increased in all three water levels, except in stems in the - 40 cm water level. Under Cd stress, cuttings in the - 40 or 0 cm water levels were characterized by a higher bioaccumulation coefficient, but a lower translocation factor, than those in the 40 cm water level. However, the bioaccumulation coefficient increased with increasing water Cd concentration in all three water levels, as did the translocation factor in the 40 cm water level. The tolerance index for the cuttings was the same among all water levels and water Cd concentrations. The results clearly indicated that the low water level increased plant growth and Cd accumulation in underground parts, while the high water level decreased plant growth but increased Cd accumulation in aboveground parts.

Trace metal metabolism in plants

Many trace metals are essential micronutrients, but also potent toxins. Due to natural and anthropogenic causes, vastly different trace metal concentrations occur in various habitats, ranging from deficient to toxic levels. Therefore, one focus of plant research is on the response to trace metals in terms of uptake, transport, sequestration, speciation, physiological use, deficiency, toxicity, and detoxification. In this review, we cover most of these aspects for the essential micronutrients copper, iron, manganese, molybdenum, nickel, and zinc to provide a broader overview than found in other recent reviews, to cross-link aspects of knowledge in this very active research field that are often seen in a separated way. For example, individual processes of metal usage, deficiency, or toxicity often were not mechanistically interconnected. Therefore, this review also aims to stimulate the communication of researchers following different approaches, such as gene expression analysis, biochemistry, or biophysics of metalloproteins. Furthermore, we highlight recent insights, emphasizing data obtained under physiologically and environmentally relevant conditions.

Genome-Wide Identification and Characterization of Four Gene Families Putatively Involved in Cadmium Uptake, Translocation and Sequestration in Mulberry

The zinc-regulated transporters, iron-regulated transporter-like proteins (ZIPs), the natural resistance and macrophage proteins (NRAMP), the heavy metal ATPases (HMAs) and the metal tolerance or transporter proteins (MTPs) families are involved in cadmium (Cd) uptake, translocation and sequestration in plants. Mulberry (Morus L.), one of the most ecologically and economically important (as a food plant for silkworm production) genera of perennial trees, exhibits excellent potential for remediating Cd-contaminated soils. However, there is no detailed information about the genes involved in Cd²⁺ transport in mulberry. In this study, we identified 31 genes based on a genome-wide analysis of the Morus notabilis genome database. According to bioinformatics analysis, the four transporter gene families in Morus were distributed in each group of the phylogenetic tree, and the gene exon/intron structure and protein motif structure were similar among members of the same group. Subcellular localization software predicted that these transporters were mainly distributed in the plasma membrane and the vacuolar membrane, with members of the same group exhibiting similar subcellular locations. Most of the gene promoters contained abiotic stress-related cis-elements. The expression patterns of these genes in different organs were determined, and the patterns identified, allowing the categorization of these genes into four groups. Under low or high-Cd²⁺ concentrations (30 μM or 100 μM, respectively), the transcriptional regulation of the 31 genes in root, stem and leaf tissues of M. alba seedlings differed with regard to tissue and time of peak expression. Heterologous expression of MaNRAMP1, MaHMA3, MaZIP4, and MaIRT1 in Saccharomyces cerevisiae increased the sensitivity of yeast to Cd, suggested that these transporters had Cd transport activity. Subcellular localization experiment showed that the four transporters were localized to the plasma membrane of yeast and tobacco. These results provide the basis for further understanding of the Cd tolerance mechanism in Morus, which can be exploited in Cd phytoremediation.

The effects of cadmium pulse dosing on physiological traits and growth of the submerged macrophyte Vallisneria spinulosa and phytoplankton biomass: a mesocosm study

Pulse inputs of heavy metals are expected to increase with a higher frequency of extreme climate events (heavy rain), leading to stronger erosion of contaminated and fertilized farmland soils to freshwaters, with potentially adverse effects on lake ecosystems. We conducted a 5-month mesocosm study to elucidate the responses of the submerged macrophyte Vallisneria spinulosa and phytoplankton to four different doses of cadmium (Cd): 0 (control), 0.05, 0.5, and 5 g m^-2 (CK, I, II, and III, respectively) under mesotrophic conditions. We found that total phosphorus concentrations were larger in the three Cd pulse treatments, whereas total nitrogen concentrations did not differ among the four treatments. The contents of chlorophyll a and soluble sugar in macrophyte leaves decreased in III, and total biomass, ramet number, plant height, and total stolon length of macrophytes were lower in both II and III. In contrast, abundances of the three main phytoplankton taxa-Cyanophyta, Chlorophyta, and Bacillariophyta-did not differ among treatments. Total phytoplankton biomass was, however, marginally lower in CK than in the Cd treatments. We conclude that exposure to strong Cd pulses led to significantly reduced growth of macrophytes, while no obvious effect appeared for phytoplankton.

Protein Biochemistry and Expression Regulation of Cadmium/Zinc Pumping ATPases in the Hyperaccumulator Plants Arabidopsis halleri and Noccaea caerulescens

P_1B-ATPases are decisive for metal accumulation phenotypes, but mechanisms of their regulation are only partially understood. Here, we studied the Cd/Zn transporting ATPases NcHMA3 and NcHMA4 from Noccaea caerulescens as well as AhHMA3 and AhHMA4 from Arabidopsis halleri. Protein biochemistry was analyzed on HMA4 purified from roots of N. caerulescens in active state. Metal titration of NcHMA4 protein with an electrochromic dye as charge indicator suggested that HMA4 reaches maximal ATPase activity when all internal high-affinity Cd²⁺ binding sites are occupied. Although HMA4 was reported to be mainly responsible for xylem loading of heavy metals for root to shoot transport, the current study revealed high expression of NcHMA4 in shoots as well. Further, there were additional 20 and 40 kD fragments at replete Zn²⁺ and toxic Cd²⁺, but not at deficient Zn²⁺ concentrations. Altogether, the protein level expression analysis suggested a more multifunctional role of NcHMA4 than previously assumed. Organ-level transcription analysis through quantitative PCR of mRNA in N. caerulescens and A. halleri confirmed the strong shoot expression of both NcHMA4 and AhHMA4. Further, in shoots NcHMA4 was more abundant in 10 μM Zn²⁺ and AhHMA4 in Zn²⁺ deficiency. In roots, NcHMA4 was up-regulated in response to deficient Zn²⁺ when compared to replete Zn²⁺ and toxic Cd²⁺ treatment. In both species, HMA3 was much more expressed in shoots than in roots, and HMA3 transcript levels remained rather constant regardless of Zn²⁺ supply, but were up-regulated by 10 μM Cd²⁺. Analysis of cellular expression by quantitative mRNA in situ hybridisation showed that in A. halleri, both HMA3 and HMA4 mRNA levels were highest in the mesophyll, while in N. caerulescens they were highest in the bundle sheath of the vein. This is likely related to the different final storage sites for hyperaccumulated metals in both species: epidermis in N. caerulescens, mesophyll in A. halleri.

Antioxidative systems, metal ion homeostasis and cadmium distribution in Iris lactea exposed to cadmium stress

Iris lactea is a perennial halophyte and is tolerant to Cd. However, the mechanisms underlying this Cd tolerance are still poorly understood. In this study, morphological, physiological and biochemical responses of I. lactea to a 21 d exposure to different concentrations of Cd (0-150mgL^-1) were investigated. I. lactea plants showed no toxicity symptoms except for a small reduction in growth at 100 and 150mgL^-1 Cd, along with the enhancement of H₂O₂ and MDA content in comparison to the control. The activities of SOD and POD were significantly enhanced and Ca accumulated with increasing Cd concentrations. Moreover, most Cd was retained in roots and only a small amount was transported to the shoots with increasing external Cd concentrations. Cd content had a negative correlation with content of K, Fe, Zn, and Mn and a positive correlation with Mg content in shoots and roots, which had no influence on these contents of mineral nutrients in shoots and chlorophyll levels with the increase of Cd concentrations. The Cd translocation factors were always less than 1 and bioaccumulation factors ranged from 3.43 to 15.6 across all treatments, suggesting that I. lactea might be effectively used in phytostabilization of Cd contaminated soils. Overall, the findings suggest that I. lactea could reduce photoinhibition and oxidative damage and maintain metal ion homeostasis in plant tissue by limiting translocation of Cd from roots to shoots and enhancing induction of antioxidant enzyme activities, thereby improving its Cd tolerance.

Protein Biochemistry and Expression Regulation of Cadmium/Zinc Pumping ATPases in the Hyperaccumulator Plants Arabidopsis halleri and Noccaea caerulescens

P_1B-ATPases are decisive for metal accumulation phenotypes, but mechanisms of their regulation are only partially understood. Here, we studied the Cd/Zn transporting ATPases NcHMA3 and NcHMA4 from Noccaea caerulescens as well as AhHMA3 and AhHMA4 from Arabidopsis halleri. Protein biochemistry was analyzed on HMA4 purified from roots of N. caerulescens in active state. Metal titration of NcHMA4 protein with an electrochromic dye as charge indicator suggested that HMA4 reaches maximal ATPase activity when all internal high-affinity Cd²⁺ binding sites are occupied. Although HMA4 was reported to be mainly responsible for xylem loading of heavy metals for root to shoot transport, the current study revealed high expression of NcHMA4 in shoots as well. Further, there were additional 20 and 40 kD fragments at replete Zn²⁺ and toxic Cd²⁺, but not at deficient Zn²⁺ concentrations. Altogether, the protein level expression analysis suggested a more multifunctional role of NcHMA4 than previously assumed. Organ-level transcription analysis through quantitative PCR of mRNA in N. caerulescens and A. halleri confirmed the strong shoot expression of both NcHMA4 and AhHMA4. Further, in shoots NcHMA4 was more abundant in 10 μM Zn²⁺ and AhHMA4 in Zn²⁺ deficiency. In roots, NcHMA4 was up-regulated in response to deficient Zn²⁺ when compared to replete Zn²⁺ and toxic Cd²⁺ treatment. In both species, HMA3 was much more expressed in shoots than in roots, and HMA3 transcript levels remained rather constant regardless of Zn²⁺ supply, but were up-regulated by 10 μM Cd²⁺. Analysis of cellular expression by quantitative mRNA in situ hybridisation showed that in A. halleri, both HMA3 and HMA4 mRNA levels were highest in the mesophyll, while in N. caerulescens they were highest in the bundle sheath of the vein. This is likely related to the different final storage sites for hyperaccumulated metals in both species: epidermis in N. caerulescens, mesophyll in A. halleri.

Transcriptional up-regulation of genes involved in photosynthesis of the Zn/Cd hyperaccumulator Sedum alfredii in response to zinc and cadmium

Zinc (Zn) and cadmium (Cd) are two closely related chemical elements with very different biological roles in photosynthesis. Zinc plays unique biochemical functions in photosynthesis. Previous studies suggested that in some Zn/Cd hyperaccumulators, many steps in photosynthesis may be Cd tolerant or even Cd stimulated. Using RNA-seq data, we found not only that Cd and Zn both up-regulated the CA1 gene, which encodes a β class carbonic anhydrase (CA) in chloroplasts, but that a large number of other Zn up-regulated genes in the photosynthetic pathway were also significantly up-regulated by Cd in leaves of the Zn/Cd hyperaccumulator Sedum alfredii. These genes also include chloroplast genes involved in transcription and translation (rps18 and rps14), electron transport and ATP synthesis (atpF and ccsA), Photosystem II (PSBI, PSBM, PSBK, PSBZ/YCF9, PSBO-1, PSBQ, LHCB1.1, LHCB1.4, LHCB2.1, LHCB4.3 and LHCB6) and Photosystem I (PSAE-1, PSAF, PSAH2, LHCA1 and LHCA4). Cadmium and Zn also up-regulated the VAR1 gene, which encodes the ATP-dependent zinc metalloprotease FTSH 5 (a member of the FtsH family), and the DAG gene, which influences chloroplast differentiation and plastid development, and the CP29 gene, which supports RNA processing in chloroplasts and has a potential role in signal-dependent co-regulation of chloroplast genes. Further morphological parameters (dry biomass, cross-sectional thickness, chloroplast size, chlorophyll content) and chlorophyll fluorescence parameters confirmed that leaf photosynthesis of S. alfredii responded to Cd much as it did to Zn, which will contribute to our understanding of the positive effects of Zn and Cd on growth of this plant.

Accumulation of Cr, Cd, Pb, Cu, and Zn by plants in tanning sludge storage sites: opportunities for contamination bioindication and phytoremediation

Tanning sludge enriched with high concentrations of Cr and other metals has adverse effects on the environment. Plants growing in the metalliferous soils may have the ability to cope with high metal concentrations. This study focuses on potentials of using native plants for bioindication and/or phytoremediation of Cr-contaminated sites. In the study, we characterized plants and soils from six tanning sludge storage sites. Soil in these sites exhibited toxic levels of Cr (averaged 16,492 mg kg^-1) and other metals (e.g., 48.3 mg Cu kg^-1, 2370 mg Zn kg^-1, 44.9 mg Pb kg^-1, and 0.59 mg Cd kg^-1). Different metal tolerance and accumulation patterns were observed among the sampled plant species. Phragmites australis, Zephyranthes candida, Cynodon dactylon, and Alternanthera philoxeroides accumulated moderate-high concentrations of Cr and other metals, which could make them good bioindicators of heavy metal pollution. High Cr and other metal concentrations (e.g., Cd and Pb) were found in Chenopodium rubrum (372 mg Cr kg^-1), Aster subulatus (310 mg Cr kg^-1), and Brassica chinensis (300 mg Cr kg^-1), being considered as metal accumulators. In addition, Nerium indicum and Z. candida were able to tolerate high concentrations of Cr and other metals, and they may be used as preferable pioneer species to grow or use for restoration in Cr-contaminated sites. This study can be useful for establishing guidelines to select the most suitable plant species to revegetate and remediate metals in tanning sludge-contaminated fields.

Physiological and biochemical mechanisms preventing Cd-toxicity in the hyperaccumulator Atriplex halimus L

The xero-halophyte Atriplex halimus L., recently described as Cd-hyperaccumulator, was examined to determine Cd toxicity threshold and the physiological mechanisms involved in Cd tolerance. An experiment was conducted to investigate the effect of cadmium from 0 to 1350 μM on chlorophyll fluorescence parameters, gas exchange, photosynthetic pigment concentrations and antioxidative enzyme activities of A. halimus. Cadmium, calcium, iron, manganese, magnesium, potassium, phosphorous, sodium and zinc concentrations were also analyzed. Plants of A. halimus were not able to survive at 1350 μM Cd and the upper tolerance limit was recorded at 650 μM Cd; although chlorosis was observed from 200 μM Cd. Cadmium accumulation increased with increase in Cd supply, reaching maxima of 0.77 and 4.65 mg g(-1) dry weight in shoots and roots, respectively, at 650 μM Cd. Dry mass, shoot length, specific leaf area, relative growth rate, net photosynthetic rate, stomatal conductance, pigments contents and chlorophyll fluorescence were significantly reduced by increasing Cd concentration. However, the activities of superoxide dismutase (SOD; EC1.15.1.1), catalase (CAT; EC1.11.1.6) and guaiacol peroxidase (GPx; EC1.11.1.7) were significantly induced by Cd. Exposures to Cd caused also a significant decrease in P contents in roots, Mg and Mn contents in shoots and Fe and K contents in roots and shoots and had no effect on Ca, Na and Zn contents. The tolerance of A. halimus to Cd stress might be related with its capacity to avoid the translocation of great amounts of Cd in its aboveground tissues and higher activities of enzymatic antioxidants in the leaf.

Stress hardening under long-term cadmium treatment is correlated with the activation of antioxidative defence and iron acquisition of chloroplasts in Populus

Cadmium (Cd), a highly toxic heavy metal affects growth and metabolic pathways in plants, including photosynthesis. Though Cd is a transition metal with no redox capacity, it generates reactive oxygen species (ROS) indirectly and causes oxidative stress. Nevertheless, the mechanisms involved in long-term Cd tolerance of poplar, candidate for Cd phytoremediation, are not well known. Hydroponically cultured poplar (Populus jacquemontiana var. glauca cv. 'Kopeczkii') plants were treated with 10 μM Cd for 4 weeks. Following a period of functional decline, the plants performed acclimation to the Cd induced oxidative stress as indicated by the decreased leaf malondialdehyde (MDA) content and the recovery of most photosynthetic parameters. The increased activity of peroxidases (PODs) could have a great impact on the elimination of hydrogen peroxide, and thus the recovery of photosynthesis, while the function of superoxide dismutase (SOD) isoforms seemed to be less important. Re-distribution of the iron content of leaf mesophyll cells into the chloroplasts contributed to the biosynthesis of the photosynthetic apparatus and some antioxidative enzymes. The delayed increase in photosynthetic activity in relation to the decline in the level of lipid peroxidation indicates that elimination of oxidative stress damage by acclimation mechanisms is required for the restoration of the photosynthetic apparatus during long-term Cd treatment.