Cadmium uptake, translocation and tolerance in the hyperaccumulator Arabidopsis halleri

Exploring Cd tolerance and detoxification strategies of Arabidopsis halleri ssp. gemmifera under high cadmium exposure

Arabidopsis halleri ssp. gemmifera is well known as a Cd hyperaccumulator. Yet, understanding how this plant survives in a high Cd environment without appearing toxicity signs is far from complete. Therefore, this study emphasized on high level of Cd to be applied to evaluate Cd detoxification and tolerance strategies in the hyperaccumulator A. halleri ssp. gemmifera. The results showed that under 300 µM of Cd exposure in a hydroponic system for 56 days, Cd tends to be transported to the stem and leaves. The availability of more than 60% of Cd mobile fractions supported Cd translocation to leaves. EPMA at the cellular level identified Cd localization at the rim of leaf cells that might be associated with the Cd-cell wall form. The Cd soluble fraction in the leaves was found as Cd-free ion and Cd-complexed compound. Interestingly, this plant can still grow despite some inhibition, such as significantly decreasing total chlorophyll and anthocyanins content in the leaves. It was predicted that Cd-free ions were sequestered into the vacuole of leaves cells, by complexing it into organic acid compounds. Therefore, tolerance strategies in A. halleri ssp. gemmifera at high Cd is proved to be associated to compartmentalization and complexation strategies.

Serendipita indica Drives Sulfur-Related Microbiota in Enhancing Growth of Hyperaccumulator Sedum alfredii and Facilitating Soil Cadmium Remediation

Endophytic fungus Serendipita indica can bolster plant growth and confer protection against various biotic and abiotic stresses. However, S. indica-reshaped rhizosphere microecology interactions and root-soil interface processes in situ at the submicrometer scale remain poorly understood. We combined amplicon sequencing and high-resolution nano X-ray fluorescence (nano-XRF) imaging of the root-soil interface to reveal cadmium (Cd) rhizosphere processes. S. indica can successfully colonize the roots of Sedum alfredii Hance, which induces a remarkable increase in shoot biomass by 211.32% and Cd accumulation by 235.72%. Nano-XRF images showed that S. indica colonization altered the Cd distribution in the rhizosphere and facilitated the proximity of more Cd and sulfur (S) to enter the roots and transport to the shoot. Furthermore, the rhizosphere-enriched microbiota demonstrated a more stable network structure after the S. indica inoculation. Keystone species were strongly associated with growth promotion and Cd absorption. For example, Comamonadaceae are closely related to the organic acid cycle and S bioavailability, which could facilitate Cd and S accumulation in plants. Meanwhile, Sphingomonadaceae could release auxin and boost plant biomass. In summary, we construct a mutualism system for beneficial fungi and hyperaccumulation plants, which facilitates high-efficient remediation of Cd-contaminated soils by restructuring the rhizosphere microbiota.

Decrypting Molecular Mechanisms Involved in Counteracting Copper and Nickel Toxicity in Jack Pine (Pinus banksiana) Based on Transcriptomic Analysis

The remediation of copper and nickel-afflicted sites is challenged by the different physiological effects imposed by each metal on a given plant system. Pinus banksiana is resilient against copper and nickel, providing an opportunity to build a valuable resource to investigate the responding gene expression toward each metal. The objectives of this study were to (1) extend the analysis of the Pinus banksiana transcriptome exposed to nickel and copper, (2) assess the differential gene expression in nickel-resistant compared to copper-resistant genotypes, and (3) identify mechanisms specific to each metal. The Illumina platform was used to sequence RNA that was extracted from seedlings treated with each of the metals. There were 449 differentially expressed genes (DEGs) between copper-resistant genotypes (RGs) and nickel-resistant genotypes (RGs) at a high stringency cut-off, indicating a distinct pattern of gene expression toward each metal. For biological processes, 19.8% of DEGs were associated with the DNA metabolic process, followed by the response to stress (13.15%) and the response to chemicals (8.59%). For metabolic function, 27.9% of DEGs were associated with nuclease activity, followed by nucleotide binding (27.64%) and kinase activity (10.16%). Overall, 21.49% of DEGs were localized to the plasma membrane, followed by the cytosol (16.26%) and chloroplast (12.43%). Annotation of the top upregulated genes in copper RG compared to nickel RG identified genes and mechanisms that were specific to copper and not to nickel. NtPDR, AtHIPP10, and YSL1 were identified as genes associated with copper resistance. Various genes related to cell wall metabolism were identified, and they included genes encoding for HCT, CslE6, MPG, and polygalacturonase. Annotation of the top downregulated genes in copper RG compared to nickel RG revealed genes and mechanisms that were specific to nickel and not copper. Various regulatory and signaling-related genes associated with the stress response were identified. They included UGT, TIFY, ACC, dirigent protein, peroxidase, and glyoxyalase I. Additional research is needed to determine the specific functions of signaling and stress response mechanisms in nickel-resistant plants.

Responses of rhizosphere microbial community structure and metabolic function to heavy metal coinhibition

Metal mineral mining results in releases of large amounts of heavy metals into the environment, and it is necessary to better understand the response of rhizosphere microbial communities to simultaneous stress from multiple heavy metals (HMs), which directly impacts plant growth and human health. In this study, by adding different concentrations of cadmium (Cd) to a soil with high background concentrations of vanadium (V) and chromium (Cr), the growth of maize during the jointing stage was explored under limiting conditions. High-throughput sequencing was used to explore the response and survival strategies of rhizosphere soil microbial communities to complex HM stress. The results showed that complex HMs inhibited the growth of maize at the jointing stage, and the diversity and abundance of maize rhizosphere soil microorganisms were significantly different at different metal enrichment levels. In addition, according to the different stress levels, the maize rhizosphere attracted many tolerant colonizing bacteria, and cooccurrence network analysis showed that these bacteria interacted very closely. The effects of residual heavy metals on beneficial microorganisms (such as Xanthomonas, Sphingomonas, and lysozyme) were significantly stronger than those of bioavailable metals and soil physical and chemical properties. PICRUSt analysis revealed that the different forms of V and Cd had significantly greater effects on microbial metabolic pathways than all forms of Cr. Cr mainly affected the two major metabolic pathways: microbial cell growth and division and environmental information transmission. In addition, significant differences in rhizosphere microbial metabolism under different concentrations were found, and this can serve as a reference for subsequent metagenomic analysis. This study is helpful for exploring the threshold for the growth of crops in toxic HM soils in mining areas and achieving further biological remediation.

Human Health Risk Assessment to the Consumption of Medicinal Plants with Melliferous Potential from the Romanian South-Eastern Region

This study presents the impact on human health by consuming medicinal herbs with high melliferous potential (HMPs) from botanical areas with different pollution levels. First, the bioaccumulation of the plants' parts has been determined. The study assessed the potential health risks associated with the ingestion of various mineral species (macroelements-K, Ca, Mg, Na; microelements-Fe, Mn, Cu, Zn, and one trace element Cd) from three types of HMPs (Sambucus nigra (SnL), Hypericum perforatum (Hp), and Tilia tomentosa (Tt)). The average concentrations of these elements were not similar even in the same type of HMPs. Nevertheless, all samples contained detectable levels of the studied elements. The average concentrations of the studied elements were very low (significantly lower than the legal limit set by the WHO). The study's findings indicated that the potential health risks associated with ingesting the elements in HMPs were within acceptable limits for children and adults. The hazard quotient (HQ) for Fe, Mn, Cu, Zn, and Cd and the hazard index (HI) for the minerals from HMPs were significantly lower than the acceptable limit (HQ and HI = 1). Similarly, the carcinogenic risk for chemical substances (Riskccs) were lower than or close to the acceptable limit (1 × 10^-4).

Cichorium intybus L. is a potential Cd-accumulator for phytoremediation of agricultural soil with strong tolerance and detoxification to Cd

Identifying suitable plants for phytoremediation of Cd (cadmium) contaminated agricultural soil is critical. In this study, whether chicory (Cichorium intybus L.) qualified as an ideal accumulator for phytoremediation was investigated. The hydroponic and pot experiments showed that Cd concentration in chicory leaves exceeded 100 mg kg-1 (BCF >1, TF >1) with 40 mg kg-1 Cd in pot; No significant effects on chicory growth, leaf protein and physiological and biochemical aspects when treated with ≤ 20 μM or 40 mg kg-1 Cd, because chicory could relieve Cd toxicity by increasing activities of photoprotection mechanisms, the reactive oxygen species scavenging system and concentrations of functional groups in plant tissues. In field experiment, 16.2 and 26.6 t ha-1 of chicory leaves was harvested in winter and summer, respectively. The highest Cd concentration in leaves was close to 25.0 mg kg-1 (BCF >1, TF >1) from the acid soil with 0.980 mg kg-1 Cd. Over 320 g ha-1 Cd was extracted from soil by harvesting chicory leaves both in winter and summer, with 9.24% and 12.9% of theoretical phytoremediation efficiency. Therefore, chicory can be as an ideal Cd-accumulator for phytoremediation of slight-to-moderate Cd-contaminated agricultural soil in any season.

Abscisic acid alleviates mercury toxicity in wheat (Triticum aestivum L.) by promoting cell wall formation

Mercury (Hg) is a heavy metal (HM) that affects crop growth and productivity. In a previous study, we found that application of exogenous abscisic acid (ABA) alleviated growth inhibition in Hg-stressed wheat seedlings. However, the physiological and molecular mechanisms underlying ABA-mediated Hg detoxification remained unclear. In this study, Hg exposure reduced the plant fresh and dry weights and root numbers. Exogenous ABA treatment significantly resumed the plant growth, increased the plant height and weight, and enriched the roots numbers and biomass. The application of ABA enhanced Hg absorption and raised the Hg levels in the roots. In addition, exogenous ABA decreased Hg-induced oxidative damage and significantly brought down the activities of antioxidant enzymes, such as SOD, POD and CAT. Global gene expression patterns in the roots and leaves exposed to HgCl₂ and ABA treatments were examined via RNA-Seq. The data showed that genes related to ABA-mediated Hg detoxification were enriched in functions related to cell wall formation. Weighted gene co-expression network analysis (WGCNA) further indicated that the genes implicated in Hg detoxification were related to cell wall synthesis. Under Hg stress, ABA significantly induced expression of the genes encoding cell wall synthesis enzymes, regulated the activity of hydrolase, and increased the concentrations of cellulose and hemicellulose, hence promoting cell wall synthesis. Taken together, these results suggest that exogenous ABA could alleviate Hg toxicity in wheat by promoting cell wall formation and suppressing translocation of Hg from roots to shoots.

Cadmium tolerance and hyperaccumulation in plants - A proteomic perspective of phytoremediation

Cadmium (Cd) is a major environmental pollutant and poses a risk of transfer into the food chain through contaminated plants. Mechanisms underlying Cd tolerance and hyperaccumulation in plants are not fully understood. Proteomics-based approaches facilitate an in-depth understanding of plant responses to Cd stress at the systemic level by identifying Cd-inducible differentially abundant proteins (DAPs). In this review, we summarize studies related to proteomic changes associated with Cd-tolerance mechanisms in Cd-tolerant crops and Cd-hyperaccumulating plants, especially the similarities and differences across plant species. The enhanced DAPs identified through proteomic studies can be potential targets for developing Cd-hyperaccumulators to remediate Cd-contaminated environments and Cd-tolerant crops with low Cd content in the edible organs. This is of great significance for ensuring the food security of an exponentially growing global population. Finally, we discuss the methodological drawbacks in current proteomic studies and propose that better protocols and advanced techniques should be utilized to further strengthen the reliability and applicability of future Cd-stress-related studies in plants. This review provides insights into the improvement of phytoremediation efficiency and an in-depth study of the molecular mechanisms of Cd enrichment in plants.

NaCl effect on Cd accumulation and cell compartmentalization in barley

The effect of sodium chloride (NaCl) on cadmium (Cd) tolerance, uptake, translocation, and compartmentation was investigated in 3 barley genotypes. Seedlings were cultivated hydroponically in the absence of NaCl and Cd (control), in the presence of 50 mM NaCl alone, in the presence of 10 µM Cd alone, and in the combined addition of NaCl (50 mM) and Cd (10 µM). Plants were cultivated during one month under 16 h light period at a minimal light intensity of 250 µmol m-2 s-1, a temperature of 25 ± 3 °C, and 70-80% of relative humidity. Results showed that NaCl alone did not significantly affect plant development and biomass production; however, Cd alone reduced plant development rate leading to a decline in biomass production in Raihane and Giza 127 but did not affect that in Amalou. NaCl addition in Cd-treated plants accentuated the Cd effect on plant growth. NaCl limited Cd accumulation in the roots and in the shoots in all tested barley varieties by reducing Cd-absorption efficiency and the translocation of Cd from the root to the shoot. In all Cd-treated plants, cell Cd compartmentalization showed the following gradient: organelles < cell wall < vacuole. NaCl in the medium increased Cd accumulation in the soluble fraction and reduced that in organelle and cell wall fractions. Globally our results showed that, although NaCl reduces Cd accumulation in barley, it accentuates the Cd toxic effects, hence limiting the plant yield. We advise farmers to avoid barley cultivation near mine sites and its irrigation with moderately salty water, although this plant is considered as salt tolerant.

The role of metal transporters in phytoremediation: A closer look at Arabidopsis

Pollution of the environment by heavy metals (HMs) has recently become a global issue, affecting the health of all living organisms. Continuous human activities (industrialization and urbanization) are the major causes of HM release into the environment. Over the years, two methods (physical and chemical) have been widely used to reduce HMs in polluted environment. However, these two methods are inefficient and very expensive to reduce the HMs released into the atmosphere. Alternatively, researchers are trying to remove the HMs by employing hyper-accumulator plants. This method, referred to phytoremediation, is highly efficient, cost-effective, and eco-friendly. Phytoremediation can be divided into five types: phytostabilization, phytodegradation, rhizofiltration, phytoextraction, and phytovolatilization, all of which contribute to HMs removal from the polluted environment. Brassicaceae family members (particularly Arabidopsis thaliana) can accumulate more HMs from the contaminated environment than those of other plants. This comprehensive review focuses on how HMs pollute the environment and discusses the phytoremediation measures required to reduce the impact of HMs on the environment. We discuss the role of metal transporters in phytoremediation with a focus on Arabidopsis. Then draw insights into the role of genome editing tools in enhancing phytoremediation efficiency. This review is expected to initiate further research to improve phytoremediation by biotechnological approaches to conserve the environment from pollution.

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.

Multi-leveled insights into the response of the eelgrass Zostera marina L to Cu than Cd exposure

Seagrass beds are recognized as critical and among the most vulnerable habitats on the planet; seagrass colonize the coastal waters where heavy metal pollution is a serious problem. In this study, the toxic effects of copper and cadmium in the eelgrass Zostera marina L. were observed at the individual, subcellular, physiologically biochemical, and molecular levels. Both Cu and Cd stress significantly inhibited the growth and the maximal quantum yield of photosystem II (Fv/Fm); and high temperature increased the degree of heavy metal damage, while low temperatures inhibited damage. The half-effect concentration (EC₅₀) of eelgrass was 28.9 μM for Cu and 2246.8 μM for Cd, indicating Cu was much more toxic to eelgrass than Cd. The effect of Cu and Cd on photosynthesis was synergistic. After 14 days of enrichment, the concentration of Cu in leaves and roots of Z. marina was 48 and 37 times higher than that in leaf sheath, and 14 and 11 times higher than that in rhizome; and the order of Cd concentration in the organs was root > leaf > rhizome > sheath. Heavy metal uptake mainly occurred in the organelles, and Cd enrichment also occurred to a certain extent in the cytoplasm. Transcriptome results showed that a number of photosynthesis-related KEGG enrichment pathways and GO terms were significantly down-regulated under Cd stress, suggesting that the photosynthetic system of eelgrass was severely damaged at the transcriptome level, which was consistent with the significant inhibition of Fv/Fm and leaf yellowing. Under Cu stress, the genes related to glutathione metabolic pathway were significantly up-regulated, together with the increased autioxidant enzyme activity of GSH-PX. In addition, the results of recovery experiment indicated that the damage caused by short-term Cd and Cu stress under EC₅₀ was reversible. These results provide heavy metal toxic effects at multiple levels and information relating to the heavy metal resistance strategies evolved by Z. marina to absorb and isolate heavy metals, and highlight the phytoremediation potential of this species especially for Cd.

Heavy Metal Pollution in Aquaculture: Sources, Impacts and Mitigation Techniques

Aquaculture is one of the fastest growing agro-industries as it presently accounts for nearly 50% of all fish for direct human consumption and 43% of total seafood supply. Fish provide about 20% average daily intake of animal protein for about 3.2 billion people globally. The treatment of aquaculture in recent years for the mitigation of heavy metals and other contaminants has been gaining traction due to the benefits of aquaculture to both man and the environment. This paper provides a review of the sources, impacts, and the various methods that have been deployed in recent years by various researchers for the treatment of heavy metal contaminated aquaculture. Related works of literature were obtained and compiled from academic search databases and were carefully analysed in this study. The dangers these metals pose to the sustainability of aquaculture were studied in this review. Studies indicate that some heavy metals, such as mercury, lead, and cadmium, due to their long-term persistence in the environment, allow them to accumulate in the food chain. Mitigation techniques such as adsorption, bio-sorption, and phytoremediation have been deployed for the treatment of heavy metal contaminated aquaculture. Some research gaps were also highlighted which could form the basis for future research, such as research centred on the effects of these metals on the embryonic development of aquaculture organisms and the alterations the metals caused in their stages of development.

Comparison of In Vitro and In Planta Heavy Metal Tolerance and Accumulation Potential of Different Armeria maritima Accessions from a Dry Coastal Meadow

The aim of the present study was to compare the tolerance to several heavy metals and their accumulation potential of Armeria maritima subsp. elongata accessions from relatively dry sandy soil habitats in the Baltic Sea region using both in vitro cultivated shoot explants and long-term soil-cultivated plants at the flowering stage as model systems. The hypothesis that was tested was that all accessions will show a relatively high heavy metal tolerance and a reasonable metal accumulation potential, but possibly to varying degrees. Under the conditions of the tissue culture, the explants accumulated extremely high concentration of Cd and Cu, leading to growth inhibition and eventual necrosis, but the accumulation of Pb in their tissues was limited. When grown in soil, the plants from different accessions showed a very high heavy metal tolerance, as the total biomass was not negatively affected by any of the treatments. The accumulation potential for heavy metals in soil-grown plants was high, with several significant accession- and metal-related differences. In general, the heavy metal accumulation potential in roots and older leaves was similar, except for Mn, which accumulated more in older leaves. The absolute higher values of the heavy metal concentrations reached in the leaves of soil-grown A. maritima plants (500 mg Cd kg^-1, 600 mg Cu kg^-1, 12,000 mg Mn kg^-1, 1500 mg Pb kg^-1, and 15,000 mg Zn kg^-1) exceeded the respective threshold values for hyperaccumulation. In conclusion, A. maritima can be characterized by a species-wide heavy metal tolerance and accumulation potential, but with a relatively high intraspecies diversity.

Micro- and Macroelements Content of Plants Used for Landfill Leachate Treatment Based on Phragmites australis and Ceratophyllum demersum

One of the key problems associated with the functioning of landfills is the generation of leachate. In order to reduce their negative impact on the environment, various treatment technologies are applied. Among them, solutions based on the use of phytotechnology deserve special attention. The aim of this study was to evaluate the impact of landfill leachate on the content of micro- and macroelements in plant material. The research was carried out in four municipal waste landfills located in Poland. Emergent macrophytes (P. australis) and submergent macrophytes (C. demersum) were used in this research. The migration and distribution of pollutants reaching the roots and shoots of P. australis from water solutions were also studied. The concentrations of heavy metals in the studied plants were low in all analysed cases. Higher metal contents could often be observed in roots rather than in shoots, but these differences were insignificant. The chemical composition of the studied plant samples was primarily related to the source of origin of the treated leachate (landfill), as clearly demonstrated by cluster analysis. In the conducted studies, no important differences were noted in the accumulation of the studied components between submergent plants (C. demersum) and emergent macrophytes (P. australis).

Impact of Soil Pollution on Melliferous Plants

This study aims at providing bee products and derivatives of medicinal plant consumers with a multifaceted perspective on mineral elements occurring in the soils of two forest zones in the vicinity of North Dobrogea (Romania) by (1) analyzing the pollution levels of the soils at three sites (denoted by DS, PH, and ST) in the study region, using different indicators; (2) providing the results of the transfer of metals from the soil to Sambucus nigra L. (SnL), Hypericum perforatum (Hp), and Tilia tomentosa (Tt). The statistical analysis of the series collected at these locations shows no difference between the elements' concentrations (as a whole). Still, the values of the geo-accumulation index (Igeo) classify the soils as being soils that are moderately to highly contaminated with Cd (and not contaminated with Cu, Mn, or Zn) with respect to the European background values. The cumulative indices-the degree of contamination (DC), the pollution load index (PLI), the Nemerow integrated pollution index (NIPI), and the potential ecological risk index (PERI) indicated the highest contamination in DS (which is a tourist area). To assess the accumulation of different metals in plants, the enrichment factors (EF) were computed. In over 75% of cases, EF was above 1, indicating a high degree of enrichment with different metals. The highest values were those for Cu (41.10 in DS for SnL), and Cd (12.85 in DS for Tt). The results showed that there were different degrees of accumulation between microelements and trace elements in the plants. Tt acted as a bioaccumulator for almost all of the studied elements (K, Mg, Na, Fe, Mn, Cu, Zn, and Cd).

Effects of copper and arsenic on their uptake and distribution in As-hyperaccumulator Pteris vittata

Arsenic (As) and copper (Cu) are common co-contaminates in soils. However, their interactive effects on their accumulation and distribution in As-hyperaccumulator Pteris vittata are poorly understood. A hydroponic experiment was conducted with As being 0, 5, or 50 μM and Cu being 0.32, 3.2, or 32 μM to evaluate their phytotoxicity, accumulation, and distribution in P. vittata. In addition, As and Cu uptake kinetics were examined using the Michaelis-Menten kinetics model. Total As and Cu concentrations in P. vittata were up to 487 and 1355 mg kg^-1. About 39-81% of the As was in the fronds compared to 0.6-18% for Cu. At 50 μM As, increasing Cu concentration from 0.32 to 32 μM increased root As while decreasing frond As concentrations, with the translocation factor (ratio of As in fronds to roots) being reduced from 4.0 to 0.31. In contrast, As did not affect Cu accumulation in P. vittata. Michaelis constant K_m value for As was higher than that of Cu (6.49-24.9 vs. 0.43-3.36), consistent with higher Cu uptake than As. Besides, Cu reduced root K but increased P levels in the roots, whereas As increased the K and P concentrations in the fronds. Our results suggest that P. vittata accumulated more Cu than As in the roots, contributing to its low As translocation. As such, high levels of Cu are likely to reduce As uptake by P. vittata during phytoremediation of As-contaminated sites.

Interaction of hyperaccumulating plants with Zn and Cd nanoparticles

Metal hyperaccumulating plant species are an interesting example of natural selection and environmental adaptation but they may also be useful to developing new technologies of environmental monitoring and remediation. Noccaea caerulescens and Arabidopsis halleri are both Brassicaceae and are known metal hyperaccumulators. This study evaluated tolerance, uptake and translocation of zinc sulfide quantum dots by N. cearulescens and cadmium sulfide quantum dots by A. halleri in direct comparison with the non-hyperaccumulator, genetically similar T. perfoliatum and A. thaliana. Growth media were supplied with two different concentrations of metal in either salt (ZnSO₄ and CdSO₄) or nanoscale form (ZnS QDs and CdS QDs). After 30 days of exposure, the concentration of metals in the soil, roots and leaves was determined. Uptake and localization of the metal in both nanoscale and non-nanoscale form inside plant tissues was investigated by Environmental Scanning Electron Microscopy (ESEM) equipped with an X-ray probe. Specifically, the hyperaccumulators in comparison with the non-hyperaccumulators accumulate ionic and nanoscale Zn and Cd in the aerial parts with a BCF ratio of 45.9 for Zn ion, 49.6 for nanoscale Zn, 2.64 for Cd ion and 2.54 for nanoscale Cd. Results obtained with a differential extraction analytical procedure also showed that a significant fraction of nanoscale metals remained inside the plants in a form compatible with the retention of at least a partial initial structure. The molecular consequences of the hyperaccumulation of nanoscale materials are discussed considering data obtained with hyperaccumulation of ionic metal. This is the first report of conventional hyperaccumulating plants demonstrating an ability to hyperaccumulate also engineered nanomaterials (ENMs) and suggests a potential novel strategy for not only understanding plant-nanomaterial interactions but also for potential biomonitoring in the environment to avoid their entering into the food chains.

Phytoremediation of Cadmium Polluted Soils: Current Status and Approaches for Enhancing

Cadmium (Cd) is a heavy metal present in atmosphere, rocks, sediments, and soils without a known role in plants. It is relatively mobile and can easily enter from soil into groundwater and contaminate the food chain. Its presence in food in excess amounts may cause severe conditions in humans, therefore prevention of cadmium entering the food chain and its removal from contaminated soils are important steps in preserving public health. In the last several years, several approaches for Cd remediation have been proposed, such as the use of soil amendments or biological systems for reduction of Cd contamination. One of the approaches is phytoremediation, which involves the use of plants for soil clean-up. In this review we summarized current data on the use of different plants in phytoremediation of Cd as well as information about different approaches which have been used to enhance phytoremediation. This includes data on the increasing metal bioavailability in the soil, plant biomass, and plant accumulation capacity as well as seed priming as a promising novel approach for phytoremediation enhancing.

Co-remediation of PTEs contaminated soil in mining area by heat modified sawdust and herb

Exploring efficient remediation technologies to remediate potentially toxic element (PTE) in soil around the mining area has become a trendy research topic. This study conducted material composed of sawdust ash (SA) and sawdust biochar (SB) with mass ratio of SA:SB = 1:2 in combination with Medicago sativa L. and Festuca arundinacea to remediate soil contaminated by zinc (Zn), cadmium (Cd), and arsenic (As) in a mining area. The result showed that the removal rates of Zn, Cd, and As were the highest under the treatment of Festuca arundinacea combined with 5% material with values of 22.15%, 22.05%, and 12.47%, respectively. Festuca arundinacea had the most potent ability to absorb and tolerate composite PTEs, and the co-remediation process could remarkably improve soil enzyme environment and microbial community diversity. The distribution of PTEs in plant subcellular showed that the accumulation of Zn, Cd, and As in the cell wall of Festuca arundinacea root was significantly increased by adding 2% materials. The concentrations of Zn, Cd, and As in the cell wall were 4486.25, 33.59, and 124.15 mg/kg, respectively. The combination of 2% material and Festuca arundinacea could effectively remove PTEs in soil and enhance the detoxification ability of the plant, thus effectively improving the soil environment and remediating PTEs pollution. This study provided insights into the remediation of PTE-contaminated soil in mining area by combining materials and plants.

Competition for light induces metal accumulation in a metal hyperaccumulating plant

Plants can respond to competition with a myriad of physiological or morphological changes. Competition has also been shown to affect the foraging decisions of plants belowground. However, a completely unexplored idea is that competition might also affect plants' foraging for specific elements required to inhibit the growth of their competitors. In this study, we examined the effect of simulated competition on root foraging and accumulation of heavy metals in the metal hyperaccumulating perennial plant Arabidopsis halleri, whose metal accumulation has been shown to provide allelopathic ability. A. halleri plants originating from both metalliferous and non-metalliferous soils were grown in a "split-root" setup with one root in a high-metal pot and the other in a low-metal one. The plants were then assigned to either simulated light competition or no-competition (control) treatments, using vertical green or clear plastic filters, respectively. While simulated light competition did not induce greater root allocation into the high-metal pots, it did result in enhanced metal accumulation by A. halleri, particularly in the less metal-tolerant plants, originating from non-metalliferous soils. Interestingly, this accumulation response was particularly enhanced for zinc rather than cadmium. These results provide support to the idea that the accumulation of metals by hyperaccumulating plants can be facultative and change according to their demand following competition.

Regulatory hubs and strategies for improving heavy metal tolerance in plants: Chemical messengers, omics and genetic engineering

Heavy metal (HM) accumulation in the agricultural soil and its toxicity is a major threat for plant growth and development. HMs disrupt functional integrity of the plants, induces altered phenological and physiological responses and slashes down qualitative crop yield. Chemical messengers such as phytohormones, plant growth regulators and gasotransmitters play a crucial role in regulating plant growth and development under metal toxicity in plants. Understanding the intricate network of these chemical messengers as well as interactions of genes/metabolites/proteins associated with HM toxicity in plants is necessary for deciphering insights into the regulatory circuit involved in HM tolerance. The present review describes (a) the role of chemical messengers in HM-induced toxicity mitigation, (b) possible crosstalk between phytohormones and other signaling cascades involved in plants HM tolerance and (c) the recent advancements in biotechnological interventions including genetic engineering, genome editing and omics approaches to provide a step ahead in making of improved plant against HM toxicities.

Kinetin mitigates Cd-induced damagesto growth, photosynthesis and PS II photochemistry of Trigonella seedlings by up-regulating ascorbate-glutathione cycle

Cytokinins (CKs) plays a key role in plant adaptation over a range of different stress conditions. Here, we analyze the effects of a cytokinin (i.e., kinetin, KN) on the growth, photosynthesis (rate of O2 evolution), PS II photochemistry and AsA-GSH cycle in Trigonella seedlings grown under cadmium (Cd) stress. Trigonella seeds were sown in soil amended with 0, 3 and 9 mg Cd kg-1 soil, and after 15 days resultant seedlings were sprayed with three doses of KN, i.e.,10 μM (low, KNL), 50 μM (medium, KNM) and 100 μM (high, KNH); subsequent experiments were performed after 15 days of KN application, i.e., 30 days after sowing. Cadmium toxicity induced oxidative damage as shown by decreased seedling growth and photosynthetic pigment production (Chl a, Chl b and Car), rates of O2-evolution, and photochemistry of PS II of Trigonella seedlings, all accompanied by an increase in H2O2 accumulation. Supplementation with doses of KN at KNL and KNM significantly improved the growth and photosynthetic activity by reducing H2O2 accumulation through the up-regulation AsA-GSH cycle. Notably, KNL and KNM doses stimulated the rate of enzyme activities of APX, GR and DHAR, involved in the AsA-GSH cycle thereby efficiently regulates the level of AsA and GSH in Trigonella grown under Cd stress. The study concludes that KN can mitigate the damaging effects of Cd stress on plant growth by maintaining the redox status (>ratios: AsA/DHA and GSH/GSSG) of cells through the regulation of AsA-GSH cycle at 10 and 50 μM KN under Cd stress conditions. At 100 μM KN, the down-regulation of AsA-GSH cycle did not support the growth and PS II activity of the test seedlings.

Effects of cadmium stress on growth and physiological characteristics of sassafras seedlings

The effects of cadmium stress on the growth and physiological characteristics of Sassafras tzumu Hemsl. were studied in pot experiments. Five Cd levels were tested [CT(Control Treatment) : 0 mg/kg, Cd5: 5 mg/kg, Cd20: 20 mg/kg, Cd50: 50 mg/kg, and Cd100: 100 mg/kg]. The growth and physiological characteristics of the sassafras seedlings in each level were measured. The results showed that soil Cd had negative influences on sassafras growth and reduced the net growth of plant height and the biomass of leaf, branch and root. Significant reductions were recorded in root biomass by 18.18%(Cd5), 27.35%(Cd20), 27.57%(Cd50) and 28.95%(Cd100). The contents of hydrogen peroxide decreased first then increased while malondialdehyde showed the opposite trend with increasing cadmium concentration. Decreases were found in hydrogen peroxide contents by 10.96%(Cd5), 11.82%(Cd20) and 7.02%(Cd50); increases were found in malondialdehyde contents by 15.47%(Cd5), 16.07%(Cd20) and 7.85%(Cd50), indicating that cadmium stress had a certain effect on the peroxidation of the inner cell membranes in the seedlings that resulted in damage to the cell membrane structure. Superoxide dismutase activity decreased among treatments by 17.05%(Cd5), 10,68%(Cd20), 20.85%(Cd50) and 8.91%(Cd100), while peroxidase activity increased steadily with increasing cadmium concentration; these results suggest that peroxidase is likely the main protective enzyme involved in the reactive oxygen removal system in sassafras seedlings. Upward trends were observed in proline content by 90.76%(Cd5), 74.36%(Cd20), 99.73%(Cd50) and 126.01%(Cd100). The increase in proline content with increasing cadmium concentration indicated that cadmium stress induced proline synthesis to resist osmotic stress in the seedlings. Compared to that in CT, the soluble sugar content declined under the different treatments by 32.84%(Cd5), 5.85%(Cd20), 25.55%(Cd50) and 38.69%(Cd100). Increases were observed in the soluble protein content by 2.34%(Cd5), 21.36%(Cd20), 53.15%(Cd50) and 24.22%(Cd100). At different levels of cadmium stress, the chlorophyll content in the seedlings first increased and then decreased, and it was higher in the Cd5 and Cd20 treatments than that in the CT treatment. These results reflected that cadmium had photosynthesis-promoting effects at low concentrations and photosynthesis-suppressing effects at high concentrations. The photosynthetic gas exchange parameters and photosynthetic light-response parameters showed downward trends with increasing cadmium concentration compared with those in CT; these results reflected the negative effects of cadmium stress on photosynthesis in sassafras seedlings.

Abscisic acid-mediated modifications in water transport continuum are involved in cadmium hyperaccumulation in Sedum alfredii

Abscisic acid (ABA) play a crucial role in plant acclimation to heavy-metals stresses. Nevertheless, the effects of ABA on long-distance transport and its consequences for cadmium (Cd) accumulation are insufficiently understood. Here, we investigated the effects of ABA on the development of the whole-plant water transport pathway and implications for Cd uptake and transport to the shoot of Sedum alfredii. Exposure to Cd stimulated the production of endogenous ABA levels in the non-hyperaccumulating ecotype (NHE), but not in the hyperaccumulating ecotype (HE). Increased ABA levels in NHE significantly reduced aquaporin expressions in roots, the number of xylem vessel in stem, dimensions and densities of stomata in leaves, but induced leaf osmotic adjustment. Furthermore, the ABA-driven modifications in NHE plants showed typically higher sensitivity to ABA content in leaves compared to HE, illustrating ecotype-specific responses to ABA level. In NHE, the ABA-mediated modifications primarily affected the xylem transport of Cd ions and, at the cost of considerable water delivery limitations, significantly reduced delivery of Cd ions to shoots. In contrast, maintenance of low ABA levels in HE failed to t limit transpiration rates and maximized Cd accumulation in shoots. Our results demonstrated that ABA regulates Cd hyperaccumulation of S. alfredii through specific modifications in the water transport continuum.

Enriched isotope tracing to reveal the fractionation and lability of legacy and newly introduced cadmium under different amendments

The newly introduced Cd (Cd_N) has different environmental fates than legacy Cd (Cd_L) and how to distinguish them in soil under different amendments is crucial for understanding natural aging and engineered remediation of Cd pollution in soil. In this study, enriched stable isotope tracer (¹¹²Cd) was introduced to distinguish the fate of Cd_N and Cd_L in paddy soil under pH adjustment and quicklime, slaked lime, and biochar amendments. The behaviors of Cd_N and Cd_L were studied during 56 days of flooding incubation through overlying water analysis, sequential extraction fractionation and lability (exchangeable pool probed by ¹¹⁰Cd isotopic spike) assessment. The results showed that soil pH is the main driving factor controlling the partition of both Cd_N and Cd_L in overlying water. During the incubation, Cd_N transformed quickly from soluble fraction to residual fraction under all treatments. In addition, at the end of the incubation, Cd_N concentrations in residual fraction were much higher than that of Cd_L, suggesting a more thorough aging of Cd_N than Cd_L. The labile Cd_N (ECd_N) under pH adjustment and biochar amendment decreased during incubation and ECd_N% was essentially the same with that of ECd_L% after 28 days, indicating the aging equilibrium of exchangeable pool of Cd_N.

Identification and characterization of SaeIF1 from the eukaryotic translation factor SUI1 family in cadmium hyperaccumulator Sedum alfredii

MAIN CONCLUSION

Cadmium-sensitive yeast screening resulted in the isolation of protein translation factor SaeIF1 from the hyperaccumulator Sedum alfredii which has both general and special regulatory roles in controlling cadmium accumulation. The hyperaccumulator of Sedum alfredii has the extraordinary ability to hyperaccumulate cadmium (Cd) in shoots. To investigate its underlying molecular mechanisms of Cd hyperaccumulation, a cDNA library was generated from leaf tissues of S. alfredii. SaeIF1, belonging to the eukaryotic protein translation factor SUI1 family, was identified by screening Cd-sensitive yeast transformants with this library. The full-length cDNA of SaeIF1 has 582 bp and encodes a predicted protein with 120 amino acids. Transient expression assays showed subcellular localization of SaeIF1 in the cytoplasm. SaeIF1 was constitutively and highly expressed in roots and shoots of the hyperaccumulator of S. alfredii, while its transcript levels showed over 100-fold higher expression in the hyperaccumulator of S. alfredii relative to the tissues of a nonhyperaccumulating ecotype of S. alfredii. However, the overexpression of SaeIF1 in yeast cells increased Cd accumulation, but conferred more Cd sensitivity. Transgenic Arabidopsis thaliana expressing SaeIF1 accumulated more Cd in roots and shoots without changes in the ratio of Cd content in shoots and roots, but were more sensitive to Cd stress than wild type. Both special and general roles of SaeIF1 in Cd uptake, transportation, and detoxification are discussed, and might be responsible for the hyperaccumulation characteristics of S. alfredii.

Cadmium accumulation, subcellular distribution and chemical fractionation in hydroponically grown Sesuvium portulacastrum [Aizoaceae]

Sesuvium portulacastrum is a well-known halophyte with considerable Cd accumulation and tolerance under high Cd stress. This species is also considered as a good candidate of Cd phytoremediation in the polluted soils. However, the mechanism of Cd accumulation, distribution and fractionation in different body parts still remain unknown. Seedlings of Sesuvium portulacastrum were studied hydroponically under exposure to a range of Cd concentrations (50 μM or μmol/L to 600 μM or μmol/L) for 28 days to investigate the potential accumulation capability and tolerance mechanisms of this species. Cd accumulation in roots showed that the bio-concentration factor was > 10, suggesting a strong ability to absorb and accumulate Cd. Cd fractionation in the aboveground parts showed the following order of distribution: soluble fraction > cell wall > organelle > cell membrane. In roots, soluble fraction was mostly predominant than other fractions. Cd speciation in leaves and stems was mainly contained of sodium chloride and deionised water extracted forms, suggesting a strong binding ability with pectin and protein as well as with organic acids. In the roots, inorganic form of Cd was dominant than other forms of Cd. It could be suggested that sodium chloride, deionised water and inorganic contained form of Cd are mainly responsible for the adaption of this plant in the Cd stress environment and alleviating Cd toxicity.

Cadmium Isotopic Fractionation in the Soil-Plant System during Repeated Phytoextraction with a Cadmium Hyperaccumulating Plant Species

Analysis of stable metal isotopes can provide important information on biogeochemical processes in the soil-plant system. Here, we conducted a repeated phytoextraction experiment using the cadmium (Cd) hyperaccumulator Sedum plumbizincicola X. H. Guo et S. B. Zhou ex L. H. Wu (Crassulaceae) in four different Cd-contaminated agricultural soils over five consecutive crops. Isotope composition of Cd was determined in the four soils before and after the fifth crop, in the plant shoots harvested in all soils in the first crop, and in the NH₄OAc extracts of two contrasting soils with large differences in soil pH (5.73 and 7.32) and clay content (20.4 and 31.3%) before and after repeated phytoextraction. Before phytoextraction NH₄OAc-extractable Cd showed a slight but significant negative isotope fractionation or no fractionation compared with total Cd (Δ^114/110Cd_extract-soil = -0.15 ± 0.05 (mean ± standard error) and 0.01 ± 0.01‰), and the extent of fractionation varied with soil pH and clay content. S. plumbizincicola preferentially took up heavy Cd from soils (Δ^114/110Cd_shoot-soil = 0.02-0.14‰), and heavy isotopes were significantly depleted in two soils after repeated phytoextraction (Δ^114/110Cd_{soil:P5-soil:P0} = -0.15 ± 0.02 and -0.12 ± 0.01‰). This provides evidence for the existence of specific Cd transporters in S. plumbizincicola, leading to positive isotope fractionation during uptake. After phytoextraction by five sequential crops, the NH₄OAc-extractable Cd pool was significantly enriched in heavy isotopes (Δ^114/110Cd_{extract:P5-extract:P0} = 0.07 ± 0.02 and 0.18 ± 0.05‰) despite the preferential uptake of heavy isotopes, indicating the occurrence of root-induced Cd mobilization in soils, which is supposed to favor heavy Cd in the organo-complexes with root exudates. Our results demonstrate that Cd is taken up by S. plumbizincicola via specific transporters, partly after active mobilization from the more strongly bound soil pool such as iron/manganese (hydr)oxide-bound Cd during repeated phytoextraction. This renders S. plumbizincicola a suitable plant for large-scale field phytoremediation.

A comparative study of root cadmium radial transport in seedlings of two wheat (Triticum aestivum L.) genotypes differing in grain cadmium accumulation

The radial transport of cadmium (Cd) is essential for Cd influx in roots. The role of radial transport pathway on the Cd translocation from root to shoot among wheat genotypes are still poorly understood. This study explored the role of apoplastic and symplastic pathway on root Cd uptake and root-to-shoot translocation in Zhenmai 10 (ZM10, high Cd in grains) and Aikang 58 (AK58, low Cd in grains). Under Cd treatment, the deposition of Casparian strips (CSs) and suberin lamellae (SL) initiated closer to the root apex in ZM10 than that in AK58, which resulted in the lower Cd concentration in apoplastic fluid of ZM10. Simultaneously, Cd-induced expression levels of genes related to Cd uptake in roots were significantly higher in AK58 by contrast with ZM10, contributing to the symplastic Cd accumulation in AK58 root. Moreover, the addition of metabolic inhibitor CCCP noticeably decreased the Cd accumulation in root of both genotypes. Intriguingly, compared to ZM10, greater amounts of Cd were sequestrated in the cell walls and vacuoles in roots of AK58, limiting the translocation of Cd from root to shoot. Furthermore, the elevated TaHMA2 expression in ZM10 indicates that ZM10 had a higher capacity of xylem loading Cd than AK58. All of these results herein suggest that the radial transport is significant for Cd accumulation in roots, but it cannot explain the difference in root-to-shoot translocation of Cd in wheat genotypes with contrast Cd accumulation in grains.

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.

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.

The presence of zinc reduced cadmium uptake and translocation in Cosmos bipinnatus seedlings under cadmium/zinc combined stress

Cadmium (Cd) and zinc (Zn) coexist in the environment but interact differently in plants. Cosmos bipinnatus has been potentially considered as a Cd-accumulator. Thus, this study investigated the detoxification mechanism in C. bipinnatus seedlings under Cd, Zn and Cd + Zn stresses. In the present study, the presence of Zn inhibited Cd uptake and translocation, whereas Cd merely hindered Zn uptake. The concentration of Cd in soluble fraction significantly decreased and Cd was bounded to the cell wall in root under Cd + Zn stress. Meanwhile, Zn and Cd mutually decreased their concentrations in the ethanol extractable form (F_E) and water extractable form (F_W) in roots and shoots. Furthermore, Cd + Zn stress enhanced the activities of superoxide dismutase (SOD, EC 1.15.1.1), peroxidase (POD, EC 1.11.1.7) and catalase (CAT, EC 1.11.1.6) compared to Cd stress alone. These results suggested that Zn effectively decreased Cd uptake and translocation, changed their subcellular distributions, regulated their chemical forms composition and increased antioxidative enzyme activities, thereby enhancing the tolerance to Cd in C. bipinnatus. This study physiologically revealed the interactive effect of Cd and Zn on the detoxification mechanism of Cd in C. bipinnatus and provided new information on phytoremediation of the heavy metal contaminated soils.

Endomembrane Reorganization Induced by Heavy Metals

Plant cells maintain plasmatic concentrations of essential heavy metal ions, such as iron, zinc, and copper, within the optimal functional range. To do so, several molecular mechanisms have to be committed to maintain concentrations of non-essential heavy metals and metalloids, such as cadmium, mercury and arsenic below their toxicity threshold levels. Compartmentalization is central to heavy metals homeostasis and secretory compartments, finely interconnected by traffic mechanisms, are determinant. Endomembrane reorganization can have unexpected effects on heavy metals tolerance altering in a complex way membrane permeability, storage, and detoxification ability beyond gene's expression regulation. The full understanding of endomembrane role is propaedeutic to the comprehension of translocation and hyper-accumulation mechanisms and their applicative employment. It is evident that further studies on dynamic localization of these and many more proteins may significantly contribute to the understanding of heavy metals tolerance mechanisms. The aim of this review is to provide an overview about the endomembrane alterations involved in heavy metals compartmentalization and tolerance in plants.

Exogenous abscisic acid (ABA) promotes cadmium (Cd) accumulation in Sedum alfredii Hance by regulating the expression of Cd stress response genes

Sedum alfredii Hance is a zinc (Zn) and cadmium (Cd) hyperaccumulator plant. However, the regulatory role of plant hormones in the Zn or Cd uptake and accumulation of S. alfredii remains unclear. In this work, the growth, Cd accumulation, abscisic acid (ABA) synthesis and catabolism, malonaldehyde (MDA) content, and transcriptional level of some Cd stress response genes under ABA and Cd co-treatment were investigated to reveal the impact of ABA on Cd resistance and Cd accumulation of S. alfredii. The results show that 0.2 mg/L ABA and 100 μmol/L Cd co-treatment enhanced Cd accumulation and growth in S. alfredii, whereas lower or higher ABA concentrations weaken or even reverse this effect, which was positively correlated with endogenous ABA content. The increase in endogenous ABA content might be the results of the increasing ABA synthetase activities and decreasing ABA lytic enzyme, which was induced by the application of 0.2 mg/L ABA under 100 μmol/L Cd treatment. Principal component analysis (PCA) indicated that ABA impacted the expression pattern of Cd stress response genes, which coincided with the Cd accumulation pattern in the shoots of S. alfredii. Cross-over analysis of partial least squares-discriminant analysis (PLS-DA) and correlation analysis indicated that HsfA4c, HMA4 expression in roots, and HMA2, HMA3, CAD, NAS expression in shoots were correlated with endogenous ABA, which suggests that endogenous ABA improves Cd resistance of seedlings, switches the root-to-shoot transporter from HMA2 to HMA4, and transports more Cd into apoplasts to promote Cd accumulation in the shoots of S. alfredii. Taken together, ABA plays an essential role not only in Cd resistance but also in Cd transport from root to shoot in S. alfredii under Cd stress.

Unique root exudate tartaric acid enhanced cadmium mobilization and uptake in Cd-hyperaccumulator Sedum alfredii

Low molecular weight organic acids (LMWOA) involved in heavy metal tolerance, translocation, and accumulation in plants. However, underlying mechanism of LMWOA secretion in metal mobilization and uptake in hyperaccumulator still need to be identified. In this study, a ¹³C labeling rhizobox was designed to investigate the composition and distribution of LMWOA in the rhizosphere of S. alfredii. The result showed that about 2.30%, 2.25% and 2.35% of the assimilated ¹³C was incorporated into oxalic acid, malic acid, and tartaric acid in rhizosphere of S. alfredii after ¹³CO₂ assimilation, respectively. Oxalic acid, malic acid, and tartaric acid were the predominant LMWOA in rhizosphere soil solution of hyperaccumulating ecotype (HE) S. alfredii, however, almost no tartaric acid was detected for non-hyperaccumulating ecotype (NHE). Tartaric acid was identified as the unique root exudate from HE S. alfredii which was mainly distributed within the range of rhizosphere 0-6 mm. Tartaric acid significantly increased the solubility of four Cd minerals. HE S. alfredii treated with tartrate + CdCO₃ had higher Cd contents and larger biomass than CdCO₃ treatment. Cadmium accumulation in HE S. alfredii was promoted by the exudation of tartaric acid, which was highly efficient in Cd solubilization due to the formation of soluble Cd-tartrate complexes.

Identification of high cadmium-accumulating oilseed sunflower (Helianthus annuus) cultivars for phytoremediation of an Oxisol and an Inceptisol

Forty oilseed sunflower cultivars were screened in two soil types for phytoremediation of Cd coupled with maximum biomass yield and oil production. Several cultivars exhibited a significant difference in biomass and yield with enhanced uptake in shoots and low accumulation in roots from two Cd-contaminated soil types, an Oxisol and an Iceptisol. The Transfer Factor of Cd was >1 in several cultivars in both soil types, where as a significant difference in phytoextraction of Cd was observed in the Oxisol (acidic soil), greater than in the Inceptisol (alkaline soil). The results revealed that of the 40 cultivars, S9178, Huanong 667in the Oxisol and cvs. DW 667, HN 667, Huanong 667 and 668F1 in the Inceptisol showed a high biomass, better yield and enhanced accumulation of Cd in the shoots but a lesser accumulation in oil. The screened cultivar S 9178 produced the greatest amount of oil (55.6%) with 77% oleic acid, which makes it suitable for human consumption. Cultivar Huanong 667 was found to be the highest accumulating cultivar in both soil types. It is therefore suggested that some sunflower cultivars do exhibit phytoremediation potential together with agro-production potential.

Cadmium absorption and translocation of amaranth (Amaranthus mangostanus L.) affected by iron deficiency

Amaranth (Amaranthus mangostanus L.) has superior capability for accumulating cadmium (Cd) and has the potential to be used for phytoremediation of Cd contaminated soils. Iron (Fe) is chemically similar to Cd and may mediate Cd-induced physiological or metabolic impacts in plants. The purpose was to investigate the model of time-dependent and concentration-dependent kinetics of Cd absorption under Fe deficiency, understanding the physiological mechanism of Cd absorption in amaranth roots. The kinetic characteristics of Cd uptake by amaranth grown in Cd enriched nutritional solution with or without Fe addition and with methanol-chloroform, carbonyl cyanide 3-chlorophenylhydrazone (CCCP), and lanthanum chloride (LaCl₃) were compared using ¹⁰⁹Cd²⁺ isotope labeling technique. The results showed that Cd uptake was time-dependent and about 90-93% of uptake occurred during the first 150 min. The kinetics of Cd uptake showed that two stages were involved. The saturation stage fitted the Michaelis-Menten model when concentrations of Cd were lower than 12.71 μmol/L and then the absorption of Cd by roots was increased linearly during the second stage. Only linear absorption was observed with methanol-chloroform treatment while the metabolic inhibitor CCCP inhibited only the saturation absorption process, and the Ca channel inhibitor LaCl₃ partially inhibited the two stages of absorption. These results indicated that the root absorption of ¹⁰⁹Cd²⁺ was enhanced under Fe deficiency which induced more Fe transporters in the root cell membrane, and the Ca channel, apoplastic and symplastic pathways enhanced the Cd absorption in roots.

Evolution of Abscisic Acid Signaling for Stress Responses to Toxic Metals and Metalloids

Toxic heavy metals and metalloids in agricultural ecosystems are crucial factors that limit global crop productivity and food safety. Industrial toxic heavy metals and metalloids such as cadmium, lead, and arsenic have contaminated large areas of arable land in the world and their accumulation in the edible parts of crops is causing serious health risks to humans and animals. Plants have co-evolved with various concentrations of these toxic metals and metalloids in soil and water. Some green plant species have significant innovations in key genes for the adaptation of abiotic stress tolerance pathways that are able to tolerate heavy metals and metalloids. Increasing evidence has demonstrated that phytohormone abscisic acid (ABA) plays a vital role in the alleviation of heavy metal and metalloid stresses in plants. Here, we trace the evolutionary origins of the key gene families connecting ABA signaling with tolerance to heavy metals and metalloids in green plants. We also summarize the molecular and physiological aspects of ABA in the uptake, root-to-shoot translocation, chelation, sequestration, reutilization, and accumulation of key heavy metals and metalloids in plants. The molecular evolution and interaction between the ABA signaling pathway and mechanisms for heavy metal and metalloid tolerance are highlighted in this review. Therefore, we propose that it is promising to manipulate ABA signaling in plant tissues to reduce the uptake and accumulation of toxic heavy metals and metalloids in crops through the application of ABA-producing bacteria or ABA analogues. This may lead to improvements in tolerance of major crops to heavy metals and metalloids.

Visible cellular distribution of cadmium and zinc in the hyperaccumulator Arabidopsis halleri ssp. gemmifera determined by 2-D X-ray fluorescence imaging using high-energy synchrotron radiation

SR-μ-XRF imaging has revealed that the distribution of Zn in leaves was different from that of Cd at a cellular level.

Quantitative proteomic analysis of tomato genotypes with differential cadmium tolerance

This is a report on comprehensive characterization of cadmium (Cd)-exposed root proteomes in tomato using label-free quantitative proteomic approach. Two genotypes differing in Cd tolerance, Pusa Ruby (Cd-tolerant) and Calabash Rouge (Cd-sensitive), were exposed during 4 days to assess the Cd-induced effects on root proteome. The overall changes in both genotypes in terms of differentially accumulated proteins (DAPs) were mainly associated to cell wall, redox, and stress responses. The proteome of the sensitive genotype was more responsive to Cd excess, once it presented higher number of DAPs. Contrasting protein accumulation in cellular component was observed: Cd-sensitive enhanced intracellular components, while the Cd-tolerant increased proteins of extracellular and envelope regions. Protective and regulatory mechanisms were different between genotypes, once the tolerant showed alterations of various protein groups that lead to a more efficient system to cope with Cd challenge. These findings could shed some light on the molecular basis underlying the Cd stress response in tomato, providing fundamental insights for the development of Cd-safe cultivars. Graphical abstract.

New Miscanthus hybrids cultivated at a Polish metal-contaminated site demonstrate high stomatal regulation and reduced shoot Pb and Cd concentrations

The increased bioeconomy targets for the biomass share of renewable energy production across Europe should be met using land unsuitable for food production. Miscanthus breeding programs targeted the production of plants with a diverse range of traits allowing a wider utilization of land resources for biofuel production without competing with arable crops. These traits include increasing tolerances to drought, chilling, and to metal(loid)s excess. Two novel Miscanthus hybrids, GNT41 and GNT34, were compared against Miscanthus x giganteus (Mxg) on metal-contaminated arable land in Poland. This study aimed at evaluating their yield, biomass quality and quantifying seasonal differences in photosynthetic and transpiration parameters. A secondary objective was to identify key physiological mechanisms underlying differences in metal accumulation between the investigated plants. The new hybrids produced a similar yield to Mxg (13-15 t ha^-1 yr^-1), had shorter shoots, higher Leaf Area Index and stem number. Based on gas exchange measurements, GNT34 exhibited isohydric (water-conserving) behavior. The stomatal response to light of the new hybrids was at least twice as fast as that of Mxg, a trait that is often associated with increased seasonal water use efficiency. This contributed to the almost 40% reduction in shoot Pb and Cd concentrations for the new hybrids as compared to Mxg. This suggested that promoting stomatal regulation in conjunction with improved water conservation may be a target for improving plants for wider use on metals contaminated land.

Cellular distribution of cadmium in two amaranth (Amaranthus mangostanus L.) cultivars differing in cadmium accumulation

Differences in cellular cadmium (Cd) distribution between Cd-tolerant and Cd-sensitive lines of amaranth (Amaranthus mangostanus L.) may reveal mechanisms involved in Cd tolerance and hyperaccumulation. We compared the cellular distribution and accumulation of Cd in roots, stems, and leaves between a low-Cd accumulating cultivar (Zibeixian, L-Cd) and a high-Cd accumulating cultivar (Tianxingmi, H-Cd) in a hydroponic experimental system. In all treatments, H-Cd grew better than L-Cd and accumulated more Cd. As the Cd concentration increased, the H-Cd plants grew normally and their biomass increased, except in the 60 μM Cd treatment. The biomass of L-Cd decreased with increasing Cd concentrations. The highest Cd concentration in the roots, stems, and leaves of H-Cd was 950 mg/kg, 305 mg/kg, and 205 mg/kg, respectively, compared with 269 mg/kg, 62.9 mg/kg, and 74.8 mg/kg, respectively, in L-Cd. The Cd distribution differed between the two cultivars. Scanning and transmission electron microscopy and energy-dispersive spectrometry analyses showed that Cd was distributed across the entire cross section of H-Cd roots but largely restricted to the epidermal cells and the exodermis of L-Cd roots. The main Cd storage sites were the root apoplast, cell walls, and intercellular spaces in H-Cd and the root epidermal cells and the exodermis in L-Cd. In H-Cd leaves, Cd accumulated mainly in vacuoles of epidermal cells and, at high external Cd concentrations, in the vacuoles of mesophyll cells.

Cadmium and/or copper excess induce interdependent metal accumulation, DNA methylation, induction of metal chelators and antioxidant defences in the seagrass Zostera marina

In this investigation, we assessed the effects of Cu and/or Cd excess on physiological and metabolic processes of the widespread seagrass Zostera marina. Adult were exposed to low Cd and Cu (0.89 and 0.8 μM, respectively) and high Cd and Cu (8.9 and 2.4 μM, respectively) for 6 d at: Control conditions; low Cu; high Cu; low Cd; high Cd; low Cd and low Cu; and high Cd and high Cu. Photosynthetic performance decreased under single and combined treatments, although effects were more negative under Cu than Cd. Total Cu accumulation was higher than Cd, under single and combined treatments; however, their accumulation was generally lower when applied together, suggesting competition among them. Levels of glutathione (GSH) and phytochelatins (PCs) followed patterns similar to metal accumulation, with up to PC5, displaying adaptations in tolerance. A metallothionein (MET) gene showed upregulation only at high Cd, low Cu, and high Cu. The expression of the enzymes glutathione reductase (GR), ascorbate peroxidase (APX), and catalase (CAT) was greatest at high Cu, and at high Cd and Cu together; the highest expression was under Cu, alone and combined. Both metals induced upregulation of the DNA methyltransferases CMT3 and DRM2, with the highest expression at single Cu. The DNA demethylation ROS1 was overexpressed in treatments containing high Cu, suggesting epigenetic modifications. The results show that under copper and/or cadmium, Z. marina was still biologically viable; certainly based, at least in part, on the induction of metal chelators, antioxidant defences and methylation/demethylation pathways of gene regulation.

Abscisic acid-mediated modifications of radial apoplastic transport pathway play a key role in cadmium uptake in hyperaccumulator Sedum alfredii

Abscisic acid (ABA) is a key phytohormone underlying plant resistance to toxic metals. However, regulatory effects of ABA on apoplastic transport in roots and consequences for uptake of metal ions are poorly understood. Here, we demonstrate how ABA regulates development of apoplastic barriers in roots of two ecotypes of Sedum alfredii and assess effects on cadmium (Cd) uptake. Under Cd treatment, increased endogenous ABA level was detected in roots of nonhyperaccumulating ecotype (NHE) due to up-regulated expressions of ABA biosynthesis genes (SaABA2, SaNCED), but no change was observed in hyperaccumulating ecotype (HE). Simultaneously, endodermal Casparian strips (CSs) and suberin lamellae (SL) were deposited closer to root tips of NHE compared with HE. Interestingly, the vessel-to-CSs overlap was identified as an ABA-driven anatomical trait. Results of correlation analyses and exogenous applications of ABA/Abamine indicate that ABA regulates development of both types of apoplastic barriers through promoting activities of phenylalanine ammonialyase, peroxidase, and expressions of suberin-related genes (SaCYP86A1, SaGPAT5, and SaKCS20). Using scanning ion-selected electrode technique and PTS tracer confirmed that ABA-promoted deposition of CSs and SL significantly reduced Cd entrance into root stele. Therefore, maintenance of low ABA levels in HE minimized deposition of apoplastic barriers and allowed maximization of Cd uptake via apoplastic pathway.

Sulfur nutrition level modifies the growth, micronutrient status, and cadmium distribution in cadmium-exposed spring wheat

The effect of S nutrition level (standard-2 and intensive-6 or 9 mmol S L-1) on the growth, micronutrient status, and Cd concentration of Cd-exposed (0, 0.0002, 0.02, and 0.04 mmol Cd L-1) Triticum aestivum L. 'Zebra' was examined. The hypothesis that Cd-induced micronutrient imbalance in this species is alleviated by enhanced S-sulfate (S-SO4) nutrition was tested. The intensive S nutrition, especially the dose of 6 mmol L-1, to some extent alleviated Cd-induced stress by improving the adverse changes in micronutrient status and increase of the biomass. The root and shoot Fe, Cu, Mn, and Zn concentrations of Cd-exposed wheat rose at 6 and remained unaltered at 9 mmol S L-1. Particularly noteworthy is the substantial increase of Fe bioconcentration found in Cd-stressed plants at 6 mmol S L-1. The root Cu concentration increased at 6 and decreased at 9 mmol S L-1, but did not change in shoots. Simultaneously, both the high S levels elevated the shoot Cl concentration but had no effect on the root Cl concentration. There were no substantial changes in the Mo concentration. The intensive S nutrition of the Cd-treated wheat did not affect the translocation factor (TF) of Fe and B. In turn, root-to-shoot translocation of Mo and Zn was enhanced at 6 and remained unchanged at 9 mmol S L-1. The changes in TF of Cl, Cu, and Mn varied greatly, depending on the S and Cd concentrations. Intensive S nutrition of Cd-stressed wheat, as a rule, dropped the root and increased the shoot Cd concentration as well as reduced Cd bioconcentration/bioaccumulation factor enhancing root-to-shoot Cd translocation.