Expression of the ZNT1 Zinc Transporter from the Metal Hyperaccumulator Noccaea caerulescens Confers Enhanced Zinc and Cadmium Tolerance and Accumulation to Arabidopsis thaliana

Unveiling the variability in cadmium accumulation and tolerance characteristics: a comparative study of Basma and Yunyan 87 tobacco varieties

Tobacco (Nicotiana tabacum L.) shows promise for remediating Cd-contaminated soil due to its significant Cd accumulation capabilities. Although various tobacco varieties exhibit distinct Cd bioaccumulation capacities, a comprehensive understanding of the underlying mechanisms is lacking. This study, conducted using hydroponics, explores differences in Cd accumulation and tolerance mechanisms between two tobacco varieties, Basma and Yunyan 87. The results showed that Cd stress reduced the dry weight, tolerance index, and root morphology for both varieties. Basma exhibited a relatively smaller decline in these indices compared to Yunyan 87. Moreover, Basma demonstrated a higher Cd bioconcentration factor (BCF), concentration, and accumulated content, signifying its superior tolerance and bioaccumulation capacity to Cd compared to Yunyan 87. The Carbonyl Cyanide3-ChloroPhenylhydrazone (CCCP) addition resulted in reduced Cd accumulation and BCFs in both tobacco species. This effect was more pronounced in Basma, suggesting that Basma relies more on an active transport process than Yunyan 87. This could potentially explain its enhanced bioaccumulation ability. Subcellular Cd distribution analysis revealed Basma's preference for distributing Cd in soluble fractions, while Yunyan 87 favoured the cell wall fractions. Transmission electron microscope showed that Basma's organelles were less damaged than Yunyan 87's under Cd stress, possibly contributing to the superior tolerance of Basma. Therefore, these results provided a theoretical foundation for development of Cd-contaminated soil tobacco remediation technology.

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

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

Proteomic and Genomic Studies of Micronutrient Deficiency and Toxicity in Plants

Micronutrients are essential for plants. Their growth, productivity and reproduction are directly influenced by the supply of micronutrients. Currently, there are eight trace elements considered to be essential for higher plants: Fe, Zn, Mn, Cu, Ni, B, Mo, and Cl. Possibly, other essential elements could be discovered because of recent advances in nutrient solution culture techniques and in the commercial availability of highly sensitive analytical instrumentation for elemental analysis. Much remains to be learned about the physiology of micronutrient absorption, translocation and deposition in plants, and about the functions they perform in plant growth and development. With the recent advancements in the proteomic and molecular biology tools, researchers have attempted to explore and address some of these questions. In this review, we summarize the current knowledge of micronutrients in plants and the proteomic/genomic approaches used to study plant nutrient deficiency and toxicity.

Natural Molecular Mechanisms of Plant Hyperaccumulation and Hypertolerance towards Heavy Metals

The main mechanism of plant tolerance is the avoidance of metal uptake, whereas the main mechanism of hyperaccumulation is the uptake and neutralization of metals through specific plant processes. These include the formation of symbioses with rhizosphere microorganisms, the secretion of substances into the soil and metal immobilization, cell wall modification, changes in the expression of genes encoding heavy metal transporters, heavy metal ion chelation, and sequestration, and regenerative heat-shock protein production. The aim of this work was to review the natural plant mechanisms that contribute towards increased heavy metal accumulation and tolerance, as well as a review of the hyperaccumulator phytoremediation capacity. Phytoremediation is a strategy for purifying heavy-metal-contaminated soils using higher plants species as hyperaccumulators.

Advances in Genes-Encoding Transporters for Cadmium Uptake, Translocation, and Accumulation in Plants

Cadmium (Cd) is a heavy metal that is highly toxic for plants, animals, and human beings. A better understanding of the mechanisms involved in Cd accumulation in plants is beneficial for developing strategies for either the remediation of Cd-polluted soils using hyperaccumulator plants or preventing excess Cd accumulation in the edible parts of crops and vegetables. As a ubiquitous heavy metal, the transport of Cd in plant cells is suggested to be mediated by transporters for essential elements such as Ca, Zn, K, and Mn. Identification of the genes encoding Cd transporters is important for understanding the mechanisms underlying Cd uptake, translocation, and accumulation in either crop or hyperaccumulator plants. Recent studies have shown that the transporters that mediate the uptake, transport, and accumulation of Cd in plants mainly include members of the natural resistance-associated macrophage protein (Nramp), heavy metal-transporting ATPase (HMA), zinc and iron regulated transporter protein (ZIP), ATP-binding cassette (ABC), and yellow stripe-like (YSL) families. Here, we review the latest advances in the research of these Cd transporters and lay the foundation for a systematic understanding underlying the molecular mechanisms of Cd uptake, transport, and accumulation in plants.

The molecular basis of zinc homeostasis in cereals

Plants require zinc (Zn) as an essential cofactor for diverse molecular, cellular and physiological functions. Zn is crucial for crop yield, but is one of the most limiting micronutrients in soils. Grasses like rice, wheat, maize and barley are crucial sources of food and nutrients for humans. Zn deficiency in these species therefore not only reduces annual yield but also directly results in Zn malnutrition of more than two billion people in the world. There has been good progress in understanding Zn homeostasis and Zn deficiency mechanisms in plants. However, our current knowledge of monocots, including grasses, remains insufficient. In this review, we provide a summary of our knowledge of molecular Zn homeostasis mechanisms in monocots, with a focus on important cereal crops. We additionally highlight divergences in Zn homeostasis of monocots and the dicot model Arabidopsis thaliana, as well as important gaps in our knowledge that need to be addressed in future research on Zn homeostasis in cereal monocots.

Nutritional and genetic variation in a core set of Ethiopian Tef (Eragrostis tef) varieties

BACKGROUND

Tef (Eragrostis tef) is a tropical cereal domesticated and grown in the Ethiopian highlands, where it has been a staple food of Ethiopians for many centuries. Food insecurity and nutrient deficiencies are major problems in the country, so breeding for enhanced nutritional traits, such as Zn content, could help to alleviate problems with malnutrition.

RESULTS

To understand the breeding potential of nutritional traits in tef a core set of 24 varieties were sequenced and their mineral content, levels of phytate and protein, as well as a number of nutritionally valuable phenolic compounds measured in grain. Significant variation in all these traits was found between varieties. Genome wide sequencing of the 24 tef varieties revealed 3,193,582 unique SNPs and 897,272 unique INDELs relative to the tef reference var. Dabbi. Sequence analysis of two key transporter families involved in the uptake and transport of Zn by the plant led to the identification of 32 Zinc Iron Permease (ZIP) transporters and 14 Heavy Metal Associated (HMA) transporters in tef. Further analysis identified numerous variants, of which 14.6% of EtZIP and 12.4% of EtHMA variants were non-synonymous changes. Analysis of a key enzyme in flavanol synthesis, flavonoid 3'-hydroxylase (F3'H), identified a T-G variant in the tef homologue Et_s3159-0.29-1.mrna1 that was associated with the differences observed in kaempferol glycoside and quercetin glycoside levels.

CONCLUSION

Wide genetic and phenotypic variation was found in 24 Ethiopian tef varieties which would allow for breeding gains in many nutritional traits of importance to human health.

OUP accepted manuscript

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

Trophic Transfer without Biomagnification of Cadmium in a Soybean-Dodder Parasitic System

Cadmium (Cd) is among the most available and most toxic heavy metals taken up by plants from soil. Compared to the classic plant-animal food chains, the host-parasitic plant food chains have, thus far, been largely overlooked in the studies of Cd trophic transfer. To investigate the pattern of Cd transfer during the infection of parasitic plants on Cd-contaminated hosts, we conducted a controlled experiment that grew soybeans parasitized by Chinese dodders (Cuscuta chinensis) in soil with different levels of Cd treatment, and examined the concentration, accumulation, allocation and transfer coefficients of Cd within this parasitic system. Results showed that among all components, dodders accounted for more than 40% biomass of the whole system but had the lowest Cd concentration and accumulated the least amount of Cd. The transfer coefficient of Cd between soybean stems and dodders was much lower than 1, and was also significantly lower than that between soybean stems and soybean leaves. All these features were continuously strengthened with the increase of Cd treatment levels. The results suggested no evidence of Cd biomagnification in dodders parasitizing Cd-contaminated hosts, and implied that the Cd transfer from hosts to dodders may be a selective process.

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

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

Redundant roles of four ZIP family members in zinc homeostasis and seed development in Arabidopsis thaliana

Zinc (Zn) is essential for normal plant growth and development. The Zn-regulated transporter, iron-regulated transporter (IRT)-like protein (ZIP) family members are involved in Zn transport and cellular Zn homeostasis throughout the domains of life. In this study, we have characterized four ZIP transporters from Arabidopsis thaliana (IRT3, ZIP4, ZIP6, and ZIP9) to better understand their functional roles. The four ZIP proteins can restore the growth defect of a yeast Zn uptake mutant and are upregulated under Zn deficiency. Single and double mutants show no phenotypes under Zn-sufficient or Zn-limited growth conditions. In contrast, triple and quadruple mutants show impaired growth irrespective of external Zn supply due to reduced Zn translocation from root to shoot. All four ZIP genes are highly expressed during seed development, and siliques from all single and higher-order mutants exhibited an increased number of abnormal seeds and decreased Zn levels in mature seeds relative to wild type. The seed phenotypes could be reversed by supplementing the soil with Zn. Our data demonstrate that IRT3, ZIP4, ZIP6, and ZIP9 function redundantly in maintaining Zn homeostasis and seed development in A. thaliana.

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.

The Different Faces of Arabidopsis arenosa-A Plant Species for a Special Purpose

The following review article collects information on the plant species Arabidopsis arenosa. Thus far, A. arenosa has been known as a model species for autotetraploidy studies because, apart from diploid individuals, there are also tetraploid populations, which is a unique feature of this Arabidopsis species. In addition, A arenosa has often been reported in heavy metal-contaminated sites, where it occurs together with a closely related species A. halleri, a model plant hyperaccumulator of Cd and Zn. Recent studies have shown that several populations of A. arenosa also exhibit Cd and Zn hyperaccumulation. However, it is assumed that the mechanism of hyperaccumulation differs between these two Arabidopsis species. Nevertheless, this phenomenon is still not fully understood, and thorough research is needed. In this paper, we summarize the current state of knowledge regarding research on A. arenosa.

Zinc toxicity in plants: a review

This review highlights the most recent updated information available about Zn phytotoxicity at physiological, biochemical and molecular levels, uptake mechanisms as well as excess Zn homeostasis in plants. Zinc (Zn) is a natural component of soil in terrestrial environments and is a vital element for plant growth, as it performs imperative functions in numerous metabolic pathways. However, potentially noxious levels of Zn in soils can result in various alterations in plants like reduced growth, photosynthetic and respiratory rate, imbalanced mineral nutrition and enhanced generation of reactive oxygen species. Zn enters into soils through various sources, such as weathering of rocks, forest fires, volcanoes, mining and smelting activities, manure, sewage sludge and phosphatic fertilizers. The rising alarm in environmental facet, as well as, the narrow gap between Zn essentiality and toxicity in plants has drawn the attention of the scientific community to its effects on plants and crucial role in agricultural sustainability. Hence, this review focuses on the most recent updates about various physiological and biochemical functions perturbed by high levels of Zn, its mechanisms of uptake and transport as well as molecular aspects of surplus Zn homeostasis in plants. Moreover, this review attempts to understand the mechanisms of Zn toxicity in plants and to present novel perspectives intended to drive future investigations on the topic. The findings will further throw light on various mechanisms adopted by plants to cope with Zn stress which will be of great significance to breeders for enhancing tolerance to Zn contamination.

Current understanding of plant zinc homeostasis regulation mechanisms

The essential nature of Zn and widespread Zn deficiency in plants under field conditions underlie the great interest of researchers in the regulation of plant Zn homeostasis. Here, the current knowledge of plant Zn homeostasis regulation, mainly in A. thaliana, is reviewed. The plant Zn homeostasis machinery is regulated largely at the transcriptional level. Local regulation in response to changes in cellular Zn status is based on the transcription factors bZIP19 and bZIP23, which sense changes in free Zn²⁺ concentrations in the cell. However, there are likely other unidentified ways to sense cellular free Zn²⁺ concentrations in addition to the well-known bZIP19 and bZIP23 factors. In recent years, the existence of a shoot-derived systemic Zn deficiency signal, which is involved in the upregulation of Zn transport from roots to shoots, was demonstrated. Additionally, rates of mRNA degradation of Zn homeostasis genes are likely regulated by changes in cellular Zn status. In addition to the regulation of Zn transport, other mechanisms for the regulation of plant Zn homeostasis exist. "Zn sparing" mechanisms could be involved in the decrease in plant Zn requirements under Zn deficiency. Additionally, autophagy is probably regulated by local Zn status and involved in Zn reutilization at the cellular level. Current issues related to studying Zn homeostasis regulation are discussed.

In situ phytoremediation of heavy metal-contaminated soil and groundwater: a green inventive approach

The heavy metal contamination of soil and groundwater is a serious threat to environment worldwide. The survival of human being primarily relies upon soil and groundwater sources. Therefore, the remediation of heavy metal-contaminated soil and groundwater is a matter of utmost concern. Heavy metals are non-degradable and persist in the environment and subsequently contaminate the food chain. Heavy metal pollution puts a serious impact on human health and it adversely affects our physical body. Although, numerous in situ conventional technologies have been utilized for the treatment purpose, but most of the techniques have some limitations such as high cost, deterioration of soil properties, disturbances to soil native flora and fauna and intensive labour. Despite that, in situ phytoremediation is a cost-effective, eco-friendly, solar-driven and novel approach with significant public acceptance. The past research reflects rare discussion addressing both (heavy metal in situ phytoremediation of soil and groundwater) in one platform. The present review article covers both the concepts of in situ phytoremediation of soil and groundwater with major emphasis on health risks of heavy metals, enhanced integrated approaches of in situ phytoremediation, mechanisms of in situ phytoremediation along with effective hyperaccumulator plants for heavy metals remediation, challenges and future prospects.

Wide cross-species RNA-Seq comparison reveals convergent molecular mechanisms involved in nickel hyperaccumulation across dicotyledons

The Anthropocene epoch is associated with the spreading of metals in the environment increasing oxidative and genotoxic stress on organisms. Interestingly, c. 520 plant species growing on metalliferous soils acquired the capacity to accumulate and tolerate a tremendous amount of nickel in their shoots. The wide phylogenetic distribution of these species suggests that nickel hyperaccumulation evolved multiple times independently. However, the exact nature of these mechanisms and whether they have been recruited convergently in distant species is not known. To address these questions, we have developed a cross-species RNA-Seq approach combining differential gene expression analysis and cluster of orthologous group annotation to identify genes linked to nickel hyperaccumulation in distant plant families. Our analysis reveals candidate orthologous genes encoding convergent function involved in nickel hyperaccumulation, including the biosynthesis of specialized metabolites and cell wall organization. Our data also point out that the high expression of IREG/Ferroportin transporters recurrently emerged as a mechanism involved in nickel hyperaccumulation in plants. We further provide genetic evidence in the hyperaccumulator Noccaea caerulescens for the role of the NcIREG2 transporter in nickel sequestration in vacuoles. Our results provide molecular tools to better understand the mechanisms of nickel hyperaccumulation and study their evolution in plants.

Hyperaccumulation of cadmium by scallop Chlamys farreri revealed by comparative transcriptome analysis

Cadmium (Cd) is a hazardous environmental contaminant, which has a serious effect on the ecosystem, food safety and human health. Scallop could accumulate high concentration of Cd from the environment and has been regarded as a Cd hyper-accumulator. In this work, we investigated the antioxidative defense, detoxification and transport of Cd in the kidneys of scallops by transcriptome analysis. A total of 598 differentially expressed genes including 387 up-regulated and 211 down-regulated ones were obtained during Cd exposure, and 46 up-regulated and 260 down-regulated ones were obtained during depuration. Cadmium exposure could cause oxidative stress in the kidneys, which was particularly shown in the pathways involved in proteasome and oxidative phosphorylation. The mRNA expression of 5 metallothionein (MT) genes were overexpressed under Cd exposure and significantly decreased during Cd depuration, which played a vital role in Cd chelation and detoxification. The expression of divalent metal transporter (DMT) genes were down-regulated insignificantly during accumulation and depuration of Cd, which suggested that the DMT played little roles in Cd transport in scallops. A positive relationship in the expression of the zinc transporter (ZIP6 and ZIP1) genes with Cd exposure and depuration was observed, which confirmed its important role for Cd uptake in the kidneys of scallops. 26S proteasome activities and MT expression were Cd-dependent. This study supplied the important reference on the hyperaccumulation of Cd by scallops and identified some effective bioindicators for the environmental risk assessment.

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.

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.

Physiological and molecular mechanisms of heavy metal accumulation in nonmycorrhizal versus mycorrhizal plants

Uptake, translocation, detoxification, and sequestration of heavy metals (HMs) are key processes in plants to deal with excess amounts of HM. Under natural conditions, plant roots often establish ecto- and/or arbuscular-mycorrhizae with their fungal partners, thereby altering HM accumulation in host plants. This review considers the progress in understanding the physiological and molecular mechanisms involved in HM accumulation in nonmycorrhizal versus mycorrhizal plants. In nonmycorrhizal plants, HM ions in the cells can be detoxified with the aid of several chelators. Furthermore, HMs can be sequestered in cell walls, vacuoles, and the Golgi apparatus of plants. The uptake and translocation of HMs are mediated by members of ZIPs, NRAMPs, and HMAs, and HM detoxification and sequestration are mainly modulated by members of ABCs and MTPs in nonmycorrhizal plants. Mycorrhizal-induced changes in HM accumulation in plants are mainly due to HM sequestration by fungal partners and improvements in the nutritional and antioxidative status of host plants. Furthermore, mycorrhizal fungi can trigger the differential expression of genes involved in HM accumulation in both partners. Understanding the molecular mechanisms that underlie HM accumulation in mycorrhizal plants is crucial for the utilization of fungi and their host plants to remediate HM-contaminated soils.

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.

Comparative transcriptome analysis revealed key factors for differential cadmium transport and retention in roots of two contrasting peanut cultivars

BACKGROUND

Peanut is the world's fourth largest oilseed crop that exhibits wide cultivar variations in cadmium (Cd) accumulation. To establish the mechanisms of Cd distribution and accumulation in peanut plants, eight cDNA libraries from the roots of two contrasting cultivars, Fenghua 1 (low-Cd cultivar) and Silihong (high-Cd cultivar), were constructed and sequenced by RNA-sequencing. The expression patterns of 16 candidate DEGs were validated by RT-qPCR analysis.

RESULTS

A total of 75,634 genes including 71,349 known genes and 4484 novel genes were identified in eight cDNA libraries, among which 6798 genes were found to be Cd-responsive DEGs and/or DEGs between these two cultivars. Interestingly, 183 DEGs encoding ion transport related proteins and 260 DEGs encoding cell wall related proteins were identified. Among these DEGs, nine metal transporter genes (PDR1, ABCC4 and ABCC15, IRT1, ZIP1, ZIP11, YSL7, DTX43 and MTP4) and nine cell wall related genes (PEs, PGIPs, GTs, XYT12 CYP450s, LACs, 4CL2, C4H and CASP5) showed higher expression in Fenghua 1 than in Silihong.

CONCLUSIONS

Both the metal transporters and cell wall modification might be responsible for the difference in Cd accumulation and translocation between Fenghua 1 and Silihong. These findings would be useful for further functional analysis, and reveal the molecular mechanism responsible for genotype difference in Cd accumulation.

LPCAT1 controls phosphate homeostasis in a zinc-dependent manner

All living organisms require a variety of essential elements for their basic biological functions. While the homeostasis of nutrients is highly intertwined, the molecular and genetic mechanisms of these dependencies remain poorly understood. Here, we report a discovery of a molecular pathway that controls phosphate (Pi) accumulation in plants under Zn deficiency. Using genome-wide association studies, we first identified allelic variation of the Lyso-PhosphatidylCholine (PC) AcylTransferase 1 (LPCAT1) gene as the key determinant of shoot Pi accumulation under Zn deficiency. We then show that regulatory variation at the LPCAT1 locus contributes significantly to this natural variation and we further demonstrate that the regulation of LPCAT1 expression involves bZIP23 TF, for which we identified a new binding site sequence. Finally, we show that in Zn deficient conditions loss of function of LPCAT1 increases the phospholipid Lyso-PhosphatidylCholine/PhosphatidylCholine ratio, the expression of the Pi transporter PHT1;1, and that this leads to shoot Pi accumulation.

LPCAT1 controls phosphate homeostasis in a zinc-dependent manner

All living organisms require a variety of essential elements for their basic biological functions. While the homeostasis of nutrients is highly intertwined, the molecular and genetic mechanisms of these dependencies remain poorly understood. Here, we report a discovery of a molecular pathway that controls phosphate (Pi) accumulation in plants under Zn deficiency. Using genome-wide association studies, we first identified allelic variation of the Lyso-PhosphatidylCholine (PC) AcylTransferase 1 (LPCAT1) gene as the key determinant of shoot Pi accumulation under Zn deficiency. We then show that regulatory variation at the LPCAT1 locus contributes significantly to this natural variation and we further demonstrate that the regulation of LPCAT1 expression involves bZIP23 TF, for which we identified a new binding site sequence. Finally, we show that in Zn deficient conditions loss of function of LPCAT1 increases the phospholipid Lyso-PhosphatidylCholine/PhosphatidylCholine ratio, the expression of the Pi transporter PHT1;1, and that this leads to shoot Pi accumulation.

The Arabidopsis bZIP19 and bZIP23 Activity Requires Zinc Deficiency - Insight on Regulation From Complementation Lines

All living organisms require zinc as an essential micronutrient. Maintaining appropriate intracellular zinc supply, and avoiding deficiency or toxic excess, requires a tight regulation of zinc homeostasis. In Arabidopsis, bZIP19 and bZIP23 (basic-leucine zipper) transcription factors are the central regulators of the zinc deficiency response. Their targets include members of the ZIP (Zrt/Irt-like Protein) transporter family, involved in cellular zinc uptake, which are up-regulated at zinc deficiency. However, the mechanisms by which these transcription factors are regulated by cellular zinc status are not yet known. Here, to further our insight, we took advantage of the zinc deficiency hypersensitive phenotype of the bzip19 bzip23 double mutant, and used it as background to produce complementation lines of each Arabidopsis F-bZIP transcription factor, including bZIP24. On these lines, we performed complementation and localization studies, analyzed the transcript level of a subset of putative target genes, and performed elemental tissue profiling. We find evidence supporting that the zinc-dependent activity of bZIP19 and bZIP23 is modulated by zinc at protein level, in the nucleus, where cellular zinc sufficiency represses their activity and zinc deficiency is required. In addition, we show that these two transcription factors are functionally redundant to a large extent, and that differential tissue-specific expression patterns might, at least partly, explain distinct regulatory activities. Finally, we show that bZIP24 does not play a central role in the Zn deficiency response. Overall, we provide novel information that advances our understanding of the regulatory activity of bZIP19 and bZIP23.

Recent strategies of increasing metal tolerance and phytoremediation potential using genetic transformation of plants

Avoidance and reduction of soil contamination with heavy metals is one of the most serious global challenges. Nowadays, science offers us new opportunities of utilizing plants to extract toxic elements from the soil by means of phytoremediation. Plant abilities to uptake, translocate, and transform heavy metals, as well as to limit their toxicity, may be significantly enhanced via genetic engineering. This paper provides a comprehensive review of recent strategies aimed at the improvement of plant phytoremediation potential using plant transformation and employing current achievements in nuclear and cytoplasmic genome transformation. Strategies for obtaining plants suitable for effective soil clean-up and tolerant to excessive concentrations of heavy metals are critically assessed. Promising directions in genetic manipulations, such as gene silencing and cis- and intragenesis, are also discussed. Moreover, the ways of overcoming disadvantages of phytoremediation using genetic transformation approachare proposed. The knowledge gathered here could be useful for designing new research aimed at biotechnological improvement of phytoremediation efficiency.

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.

Divergent biology of facultative heavy metal plants

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

F-group bZIPs in barley-a role in Zn deficiency

Zinc (Zn) deficiency negatively impacts the development and health of plants and affects crop yield. When experiencing low Zn, plants undergo an adaptive response to maintain Zn homeostasis. We provide further evidence for the role of F-group transcription factors, AtbZIP19 and AtbZIP23, in responding to Zn deficiency in Arabidopsis and demonstrate the sensitivity and specificity of this response. Despite their economic importance, the role of F-group bZIPs in cereal crops is largely unknown. Here, we provide new insights by functionally characterizing these in barley (Hordeum vulgare), demonstrating an expanded number of F-group bZIPs (seven) compared to Arabidopsis. The F-group barley bZIPs, HvbZIP56 and HvbZIP62, partially rescue the Zn-dependent growth phenotype and ZIP-transporter gene regulation of an Arabidopsis bzip19-4 bzip23-2 mutant. This supports a conserved mechanism of action in adapting to Zn deficiency. HvbZIP56 localizes to the cytoplasm and nucleus when expressed in Arabidopsis and tobacco. Promoter analysis demonstrates that the barley ZIP transporters that are upregulated under Zn deficiency contain cis Zn-deficiency response elements (ZDREs). ZDREs are also found in particular barley bZIP promoters. This study represents a significant step forward in understanding the mechanisms controlling Zn responses in cereal crops, and will aid in developing strategies for crop improvement.

Cadmium effects on DNA and protein metabolism in oyster (Crassostrea gigas) revealed by proteomic analyses

Marine molluscs, including oysters, can concentrate high levels of cadmium (Cd) in their soft tissues, but the molecular mechanisms of Cd toxicity remain speculative. In this study, Pacific oysters (Crassostrea gigas) were exposed to Cd for 9 days and their gills were subjected to proteomic analysis, which were further confirmed with transcriptomic analysis. A total of 4,964 proteins was quantified and 515 differentially expressed proteins were identified in response to Cd exposure. Gene Ontology enrichment analysis revealed that excess Cd affected the DNA and protein metabolism. Specifically, Cd toxicity resulted in the inhibition of DNA glycosylase and gap-filling and ligation enzymes expressions in base excision repair pathway, which may have decreased DNA repair capacity. At the protein level, Cd induced the heat shock protein response, initiation of protein refolding as well as degradation by ubiquitin proteasome pathway, among other effects. Excess Cd also induced antioxidant responses, particularly glutathione metabolism, which play important roles in Cd chelation and anti-oxidation. This study provided the first molecular mechanisms of Cd toxicity on DNA and protein metabolism at protein levels, and identified molecular biomarkers for Cd toxicity in oysters.

Phylogenetic analysis of F-bZIP transcription factors indicates conservation of the zinc deficiency response across land plants

Basic leucine zipper (bZIP) transcription factors control important developmental and physiological processes in plants. In Arabidopsis thaliana, the three gene F-bZIP subfamily has been associated with zinc deficiency and salt stress response. Benefiting from the present abundance of plant genomic data, we performed an evolutionary and structural characterization of plant F-bZIPs. We observed divergence during seed plant evolution, into two groups and inferred different selective pressures for each. Group 1 contains AtbZIP19 and AtbZIP23 and appears more conserved, whereas Group 2, containing AtbZIP24, is more prone to gene loss and expansion events. Transcriptomic and experimental data reinforced AtbZIP19/23 as pivotal regulators of the zinc deficiency response, mostly via the activation of genes from the ZIP metal transporter family, and revealed that they are the main regulatory switch of AtZIP4. A survey of AtZIP4 orthologs promoters across different plant taxa revealed an enrichment of the Zinc Deficiency Response Element (ZDRE) to which both AtbZIP19/23 bind. Overall, our results indicate that while the AtbZIP24 function in the regulation of the salt stress response may be the result of neo-functionalization, the AtbZIP19/23 function in the regulation of the zinc deficiency response may be conserved in land plants (Embryophytes).