Phylogenetic and functional analysis of the Cation Diffusion Facilitator (CDF) family: improved signature and prediction of substrate specificity

Localization of the MTP4 transporter to trans-Golgi network in pollen tubes of Arabidopsis thaliana

Zinc (Zn) is an essential element for plants. Numerous proteins in different cellular compartments require Zn for their structure and function. Zn can be toxic when it accumulates in high levels in the cytoplasm. Therefore, Zn homeostasis at tissue, cell, and organelle levels is vital for plant growth. A part of the metal tolerance protein (MTP) / Cation Diffusion Facilitator (CDF) transporters functions as Zn transporters, exporting Zn from the cytosol to various membrane compartments. In Arabidopsis thaliana, MTP1, MTP2, MTP3, MTP4, MTP5, and MTP12 are classified as Zn transporters (Zn-CDF). In this study, we systematically analyzed the localization of GFP-fused Zn-CDFs in the leaf epidermal cells of Nicotiana benthamiana. As previously reported, MTP1 and MTP3 were localized to tonoplast, MTP2 to endoplasmic reticulum, and MTP5 to Golgi. In addition, we identified the localization of MTP4 to trans-Golgi Network (TGN). Since MTP4 is specifically expressed in pollen, we analyzed the localization of MTP4-GFP in the Arabidopsis pollen tubes and confirmed that it is in the TGN. We also showed the Zn transport capability of MTP4 in yeast cells. We then analyzed the phenotype of an mtp4 T-DNA insertion mutant under both limited and excess Zn conditions. We found that their growth and fertility were not largely different from the wild-type. Our study has paved the way for investigating the possible roles of MTP4 in metallating proteins in the secretory pathway or in exporting excess Zn through exocytosis. In addition, our system of GFP-fused MTPs will help study the mechanisms for targeting transporters to specific membrane compartments.

HcZnT2 is a highly mycorrhiza-induced zinc transporter from Hebeloma cylindrosporum in association with pine

Zinc (Zn) shortage is a common micronutrient deficiency affecting plants worldwide, while Zn toxicity may occur when this metal is in excess. Ectomycorrhizal (ECM) fungi are known to be able to modulate the transfer of macro- and microelements, among them Zn, to the plant. However, the underlying mechanisms are not well understood. We identified the HcZnT2 gene from the ECM fungus Hebeloma cylindrosporum, encoding a member of the Cation Diffusion Facilitator (CDF) family including Zn transporters, and analyzed its transcriptional regulation, the transport function by yeast complementation experiments, and its subcellular localization using a GFP fusion protein in yeast. HcZnT2 is highly induced during mycorrhization of Pinus pinaster, and upregulated in presence of the host plant root even without any direct contact. However, HcZnT2 is repressed by Zn excess conditions. By functional expression in yeast, our results strongly support the ability of HcZnT2 to transport Zn and, to a lesser extent, manganese. HcZnT2 localization was associated with the endoplasmic reticulum of yeast. Mycorrhizal gene activation at low external Zn suggests that the Zn transporter HcZnT2 might be important for the early establishment of the ECM symbiosis during Zn deficiency, rather than under Zn excess. HcZnT2 arises as an extremely remarkable candidate playing a key role in Zn homeostasis and regulation in ectomycorrhiza.

Biotechnological approaches for enhancement of heavy metal phytoremediation capacity of plants

Heavy metals are extremely hazardous for human health due to their toxic effects. They are non-biodegradable in nature, thus remain in the environment and enter and accumulate in the human body through biomagnification; hence, there is a serious need of their remediation. Phytoremediation has emerged as a green, sustainable, and effective solution for heavy metal removal and many plant species could be employed for this purpose. Plants are able to sequester substantial quantity of heavy metals, in some cases thousands of ppm, due to their robust physiology enabling high metal tolerance and anatomy supporting metal ion accumulation. Identification and modification of potential target genes involved in heavy metal accumulation have led to improved phytoremediation capacity of plants at the molecular level. The introduction of foreign genes through genetic engineering approaches has further enhanced phytoremediation capacity manifolds. This review gives an insight towards improving the phytoremediation efficiency through a better understanding of molecular mechanisms involved, expression of different proteins, genetic engineering approaches for transgenic production, and genetic modifications. It also comprehends novel omics tools such as genomics, metabolomics, proteomics, transcriptomics, and genome editing technologies for improvement of phytoremediation ability of plants.

Molecular evolution and functional diversification of metal tolerance protein families in cereals plants and function of maize MTP protein

Plants employ metal tolerance proteins (MTPs) to confer tolerance by sequestering excess ions into vacuoles. MTPs belong to the cation diffusion facilitator (CDF) family, which facilitates the transport of divalent transition metal cations. In this study, we conducted a comprehensive analysis of the MTP gene families across 21 plant species, including maize (Zea mays). A total of 247 MTP genes were identified within these plant genomes and categorized into distinct subgroups, namely Zn-CDF, Mn-CDF, and Fe/Zn-CDF, based on phylogenetic analyses. This investigation encompassed the characterization of genomic distribution, gene structures, cis-regulatory elements, collinearity relationships, and gene ontology functions associated with MTPs. Transcriptomic analyses unveiled stress-specific expression patterns of MTP genes under various abiotic stresses. Moreover, quantitative RT-PCR assays were employed to assess maize MTP gene responses to diverse heavy metal stress conditions. Functional validation of metal tolerance roles was achieved through heterologous expression in yeast. This integrated evolutionary scrutiny of MTP families in cereals furnishes a valuable framework for the elucidation of MTP functions in subsequent studies. Notably, the prioritized MTP gene ZmMTP6 emerged as a positive regulator of plant Cd tolerance, thereby offering a pivotal genetic asset for the development of Cd-tolerant crops, particularly maize cultivars.

Molecular characterization and expression patterns of MTP genes under heavy metal stress in mustard (Brassica juncea L.)

Members of the Metal Tolerance Protein (MTP) family are critical in mediating the transport and tolerance of divalent metal cations. Despite their significance, the understanding of MTP genes in mustard (Brassica juncea) remains limited, especially regarding their response to heavy metal (HM) stress. In our study, we identified MTP gene sets in Brassica rapa (17 genes), Brassica nigra (18 genes), and B. juncea (33 genes) using the HMMER (Cation_efflux; PF01545) and BLAST analysis. For the 33 BjMTPs, a comprehensive bioinformatics analysis covering the physicochemical properties, phylogenetic relationships, conserved motifs, protein structures, collinearity, spatiotemporal RNA-seq expression, GO enrichment, and expression profiling under six HM stresses (Mn2+, Fe2+, Zn2+, Cd2+, Sb3+, and Pb2+) were carried out. According to the findings of physicochemical characteristics, phylogenetic tree, and collinearity, the allopolyploid B. juncea's MTP genes were inherited from its progenitors, B. rapa and B. nigra, with minimal gene loss during polyploidization. Members of the BjMTP family exhibited conserved motifs, promoter elements, and expression patterns across subgroups, consistent with the seven evolutionary branches (G1, G4-G9, and G12) of the MTPs. Further, spatiotemporal expression profiling under HM stresses successfully identified specific genes and crucial cis-regulatory elements associated with the response of BjMTPs to HM stresses. These findings may contribute to the genetic improvement of B. juncea for enhanced HM tolerance, facilitating the remediation of HM-contaminated areas.

MTP family analysis and association study reveal the role of ZmMTP11 in lead (Pb) accumulation

The metal tolerance protein (MTP) gene family plays an essential role in the transport of heavy metals, however the function of the MTP family in transporting lead (Pb) was still unclear in plants. In this study, we identified and characterized 12 ZmMTPs in the whole genome of maize. These ZmMTP genes were divided into three subfamilies in evolution, namely Zn-CDF, Zn/Fe-CDF, Mn-CDF subfamilies, which showed diverse expression patterns in different tissues of maize. Using gene-based association analyses, we identified a Pb accumulation-related MTP member in maize, ZmMTP11, which was located in plasma membrane and had the potential of transporting Pb ion. Under the Pb treatment, ZmMTP11 showed a generally decreased expression relative to the normal conditions. Heterologous expressions of ZmMTP11 in yeast, Arabidopsis, and rice demonstrated that ZmMTP11 enhanced Pb accumulation in the cells without affecting yeast and plant growth under Pb stress. Remarkably, the increased Pb concentration in the plant roots did not cause changes in Pb content in the shoots. Our study provides new insights into the genetic improvement of heavy metal tolerance in plants and contributes to bioremediation of Pb-contaminant soils.

The genes mgtE and spoVG are involved in zinc tolerance of Staphylococcus aureus

Metals are essential for all living organisms, but the type of metal and its concentration determines its action. Even low concentrations of metals may have toxic effects on organisms and therefore exhibit antimicrobial activities. In this study, we investigate the evolutionary adaptation processes of Staphylococcus aureus to metals and common genes for metal tolerance. Laboratory and clinical isolates were treated with manganese, cobalt, zinc, or nickel metal salts to generate growth-adapted mutants. After growth in medium supplemented with zinc, whole-genome sequencing identified, among others, two genes, mgtE (SAUSA300_0910), a putative magnesium transporter and spoVG (SAUSA300_0475), a global transcriptional regulator, as hot spots for stress-induced single-nucleotide polymorphisms (SNPs). SNPs in mgtE were also detected in mutants treated with high levels of cobalt or nickel salts. To investigate the effect of these genes on metal tolerance, deletion mutants and complementation strains in an S. aureus USA300 LAC* laboratory strain were generated. Both, the mgtE and spoVG deletion strains were more tolerant to cobalt, manganese, and zinc. The mgtE mutant was also more tolerant to nickel exposure. Inductively coupled plasma mass spectrometry analysis demonstrated that the mgtE deletion mutant accumulated less intracellular zinc than the wild type, explaining increased tolerance. From these results, we conclude that mgtE gene inactivation increases zinc tolerance presumably due to reduced uptake of zinc. For the SpoVG mutant, no direct effect on the intracellular zinc concentration was detected, indicating toward different pathways to increase tolerance. Importantly, inactivation of these genes offers a growth advantage in environments containing certain metals, pointing toward a common tolerance mechanism.

IMPORTANCE

Staphylococcus aureus is an opportunistic pathogen causing tremendous public health burden and high mortality in invasive infections. Treatment is becoming increasingly difficult due to antimicrobial resistances. The use of metals in animal husbandry and aquaculture to reduce bacterial growth and subsequent acquisition of metal resistances has been shown to co-select for antimicrobial resistance. Therefore, understanding adaptive mechanisms that help S. aureus to survive metal exposure is essential. Using a screening approach, we were able to identify two genes encoding the transporter MgtE and the transcriptional regulator SpoVG, which conferred increased tolerance to specific metals such as zinc when inactivated. Further testing showed that the deletion of mgtE leads to reduced intracellular zinc levels, suggesting a role in zinc uptake. The accumulation of mutations in these genes when exposed to other metals suggests that inactivation of these genes could be a common mechanism for intrinsic tolerance to certain metals.

Specific of accumulation of manganese in organs and tissues of Hereford cattle

The elemental status of cattle is one of the important factors, which determine its growth, fertility, fetal development, meat and dairy production, etc. Therefore, the study of content of different elements in cattle organs and tissues and its correlation with cattle characteristics and diet is urgent task. It is also important to develop intravital and low-invasive methods to analyze element content in cattle to regulate its diet during lifetime. In the present work, we have studied the content and distribution of manganese in Hereford cattle from an ecologically clean zone of Western Siberia (Russia). 252 samples were taken from 31 bulls aged 15-18 months. They were collected from various livestock farms in the region and analyzed using atomic absorption spectrophotometry (organs and muscle tissue) and inductively coupled plasma atomic emission spectrometry (hair). The median values of manganese concentration obtained in natural moisture for hair, heart, kidneys, liver, lungs, muscles, spleen, testes, and brain were 25, 0.37, 1.0, 2.6, 0.4, 0.2, 0.4, 0.5, and 0.5 ppm. Accordingly, the concentration of manganese differs significantly in the organs and tissues of animals (H = 188.6, df = 8, p <0.0001). Statistically significant associations of manganese were revealed in pairs: liver-testis, hair-testis, spleen-testis, and heart-brain. The classification of organs and tissues of animals according to the level of content and variability of manganese is carried out. The concentration of manganese in the body is not uniform, most of all it is deposited in the hair and excretory organs of the liver and kidneys. In other organs and muscle tissues, the distribution of manganese is more even and is in the range of 0.2-0.5 ppm. The resulting ranges can be used as a guideline for Hereford cattle bred in Western Siberia.

Detoxification of copper and zinc from anaerobic digestate effluent by indigenous bacteria: Mechanisms, pathways and metagenomic analysis

The presence of organic-complexed copper and zinc in anaerobic digestate effluent (ADE) poses persistent ecological toxicity. This study investigated the detoxification performance and biotic responses of indigenous bacteria against ethylene diamine tetraacetic acid (EDTA)-complexed Cu(II) and Zn(II). Heavy metals (HMs) stress induced reactive oxygen species (ROS) generation and enhanced extracellular polymeric substances (EPS) secretion. At a Cu(II) influent concentration of 20.0 mg·L-1, indigenous bacteria removed 88.2% of Cu(II) within nine days. The majority of copper and zinc sequestered by bacteria were stored in the cell envelope, with over 50% of copper and 60% of zinc being immobilized. Transmission electron microscopy mapping (TEM-mapping) revealed significant mineralization of copper and zinc on the cell wall. Proteins abundant in EPS, alongside humic acid-like substances, effectively adsorbed HMs. Indigenous bacteria exhibited the capacity to reduce cupric to the cuprous state and cupric is preferentially reduced to cuprous before reaching reducing capacity saturation. Sulfur precipitation emerges as a crucial pathway for Zn(II) removal. Metagenomic analysis indicated that indigenous bacteria upregulated genes related to HMs homeostasis, efflux, and DNA repair, enhancing its resistance to high concentrations HMs. This study provided theoretical guidance for employing bacterial consortia to eliminate HMs in complex aquatic environments.

The Bee Gut Microbiota: Bridging Infective Agents Potential in the One Health Context

The bee gut microbiota plays an important role in the services the bees pay to the environment, humans and animals. Alongside, gut-associated microorganisms are vehiculated between apparently remote habitats, promoting microbial heterogeneity of the visited microcosms and the transfer of the microbial genetic elements. To date, no metaproteomics studies dealing with the functional bee microbiota are available. Here, we employ a metaproteomics approach to explore a fraction of the bacterial, fungal, and unicellular parasites inhabiting the bee gut. The bacterial community portrays a dynamic composition, accounting for specimens of human and animal concern. Their functional features highlight the vehiculation of virulence and antimicrobial resistance traits. The fungal and unicellular parasite fractions include environment- and animal-related specimens, whose metabolic activities support the spatial spreading of functional features. Host proteome depicts the major bee physiological activities, supporting the metaproteomics strategy for the simultaneous study of multiple microbial specimens and their host-crosstalks. Altogether, the present study provides a better definition of the structure and function of the bee gut microbiota, highlighting its impact in a variety of strategies aimed at improving/overcoming several current hot topic issues such as antimicrobial resistance, environmental pollution and the promotion of environmental health.

Structures, Mechanisms, and Physiological Functions of Zinc Transporters in Different Biological Kingdoms

Zinc transporters take up/release zinc ions (Zn2+) across biological membranes and maintain intracellular and intra-organellar Zn2+ homeostasis. Since this process requires a series of conformational changes in the transporters, detailed information about the structures of different reaction intermediates is required for a comprehensive understanding of their Zn2+ transport mechanisms. Recently, various Zn2+ transport systems have been identified in bacteria, yeasts, plants, and humans. Based on structural analyses of human ZnT7, human ZnT8, and bacterial YiiP, we propose updated models explaining their mechanisms of action to ensure efficient Zn2+ transport. We place particular focus on the mechanistic roles of the histidine-rich loop shared by several zinc transporters, which facilitates Zn2+ recruitment to the transmembrane Zn2+-binding site. This review provides an extensive overview of the structures, mechanisms, and physiological functions of zinc transporters in different biological kingdoms.

Genome-wide identification, expression and function analysis of the MTP gene family in tulip (Tulipa gesneriana)

Currently, soil heavy metal contamination is a severe issue, particularly with Cd pollution. The metal tolerance protein (MTP) proteins, as plant divalent cation transporters, play a crucial role in the transport and tolerance of heavy metals in plants. This study conducted comprehensive identification and characterization of the MTP gene family in the tulip. A total of 11 TgMTP genes were identified and phylogenetically classified into three subfamilies. Conserved motif and gene structure analyses unveiled commonalities and variations among subfamily members. Expression profiling demonstrated several TgMTPs were markedly upregulated under Cd exposure, including the TgMTP7.1. Heterologous expression in yeast validated that TgMTP7.1 could ameliorate Cd sensitivity and enhance its tolerance. These results provide primary insights into the MTP gene family in tulip. Phylogenetic relationships and functional analyses establish a framework for elucidating the transporters and molecular mechanisms governing Cd accumulation and distribution in tulip. Key TgMTPs identified, exemplified by TgMTP7.1, may illuminate molecular breeding efforts aimed at developing Cd-tolerant cultivars for the remediation of soil Cd contamination.

Characterization of three predicted zinc exporters in Brucella ovis identifies ZntR-ZntA as a powerful zinc and cadmium efflux system not required for virulence and unveils pathogenic Brucellae heterogeneity in zinc homeostasis

Brucella ovis causes non-zoonotic ovine brucellosis of worldwide distribution and is responsible for important economic losses mainly derived from male genital lesions and reproductive fails. Studies about the virulence mechanisms of this rough species (lacking lipopolysaccharide O-chains) are underrepresented when compared to the main zoonotic Brucella species that are smooth (with O-chains). Zinc intoxication constitutes a defense mechanism of the host against bacterial pathogens, which have developed efflux systems to counterbalance toxicity. In this study, we have characterized three potential B. ovis zinc exporters, including the ZntA ortholog previously studied in B. abortus. Despite an in-frame deletion removing 100 amino acids from B. ovis ZntA, the protein retained strong zinc efflux properties. Only indirect evidence suggested a higher exporter activity for B. abortus ZntA, which, together with differences in ZntR-mediated regulation of zntA expression between B. ovis and B. abortus, could contribute to explaining why the ΔzntR mutant of B. abortus is attenuated while that of B. ovis is virulent. Additionally, B. ovis ZntA was revealed as a powerful cadmium exporter contributing to cobalt, copper, and nickel detoxification, properties not previously described for the B. abortus ortholog. Deletion mutants for BOV_0501 and BOV_A1100, also identified as potential zinc exporters and pseudogenes in B. abortus, behaved as the B. ovis parental strain in all tests performed. However, their overexpression in the ΔzntA mutant allowed the detection of discrete zinc and cobalt efflux activity for BOV_0501 and BOV_A1100, respectively. Nevertheless, considering their low expression levels and the stronger activity of ZntA as a zinc and cobalt exporter, the biological role of BOV_0501 and BOV_A1100 is questionable. Results presented in this study evidence heterogeneity among pathogenic Brucellae regarding zinc export and, considering the virulence of B. ovis ΔzntA, suggest that host-mediated zinc intoxication is not a relevant mechanism to control B. ovis infection.

Energy coupling and stoichiometry of Zn2+/H+ antiport by the prokaryotic cation diffusion facilitator YiiP

YiiP from Shewanella oneidensis is a prokaryotic Zn2+/H+ antiporter that serves as a model for the Cation Diffusion Facilitator (CDF) superfamily, members of which are generally responsible for homeostasis of transition metal ions. Previous studies of YiiP as well as related CDF transporters have established a homodimeric architecture and the presence of three distinct Zn2+ binding sites named A, B, and C. In this study, we use cryo-EM, microscale thermophoresis and molecular dynamics simulations to address the structural and functional roles of individual sites as well as the interplay between Zn2+ binding and protonation. Structural studies indicate that site C in the cytoplasmic domain is primarily responsible for stabilizing the dimer and that site B at the cytoplasmic membrane surface controls the structural transition from an inward facing conformation to an occluded conformation. Binding data show that intramembrane site A, which is directly responsible for transport, has a dramatic pH dependence consistent with coupling to the proton motive force. A comprehensive thermodynamic model encompassing Zn2+ binding and protonation states of individual residues indicates a transport stoichiometry of 1 Zn2+ to 2-3 H+ depending on the external pH. This stoichiometry would be favorable in a physiological context, allowing the cell to use the proton gradient as well as the membrane potential to drive the export of Zn2+.

Group A Streptococcus cation diffusion facilitator proteins contribute to immune evasion by regulating intracellular metal concentrations

Cation diffusion facilitators (CDFs) are a large family of divalent metal transporters with broad specificities that contribute to intracellular metal homeostasis and toxicity in bacterial pathogens. Streptococcus pyogenes (Group A Streptococcus [GAS]) expresses two homologous CDF efflux transporters, MntE and CzcD, which selectively transport Mn and Zn, respectively. We discovered that the MntE- and CzcD-deficient strains exhibited a marked decrease in the viability of macrophage-differentiated THP-1 cells and neutrophils. In addition, the viability of mice infected with both deficient strains markedly increased. Consistent with a previous study, our results suggest that MntE regulates the PerR-dependent oxidative stress response by maintaining intracellular Mn levels and contributing to the growth of GAS. The maturation and proteolytic activity of streptococcal cysteine protease (SpeB), an important virulence factor in GAS, has been reported to be abrogated by zinc and copper. Zn inhibited the maturation and proteolytic activity of SpeB in the culture supernatant of the CzcD-deficient strain. Furthermore, Mn inhibited SpeB maturation and proteolytic activity in a MntE-deficient strain. Since the host pathogenicity of the SpeB-deficient strain was significantly reduced, maintenance of intracellular manganese and zinc levels in the GAS via MntE and CzcD may not only confer metal resistance to the bacterium, but may also play an essential role in its virulence. These findings provide new insights into the molecular mechanisms of pathogenicity, which allow pathogens to survive under stressful conditions associated with elevated metal ion concentrations during host infection.

Insights into heavy metal tolerance mechanisms of Brassica species: physiological, biochemical, and molecular interventions

Heavy metal (HM) contamination of soil due to anthropogenic activities has led to bioaccumulation and biomagnification, posing toxic effects on plants by interacting with vital cellular biomolecules such as DNA and proteins. Brassica species have developed complex physiological, biochemical, and molecular mechanisms for adaptability, tolerance, and survival under these conditions. This review summarizes the HM tolerance strategies of Brassica species, covering the role of root exudates, microorganisms, cell walls, cell membranes, and organelle-specific proteins. The first line of defence against HM stress in Brassica species is the avoidance strategy, which involves metal ion precipitation, root sorption, and metal exclusion. The use of plant growth-promoting microbes, Pseudomonas, Psychrobacter, and Rhizobium species effectively immobilizes HMs and reduces their uptake by Brassica roots. The roots of Brassica species efficiently detoxify metals, particularly by flavonoid glycoside exudation. The composition of the cell wall and callose deposition also plays a crucial role in enhancing HMs resistance in Brassica species. Furthermore, plasma membrane-associated transporters, BjCET, BjPCR, BjYSL, and BnMTP, reduce HM concentration by stimulating the efflux mechanism. Brassica species also respond to stress by up-regulating existing protein pools or synthesizing novel proteins associated with HM stress tolerance. This review provides new insights into the HM tolerance mechanisms of Brassica species, which are necessary for future development of HM-resistant crops.

Thermodynamically consistent determination of free energies and rates in kinetic cycle models

Kinetic and thermodynamic models of biological systems are commonly used to connect microscopic features to system function in a bottom-up multiscale approach. The parameters of such models-free energy differences for equilibrium properties and in general rates for equilibrium and out-of-equilibrium observables-have to be measured by different experiments or calculated from multiple computer simulations. All such parameters necessarily come with uncertainties so that when they are naively combined in a full model of the process of interest, they will generally violate fundamental statistical mechanical equalities, namely detailed balance and an equality of forward/backward rate products in cycles due to Hill. If left uncorrected, such models can produce arbitrary outputs that are physically inconsistent. Here, we develop a maximum likelihood approach (named multibind) based on the so-called potential graph to combine kinetic or thermodynamic measurements to yield state-resolved models that are thermodynamically consistent while being most consistent with the provided data and their uncertainties. We demonstrate the approach with two theoretical models, a generic two-proton binding site and a simplified model of a sodium/proton antiporter. We also describe an algorithm to use the multibind approach to solve the inverse problem of determining microscopic quantities from macroscopic measurements and, as an example, we predict the microscopic p K a values and protonation states of a small organic molecule from 1D NMR data. The multibind approach is applicable to any thermodynamic or kinetic model that describes a system as transitions between well-defined states with associated free energy differences or rates between these states. A Python package multibind, which implements the approach described here, is made publicly available under the MIT Open Source license.

Genome-Wide Identification and Expression Analysis of MTP (Metal Ion Transport Proteins) Genes in the Common Bean

MTP/CDF carriers, called metal ion transport proteins, act as substrates for the transmission of micronutrients such as iron (Fe), zinc (Zn), and manganese (Mn) to membrane carriers in plants. In this study, genome-wide analysis of the MTP gene family in the common bean genome, expression analysis of the PvMTP4, PvMTP5, and PvMTP12 genes after Fe and Zn treatments, and the effects of Fe and Zn applications on iron and zinc content were investigated. This study used common bean genotypes assumed to have high or low Fe and Zn accumulation ability. PvMTP genes were defined as containing conserved catalytic domains with molecular weights and protein lengths ranging from 41.35 to 91.05 kDa and from 369 to 813 amino acids (aa), respectively. As a result of the phylogenetic analysis, three main clusters containing seven subgroups were formed. In this study, the first characterization of the MTP gene family of beans was performed, and the responses of three different PvMTP genes in the Zn-CDF group to Fe and Zn applications were revealed. The obtained findings are thought to constitute pioneering resources for future research on common bean biofortification studies, plant breeding related to Fe and Zn, and the functional characterization of the MTP gene family.

Thermodynamically consistent determination of free energies and rates in kinetic cycle models

Kinetic and thermodynamic models of biological systems are commonly used to connect microscopic features to system function in a bottom-up multiscale approach. The parameters of such models-free energy differences for equilibrium properties and in general rates for equilibrium and out-of-equilibrium observables-have to be measured by different experiments or calculated from multiple computer simulations. All such parameters necessarily come with uncertainties so that when they are naively combined in a full model of the process of interest, they will generally violate fundamental statistical mechanical equalities, namely detailed balance and an equality of forward/backward rate products in cycles due to T. Hill. If left uncorrected, such models can produce arbitrary outputs that are physically inconsistent. Here we develop a maximum likelihood approach (named multibind ) based on the so-called potential graph to combine kinetic or thermodynamic measurements to yield state resolved models that are thermodynamically consistent while being most consistent with the provided data and their uncertainties. We demonstrate the approach with two theoretical models, a generic two-proton binding site and a simplified model of a sodium/proton antiporter. We also describe an algorithm to use the multibind approach to solve the inverse problem of determining microscopic quantities from macroscopic measurements and as an example we predict the microscopic p K a s and protonation states of a small organic molecule from 1D NMR data. The multibind approach is applicable to any thermodynamic or kinetic model that describes a system as transitions between well-defined states with associated free energy differences or rates between these states. A Python package multibind , which implements the approach described here, is made publicly available under the MIT Open Source license.

WHY IT MATTERS

The increase in computational efficiency and rapid advances in methodology for quantitative free energy and rate calculations has allowed for the construction of increasingly complex thermodynamic or kinetic "bottom-up" models of chemical and biological processes. These multi-scale models serve as a framework for analyzing aspects of cellular function in terms of microscopic, molecular properties and provide an opportunity to connect molecular mechanisms to cellular function. The underlying model parameters-free energy differences or rates-are constrained by thermodynamic identities over cycles of states but these identities are not necessarily obeyed during model construction, thus potentially leading to inconsistent models. We address these inconsistencies through the use of a maximum likelihood approach for free energies and rates to adjust the model parameters in such a way that they are maximally consistent with the input parameters and exactly fulfill the thermodynamic cycle constraints. This approach enables formulation of thermodynamically consistent multi-scale models from simulated or experimental measurements.

Tea-based biochar-mediated changes in cation diffusion homeostasis in rice grown in heavy metal (loid) contaminated mining soil

Foreseeable future scenarios highlight the urgency of applying eco-safe avoidance methods or tolerance to heavy metal(loid) (HM) stress in agricultural production areas of contamination. The analyses show that the Ni, Mn, As, and Cr concentrations detected in the soils of the paddy fields in the Black Sea region vary between 123.60 and 263.30; 687-1271; 8.90-14.50; 162.00-340.00 mg kg-1 proving high accumulation of Ni, Mn, As, Cr in rice. Overconsumption of rice farmed extensively on these soils might also lead to human HM-related health problems. Therefore, in the current study, the approach of using tea-based biochar (BC) proven to have one of the most significant potentials as a soil amendment to reduce HM transmission to in-vitro-grown rice plants was investigated in the soil medium naturally contaminated with HMs. The tea-BC was produced from readily available local black tea waste of a conventional fermentation process and applied in the in-vitro experiments. Among the tested doses examined, 1% tea-BC showed a more positive effect on rice plant growth and development characterized by a better relative growth rate (59.7 and 84 mg g-1 d-1 for root and shoot tissues), photosynthetic pigment intactness (62.48 μg mL-1), cellular membrane integrity (93%), and relative water (96%) than the other rates (0% BC, 3%BC, 5%BC). The mRNA expression data highlights the probability of a cation diffusion facilitator (CDF) (OsMTP11) in concert with catalase isozyme (CATa) and dehydration-responsive element binding protein (DREB1a) linking the HM detoxification, oxidative defense, and dehydration pathways with the help of tea-BC. At the optimum concentration (1%BC), this approach might reduce HM accumulation levels of crops planted in HM-contaminated farmlands.

Energy Coupling and Stoichiometry of Zn2+/H+ Antiport by the Cation Diffusion Facilitator YiiP

YiiP is a prokaryotic Zn2+/H+ antiporter that serves as a model for the Cation Diffusion Facilitator (CDF) superfamily, members of which are generally responsible for homeostasis of transition metal ions. Previous studies of YiiP as well as related CDF transporters have established a homodimeric architecture and the presence of three distinct Zn2+ binding sites named A, B, and C. In this study, we use cryo-EM, microscale thermophoresis and molecular dynamics simulations to address the structural and functional roles of individual sites as well as the interplay between Zn2+ binding and protonation. Structural studies indicate that site C in the cytoplasmic domain is primarily responsible for stabilizing the dimer and that site B at the cytoplasmic membrane surface controls the structural transition from an inward facing conformation to an occluded conformation. Binding data show that intramembrane site A, which is directly responsible for transport, has a dramatic pH dependence consistent with coupling to the proton motive force. A comprehensive thermodynamic model encompassing Zn2+ binding and protonation states of individual residues indicates a transport stoichiometry of 1 Zn2+ to 2-3 H+ depending on the external pH. This stoichiometry would be favorable in a physiological context, allowing the cell to use the proton gradient as well as the membrane potential to drive the export of Zn2+.

Deciphering the functional roles of transporter proteins in subcellular metal transportation of plants

MAIN CONCLUSION

The role of transporters in subcellular metal transport is of great significance for plants in coping with heavy metal stress and maintaining their proper growth and development. Heavy metal toxicity is a serious long-term threat to plant growth and agricultural production, becoming a global environmental concern. Excessive heavy metal accumulation not only damages the biochemical and physiological functions of plants but also causes chronic health hazard to human beings through the food chain. To deal with heavy metal stress, plants have evolved a series of elaborate mechanisms, especially a variety of spatially distributed transporters, to strictly regulate heavy metal uptake and distribution. Deciphering the subcellular role of transporter proteins in controlling metal absorption, transport and separation is of great significance for understanding how plants cope with heavy metal stress and improving their adaptability to environmental changes. Hence, we herein introduce the detrimental effects of excessive common essential and non-essential heavy metals on plant growth, and describe the structural and functional characteristics of transporter family members, with a particular emphasis on their roles in maintaining heavy metal homeostasis in various organelles. Besides, we discuss the potential of controlling transporter gene expression by transgenic approaches in response to heavy metal stress. This review will be valuable to researchers and breeders for enhancing plant tolerance to heavy metal contamination.

Investigation of the role of TmMYB16/123 and their targets (TmMTP1/11) in the tolerance of Taxus media to cadmium

The toxicity and stress caused by heavy metal contamination has become an important constraint to the growth and flourishing of trees. In particular, species belonging to the genus Taxus, which are the only natural source for the anti-tumor medicine paclitaxel, are known to be highly sensitive to environmental changes. To investigate the response of Taxus spp. to heavy metal stress, we analyzed the transcriptomic profiles of Taxus media trees exposed to cadmium (Cd2+). In total, six putative genes from the metal tolerance protein (MTP) family were identified in T. media, including two Cd2+ stress inducible TMP genes (TmMTP1, TmMTP11 and Taxus media). Secondary structure analyses predicted that TmMTP1 and TmMTP11, which are members of the Zn-CDF and Mn-CDF subfamily proteins, respectively, contained six and four classic transmembrane domains, respectively. The introduction of TmMTP1/11 into the ∆ycf1 yeast cadmium-sensitive mutant strain showed that TmMTP1/11 might regulate the accumulation of Cd2+ to yeast cells. To screen the upstream regulators, partial promoter sequences of the TmMTP1/11 genes were isolated using the chromosome walking method. Several myeloblastosis (MYB) recognition elements were identified in the promoters of these genes. Furthermore, two Cd2+-induced R2R3-MYB TFs, TmMYB16 and TmMYB123, were identified. Both in vitro and in vivo assays confirmed that TmMTB16/123 play a role in Cd2+ tolerance by activating and repressing the expression of TmMTP1/11 genes. The present study elucidated new regulatory mechanisms underlying the response to Cd stress and can contribute to the breeding of Taxus species with high environmental adaptability.

Getting zinc into and out of cells

Ion channels are specialized proteins located on the plasma membrane and control the movement of ions across the membrane. Zn ion plays an indispensable role as a structural constituent of various proteins, moreover, it plays an important dynamic role in cell signaling. In this chapter, we discuss computational insights into zinc efflux and influx mechanism through YiiP (from Escherichia coli and Shewanella oneidensis) and BbZIP (Bordetella bronchiseptica) transporters, respectively. Gaining knowledge about the mechanism of zinc transport at the molecular level can aid in developing treatments for conditions such as diabetes and cancer by manipulating extracellular and intracellular levels of zinc ions.

Identification and Characterization of Dmct: A Cation Transporter in Yarrowia lipolytica Involved in Metal Tolerance

Yarrowia lipolytica is a dimorphic fungus used as a model organism to investigate diverse biotechnological and biological processes, such as cell differentiation, heterologous protein production, and bioremediation strategies. However, little is known about the biological processes responsible for cation concentration homeostasis. Metals play pivotal roles in critical biochemical processes, and some are toxic at unbalanced intracellular concentrations. Membrane transport proteins control intracellular cation concentrations. Analysis of the Y. lipolytica genome revealed a characteristic functional domain of the cation efflux protein family, i.e., YALI0F19734g, which encodes YALI0F19734p (a putative Yl-Dmct protein), which is related to divalent metal cation tolerance. We report the in silico analysis of the putative Yl-Dmct protein's characteristics and the phenotypic response to divalent cations (Ca²⁺, Cu²⁺, Fe²⁺, and Zn²⁺) in the presence of mutant strains, Δdmct and Rdmct, constructed by deletion and reinsertion of the DMCT gene, respectively. The absence of the Yl-Dmct protein induces cellular and growth rate changes, as well as dimorphism differences, when calcium, copper, iron, and zinc are added to the cultured medium. Interestingly, the parental and mutant strains were able to internalize the ions. Our results suggest that the protein encoded by the DMCT gene is involved in cell development and cation homeostasis in Y. lipolytica.

Detoxifying the heavy metals: a multipronged study of tolerance strategies against heavy metals toxicity in plants

Heavy metal concentrations exceeding permissible limits threaten human life, plant life, and all other life forms. Different natural and anthropogenic activities emit toxic heavy metals in the soil, air, and water. Plants consume toxic heavy metals from their roots and foliar part inside the plant. Heavy metals may interfere with various aspects of the plants, such as biochemistry, bio-molecules, and physiological processes, which usually translate into morphological and anatomical changes. They use various strategies to deal with the toxic effects of heavy metal contamination. Some of these strategies include restricting heavy metals to the cell wall, vascular sequestration, and synthesis of various biochemical compounds, such as phyto-chelators and organic acids, to bind the free moving heavy metal ions so that the toxic effects are minimized. This review focuses on several aspects of genetics, molecular, and cell signaling levels, which integrate to produce a coordinated response to heavy metal toxicity and interpret the exact strategies behind the tolerance of heavy metals stress. It is suggested that various aspects of some model plant species must be thoroughly studied to comprehend the approaches of heavy metal tolerance to put that knowledge into practical use.

Genome-wide identification and expression profile analysis of metal tolerance protein gene family in Eucalyptus grandis under metal stresses

Metal tolerance proteins (MTPs) as Me²⁺/H⁺(K⁺) antiporters participate in the transport of divalent cations, leading to heavy metal stress resistance and mineral utilization in plants. In the present study, to obtain better knowledge of the biological functions of the MTPs family, 20 potential EgMTPs genes were identified in Eucalyptus grandis and classified into seven groups belonging to three cation diffusion facilitator groups (Mn-CDFs, Zn/Fe-CDFs, and Zn-CDFs) and seven groups. EgMTP-encoded amino acids ranged from 315 to 884, and most of them contained 4-6 recognized transmembrane domains and were clearly prognosticated to localize into the cell vacuole. Almost all EgMTP genes experienced gene duplication events, in which some might be uniformly distributed in the genome. The numbers of cation efflux and the zinc transporter dimerization domain were highest in EgMTP proteins. The promoter regions of EgMTP genes have different cis-regulatory elements, indicating that the transcription rate of EgMTP genes can be a controlled response to different stimuli in multiple pathways. Our findings provide accurate perception on the role of the predicted miRNAs and the presence of SSR marker in the Eucalyptus genome and clarify their functions in metal tolerance regulation and marker-assisted selection, respectively. Gene expression profiling based on previous RNA-seq data indicates a probable function for EgMTP genes during development and responses to biotic stress. Additionally, the upregulation of EgMTP6, EgMTP5, and EgMTP11.1 to excess Cd²⁺ and Cu²⁺ exposure might be responsible for metal translocation from roots to leaves.

Dissecting the relative contribution of ECA3 and group 8/9 cation diffusion facilitators to manganese homeostasis in Arabidopsis thaliana

Manganese (Mn) is an essential micronutrient for plant growth but becomes toxic when present in excess. A number of Arabidopsis proteins are involved in Mn transport including ECA3, MTPs, and NRAMPs; however, their relative contributions to Mn homeostasis remain to be demonstrated. A major focus here was to clarify the importance of ECA3 in responding to Mn deficiency and toxicity using a range of mutants. We show that ECA3 localizes to the trans-Golgi and plays a major role in response to Mn deficiency with severe effects seen in eca3 nramp1 nramp2 under low Mn supply. ECA3 plays a minor role in Mn-toxicity tolerance, but only when the cis-Golgi-localized MTP11 is non-functional. We also use mutants and overexpressors to determine the relative contributions of MTP members to Mn homeostasis. The trans-Golgi-localized MTP10 plays a role in Mn-toxicity tolerance, but this is only revealed in mutants when MTP8 and MTP11 are non-functional and when overexpressed in mtp11 mutants. MTP8 and MTP10 confer greater Mn-toxicity resistance to the pmr1 yeast mutant than MTP11, and an important role for the first aspartate in the fifth transmembrane domain DxxxD motif is demonstrated. Overall, new insight into the relative influence of key transporters in Mn homeostasis is provided.

Cation diffusion facilitator proteins of Beta vulgaris reveal diversity of metal handling in dicotyledons

Manganese (Mn), iron (Fe), and zinc (Zn) are essential for diverse processes in plants, but their availability is often limiting or excessive. Cation diffusion facilitator (CDF) proteins have been implicated in the allocation of those metals in plants, whereby most of our mechanistic understanding has been obtained in Arabidopsis. It is unclear to what extent this can be generalized to other dicots. We characterized all CDFs/metal tolerance proteins of sugar beet (Beta vulgaris spp. vulgaris), which is phylogenetically distant from Arabidopsis. Analysis of subcellular localization, substrate selectivities, and transcriptional regulation upon exposure to metal deficiencies and toxicities revealed unexpected deviations from their Arabidopsis counterparts. Localization and selectivity of some members were modulated by alternative splicing. Notably, unlike in Arabidopsis, Mn- and Zn-sequestrating members were not induced in Fe-deficient roots, pointing to differences in the Fe acquisition machinery. This was supported by low Zn and Mn accumulation under Fe deficiency and a strikingly increased Fe accumulation under Mn and Zn excess, coinciding with an induction of BvIRT1. High Zn load caused a massive upregulation of Zn-BvMTPs. The results suggest that the employment of the CDF toolbox is highly diverse amongst dicots, which questions the general applicability of metal homeostasis models derived from Arabidopsis.

Arabidopsis COPT1 copper transporter uses a single histidine to regulate transport activity and protein stability

Copper acquisition and subsequent delivery to target proteins are essential for many biological processes. However, the cellular levels of this trace element must be controlled because of its potential toxicity. The COPT1 protein rich in potential metal-binding amino acids functions in high affinity copper uptake at the plasma membrane of Arabidopsis cells. The functional role of these putative metal-binding residues is largely unknown. Through truncations and site-directed mutagenesis, we identified His43, a single residue within the extracellular N-terminal domain as absolutely critical for copper uptake of COPT1. Substitution of this residue with leucine, methionine or cysteine almost inactivated transport function of COPT1, implying that His43 fails to serves as a copper ligand in the regulation of COPT1 activity. Deletion of all extracellular N-terminal metal-binding residues completely blocked copper-stimulated degradation but did not alter the subcellular distribution and multimerization of COPT1. Although mutation of His43 to alanine and serine retained the transporter activity in yeast cells, the mutant protein was unstable and degraded in the proteasome in Arabidopsis cells. Our results demonstrate a pivotal role for the extracellular residue His43 in high affinity copper transport activity, and suggest common molecular mechanisms for regulating both metal transport and protein stability of COPT1.

Novel biofortification candidate: MTP1 increases microelement contents and decreases toxic heavy metal accumulation in grains

Decreases in microelement contents and increases in toxic element levels seriously affect crop growth and human health. Thus, improving the elemental content of food crops is an important environmental issue for enhancing crop production and quality. Previous research showed that metal tolerance protein 1 (MTP1) is localized at the vacuole membrane, wherein it mediates the translocation of heavy metal ions. Therefore, LmMTP1 was isolated from annual ryegrass (Lolium multiflorum). Real-time quantitative PCR analyses revealed LmMTP1 expression increased significantly in the roots after Zn, Co, and Cd treatments. Confocal microscopy images indicated LmMTP1 was localized at the vacuole membrane. The expression of LmMTP1 in transgenic yeast and rice resulted in increased Zn, Co, and Cd tolerance. The examination of heavy metal contents detected increases in the Zn and Co contents, but decreases in the Cd contents, of yeast and rice. Moreover, the grains of LmMTP1-expressing transgenic rice had higher Zn/Co contents and lower Cd contents than wild-type rice grains. These results imply that LmMTP1 influences Zn, Co, and Cd tolerance and accumulation. Furthermore, LmMTP1 might be a novel biofortification-related candidate gene useful for improving the storage of essential elements and eliminating toxic heavy metals from crops.

Cell-Surface Autoantibody Targets Zinc Transporter-8 (ZnT8) for In Vivo β-Cell Imaging and Islet-Specific Therapies

Type 1 diabetes (T1D) is a disease in which autoimmune attacks are directed at the insulin-producing β-cell in the pancreatic islet. Autoantigens on the β-cell surface membrane are specific markers for molecular recognition and targets for engagement by autoreactive B lymphocytes, which produce islet cell surface autoantibody (ICSA) upon activation. We report the cloning of an ICSA (mAb43) that recognizes a major T1D autoantigen, ZnT8, with a subnanomolar binding affinity and conformation specificity. We demonstrate that cell-surface binding of mAb43 protects the extracellular epitope of ZnT8 against immunolabeling by serum ICSA from a patient with T1D. Furthermore, mAb43 exhibits in vitro and ex vivo specificity for islet cells, mirroring the exquisite specificity of islet autoimmunity in T1D. Systemic administration of mAb43 yields a pancreas-specific biodistribution in mice and islet homing of an mAb43-linked imaging payload through the pancreatic vasculature, thereby validating the in vivo specificity of mAb43. Identifying ZnT8 as a major antigenic target of ICSA allows for research into the molecular recognition and engagement of autoreactive B cells in the chronic phase of T1D progression. The in vivo islet specificity of mAb43 could be further exploited to develop in vivo imaging and islet-specific immunotherapies.

Genome-wide identification and expression analysis of metal tolerance protein (MTP) gene family in soybean (Glycine max) under heavy metal stress

AIM

Plant metal tolerance proteins (MTPs) are plant membrane divalent cation transporters that specifically contribute to heavy metal stress resistance and mineral uptake. However, little is known about this family's molecular behaviors and biological activities in soybean.

METHODS AND RESULTS

A total of 20 potential MTP candidate genes were identified and studied in the soybean genome for phylogenetic relationships, chromosomal distributions, gene structures, gene ontology, cis-elements, and previous gene expression. Furthermore, the expression of MTPs has been investigated under different heavy metals treatments. All identified soybean MTPs (GmaMTPs) contain a cation efflux domain or a ZT dimer and are further divided into three primary cation diffusion facilitator (CDF) groups: Mn-CDFs, Zn-CDFs, and Fe/Zn-CDFs. The developmental analysis reveals that segmental duplication contributes to the GmaMTP family's expansion. Tissue-specific expression profiling revealed comparative expression profiling in similar groups, although gene expression differed between groups. GmaMTP genes displayed biased responses in either plant leaves or roots when treated with heavy metal. In the leaves and roots, nine and ten GmaMTPs responded to at least one metal ion treatment. Furthermore, in most heavy metal treatments, GmaMTP1.1, GmaMTP1.2, GmaMTP3.1, GmaMTP3.2, GmaMTP4.1, and GmaMTP4.3 exhibited significant expression responses.

CONCLUSION

Our findings provided insight into the evolution of MTPs in soybean. Overall, our findings shed light on the evolution of the MTP gene family in soybean and pave the path for further functional characterization of this gene family.

Genetic Regulation Mechanism of Cadmium Accumulation and Its Utilization in Rice Breeding

Cadmium (Cd) is a heavy metal whose pollution in rice fields leads to varying degrees of Cd accumulation in rice. Furthermore, the long-term consumption of Cd-contaminated rice is harmful to human health. Therefore, it is of great theoretical significance and application value to clarify the genetic regulation mechanism of Cd accumulation in rice and cultivate rice varieties with low Cd accumulation for the safe use of Cd-contaminated soils. This review summarizes the effects of Cd on rice growth, yield, and quality; the physiological and molecular mechanisms of Cd absorption in the roots, loading, and transport of Cd in the xylem, the distribution of Cd in nodes, redistribution of Cd in leaves, and accumulation of Cd in the grains; the regulation mechanism of the Cd stress response; and the breeding of rice with low Cd accumulation. Future directions on the genetic regulation of Cd in rice and application are also discussed. This review provides a theoretical basis for studies exploring the genetic regulation of Cd stress in rice. It also offers a basis for formulating effective strategies to reduce the Cd content in rice.

Iron homeostasis in Bacillus subtilis relies on three differentially expressed efflux systems

In Bacillus subtilis, iron homeostasis is maintained by the ferric uptake regulator (Fur) and manganese homeostasis relies on the manganese transport regulator (MntR). Both Fur and MntR function as bi-functional metalloregulators that repress import and activate metal ion efflux systems. The ferrous iron efflux ATPase, PfeT, is derepressed by hydrogen peroxide (H₂O₂) as sensed by PerR and induced by iron as sensed by Fur. Mutants lacking PfeT are sensitive to iron intoxication. Here, we show that mntR mutants are also iron-sensitive, largely due to decreased expression of the MntR-activated MneP and MneS cation diffusion facilitator (CDF) proteins previously defined for their role in Mn²⁺ export. The ability of MneP and MneS to export iron is apparent even when their expression is not induced by Mn²⁺. Our results demonstrate that PfeT, MneP and MneS each contribute to iron homeostasis, and a triple mutant lacking all three is more iron-sensitive than any single mutant. We further show that sensitivity to H₂O₂ does not correlate with iron sensitivity. For example, an mntR mutant is H₂O₂-sensitive due to elevated Mn(II) that increases PerR-mediated repression of peroxide resistance genes, and this repression is antagonized by elevated Fe²⁺ in an mntR pfeT mutant. Thus, H₂O₂-sensitivity reflects the relative levels of Mn²⁺ and Fe²⁺ as sensed by the PerR regulatory protein. These results underscore the complex interplay between manganese, iron and oxidative stress in B. subtilis.

Metal tolerance gene family in barley: an in silico comprehensive analysis

Metal-tolerance proteins (MTPs) are divalent cation transporters that play critical roles in metal tolerance and ion homeostasis in plants. However, a comprehensive study of MTPs is still lacking in crop plants. The current study aimed to comprehensively identify and characterize the MTP gene family in barley (Hordeum vulgare, Hv), an important crop. In total, 12 HvMTPs were identified in the barley genome in this study. They were divided into three phylogenetic groups (Zn-cation diffusion facilitator proteins [CDFs], Fe/Zn-CDFs, and Mn-CDFs) and further subdivided into seven groups (G1, G5, G6, G7, G8, G9, and G12). The majority of MTPs were hydrophobic proteins found in the vacuolar membrane. Gene duplication analysis of HvMTPs revealed one pair of segmental-like duplications in the barley genome. Evolutionary analysis suggested that barley MTPs underwent purifying natural selection. Additionally, the HvMTPs were analyzed in the pan-genome sequences of barley (20 accessions), which suggests that HvMTPs are highly conserved in barley evolution. Cis-acting regulatory elements, microRNA target sites, and protein-protein interaction analysis indicated the role of HvMTPs in a variety of biological processes. Expression profiling suggests that HvMTPs play an active role in maintaining barley nutrient homeostasis throughout its life cycle, and their expression levels were not significantly altered by abiotic stresses like cold, drought, or heat. The expression of barley HvMTP genes in the presence of heavy metals such as Zn²⁺, Cu²⁺, As³⁺, and Cd²⁺ revealed that these MTPs were induced by at least one metal ion, implying their involvement in metal tolerance or transportation. The identification and comprehensive investigation of MTP gene family members will provide important gene resources for the genetic improvement of crops for metal tolerance, bioremediation, or biofortification of staple crops.

Multi-Omics Uncover the Mechanism of Wheat under Heavy Metal Stress

Environmental pollution of heavy metals has received growing attention in recent years. Heavy metals such as cadmium, lead and mercury can cause physiological and morphological disturbances which adversely affect the growth and quality of crops. Wheat (Triticum aestivum L.) can accumulate high contents of heavy metals in its edible parts. Understanding wheat response to heavy metal stress and its management in decreasing heavy metal uptake and accumulation may help to improve its growth and grain quality. Very recently, emerging advances in heavy metal toxicity and phytoremediation methods to reduce heavy metal pollution have been made in wheat. Especially, the molecular mechanisms of wheat under heavy metal stress are increasingly being recognized. In this review, we focus on the recently described epigenomics, transcriptomics, proteomics, metabolomics, ionomics and multi-omics combination, as well as functional genes uncovering heavy metal stress in wheat. The findings in this review provide some insights into challenges and future recommendations for wheat under heavy metal stress.

Arbuscular mycorrhiza mitigates zinc stress on Eucalyptus grandis through regulating metal tolerance protein gene expression and ionome uptake

Arbuscular mycorrhizal (AM) fungi are symbionts of most terrestrial plants and enhance their adaptability in metal-contaminated soils. In this study, mycorrhized and non-mycorrhized Eucalyptus grandis were grown under different Zn treatments. After 6 weeks of treatment, the growing status and ionome content of plants as well as the expression patterns of metal tolerance proteins and auxin biosynthesis-related genes were measured. In this study, mycorrhized E. grandis showed higher biomass and height at a high level of Zn compared with non-mycorrhized plants. In addition, AM plants accumulated P, Mg, and Mn in roots and P, Fe, and Cu in shoots, which indicate that AM fungi facilitate the uptake of ionome nutrients to promote plant growth. In addition, mycorrhiza upregulated the expression of EgMTP1 and EgMTP7, whose encoding proteins were predicted to be located at the vacuolar membrane. Meanwhile, Golgi membrane transporter EgMTP5 was also induced in AM shoot. Our results suggest that AM likely mitigates Zn toxicity through sequestrating excess Zn into vacuolar and Golgi. Furthermore, the expression of auxin biosynthesis-related genes was facilitated by AM, and this is probably another approach for Zn tolerance.

The vacuolar transporter LaMTP8.1 detoxifies manganese in leaves of Lupinus albus

Manganese (Mn) is an essential microelement, but overaccumulation is harmful to many plant species. Most plants have similar minimal Mn requirements, but the tolerance to elevated Mn varies considerably. Mobilization of phosphate (P) by plant roots leads to increased Mn uptake, and shoot Mn levels have been reported to serve as an indicator for P mobilization efficiency in the presence of P deficiency. White lupin (Lupinus albus L.) mobilizes P and Mn with outstanding efficiency due to the formation of determinate cluster roots that release carboxylates. The high Mn tolerance of L. albus goes along with shoot Mn accumulation, but the molecular basis of this detoxification mechanism has been unknown. In this study, we identify LaMTP8.1 as the transporter mediating vacuolar sequestration of Mn in the shoot of white lupin. The function of Mn transport was demonstrated by yeast complementation analysis, in which LaMTP8.1 detoxified Mn in pmr1∆ mutant cells upon elevated Mn supply. In addition, LaMTP8.1 also functioned as an iron (Fe) transporter in yeast assays. The expression of LaMTP8.1 was particularly high in old leaves under high Mn stress. However, low P availability per se did not result in transcriptional upregulation of LaMTP8.1. Moreover, LaMTP8.1 expression was strongly upregulated under Fe deficiency, where it was accompanied by Mn accumulation, indicating a role in the interaction of these micronutrients in L. albus. In conclusion, the tonoplast-localized Mn transporter LaMTP8.1 mediates Mn detoxification in leaf vacuoles, providing a mechanistic explanation for the high Mn accumulation and Mn tolerance in this species.

The Role of Sulfur in Agronomic Biofortification with Essential Micronutrients

Sulfur (S) is an essential macronutrient for plants, being necessary for their growth and metabolism and exhibiting diverse roles throughout their life cycles. Inside the plant body, S is present either in one of its inorganic forms or incorporated in an organic compound. Moreover, organic S compounds may contain S in its reduced or oxidized form. Among others, S plays roles in maintaining the homeostasis of essential micronutrients, e.g., iron (Fe), copper (Cu), zinc (Zn), and manganese (Mn). One of the most well-known connections is homeostasis between S and Fe, mainly in terms of the role of S in uptake, transportation, and distribution of Fe, as well as the functional interactions of S with Fe in the Fe-S clusters. This review reports the available information describing the connections between the homeostasis of S and Fe, Cu, Zn, and Mn in plants. The roles of S- or sulfur-derived organic ligands in metal uptake and translocation within the plant are highlighted. Moreover, the roles of these micronutrients in S homeostasis are also discussed.

Genome-Wide Identification of Strawberry Metal Tolerance Proteins and Their Expression under Cadmium Toxicity

Metal tolerance proteins (MTPs) are divalent cation transporters, known to upkeep the mineral nutrition of plants and heavy metal transport at cell, tissue, or whole plant levels. However, information related to evolutionary relationships and biological functions of MTP genes in strawberry (Fragaria vesca L.) remain elusive. Herein, we identified 12 MTP genes from the strawberry genome and divided them into three main groups (i.e., Zn-MTP, Fe/Zn MTP, and Mn-MTP), which is similar to MTP grouping in Arabidopsis and rice. The strawberry MTPs (FvMTPs) are predicted to be localized in the vacuole, while open reading frame (ORF) length ranged from 1113 to 2589 bp with 370 to 862 amino acids, and possess 4 to 6 transmembrane domains (TMDs), except for FvMTP12 that possessed 16 TMDs. All the FvMTP genes had putative cation efflux and cation diffusion facilitator domains along with a zinc dimerization (ZT-dimer) domain in Mn-MTPs. The collinear analysis suggested their conservation between strawberry and Arabidopsis MTPs. Promoter analysis also demonstrated that some of them might possibly be regulated by hormones and abiotic stress factors. Moreover, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis proposed that FvMTP genes are involved in cation transport and homeostasis. The expression analysis showed that FvMTP1, FvMTP1.1, and FvMTP4 were significantly induced in leaf samples, while FvMTP1.1 and FvMTP4 were significantly regulated in roots of cadmium (Cd)-treated strawberry plants during progressive stress duration. The findings of Cd accumulation depicted that Cd contents were significantly higher in root tissues than that of leaf tissues of strawberry. These results are indicative of their response during the specific duration in Cd detoxification, while further functional studies can accurately verify their specific role.

MTP8 from Triticum urartu Is Primarily Responsible for Manganese Tolerance

Mineral nutrients, such as manganese (Mn) and iron (Fe), play essential roles in many biological processes in plants but their over-enrichment is harmful for the metabolism. Metal tolerance proteins (MTPs) are involved in cellular Mn and Fe homeostasis. However, the transporter responsible for the transport of Mn in wheat is unknown. In our study, TuMTP8, a Mn-CDF transporter from diploid wheat (Triticum urartu), was identified. Expression of TuMTP8 in yeast strains of Δccc1 and Δsmf1 and Arabidopsis conferred tolerance to elevated Mn and Fe, but not to other metals (zinc, cobalt, copper, nickel, or cadmium). Compared with TuVIT1 (vacuole Fe transporter), TuMTP8 shows a significantly higher proportion in Mn transport and a smaller proportion in Fe transport. The transient analysis in tobacco epidermal cells revealed that TuMTP8 localizes to vacuolar membrane. The highest transcript levels of TuMTP8 were in the sheath of the oldest leaf and the awn, suggesting that TuMTP8 sequesters excess Mn into the vacuole in these organs, away from more sensitive tissues. These findings indicate that TuMTP8, a tonoplast-localized Mn/Fe transporter, functions as a primary balancer to regulate Mn distribution in T. urartu under elevated Mn conditions and participates in the intracellular transport and storage of excess Mn as a detoxification mechanism, thereby conferring Mn tolerance.

Changes in physiological responses and MTP (metal tolerance protein) transcripts in soybean (Glycine max) exposed to differential iron availability

Members of MTP (metal tolerance protein) family are potential metal ion transporters, but little is known about how their responses and expression are altered in response to the deficiency and excess of Fe in soybean. In this study, root and shoot length and biomass in addition to leaf chlorophyll score, PSII efficiency and photosynthetic performance index were adversely affected by Fe-deficiency and excess Fe. Fe and S concentrations in the root and shoot, as well as the increased root FCR activity, consistently decreased and increased, respectively, accompanied by elevated Zn levels under Fe deficiency and Fe toxicity. This implies that Fe-uptake of plants subjected to differential Fe availability are likely determined by S and Zn nutritional status. In qPCR analysis, GmMTP5, GmMTP7, GmMTP8, and GmMTP10 genes showed downregulation under Fe shortage, whereas GmMTP6 and GmMTP11 were significantly upregulated due to Fe-toxicity. Further, GmMTP1, GmMTP3, GmMTP6, GmMTP7, and GmMTP10 were significantly induced in response to Fe toxicity, indicating their potential role in metal tolerance. Bioinformatics analysis showed that soybean MTP genes possessed a close relationship with certain Arabidopsis genes (i.e. ZAT, MTPB1) involved in solute transport and metal sequestration. Furthermore, top five motifs of soybean MTP protein correspond to the cation efflux family exhibited strong amino acid and evolutionary similarities with Arabidopsisthaliana. These findings shed light on Fe homeostasis mechanisms in soybean and could be used to regulate Fe uptake through breeding or transgenic manipulations of MTP genes.

Heavy metal (loid)s phytotoxicity in crops and its mitigation through seed priming technology

Unexpected bioaccumulation and biomagnification of heavy metal(loid)s (HMs) in the environment have become a predicament for all living organisms, including plants. The presence of these HMs in the plant system raised the level of reactive oxygen species (ROS) and remodeled several vital cellular biomolecules. These lead to several morphological, physiological, metabolic, and molecular aberrations in plants ranging from chlorosis of leaves to the lipid peroxidation of membranes, and degradation of proteins and nucleic acid including the modulation of the enzymatic system, which ultimately affects the plant growth and productivity. Plants are equipped with several mechanisms to counteract the HMs toxicity. Among them, seed priming (SP) technology has been widely tested with the use of several inorganic chemicals, plant growth regulators (PGRs), gasotransmitters, nanoparticles, living organisms, and plant leaf extracts. The use of these compounds has the potential to alleviate the HMs toxicity through the strengthening of the antioxidant defense system, generation of low molecular weight metallothionein's (MTs), and phytochelatins (PCs), and improving seedling vigor during early growth stages. This review presents an account of the sources, uptake and transport, and phytotoxic effects of HMs with special attention to different mechanism/s, occurring to mitigate the HMs toxicity in plants employing SP technology.Novelty statement: To the best of our knowledge, this review has delineated the consequences of HMs on the crucial plant processes, which ultimately affect plant growth and development. This review also compiled the up to dated information on phytotoxicity of HMs through the use of SP technology, this review discussed how different types of SP approaches help in diminishing the concentration HMs in plant systems. Also, we depicted mechanisms, represent how HMs transport and their actions on cellular levels, and emphasized, how diverse SP technology effectiveness in the mitigation of plants' phytotoxicity in unique ways.

Hydrogen Sulfide Alleviates Manganese Stress in Arabidopsis

Hydrogen sulfide (H₂S) has been shown to participate in various stress responses in plants, including drought, salinity, extreme temperatures, osmotic stress, and heavy metal stress. Manganese (Mn), as a necessary nutrient for plant growth, plays an important role in photosynthesis, growth, development, and enzymatic activation of plants. However, excessive Mn²⁺ in the soil can critically affect plant growth, particularly in acidic soil. In this study, the model plant Arabidopsis thaliana was used to explore the mechanism of H₂S participation and alleviation of Mn stress. First, using wild-type Arabidopsis with excessive Mn²⁺ treatment, the following factors were increased: H₂S content, the main H₂S synthetase L-cysteine desulfhydrase enzyme (AtLCD) activity, and the expression level of the AtLCD gene. Further, using the wild-type, AtLCD deletion mutant (lcd) and overexpression lines (OE5 and OE32) as materials, the phenotype of Arabidopsis seedlings was observed by exogenous application of hydrogen sulfide donor sodium hydrosulfide (NaHS) and scavenger hypotaurine (HT) under excessive Mn²⁺ treatment. The results showed that NaHS can significantly alleviate the stress caused by Mn²⁺, whereas HT aggravates this stress. The lcd mutant is more sensitive to Mn stress than the wild type, and the overexpression lines are more resistant. Moreover, the mechanism of H₂S alleviating Mn stress was determined. The Mn²⁺ content and the expression of the Mn transporter gene in the mutant were significantly higher than those of the wild-type and overexpression lines. The accumulation of reactive oxygen species was significantly reduced in NaHS-treated Arabidopsis seedlings and AtLCD overexpression lines, and the activities of various antioxidant enzymes (SOD, POD, CAT, APX) also significantly increased. In summary, H₂S is involved in the response of Arabidopsis to Mn stress and may alleviate the inhibition of Mn stress on Arabidopsis seedling growth by reducing Mn²⁺ content, reducing reactive oxygen species content, and enhancing antioxidant enzyme activity. This study provides an important basis for further study of plant resistance to heavy metal stress.

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.

Genome-Wide Identification and Expressional Profiling of the Metal Tolerance Protein Gene Family in Brassica napus

The Cation Diffusion Facilitator (CDF) family, also named Metal Tolerance Protein (MTP), is one of the gene families involved in heavy metal transport in plants. However, a comprehensive study of MTPs in Brassica napus has not been reported yet. In the present study, we identified 33 BnMTP genes from the rapeseed genome using bioinformatic analyses. Subsequently, we analyzed the phylogenetic relationship, gene structure, chromosome distribution, conserved domains, and motifs of the BnMTP gene family. The 33 BnMTPs were phylogenetically divided into three major clusters (Zn-CDFs, Fe/Zn-CDFs, and Mn-CDFs) and seven groups (group 1, 5, 6, 7, 8, 9, and 12). The structural characteristics of the BnMTP members were similar in the same group, but different among groups. Evolutionary analysis indicated that the BnMTP gene family mainly expanded through whole-genome duplication (WGD) and segmental duplication events. Moreover, the prediction of cis-acting elements and microRNA target sites suggested that BnMTPs might be involved in plant growth, development, and stress responses. In addition, we found the expression of 24 BnMTPs in rapeseed leaves or roots could respond to heavy metal ion treatments. These results provided an important basis for clarifying the biological functions of BnMTPs, especially in heavy metal detoxification, and will be helpful in the phytoremediation of heavy metal pollution in soil.

Identification and functional analysis of cation-efflux transporter 1 from Brassica juncea L

BACKGROUND

Brassica juncea behaves as a moderate-level accumulator of various heavy metal ions and is frequently used for remediation. To investigate the roles of metal ion transporters in B. juncea, a cation-efflux family gene, BjCET1, was cloned and functionally characterized.

RESULTS

BjCET1 contains 382 amino acid residues, including a signature motif of the cation diffusion facilitator protein family, six classic trans-membrane-spanning structures and a cation-efflux domain. A phylogenetic analysis showed that BjCET1 has a high similarity level with metal tolerance proteins from other Brassica plants, indicating that this protein family is highly conserved in Brassica. BjCET1 expression significantly increased at very early stages during both cadmium and zinc treatments. Green fluorescence detection in transgenic tobacco leaves revealed that BjCET1 is a plasma membrane-localized protein. The heterologous expression of BjCET1 in a yeast mutant increased the heavy-metal tolerance and decreased the cadmium or zinc accumulations in yeast cells, suggesting that BjCET1 is a metal ion transporter. The constitutive expression of BjCET1 rescued the heavy-metal tolerance capability of transgenic tobacco plants.

CONCLUSIONS

The data suggest that BjCET1 is a membrane-localized efflux transporter that plays essential roles in heavy metal ion homeostasis and hyper-accumulation.