Golgi localized barley MTP8 proteins facilitate Mn transport

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 Malus domestica metal tolerance protein MdMTP11.1 was involved in the detoxification of excess manganese in Arabidopsis thaliana

Ion homeostasis is maintained in plant cells by specialized transporters. However, functional studies on Mn transporters in apple trees have not been reported. MdMTP11.1, which encodes a putative Mn-MTP transporter in Malus domestica, was expressed highly in leaves and induced by Mn stress. Subcellular localization analysis of the MdMTP11.1-GFP fusion protein indicated that MdMTP11.1 was targeted to the Golgi. Meanwhile, overexpression of MdMTP11.1 in Arabidopsis thaliana conferred increased resistance to plants under toxic Mn levels, as evidenced by increased biomass of whole plant and length of primary root. Analysis of Mn bioaccumulation indicated that overexpression of MdMTP11.1 effectively reduced the content of Mn in every subcellular component and chemical forms when the plants were subjected with Mn stress. The majority of Mn of action were bound to cell wall and combined with un-dissolved phosphate. Besides, contents of malondialdehyde (MDA), proline and hydrogen peroxide (H2O2) were significantly lower, while content of chlorophyll and activities of CAT, SOD, POD and APX were significantly higher in MdMTP11.1-over-expressing plants compared with that in wild type plants under Mn stress. Taken together, these results suggest that MdMTP11.1 is a Mn specific transporter localized to the Golgi can maintain the phenotype, reduce the Mn accumulation and alleviate damage of oxidative stress, conferring the positive role of Mn tolerance.

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.

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.

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.

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.

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.

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.

Genome-wide Identification of Metal Tolerance Protein Genes in Peanut: Differential Expression in the Root of Two Contrasting Cultivars Under Metal Stresses

Metal tolerance proteins (MTP) are Me²⁺/H⁺(K⁺) antiporters that play important roles in the transport of divalent cations in plants. However, their functions in peanut are unknown. In the present study, a total of 24 AhMTP genes were identified in peanut, which were divided into seven groups belonging to three substrate-specific clusters (Zn-CDFs, Zn/Fe-CDFs, and Mn-CDFs). All AhMTP genes underwent whole genome or segmental gene duplication events except AhMTP12. Most AhMTP members within the same subfamily or group generally have similar gene and protein structural characteristics. However, some genes, such as AhMTP1.3, AhMTP2.4, and AhMTP12, showed wide divergences. Most of AhMTP genes preferentially expressed in reproductive tissues, suggesting that these genes might play roles in metal transport during the pod and seed development stages. Excess metal exposure induced expressions for most of AhMTP genes in peanut roots depending on cultivars. By contrast, AhMTP genes in the root of Fenghua 1 were more sensitive to excess Fe, Cd, and Zn exposure than that of Silihong. Stepwise linear regression analysis showed that the percentage of Fe in shoots significantly and positively correlated with the expression of AhMTP4.1, AhMTP9.1, and AhMTPC4.1, but negatively correlated with that of AhMTPC2.1 and AhMTP12. The expression of AhMTP1.1 showed a significant and negative correlation with the percentage of Mn in shoots. The percentage of Zn in shoots was significantly and positively correlated with the expression of AhMTP2.1 but was negatively correlated with that of AhMTPC2.1. The differential responses of AhMTP genes to metal exposure might be, at least partially, responsible for the different metal translocation from roots to shoots between Fenghua 1 and Silihong.

Metal Tolerance Protein Encoding Gene Family in Fagopyrum tartaricum: Genome-Wide Identification, Characterization and Expression under Multiple Metal Stresses

Metal tolerance proteins (MTP) as divalent cation transporters are essential for plant metal tolerance and homeostasis. However, the characterization and the definitive phylogeny of the MTP gene family in Fagopyrum tartaricum, and their roles in response to metal stress are still unknown. In the present study, MTP genes in Fagopyrum tartaricum were identified, and their phylogenetic relationships, structural characteristics, physicochemical parameters, as well as expression profiles under five metal stresses including Fe, Mn, Cu, Zn, and Cd were also investigated. Phylogenetic relationship analysis showed that 12 Fagopyrum tartaricum MTP genes were classified into three major clusters and seven groups. All FtMTPs had typical structural features of the MTP gene family and were predicted to be located in the cell vacuole. The upstream region of FtMTPs contained abundant cis-acting elements, implying their functions in development progress and stress response. Tissue-specific expression analysis results indicated the regulation of FtMTPs in the growth and development of Fagopyrum tataricum. Besides, the expression of most FtMTP genes could be induced by multiple metals and showed different expression patterns under at least two metal stresses. These findings provide useful information for the research of the metal tolerance mechanism and genetic improvement of Fagopyrum tataricum.

Manganese transport in mammals by zinc transporter family proteins, ZNT and ZIP

Manganese (Mn) is an essential trace element required for various biological processes. However, excess Mn causes serious side effects in humans, including parkinsonism. Thus, elucidation of Mn homeostasis at the systemic, cellular, and molecular levels is important. Many metal transporters and channels can be involved in the transport and homeostasis of Mn, and an increasing body of evidence shows that several zinc (Zn) transporters belonging to the ZIP and ZNT families, specifically, ZNT10, ZIP8, and ZIP14, play pivotal roles in Mn metabolism. Mutations in the genes encoding these transporter proteins are associated with congenital disorders related to dysregulated Mn homeostasis in humans. Moreover, single nucleotide polymorphisms of ZIP8 are associated with multiple clinical phenotypes. In this review, we discuss the recent literature on the structural and biochemical features of ZNT10, ZIP8, and ZIP14, including transport mechanisms, regulation of expression, and pathophysiological functions. Because a disturbance in Mn homeostasis is closely associated with a variety of phenotypes and risk of human diseases, these transporters constitute a significant target for drug development. An understanding of the roles of these key transporters in Mn metabolism should provide new insights into pharmacological applications of their inhibitors and enhancers in human diseases.

Characterization of Metal Tolerance Proteins and Functional Analysis of GmMTP8.1 Involved in Manganese Tolerance in Soybean

Manganese is an essential micronutrient for plant growth but can be toxic to plants when it reaches excessive levels. Although metal tolerance proteins (MTPs), which belong to the cation diffusion facilitator (CDF) family, have been demonstrated to play critical roles in manganese (Mn) tolerance in plants, the characteristics and functions of GmMTP members in the response of soybean (Glycine max) to Mn toxicity have not been documented. In this study, growth inhibition was observed in soybean plants that were exposed to a toxic level of Mn in hydroponics, as reflected by the generation of brown spots, and decreased leaf chlorophyll concentration and plant fresh weight. Subsequent genome-wide analysis resulted in the identification of a total of 14 GmMTP genes in the soybean genome. Among these GmMTPs, 9 and 12 were found to be regulated by excess Mn in leaves and roots, respectively. Furthermore, the function of GmMTP8.1, a Mn-CDF homologue of ShMTP8 identified in the legume Stylosanthes hamata that is involved in Mn detoxification, was characterized. Subcellular localization analysis showed that GmMTP8.1 was localized to the endoplasmic reticulum (ER). Heterologous expression of GmMTP8.1 led to the restoration of growth of the Mn-hypersensitive yeast (Saccharomyces cerevisiae) mutant Δpmr1, which is made defective in Mn transport into the Golgi apparatus by P-type Ca/Mn-ATPase. Furthermore, GmMTP8.1 overexpression conferred tolerance to the toxic level of Mn in Arabidopsis (Arabidopsis thaliana). Under excess Mn conditions, concentrations of Mn in shoots but not roots were decreased in transgenic Arabidopsis, overexpressing GmMTP8.1 compared to the wild type. The overexpression of GmMTP8.1 also led to the upregulation of several transporter genes responsible for Mn efflux and sequestration in Arabidopsis, such as AtMTP8/11. Taken together, these results suggest that GmMTP8.1 is an ER-localized Mn transporter contributing to confer Mn tolerance by stimulating the export of Mn out of leaf cells and increasing the sequestration of Mn into intracellular compartments.

Characterization of the gene family encoding metal tolerance proteins in Triticum urartu: Phylogenetic, transcriptional, and functional analyses

Homeostasis of microelements in organisms is vital for normal metabolism. In plants, the cation diffusion facilitator (CDF) protein family, also known as metal tolerance proteins (MTPs), play critical roles in maintaining trace metal homeostasis. However, little is known about these proteins in wheat. In this study, we characterized the MTP family of Triticum urartu, the donor of 'A' genome of Triticum aestivum, and analysed their phylogenetic relationships, sequence signatures, spatial expression patterns in the diploid wheat, and their transport activity when heterologously expressed. Nine MTPs were identified in the T. urartu genome database, and were classified and designated based on their sequence similarity to Arabidopsis thaliana (Arabidopsis) and Oryza sativa MTPs. Phylogenetic and sequence analyses indicated that the triticum urartu metal tolerance protein (TuMTP)s comprise three Zn-CDFs, two Fe/Zn-CDFs, and four Mn-CDFs; and can be further classified into six subgroups. Among the TuMTPs, there are no MTP2-5 and MTP9-10 counterparts but two MTP1/8/11 orthologs in relation to AtMTPs. It was also shown that members of the same cluster share similar sequence characteristic, i.e. number of introns, predicted transmembrane domains, and motifs. When expressed in yeast, TuMTP1 and TuMTP1.1 conferred tolerance to Zn and Co but not to other metal ions; while TuMTP8, TuMTP8.1, TuMTP11, and TuMTP11.1 conferred tolerance to Mn. When expressed in Arabidopsis, TuMTP1 localized to the tonoplast and significantly enhanced Zn and Co tolerance. TuMTPs showed diverse tissue-specific expression patterns. Taken together, the closely clustered TuMTPs share structural features and metal specificity but play diverse roles in the homeostasis of microelements in plant cells.

Tonoplast-associated calcium signaling regulates manganese homeostasis in Arabidopsis

Manganese (Mn) is an essential micronutrient in plants. However, excessive Mn absorption in acidic and waterlogged soils can lead to Mn toxicity. Despite their essential roles in Mn homeostasis, transcriptional and post-transcriptional modifications of Mn transporters remain poorly understood. Here, we demonstrated that high-Mn stress induces an obvious Ca²⁺ signature in Arabidopsis. We identified four calcium-dependent protein kinases, CPK4/5/6/11, that interact with the tonoplast-localized Mn and iron (Fe) transporter MTP8 in vitro and in vivo. The cpk4/5/6/11 quadruple mutant displayed a dramatic high-Mn-sensitive phenotype similar to that of the mtp8 mutant. CPKs phosphorylated the N-terminal domain of MTP8 primarily at the Ser31 and Ser32 residues. Transport assays combined with multiple physiological experiments on phospho-dead variant MTP8^S31/32A and phospho-mimetic variant MTP8^S31/32D plants under different Mn and Fe conditions suggested that Ser31 and Ser32 are crucial for MTP8 function. In addition, genetic analysis showed that CPKs functioned upstream of MTP8. In summary, we identified a tonoplast-associated calcium signaling cascade that orchestrates Mn homeostasis and links Mn toxicity, Ca²⁺ signaling, and Mn transporters. These findings provide new insight into Mn homeostasis mechanisms and Ca²⁺ signaling pathways in plants, providing potential targets for engineering heavy metal toxicity-tolerant plants.

Roles of subcellular metal homeostasis in crop improvement

Improvement of crop production in response to rapidly changing environmental conditions is a serious challenge facing plant breeders and biotechnologists. Iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu) are essential micronutrients for plant growth and reproduction. These minerals are critical to several cellular processes including metabolism, photosynthesis, and cellular respiration. Regulating the uptake and distribution of these minerals could significantly improve plant growth and development, ultimately leading to increased crop production. Plant growth is limited by mineral deficiency, but on the other hand, excess Fe, Mn, Cu, and Zn can be toxic to plants; therefore, their uptake and distribution must be strictly regulated. Moreover, the distribution of these metals among subcellular organelles is extremely important for maintaining optimal cellular metabolism. Understanding the mechanisms controlling subcellular metal distribution and availability would enable development of crop plants that are better adapted to challenging and rapidly changing environmental conditions. Here, we describe advances in understanding of subcellular metal homeostasis, with a particular emphasis on cellular Fe homeostasis in Arabidopsis and rice, and discuss strategies for regulating cellular metabolism to improve plant production.

Expression of a Brassica napus metal transport protein (BnMTP3) in Arabidopsis thaliana confers tolerance to Zn and Mn

The essential micronutrient elements zinc (Zn) and manganese (Mn) are crucial for plant growth and development. As an important oil crop, the yield and quality of rapeseed are affected by Zn and Mn toxicity. The cation diffusion facilitator (CDF) family of proteins play significant roles in maintaining intracellular ionic homeostasis and tolerance in plants. However, research on CDF proteins in rapeseed is lacking. In this study, the function of a Brassica napus cation diffusion facilitator/ metal tolerance protein (CDF/MTP) was investigated. The protein, abbreviated BnMTP3 is homologous to the Arabidopsis thaliana MTP3 (AtMTP3). Heterologous expression of BnMTP3 in yeast enhanced tolerance and intracellular sequestration of Zn and Mn. Expression of BnMTP3 in A. thaliana increased Zn and Mn tolerance and markedly increased Zn accumulation in roots. Quantitative RT-PCR analysis showed that BnMTP3 is primarily expressed in roots, and subcellular localization suggested that BnMTP3 is localized in the trans-Golgi network (TGN) and the prevacuolar compartment (PVC) in Arabidopsis and rape protoplast. After treatment with Zn and Mn, BnMTP3 was observed on the vacuolar membrane in transgenic Arabidopsis lines. These findings suggest that BnMTP3 confers Zn and Mn tolerance by sequestering Zn and/or Mn into the vacuole.

Pear metal transport protein PbMTP8.1 confers manganese tolerance when expressed in yeast and Arabidopsis thaliana

Manganese (Mn) is demonstrated to be essential for plants. Ion homeostasis is maintained in plant cells by specialized transporters. PbMTP8.1, which encodes a putative Mn-CDF transporter in Pyrus bretschneideri Rehd, was expressed mainly in leaves and complemented the Mn hypersensitivity of the Mn-sensitive yeast mutant △pmr1 in previous research conducted by our laboratory. In the present study, we report that the expression of PbMTP8.1 can enhance Mn tolerance and accumulation in Saccharomyces cerevisiae. Subcellular localization analysis of the PbMTP8.1-GFP fusion protein indicated that PbMTP8.1 was targeted to the pre-vacuolar compartment (PVC). In addition, the overexpression of PbMTP8.1 in Arabidopsis thaliana conferred increased resistance to plants under toxic Mn levels, as indicated by increased fresh and dry weights of shoots and roots. Mn accumulation in vacuoles of PbMTP8.1-overexpressing plants was significantly increased when compared with that in wild-type plants under Mn stress. This suggests that a considerable proportion of Mn enters into the vacuoles through a PbMTP8.1-dependent mechanism. Taken together, these results indicate PbMTP8.1 is a Mn-specific transporter that is localized to the PVC, and confers Mn tolerance by sequestering Mn into the vacuole.

Molecular mechanisms of plant adaptive responses to heavy metals stress

Heavy metals (HMs) are among the main environmental pollutants that can enter the soil, water bodies, and the atmosphere as a result of natural processes (weathering of rocks, volcanic activity), and also as a result of human activities (mining, metallurgical and chemical industries, transport, application of mineral fertilizers). Plants counteract the HMs stresses through morphological and physiological adaptations, which are imparted through well-coordinated molecular mechanisms. New approaches, which include transcriptomics, genomics, proteomics, and metabolomics analyses, have opened the paths to understand such complex networks. This review sheds light on molecular mechanisms included in plant adaptive and defense responses during metal stress. It is focused on the entry of HMs into plants, its transport and accumulation, effects on the main physiological processes, gene expressions included in plant adaptive and defense responses during HM stress. Analysis of new data allowed the authors to conclude that the most important mechanism of HM tolerance is extracellular and intracellular HM sequestration. Organic anions (malate, oxalate, etc.) provide extracellular sequestration of HM ions. Intracellular HM sequestration depends not only on a direct binding mechanism with different polymers (pectin, lignin, cellulose, hemicellulose, etc.) or organic anions but also on the action of cellular receptors and transmembrane transporters. We focused on the functioning chloroplasts, mitochondria, and the Golgi complex under HM stress. The currently known molecular mechanisms of plant tolerance to the toxic effects of HMs are analyzed.

Identification of MTP gene family in tea plant (Camellia sinensis L.) and characterization of CsMTP8.2 in manganese toxicity

Cation diffusion facilitators (CDFs) play central roles in metal homeostasis and tolerance in plants, but the specific functions of Camellia sinensis CDF-encoding genes and the underlying mechanisms remain unknown. Previously, transcriptome sequencing results in our lab indicated that the expression of CsMTP8.2 in tea plant shoots was down-regulated exposed to excessive amount of Mn²⁺ conditions. To elucidate the possible mechanisms involved, we systematically identified 13 C. sinensis CsMTP genes from three subfamilies and characterized their phylogeny, structures, and the features of the encoded proteins. The transcription of CsMTP genes was differentially regulated in C. sinensis shoots and roots in responses to high concentrations of Mn, Zn, Fe, and Al. Differences in the cis-acting regulatory elements in the CsMTP8.1 and CsMTP8.2 promoters suggested the expression of these two genes may be differentially regulated. Transient expression analysis indicated that CsMTP8.2 was localized to the plasma membrane in tobacco and onion epidermal cells. Moreover, when heterologously expressed in yeast, CsMTP8.2 conferred tolerance to Ni and Mn but not to Zn. Additionally, heterologous expression of CsMTP8.2 in Arabidopsis thaliana revealed that CsMTP8.2 positively regulated the response to manganese toxicity by decreasing the accumulation of Mn in plants. However, there was no difference in the accumulation of other metals, including Cu, Fe, and Zn. These results suggest that CsMTP8.2 is a Mn-specific transporter that contributes to the efflux of excess Mn²⁺ from plant cells.

Manganese in Plants: From Acquisition to Subcellular Allocation

Manganese (Mn) is an important micronutrient for plant growth and development and sustains metabolic roles within different plant cell compartments. The metal is an essential cofactor for the oxygen-evolving complex (OEC) of the photosynthetic machinery, catalyzing the water-splitting reaction in photosystem II (PSII). Despite the importance of Mn for photosynthesis and other processes, the physiological relevance of Mn uptake and compartmentation in plants has been underrated. The subcellular Mn homeostasis to maintain compartmented Mn-dependent metabolic processes like glycosylation, ROS scavenging, and photosynthesis is mediated by a multitude of transport proteins from diverse gene families. However, Mn homeostasis may be disturbed under suboptimal or excessive Mn availability. Mn deficiency is a serious, widespread plant nutritional disorder in dry, well-aerated and calcareous soils, as well as in soils containing high amounts of organic matter, where bio-availability of Mn can decrease far below the level that is required for normal plant growth. By contrast, Mn toxicity occurs on poorly drained and acidic soils in which high amounts of Mn are rendered available. Consequently, plants have evolved mechanisms to tightly regulate Mn uptake, trafficking, and storage. This review provides a comprehensive overview, with a focus on recent advances, on the multiple functions of transporters involved in Mn homeostasis, as well as their regulatory mechanisms in the plant's response to different conditions of Mn availability.

Advances in the Mechanisms of Plant Tolerance to Manganese Toxicity

Manganese (Mn) is an essential element for plant growth due to its participation in a series of physiological and metabolic processes. Mn is also considered a heavy metal that causes phytotoxicity when present in excess, disrupting photosynthesis and enzyme activity in plants. Thus, Mn toxicity is a major constraint limiting plant growth and production, especially in acid soils. To cope with Mn toxicity, plants have evolved a wide range of adaptive strategies to improve their growth under this stress. Mn tolerance mechanisms include activation of the antioxidant system, regulation of Mn uptake and homeostasis, and compartmentalization of Mn into subcellular compartments (e.g., vacuoles, endoplasmic reticulum, Golgi apparatus, and cell walls). In this regard, numerous genes are involved in specific pathways controlling Mn detoxification. Here, we summarize the recent advances in the mechanisms of Mn toxicity tolerance in plants and highlight the roles of genes responsible for Mn uptake, translocation, and distribution, contributing to Mn detoxification. We hope this review will provide a comprehensive understanding of the adaptive strategies of plants to Mn toxicity through gene regulation, which will aid in breeding crop varieties with Mn tolerance via genetic improvement approaches, enhancing the yield and quality of crops.

Ancient barley landraces adapted to marginal soils demonstrate exceptional tolerance to manganese limitation

BACKGROUND AND AIMS

Micronutrient deficiency in cereals is a problem of global significance, severely reducing grain yield and quality in marginal soils. Ancient landraces represent, through hundreds of years of local adaptation to adverse soil conditions, a unique reservoir of genes and unexplored traits for enhancing yield and abiotic stress tolerance. Here we explored and compared the genetic variation in a population of Northern European barley landraces and modern elite varieties, and their tolerance to manganese (Mn) limitation.

METHODS

A total of 135 barley accessions were genotyped and the genetic diversity was explored using Neighbor-Joining clustering. Based on this analysis, a sub-population of genetically diverse landraces and modern elite control lines were evaluated phenotypically for their ability to cope with Mn-deficient conditions, across three different environments increasing in complexity from hydroponics through pot experiments to regional field trials.

KEY RESULTS

Genetically a group of Scottish barley landraces (Bere barley) were found to cluster according to their island of origin, and accessions adapted to distinct biogeographical zones with reduced soil fertility had particularly larger Mn, but also zinc (Zn) and copper (Cu) concentrations in the shoot. Strikingly, when grown in an alkaline sandy soil in the field, the locally adapted landraces demonstrated an exceptional ability to acquire and translocate Mn to developing leaves, maintain photosynthesis and generate robust grain yields, whereas modern elite varieties totally failed to complete their life cycle.

CONCLUSIONS

Our results highlight the importance of gene pools of local adaptation and the value of ancient landrace material to identify and characterize genes that control nutrient use efficiency traits in adverse environments to raise future crop production and improve agricultural sustainability in marginal soils. We propose and discuss a model summarizing the physiological mechanisms involved in the complex trait of tolerance to Mn limitation.

Monochromatic lights-induced trends in antioxidant and antidiabetic polyphenol accumulation in in vitro callus cultures of Lepidium sativum L

Lepidium sativum L. is an important edible, herbaceous plant with huge medicinal value as cardio-protective, hepatoprotective and antitumor agent. This study was designed and performed to investigate biosynthesis of plant's active ingredients in callus cultures of L. sativum in response to the exposure of multi spectral lights. Optimum biomass accumulation (15.36 g/L DW), total phenolic and flavonoid contents (TPC; 47.43 mg/g; TFC; 9.41 mg/g) were recorded in calli placed under white light (24 h) compared to rest of the treatments. Antioxidant enzymatic activities i.e. superoxide dismutase and peroxidase were found optimum in cultures exposed to green light (SOD; 0.054 nM/min/mg FW, POD; 0.501 nM/min/mg FW). Phytochemical analysis further confirmed the potential influence of white light exposure on enhanced production of plant's metabolites. Significant enhancement level of major metabolic compounds such as chlorogenic acid (7.20 mg/g DW), quercetin (22.08 mg/g DW), kaempferol (7.77 mg/g DW) and minor compounds including ferulic acid, sinapic acid, protocatechuic acid, vanillic acid and caffeic acid were recorded in white light compared to control (photoperiod), whereas blue light increased the p-coumaric acid accumulation. Moreover, callus cultures of this plant under white light (24 h) showed highest in vitro based anti-diabetic and antioxidant activities compared to other conditions. Finding of our current study revealed that multi spectral lights are proved to be an effective strategy for enhancing metabolic quantity of antioxidant and anti-diabetic bioactive compounds in callus cultures of L. sativum L.

Genome-wide identification and characterization of the metal tolerance protein (MTP) family in grape (Vitis vinifera L.)

Metal tolerance proteins (MTPs) play an important role in the transport of metals at the cellular, tissue and whole plant levels. In the present study, 11 MTP genes were identified and these clustered in three major sub-families Fe/Zn-MTP, Zn-MTP, and Mn-MTP, and seven groups, which are similar to the grouping of MTP genes in both Arabidopsis and rice. Vitis vinifera metal tolerance proteins (VvMTP) ranged from 366 to 1092 amino acids, were predicted to be located in the cell vacuole, and had four to six putative TMDs, except for VvtMTP12 and VvMTP1. The VvMTPs had putative cation diffusion facilitator (CDF) domains and the putative Mn-MTPs also had zinc transporter dimerization domains (ZD-domains). V. vinifera Mn-MTPs had gene structures and motif distributions similar to those of the Fe/Zn-MTP and Zn-MTP sub-families. The upstream regions of VvMTP genes had variable frequencies of cis-regulatory elements that could indicate regulation at different developmental stages and/or differential regulation in response to stress. Comparison of the VvMTP coding sequences with known miRNAs found in various plant species indicated the presence of 13 putative miRNAs, with 7 of these associated with VvMTPs. Temporal and spatial expression profiling indicates a potential role for VvMTP genes during growth and development in grape plants, as well as the involvement of these genes in plant responses to environmental stress, especially osmotic stress. The data generated from this study provides a basis for further investigation of the roles of MTP genes in grapes.

The maize bHLH transcription factor bHLH105 confers manganese tolerance in transgenic tobacco

Manganese (Mn) toxicity is an important limiting factor for crop production in acidic soils. The basic helix-loop-helix (bHLH) transcription factors are involved in a variety of physiological processes. However, whether the bHLHs are involved in excess Mn stress response is largely unknown. Here, we report the functional characterization of ZmbHLH105 isolated from maize (Zea mays). The transcript levels of ZmbHLH105 were higher in leaves, and were markedly up-regulated under excess Mn stress in maize. ZmbHLH105 was localized in the nucleus with transactivation activity. Ectopic expression of ZmbHLH105 enhanced Mn tolerance in Saccharomyces cerevisiae cells. ZmbHLH105-overexpressing (OE) plants showed improved excess Mn tolerance in transgenic tobacco. The stress-tolerant phenotypes of these OE tobacco lines were accompanied by increases of key antioxidant enzyme activities, but decreases of reactive oxygen species (ROS) accumulations. Importantly, the OE plants had less increases than the wild-type in toxic Mn accumulation. Moreover, the transcript levels of Mn/Fe-related transporters in the OE lines displayed remarkable decreases compared with the wild-type under Mn stress, suggesting that ZmbHLH105 reduced Mn accumulation in plants largely by repressing expression of Mn/Fe-regulated transporter genes. Taken together, these results indicate that ZmbHLH105 confers improved Mn stress tolerance possibly by regulating antioxidant machinery-mediated ROS scavenging and expression of Mn/Fe-related transporters in plants. ZmbHLH105 could be exploited for developing drought-tolerant maize varieties.

Silicon Alleviates Iron Deficiency in Barley by Enhancing Expression of Strategy II Genes and Metal Redistribution

The beneficial effects of silicon (Si) have been shown on plants using reduction-based strategy for iron (Fe) acquisition. Here we investigated the influence of Si on Fe deficiency stress alleviation in barley (Hordeum vulgare), a crop plant which uses the chelation-based strategy for Fe acquisition. Analyses of chlorophyll content, ROS accumulation, antioxidative status, concentrations of Fe and other micronutrients, along with the expression of Strategy II genes were studied in response to Si supply. Si successfully ameliorated Fe deficiency in barley, diminishing chlorophyll and biomass loss, and improving the activity of antioxidative enzymes, resulting in lowered reactive oxidative species accumulation in the youngest leaves. Alleviation of Fe deficiency stress correlated well with the Si-induced increase of Fe content in the youngest leaves, while it was decreased in root. Moreover, Si nutrition lowered accumulation of other micronutrients in the youngest leaves of Fe deprived plants, by retaining them in the root. On the transcriptional level, Si led to an expedient increase in the expression of genes involved in Strategy II Fe acquisition in roots at the early stage of Fe deficiency stress, while decreasing their expression in a prolonged stress response. Expression of Strategy II genes was remarkably upregulated in the leaves of Si supplied plants. This study broadens the perspective of mechanisms of Si action, providing evidence for ameliorative effects of Si on Strategy II plants, including its influence on accumulation and distribution of microelements, as well as on the expression of the Strategy II genes.

Metal tolerance protein MTP6 affects mitochondrial iron and manganese homeostasis in cucumber

Members of the cation diffusion facilitator (CDF) family have been identified in all kingdoms of life. They have been divided into three subgroups, namely Zn-CDF, Fe/Zn-CDF, and Mn-CDF, based on their putative specificity to transported metal ions. The plant metal tolerance protein 6 (MTP6) proteins fall into the Fe/Zn-CDF subgroup; however, their function in iron/zinc transport has not yet been confirmed. Here, we characterized the MTP6 protein from cucumber, Cucumis sativus. When expressed in yeast and in protoplasts isolated from Arabidopsis cells, CsMTP6 localized in mitochondria and contributed to the efflux of Fe and Mn from these organelles. Immunolocalization of CsMTP6 in cucumber membranes confirmed this association with mitochondria. Root expression and protein levels of CsMTP6 were significantly up-regulated in conditions of Fe deficiency and excess, but were not affected by Mn availability. These results indicate that MTP6 proteins contribute to the distribution of Fe and Mn between the cytosol and mitochondria of plant cells, and are regulated by Fe to maintain mitochondrial and cytosolic iron homeostasis under varying conditions of Fe availability.

Synergistic effects of melatonin and distinct spectral lights for enhanced production of anti-cancerous compounds in callus cultures of Fagonia indica

Fagonia indica is one of the commercially vital medicinal plant species. It is well-known for biosynthesis of anticancer phenolics and flavonoids metabolites. The plant has been exploited for in vitro studies and production of vital phytochemicals, however, the synergistic effects of melatonin and lights remains to be investigated. In current study, we have evaluated the synergistic effects of melatonin and different light emitting diodes (LEDs) in callus cultures of F. indica. Both, light and melatonin play vital role in physiological and biochemical processes of plant cell. The highest Fresh weight (FW: 320 g/L) and Dry weight (DW: 20 g/L) was recorded in cultures under white LEDs. Optimum total phenolics content (11.3 μg GAE/mg), total flavonoids content (4.02 μg QAE/mg) and Free radical scavenging activity (97%) was found in cultures grown under white LED and melatonin. Furthermore, cultures maintained under white light were also found with highest levels of phenolic and flavonoids production (total phenolic production; 226.9 μg GAE/mg, Total flavonoid production; 81 μg QAE/mg) than other LED-grown cultures. However, the antioxidant enzymes; Superoxide dismutase (SOD: 0.53 nM/min/mg FW) and Peroxidase (POD:1.18 nM/min/mg FW) were found optimum in cultures grown under blue LED. The HPLC data showed that enhanced total production of metabolites was recorded in cultures under white LED (6.765 μg/mg DW) than other lights and control. The findings of this study comprehend the role of melatonin and influence of light quality on biomass accumulation and production of phytochemicals in callus cultures of F. indica.

Cucumber metal tolerance protein 7 (CsMTP7) is involved in the accumulation of Fe in mitochondria under Fe excess

The plant metal tolerance protein family (MTP) includes 12 members that have been classified into three phylogenetically different subgroups - Zn-cation diffusion facilitator (CDF), Fe/Zn-CDF and Mn-CDF - based on their putative metal specificity. To date, only members belonging to the Zn-CDF or Mn-CDF group have been characterized functionally. The plant Fe/Zn-CDF subgroup includes two proteins, MTP6 and MTP7, but their function and metal specificity have not been confirmed. In this study we showed that cucumber CsMTP7 is a highly specific mitochondrial Fe importer that is able to confer yeast tolerance to Fe excess through increased accumulation of Fe in the mitochondria. We also demonstrated that CsMTP7 contributes to the increased accumulation of Fe in the mitochondria of Arabidopsis thaliana protoplasts. The transcripts and mitochondrial levels of CsMTP7 and ferritin - the iron-storing protein - are significantly increased in cucumber roots in response to Fe excess. This finding suggests that CsMTP7 and ferritin work in concert to accumulate Fe in plant mitochondria. As genes that encode orthologous proteins have been identified in phylogenetically distant organisms, including Archaea, cyanobacteria, humans and plants, but not in yeast, we concluded that the MTP7-mediated mitochondrial Fe accumulation may be conserved in the species, and express mitochondrial ferritin for mitochondrial Fe storage.

OsMTP11, a trans-Golgi network localized transporter, is involved in manganese tolerance in rice

Metal tolerance proteins (MTPs) belong to the cation diffusion facilitator family (CDF) and have been implicated in metal transport and homeostasis in different plant species. Here we report on the rice gene OsMTP11 that encodes a putative CDF transporter that is homologous to members of the Mn-CDF cluster. The expression of OsMTP11 was found to enhance Mn tolerance in the Mn-sensitive yeast mutant pmr1. Knockdown of OsMTP11 resulted in growth inhibition in the presence of high concentrations of Mn, and also led to increased accumulation of Mn in the shoots and roots. The overexpression of OsMTP11 was found to enhance Mn tolerance in rice, and under supplementation with a toxic level of Mn, decreased Mn concentration was observed in the shoots and roots. Subcellular localization in rice protoplasts and tobacco epidermal cells revealed that OsMTP11 localizes to the trans-Golgi network (TGN), and a significant relocalization to the plasma membrane can be triggered by high extracellular Mn in tobacco epidermal cells. These findings suggest that OsMTP11 is a TGN-localized Mn transporter that is required for Mn homeostasis and contributes towards Mn tolerance in rice.

Trace metal metabolism in plants

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

Genome-wide association study to identify candidate loci and genes for Mn toxicity tolerance in rice

Manganese (Mn) is an essential micro-nutrient for plants, but flooded rice fields can accumulate high levels of Mn²⁺ leading to Mn toxicity. Here, we present a genome-wide association study (GWAS) to identify candidate loci conferring Mn toxicity tolerance in rice (Oryza sativa L.). A diversity panel of 288 genotypes was grown in hydroponic solutions in a greenhouse under optimal and toxic Mn concentrations. We applied a Mn toxicity treatment (5 ppm Mn²⁺, 3 weeks) at twelve days after transplanting. Mn toxicity caused moderate damage in rice in terms of biomass loss and symptom formation despite extremely high shoot Mn concentrations ranging from 2.4 to 17.4 mg g^-1. The tropical japonica subpopulation was more sensitive to Mn toxicity than other subpopulations. Leaf damage symptoms were significantly correlated with Mn uptake into shoots. Association mapping was conducted for seven traits using 416741 single nucleotide polymorphism (SNP) markers using a mixed linear model, and detected six significant associations for the traits shoot manganese concentration and relative shoot length. Candidate regions contained genes coding for a heavy metal transporter, peroxidase precursor and Mn²⁺ ion binding proteins. The significant marker SNP-2.22465867 caused an amino acid change in a gene (LOC_Os02g37170) with unknown function. This study demonstrated significant natural variation in rice for Mn toxicity tolerance and the possibility of using GWAS to unravel genetic factors responsible for such complex traits.

Intracellular Distribution of Manganese by the Trans-Golgi Network Transporter NRAMP2 Is Critical for Photosynthesis and Cellular Redox Homeostasis

Plants require trace levels of manganese (Mn) for survival, as it is an essential cofactor in oxygen metabolism, especially O2 production via photosynthesis and the disposal of superoxide radicals. These processes occur in specialized organelles, requiring membrane-bound intracellular transporters to partition Mn between cell compartments. We identified an Arabidopsis thaliana member of the NRAMP family of divalent metal transporters, NRAMP2, which functions in the intracellular distribution of Mn. Two knockdown alleles of NRAMP2 showed decreased activity of photosystem II and increased oxidative stress under Mn-deficient conditions, yet total Mn content remained unchanged. At the subcellular level, these phenotypes were associated with a loss of Mn content in vacuoles and chloroplasts. NRAMP2 was able to rescue the mitochondrial yeast mutant mtm1∆ In plants, NRAMP2 is a resident protein of the trans-Golgi network. NRAMP2 may act indirectly on downstream organelles by building up a cytosolic pool that is used to feed target compartments. Moreover, not only does the nramp2 mutant accumulate superoxide ions, but NRAMP2 can functionally replace cytosolic superoxide dismutase in yeast, indicating that the pool of Mn displaced by NRAMP2 is required for the detoxification of reactive oxygen species.

OsMTP11 is localised at the Golgi and contributes to Mn tolerance

Membrane transporters play a key role in obtaining sufficient quantities of manganese (Mn) but also in protecting against Mn toxicity. We have characterized OsMTP11, a member of the Cation Diffusion Facilitator/Metal Tolerance Protein (CDF/MTP) family of metal cation transporters in Oryza sativa. We demonstrate that OsMTP11 functions in alleviating Mn toxicity as its expression can rescue the Mn-sensitive phenotype of the Arabidopsis mtp11-3 knockout mutant. When expressed stably in Arabidopsis and transiently in rice and tobacco, it localises to the Golgi. OsMTP11 partially rescues the Mn-hypersensitivity of the pmr1 yeast mutant but only slightly alleviates the Zn sensitivity of the zrc1 cot1 yeast mutant. Overall, these results suggest that OsMTP11 predominantly functions as a Mn-transporting CDF with lower affinity for Zn. Site-directed mutagenesis studies revealed four substitutions in OsMTP11 that appear to alter its transport activity. OsMTP11 harbouring a substitution of leucine 150 to a serine fully rescued pmr1 Mn-sensitivity at all concentrations tested. The other substitutions, including those at conserved DxxxD domains, reduced complementation of pmr1 to different levels. This indicates their importance for OsMTP11 function and is a starting point for refining transporter activity/specificity.

The Tonoplast-Localized Transporter MTP8.2 Contributes to Manganese Detoxification in the Shoots and Roots of Oryza sativa L

Manganese (Mn) cation diffusion facilitators (Mn-CDFs) play important roles in the Mn homeostasis of plants. In rice, the tonoplast-localized Mn-CDF metal tolerance protein 8.1 (MTP8.1) is involved in Mn detoxification in the shoots. This study functionally characterized the Mn-CDF MTP8.2 and determined its contribution to Mn tolerance. MTP8.2 was found to share 68% identity with MTP8.1 and was expressed in both the shoots and roots, but its transcription level was lower than that of MTP8.1. Transient expression of the MTP8.2:green fluorescent protein (GFP) fusion protein and immunoblotting studies indicated that MTP8.2 was also localized to the tonoplast. MTP8.2 expression in yeast conferred tolerance to Mn but not to Fe, Zn, Co, Ni or Cd. MTP8.2 knockdown caused further growth reduction of shoots and roots in the mtp8.1 mutant, which already exhibits stunted growth under conditions of excess Mn. In the presence of high Mn, the MTP8.2 knockdown lines of the mtp8.1 mutant showed lower root Mn concentrations, as well as lower root:total Mn ratios, than those of wild-type rice and the mtp8.1 mutant. These findings indicate that MTP8.2 mediates Mn tolerance along with MTP8.1 through the sequestration of Mn into the shoot and root vacuoles.

Genome-wide identification of sweet orange (Citrus sinensis) metal tolerance proteins and analysis of their expression patterns under zinc, manganese, copper, and cadmium toxicity

Plant metal tolerance proteins (MTPs) play important roles in heavy metal homeostasis; however, related information in citrus plants is limited. Citrus genome sequencing and assembly have enabled us to perform a systematic analysis of the MTP gene family. We identified 12 MTP genes in sweet orange, which we have named as CitMTP1 and CitMTP3 to CitMTP12 based on their sequence similarity to Arabidopsis thaliana MTPs. The CitMTPs were predicted to encode proteins of 864 to 2556 amino acids in length that included 4 to 6 putative transmembrane domains (TMDs). Furthermore, all the CitMTPs contained a highly conserved signature sequence encompassing the TMD-II and the start of the TMD-III. Phylogenetic analysis further classified the CitMTPs into Fe/Zn-MTP, Mn-MTP, and Zn-MTP subgroups, which coincided with the MTPs of A. thaliana and rice. The closely clustered CitMTPs shared a similar gene structure. Expression analysis indicated that most CitMTP transcripts were upregulated to various extents under heavy metal stress. Among these, CitMTP5 in the roots and CitMTP11 in the leaves during Zn stress, CitMTP8 in the roots and CitMTP8.1 in the leaves during Mn stress, CitMTP12 in the roots and CitMTP1 in the leaves during Cu stress, and CitMTP11 in the roots and CitMTP1 in the leaves during Cd stress showed the highest extent of upregulation. These findings are suggestive of their individual roles in heavy metal detoxification.

Metal Tolerance Protein 8 Mediates Manganese Homeostasis and Iron Reallocation during Seed Development and Germination

Metal accumulation in seeds is a prerequisite for germination and establishment of plants but also for micronutrient delivery to humans. To investigate metal transport processes and their interactions in seeds, we focused on METAL TOLERANCE PROTEIN8 (MTP8), a tonoplast transporter of the manganese (Mn) subclade of cation diffusion facilitators, which in Arabidopsis (Arabidopsis thaliana) is expressed in embryos of seeds. The x-ray fluorescence imaging showed that expression of MTP8 was responsible for Mn localization in subepidermal cells on the abaxial side of the cotyledons and in cortical cells of the hypocotyl. Accordingly, under low Mn availability, MTP8 increased seed stores of Mn, required for efficient seed germination. In mutant embryos lacking expression of VACUOLAR IRON TRANSPORTER1 (VIT1), MTP8 built up iron (Fe) hotspots in MTP8-expressing cells types, suggesting that MTP8 transports Fe in addition to Mn. In mtp8 vit1 double mutant seeds, Mn and Fe were distributed in all cell types of the embryo. An Fe transport function of MTP8 was confirmed by its ability to complement Fe hypersensitivity of a yeast mutant defective in vacuolar Fe transport. Imbibing mtp8-1 mutant seeds in the presence of Mn or subjecting seeds to wet-dry cycles showed that MTP8 conferred Mn tolerance. During germination, MTP8 promoted reallocation of Fe from the vasculature. These results indicate that cell type-specific accumulation of Mn and Fe in seeds depends on MTP8 and that this transporter plays an important role in the generation of seed metal stores as well as for metal homeostasis and germination efficiency under challenging environmental conditions.

Metal transport protein 8 in Camellia sinensis confers superior manganese tolerance when expressed in yeast and Arabidopsis thaliana

Manganese (Mn) is an important micronutrient element required for plant growth and development, playing catalytic roles in enzymes, membranes and DNA replication. The tea plant (Camellia sinensis) is able to accumulate high concentration of Mn without showing signs of toxicity, but the molecular mechanisms underlying this remain largely unknown. In this study, the C. sinensis cultivar ‘LJCY’ had higher Mn tolerance than cultivar ‘YS’, because chlorophyll content reduction was lower under the high Mn treatment. Proteomic analysis of the leaves revealed that C. sinensis Metal Tolerance Protein 8 (CsMTP8) accumulated in response to Mn toxicity in cultivar ‘LJCY’. The gene encoding CsMTP8, designated as CsMTP8 was also isolated, and its expression enhanced Mn tolerance in Saccharomyces cerevisiae. Similarly, the overexpression of CsMTP8 in Arabidopsis thaliana increased plant tolerance and reduced Mn accumulation in plant tissues under excess Mn conditions. Subcellular localization analysis of green florescence fused protein indicated that CsMTP8 was localized to the plasma membranes. Taken together, the results suggest that CsMTP8 is a Mn-specific transporter, which is localized in the plasma membrane, and transports excess Mn out of plant cells. The results also suggest that it is needed for Mn tolerance in shoots.

Regulating Subcellular Metal Homeostasis: The Key to Crop Improvement

Iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu) are essential micronutrient mineral elements for living organisms, as they regulate essential cellular processes, such as chlorophyll synthesis and photosynthesis (Fe, Cu, and Mn), respiration (Fe and Cu), and transcription (Zn). The storage and distribution of these minerals in various cellular organelles is strictly regulated to ensure optimal metabolic rates. Alteration of the balance in uptake, distribution, and/or storage of these minerals severely impairs cellular metabolism and significantly affects plant growth and development. Thus, any change in the metal profile of a cellular compartment significantly affects metabolism. Different subcellular compartments are suggested to be linked through complex retrograde signaling networks to regulate cellular metal homeostasis. Various genes regulating cellular and subcellular metal distribution have been identified and characterized. Understanding the role of these transporters is extremely important to elaborate the signaling between various subcellular compartments. Moreover, modulation of the proteins involved in cellular metal homeostasis may help in the regulation of metabolism, adaptability to a diverse range of environmental conditions, and biofortification. Here, we review progress in the understanding of different subcellular metal transport components in plants and discuss the prospects of regulating cellular metabolism and strategies to develop biofortified crop plants.

Direct Comparison of Manganese Detoxification/Efflux Proteins and Molecular Characterization of ZnT10 Protein as a Manganese Transporter

Manganese homeostasis involves coordinated regulation of specific proteins involved in manganese influx and efflux. However, the proteins that are involved in detoxification/efflux have not been completely resolved nor has the basis by which they select their metal substrate. Here, we compared six proteins, which were reported to be involved in manganese detoxification/efflux, by evaluating their ability to reduce manganese toxicity in chicken DT40 cells, finding that human ZnT10 (hZnT10) was the most significant contributor. A domain swapping and substitution analysis between hZnT10 and the zinc-specific transporter hZnT1 showed that residue Asn(43), which corresponds to the His residue constituting the potential intramembranous zinc coordination site in other ZnT transporters, is necessary to impart hZnT10's unique manganese mobilization activity; residues Cys(52) and Leu(242) in transmembrane domains II and V play a subtler role in controlling the metal specificity of hZnT10. Interestingly, the His → Asn reversion mutant in hZnT1 conferred manganese transport activity and loss of zinc transport activity. These results provide important information about manganese detoxification/efflux mechanisms in vertebrate cells as well as the molecular characterization of hZnT10 as a manganese transporter.

The Vacuolar Manganese Transporter MTP8 Determines Tolerance to Iron Deficiency-Induced Chlorosis in Arabidopsis

Iron (Fe) deficiency is a widespread nutritional disorder on calcareous soils. To identify genes involved in the Fe deficiency response, Arabidopsis (Arabidopsis thaliana) transfer DNA insertion lines were screened on a high-pH medium with low Fe availability. This approach identified METAL TOLERANCE PROTEIN8 (MTP8), a member of the Cation Diffusion Facilitator family, as a critical determinant for the tolerance to Fe deficiency-induced chlorosis, also on soil substrate. Subcellular localization to the tonoplast, complementation of a manganese (Mn)-sensitive Saccharomyces cerevisiae yeast strain, and Mn sensitivity of mtp8 knockout mutants characterized the protein as a vacuolar Mn transporter suitable to prevent plant cells from Mn toxicity. MTP8 expression was strongly induced on low-Fe as well as high-Mn medium, which were both strictly dependent on the transcription factor FIT, indicating that high-Mn stress induces Fe deficiency. mtp8 mutants were only hypersensitive to Fe deficiency when Mn was present in the medium, which further suggested an Mn-specific role of MTP8 during Fe limitation. Under those conditions, mtp8 mutants not only translocated more Mn to the shoot than did wild-type plants but suffered in particular from critically low Fe concentrations and, hence, Fe chlorosis, although the transcriptional Fe deficiency response was up-regulated more strongly in mtp8. The diminished uptake of Fe from Mn-containing low-Fe medium by mtp8 mutants was caused by an impaired ability to boost the ferric chelate reductase activity, which is an essential process in Fe acquisition. These findings provide a mechanistic explanation for the long-known interference of Mn in Fe nutrition and define the molecular processes by which plants alleviate this antagonism.

Cucumber metal tolerance protein CsMTP9 is a plasma membrane H⁺-coupled antiporter involved in the Mn²⁺ and Cd²⁺ efflux from root cells

Members of the plant metal tolerance protein (MTP) family have been classified into three major groups - Zn-CDF, Mn-CDF and Zn/Fe-CDF - however, the selectivity of most of the MTPs has not been confirmed yet. Cucumber gene CsMTP9 encoding a putative CDF transporter homologous to members of the Mn-CDF cluster is expressed exclusively in roots. The relative abundance of CsMTP9 transcript and protein in roots is significantly increased under Mn excess and Cd. Immunolocalization with specific antibodies revealed that CsMTP9 is a plasma membrane transporter that localizes to the inner PM domain of root endodermal cells. The plasma membrane localization of CsMTP9 was confirmed by the expression of the fusion proteins of GFP (green fluorescent protein) and CsMTP9 in yeast and protoplasts prepared from Arabidopsis cells. In yeast, CsMTP9 transports Mn(2+) and Cd(2+) via a proton-antiport mechanism with an apparent Km values of approximately 10 μm and 2.5 μm for Mn(2+) and Cd(2+) , respectively. In addition, CsMTP9 expression in yeast rescues the Mn- and Cd-hypersensitive phenotypes through the enhanced efflux of Mn(2+) and Cd(2+) from yeast cells. Similarly, the overexpression of CsMTP9 in A. thaliana confers increased resistance of plants to Mn excess and Cd but not to other heavy metals and leads to the enhanced translocation of manganese and cadmium from roots to shoots. These findings indicate that CsMTP9 is a plasma membrane H(+) -coupled Mn(2+) and Cd(2+) antiporter involved in the efflux of manganese and cadmium from cucumber root cells by the transport of both metals from endodermis into vascular cylinder.