Functional analysis of the rice vacuolar zinc transporter OsMTP1

Transcriptional effects of cadmium on iron homeostasis differ in calamine accessions of Noccaea caerulescens

Calamine accessions of the zinc/cadmium/nickel hyperaccumulator, Noccaea caerulescens, exhibit striking variation in foliar cadmium accumulation in nature. The Ganges accession (GA) from Southern France displays foliar cadmium hyperaccumulation (>1000 μg g^-1 DW), whereas the accession La Calamine (LC) from Belgium, with similar local soil metal composition, does not (<100 μg g^-1 DW). All calamine accessions are cadmium hypertolerant. To find out the differences between LC and GA in their basic adaptation mechanisms, we bypassed the cadmium excluding phenotype of LC by exposing the plants to 50 μm cadmium in hydroponics, achieving equal cadmium accumulation in the shoots. The iron content increased in the roots of both accessions. GA exhibited significant decreases in manganese and zinc contents in the roots and shoots, approaching those in LC. Altogether 702 genes responded differently to cadmium exposure between the accessions, 157 and 545 in the roots and shoots, respectively. Cadmium-exposed LC showed a stress response and had decreased levels of a wide range of photosynthesis-related transcripts. GA showed less changes, mainly exhibiting an iron deficiency-like response. This included increased expression of genes encoding five iron deficiency-regulated bHLH transcription factors, ferric reduction oxidase FRO2, iron transporters IRT1 and OPT3, and nicotianamine synthase NAS1, and decreased expression of genes encoding ferritins and NEET (a NEET family iron-sulfur protein), which is possibly involved in iron transfer, distribution and/or management. The function of the IRT1 gene in the accessions was compared. We conclude that the major difference between the two accessions is in the way they cope with iron under cadmium exposure.

Cucumber Golgi protein CsMTP5 forms a Zn-transporting heterodimer with high molecular mass protein CsMTP12

Heterodimeric complexes formed by members of the cation facilitator (CDF) family catalyse the import of Zn into the secretory pathway of yeast and vertebrate cells. Orthologous proteins AtMTP5 and AtMTP12 from Arabidopsis have also been shown to form a heterodimeric complex at the Golgi compartment of plant cells that possibly transport Zn. In this study we show that cucumber proteins CsMTP5 and CsMTP12 form a functional heterodimer that is involved in the loading of Zn into the ER lumen under low Zn, and not in the detoxification of yeast from Zn excess through vesicle-mediated exocytosis. Using specific antibodies, we demonstrate that CsMTP5 is localized at the Golgi compartment of cucumber cells and is markedly up-regulated upon Zn deficiency. The level of CsMTP5 transcript in cucumber is also significantly elevated in Zn-limiting conditions, whereas the expression of CsMTP12 is independent of the availability of Zn. Therefore we propose that the cucumber heterodimeric complex CsMTP5-CsMTP12 functions to deliver Zn to Zn-dependent proteins of the Golgi compartment and is regulated by zinc at the level of CsMTP5 transcription.

Molecular processes in iron and zinc homeostasis and their modulation for biofortification in rice

More than a billion people suffer from iron or zinc deficiencies globally. Rice (Oryza sativa L.) iron and zinc biofortification; i.e., intrinsic iron and zinc enrichment of rice grains, is considered the most effective way to tackle these deficiencies. However, rice iron biofortification, by means of conventional breeding, proves difficult due to lack of sufficient genetic variation. Meanwhile, genetic engineering has led to a significant increase in the iron concentration along with zinc concentration in rice grains. The design of impactful genetic engineering biofortification strategies relies upon vast scientific knowledge of precise functions of different genes involved in iron and zinc uptake, translocation and storage. In this review, we present an overview of molecular processes controlling iron and zinc homeostasis in rice. Further, the genetic engineering approaches adopted so far to increase the iron and zinc concentrations in polished rice grains are discussed in detail, highlighting the limitations and/or success of individual strategies. Recent insight suggests that a few genetic engineering strategies are commonly utilized for elevating iron and zinc concentrations in different genetic backgrounds, and thus, it is of great importance to accumulate scientific evidence for diverse genetic engineering strategies to expand the pool of options for biofortifying farmer-preferred cultivars.

Triticum urartu MTP1: its ability to maintain Zn2+ and Co2+ homeostasis and metal selectivity determinants

TuMTP1 maintains Zn²⁺ and Co²⁺ homeostasis by sequestering excess Zn²⁺ and Co²⁺ into vacuoles. The mutations NSEDD/VTVTT in the His-rich loop and I119F in TMD3 of TuMTP1 restrict metal selectivity. Mineral nutrients, such as zinc (Zn) and cobalt (Co), are essential or beneficial for plants but can be toxic at elevated levels. Metal tolerance proteins (MTPs) are plant members of the cation diffusion facilitator (CDF) transporter family involved in cellular metal homeostasis. However, the determinants of substrate selectivity have not been clarified due to the diversity of MTP1 substrates in various plants. In this study, Triticum urartu MTP1 was characterized. When expressed in yeast, TuMTP1 conferred tolerance to Zn²⁺ and Co²⁺ but not Fe²⁺, Cu²⁺, Ni²⁺ or Cd²⁺ in solid and liquid culture and localized on the vacuolar membrane. Furthermore, TuMTP1-expressing yeast accumulated more Zn²⁺ and Co²⁺ when treated. TuMTP1 expression in T. urartu roots was significantly increased under Zn²⁺ and Co²⁺ stresses. Determinants of substrate selectivity were then examined through site-directed mutagenesis. The exchange of NSEDD with VTVTT in the His-rich loop of TuMTP1 restricted its metal selectivity to Zn²⁺, whereas the I119F mutation confined specificity to Co²⁺. The mutations H74, D78, H268 and D272 (in the Zn²⁺-binding site) and Leu322 (in the C-terminal Leu-zipper) partially or completely abolished the transport function of TuMTP1. These results show that TuMTP1 might sequester excess cytosolic Zn²⁺ and Co²⁺ into yeast vacuoles to maintain Zn²⁺ and Co²⁺ homeostasis. The NSEDD/VTVTT and I119F mutations are crucially important for restricting the substrate specificity of TuMTP1, and the Zn²⁺-binding site and Leu322 are essential for its ion selectivity and transport function. These results can be employed to change metal selectivity for biofortification or phytoremediation applications.

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.

Graphene Oxide-Upconversion Nanoparticle Based Portable Sensors for Assessing Nutritional Deficiencies in Crops

The development of innovative technologies to rapidly detect biomarkers associated with nutritional deficiencies in crops is highly relevant to agriculture and thus could impact the future of food security. Zinc (Zn) is an important micronutrient in plants, and deficiency leads to poor health, quality, and yield of crops. We have developed portable sensors, based on graphene oxide and upconversion nanoparticles, which could be used in the early detection of Zn deficiency in crops, sensing mRNAs encoding members of the ZIP-transporter family in crops. ZIPs are membrane transport proteins, some of which are up-regulated at the early stages of Zn deficiency, and they are part of the biological mechanism by which crops respond to nutritional deficiency. The principle of these sensors is based on the intensity of the optical output resulting from the interaction of oligonucleotide-coated upconversion nanoparticles and graphene oxide in the absence or presence of a specific oligonucleotide target. The sensors can reliably detect mRNAs in RNA extracts from plants using a smartphone camera. Our work introduces the development of accurate and highly sensitive sensors for use in the field to determine crop nutrient status and ultimately facilitate economically important nutrient management decisions.

Zinc, zinc nanoparticles and plants

Zinc belongs to the mineral elements, the so-called micronutrients, which are essential for all types of plants. Embedding itself into the enzymes associated with proteosynthesis and energy processes, zinc is necessary for maintaining the integrity of biomembranes and also plays an important role in the development of seeds and generative organs. This review focuses on summarising the findings on the interaction of zinc and plants and translates into the knowledge of the effect of zinc nanoparticles on plants. The findings include an overview of both positive and negative effects on plants. In conclusion there is a great interest in nano-zinc as improving the knowledge about individual forms of zinc and their uptake and assimilation within higher plants may be the first step towards a wider involvement of zinc nanoparticles into agriculture.

The potential of genetic engineering of plants for the remediation of soils contaminated with heavy metals

The genetic engineering of plants to facilitate the reclamation of soils and waters contaminated with inorganic pollutants is a relatively new and evolving field, benefiting from the heterologous expression of genes that increase the capacity of plants to mobilize, stabilize and/or accumulate metals. The efficiency of phytoremediation relies on the mechanisms underlying metal accumulation and tolerance, such as metal uptake, translocation and detoxification. The transfer of genes involved in any of these processes into fast-growing, high-biomass crops may improve their reclamation potential. The successful phytoextraction of metals/metalloids and their accumulation in aerial organs have been achieved by expressing metal ligands or transporters, enzymes involved in sulfur metabolism, enzymes that alter the chemical form or redox state of metals/metalloids and even the components of primary metabolism. This review article considers the potential of genetic engineering as a strategy to improve the phytoremediation capacity of plants in the context of heavy metals and metalloids, using recent case studies to demonstrate the practical application of this approach in the field.

Genome-Wide Identification and Expression Analysis of the Cation Diffusion Facilitator Gene Family in Turnip Under Diverse Metal Ion Stresses

The cation diffusion facilitator (CDF) family is one of the gene families involved in metal ion uptake and transport in plants, but the understanding of the definite roles and mechanisms of most CDF genes remain limited. In the present study, we identified 18 candidate CDF genes from the turnip genome and named them BrrMTP1.1-BrrMTP12. Then, we performed a comparative genomic analysis on the phylogenetic relationships, gene structures and chromosome distributions, conserved domains, and motifs of turnip CDFs. The constructed phylogenetic tree indicated that the BrrMTPs were divided into seven groups (groups 1, 5, 6, 7, 8, 9, and 12) and formed three major clusters (Zn-CDFs, Fe/Zn-CDFs, and Mn-CDFs). Moreover, the structural characteristics of the BrrMTP members in the same group were similar but varied among groups. To investigate the potential roles of BrrMTPs in turnip, we conducted an expression analysis on all BrrMTP genes under Mg, Zn, Cu, Mn, Fe, Co, Na, and Cd stresses. Results showed that the expression levels of all BrrMTP members were induced by at least one metal ion, indicating that these genes may be related to the tolerance or transport of those metal ions. Based on the roles of different metal ions for plants, we hypothesized that BrrMTP genes are possibly involved in heavy metal accumulation and tolerance to salt stress apart from their roles in the maintenance of mineral nutrient homeostasis in turnip. These findings are helpful to understand the roles of MTPs in plants and provide preliminary information for the study of the functions of BrrMTP genes.

Vacuolar Transporters - Companions on a Longtime Journey

Biochemical and electrophysiological studies on plant vacuolar transporters became feasible in the late 1970s and early 1980s, when methods to isolate large quantities of intact vacuoles and purified vacuolar membrane vesicles were established. However, with the exception of the H⁺-ATPase and H⁺-PPase, which could be followed due to their hydrolytic activities, attempts to purify tonoplast transporters were for a long time not successful. Heterologous complementation, T-DNA insertion mutants, and later proteomic studies allowed the next steps, starting from the 1990s. Nowadays, our knowledge about vacuolar transporters has increased greatly. Nevertheless, there are several transporters of central importance that have still to be identified at the molecular level or have even not been characterized biochemically. Furthermore, our knowledge about regulation of the vacuolar transporters is very limited, and much work is needed to get a holistic view about the interplay of the vacuolar transportome. The huge amount of information generated during the last 35 years cannot be summarized in such a review. Therefore, I decided to concentrate on some aspects where we were involved during my research on vacuolar transporters, for some our laboratories contributed more, while others contributed less.

You Shall Not Pass: Root Vacuoles as a Symplastic Checkpoint for Metal Translocation to Shoots and Possible Application to Grain Nutritional Quality

Plant nutrient uptake is performed mostly by roots, which have to acquire nutrients while avoiding excessive amounts of essential and toxic elements. Apoplastic barriers such as the casparian strip and suberin deposition block free diffusion from the rhizosphere into the xylem, making selective plasma membrane transporters able to control elemental influx into the root symplast, efflux into the xylem and therefore shoot translocation. Additionally, transporters localized to the tonoplast of root cells have been demonstrated to regulate the shoot ionome, and may be important for seed elemental translocation. Here we review the role of vacuolar transporters in the detoxification of elements such as zinc (Zn), manganese (Mn), cadmium (Cd), cobalt (Co) and nickel (Ni) that are co-transported with iron (Fe) during the Fe deficiency response in Arabidopsis thaliana, and the possible conservation of this mechanism in rice (Oryza sativa). We also discuss the evidence that vacuolar transporters are linked to natural variation in shoot ionome in Arabidopsis and rice, indicating that vacuolar storage might be amenable to genetic engineering without strong phenotypical changes. Finally, we discuss the possible use of root's vacuolar transporters to increase the nutritional quality of crop grains.

Contribution of NtZIP1-Like to the Regulation of Zn Homeostasis

Tobacco has frequently been suggested as a candidate plant species for use in phytoremediation of metal contaminated soil but knowledge on the regulation of its metal-homeostasis is still in the infancy. To identify new tobacco metal transport genes that are involved in Zn homeostasis a bioinformatics study using the tobacco genome information together with expression analysis was performed. Ten new tobacco metal transport genes from the ZIP, NRAMP, MTP, and MRP/ABCC families were identified with expression levels in leaves that were modified by exposure to Zn excess. Following exposure to high Zn there was upregulation of NtZIP11-like, NtNRAMP3, three isoforms of NtMTP2, three MRP/ABCC genes (NtMRP5-like, NtMRP10-like, and NtMRP14 like) and downregulation of NtZIP1-like and NtZIP4. This suggests their involvement in several processes governing the response to Zn-related stress and in the efficiency of Zn accumulation (uptake, sequestration, and redistribution). Further detailed analysis of NtZIP1-like provided evidence that it is localized at the plasma membrane and is involved in Zn but not Fe and Cd transport. NtZIP1-like is expressed in the roots and shoots, and is regulated developmentally and in a tissue-specific manner. It is highly upregulated by Zn deficiency in the leaves and the root basal region but not in the root apical zone (region of maturation and absorption containing root hairs). Thus NtZIP1-like is unlikely to be responsible for Zn uptake by the root apical region but rather in the uptake by root cells within the already mature basal zone. It is downregulated by Zn excess suggesting it is involved in a mechanism to protect the root and leaf cells from accumulating excess Zn.

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.

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

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

Phytosiderophores determine thresholds for iron and zinc accumulation in biofortified rice endosperm while inhibiting the accumulation of cadmium

Nicotianamine (NA) and 2'-deoxymugenic acid (DMA) are metal-chelating ligands that promote the accumulation of metals in rice endosperm, but it is unclear how these phytosiderophores regulate the levels of different metals and limit their accumulation. In this study, transgenic rice plants producing high levels of NA and DMA accumulated up to 4-fold more iron (Fe) and 2-fold more zinc (Zn) in the endosperm compared with wild-type plants. The distribution of Fe and Zn in vegetative tissues suggested that both metals are sequestered as a buffering mechanism to avoid overloading the seeds. The buffering mechanism involves the modulation of genes encoding metal transporters in the roots and aboveground vegetative tissues. As well as accumulating more Fe and Zn, the endosperm of the transgenic plants accumulated less cadmium (Cd), suggesting that higher levels of Fe and Zn competitively inhibit Cd accumulation. Our data show that although there is a strict upper limit for Fe (~22.5 µg g-1 dry weight) and Zn (~84 µg g-1 dry weight) accumulation in the endosperm, the careful selection of strategies to increase endosperm loading with essential minerals can also limit the accumulation of toxic metals such as Cd, thus further increasing the nutritional value of rice.

Cadmium and zinc activate adaptive mechanisms in Nicotiana tabacum similar to those observed in metal tolerant plants

MAIN CONCLUSION

Tobacco germinated and grew in the presence of high concentrations of cadmium and zinc without toxic symptoms. Evidence suggests that these ions are sequestered into the vacuole by heavy metal/H + exchanger mechanisms. Heavy metal hyperaccumulation and hypertolerance are traits shared by a small set of plants which show specialized physiological and molecular adaptations allowing them to accumulate and sequester toxic metal ions. Nicotiana tabacum was used to test its potential as a metal-accumulator in a glass house experiment. Seed germination was not affected in the presence of increasing concentrations of zinc and cadmium. Juvenile and adult plants could concentrate CdCl2 and ZnSO4 to levels exceeding those in the hydroponic growth medium and maintained or increased their leaf dry weight when treated with 0.5- or 1-mM CdCl2 or 1-mM ZnSO4 for 5 days. Accumulation of heavy metals did not affect the chlorophyll and carotenoid levels, while variable effects were observed in cell sap osmolarity. Heavy metal-dependent H+ transport across the vacuole membrane was monitored using quinacrine fluorescence quenching. Cadmium- or zinc-dependent fluorescence recovery revealed that increasing concentrations of heavy metals stimulated the activities of the tonoplast Cd2+ or Zn2+/H+ exchangers. Immunodetection of the V-ATPase subunits showed that the increased proton transport by zinc was not due to changes in protein amount. MTP1 and MTP4 immunodetection and semiquantitative RT-PCR of NtMTP1, NtNRAMP1, and NtZIP1 helped to identify the genes that are likely involved in sequestration of cadmium and zinc in the leaf and root tissue. Finally, we demonstrated that cadmium and zinc treatments induced an accumulation of zinc in leaf tissues. This study shows that N. tabacum possesses a hyperaccumulation response, and thus could be used for phytoremediation purposes.

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.

The effects of endophytic bacterium SaMR12 on Sedum alfredii Hance metal ion uptake and the expression of three transporter family genes after cadmium exposure

A hydroponic experiment was conducted to investigate the effects of an endophytic bacterium SaMR12 on Sedum alfredii Hance metal ion accumulation, chlorophyll concentration, and the expression of three metal transporter families, zinc-regulated transporters, iron-regulated transporter-like protein (ZIP); natural resistance-associated macrophage protein; and heavy metal ATPase (HMA) at different Cd treatment levels. The results showed that at relatively low Cd conditions (≤25 μM), SaMR12 demonstrated a 19.5-27.5% increase in Fe, a 46.7-90.7% increase in Zn, and a 7.9-43.7% increase in Cu content in the shoot and elevated expression of SaIRT1, SaZIP3, SaHMA2, and SaNramp3 in the shoot and SaZIP1, SaHMA2, SaNramp1, and SaNramp3 in the root. At high Cd conditions (100 and 400 μM), SaMR12 demonstrated a 16.4-18.5% increase in leaf chlorophyll concentration, a 18.9-23.2% increase in Fe, and a 15.4-17.5% increase in Mg content in the shoot and elevated expression of SaZIP3, SaNramp6, SaHMA2, and SaHMA3 in the shoot and SaZIP3, SaNarmp1, SaNarmp3, and SaNarmp6 in the root. These results indicated that SaMR12 can elevate essential metal ion uptake and regulate the expression of transport genes to promote plant growth and enhance Cd tolerance and uptake to improve Cd accumulation up to 118-130%.

Genome-wide exploration of metal tolerance protein (MTP) genes in common wheat (Triticum aestivum): insights into metal homeostasis and biofortification

Metal transport process in plants is a determinant of quality and quantity of the harvest. Although it is among the most important of staple crops, knowledge about genes that encode for membrane-bound metal transporters is scarce in wheat. Metal tolerance proteins (MTPs) are involved in trace metal homeostasis at the sub-cellular level, usually by providing metal efflux out of the cytosol. Here, by using various bioinformatics approaches, genes that encode for MTPs in the hexaploid wheat genome (Triticum aestivum, abbreviated as Ta) were identified and characterized. Based on the comparison with known rice MTPs, the wheat genome contained 20 MTP sequences; named as TaMTP1-8A, B and D. All TaMTPs contained a cation diffusion facilitator (CDF) family domain and most members harbored a zinc transporter dimerization domain. Based on motif, phylogeny and alignment analysis, A, B and D genomes of TaMTP3-7 sequences demonstrated higher homology compared to TaMTP1, 2 and 8. With reference to their rice orthologs, TaMTP1s and TaMTP8s belonged to Zn-CDFs, TaMTP2s to Fe/Zn-CDFs and TaMTP3-7s to Mn-CDFs. Upstream regions of TaMTP genes included diverse cis-regulatory motifs, indicating regulation by developmental stage, tissue type and stresses. A scan of the coding sequences of 20 TaMTPs against published miRNAs predicted a total of 14 potential miRNAs, mainly targeting the members of most diverged groups. Expression analysis showed that several TaMTPs were temporally and spatially regulated during the developmental time-course. In grains, MTPs were preferentially expressed in the aleurone layer, which is known as a reservoir for high concentrations of iron and zinc. The work identified and characterized metal tolerance proteins in common wheat and revealed a potential involvement of MTPs in providing a sink for trace element storage in wheat grains.

Morphophysiological characteristic analysis demonstrated the potential of sweet sorghum (Sorghum bicolor (L.) Moench) in the phytoremediation of cadmium-contaminated soils

Cadmium (Cd) contamination is a worldwide environmental problem, and remediation of Cd pollution is of great significance for food production as well as human health. Here, the responses of sweet sorghum cv. 'M-81E' to cadmium stress were studied for its potential as an energy plant in restoring soils contaminated by cadmium. In hydroponic experiments, the biomass of 'M-81E' showed no obvious change under 10 μM cadmium treatment. Cadmium concentration was the highest in roots of seedlings as well as mature plants, but in agricultural practice, the valuable and harvested parts of sweet sorghum are shoots, so promoting the translocation of cadmium to shoots is of great importance in order to improve its phytoremediation capacity. Further histochemical assays with dithizone staining revealed that cadmium was mainly concentrated in the stele of roots and scattered in intercellular space of caulicles. Moreover, the correlation analysis showed that Cd had a negative relationship with iron (Fe), zinc (Zn), and manganese (Mn) in caulicles and leaves and a positive relationship with Fe in roots. These results implied that cadmium might compete with Fe, Zn, and Mn for the transport binding sites and further prevent their translocation to shoots. In addition, transmission electron microscopic observations showed that under 100 μM cadmium treatment, the structure of chloroplast was impaired and the cell wall of vascular bundle cells in leaves and xylem and phloem cells in roots turned thicker compared to control. In summary, morphophysiological characteristic analysis demonstrated sweet sorghum can absorb cadmium and the growth is not negatively affected by mild level cadmium stress; thus, it is a promising material for the phytoremediation of cadmium-contaminated soils considering its economic benefit. This study also points out potential strategies to improve the phytoremediation capacity of sweet sorghum through genetic modification of transporters and cell wall components.

A mutagenic study identifying critical residues for the structure and function of rice manganese transporter OsMTP8.1

Rice (Oryza sativa) MTP8.1 (OsMTP8.1) is a tonoplast-localized manganese transporter of the cation diffusion facilitator family. Here we present a structure-function analysis of OsMTP8.1 based on the site-directed and random mutagenesis and complementation assays in manganese hypersensitive yeast, in combination with three-dimensional (3D) structure modeling based on the crystal structure of the Escherichia coli CDF family member, EcYiiP. Two metal-binding sites are conserved in OsMTP8.1 with EcYiiP, one is between transmembrane helices TM2 and TM5, the other is the cytoplasmic C-terminus. In addition to these two metal-binding sites, there may exist other Mn-binding sites such as that at the very end of the CTD. Two residues (R167 and L296) may play an important role for the hinge-like movement of CTDs. Several mutations such as E357A and V374D may affect dimer formation, and S132A may induce a conformational change, resulting in a loss of transport function or modification in metal selectivity. The N-terminus of OsMTP8.1 was not functional for Mn transport activity, and the real function of NTD remains to be investigated in the future. The findings of the present study illustrate the structure-function relationship of OsMTP8.1 in Mn transport activity, which may also be applied to other plant Mn-CDF proteins.

Enhanced cadmium accumulation and tolerance in transgenic tobacco overexpressing rice metal tolerance protein gene OsMTP1 is promising for phytoremediation

One of the most grievous heavy metal pollutants in the environment is cadmium (Cd), which is not only responsible for the crop yield loss owing to its phytotoxicity, but also for the human health hazards as the toxic elements usually accumulate in the consumable parts of crop plants. In the present study, we aimed to isolate and functionally characterize the OsMTP1 gene from indica rice (Oryza sativa L. cv. IR64) to study its potential application for efficient phytoremediation of Cd. The 1257 bp coding DNA sequence (CDS) of OsMTP1 encodes a ∼46 kDa protein belonging to the cation diffusion facilitator (CDF) or metal tolerance/transport protein (MTP) family. The OsMTP1 transcript in rice plant was found to respond during external Cd stress. Heterologous expression of OsMTP1 in tobacco resulted in the reduction of Cd stress-induced phytotoxic effects, including growth inhibition, lipid peroxidation, and cell death. Compared to untransformed control, the transgenic tobacco plants showed enhanced vacuolar thiol content, indicating vacuolar localization of the sequestered Cd. The transgenic tobacco plants exhibited significantly higher biomass growth (2.2-2.8-folds) and hyperaccumulation of Cd (1.96-2.22-folds) compared to untransformed control under Cd exposure. The transgenic plants also showed moderate tolerance and accumulation of arsenic (As) upon exogenous As stress, signifying broad substrate specificity of OsMTP1. Together, findings of our research suggest that the transgenic tobacco plants overexpressing OsMTP1 with its hyperaccumulating activity and increased growth rate could be useful for future phytoremediation applications to clean up the Cd-contaminated soil.

The Challenges and Solutions for Cadmium-contaminated Rice in China: A Critical Review

The wide occurrence of Cd-contaminated rice in southern China poses significant public health risk and deserves immediate action, which arises primarily from extensive metal (including Cd) contamination of paddies with the fast expansion of nonferrous metal mining and smelting activities. Accumulation of Cd in rice grains can be reduced by removing Cd from the contaminated paddy soils, reducing its bioavailability, and controlling its uptake by rice plants. Although a range of measures can be taken to rehabilitate Cd-contaminated lands, including soil replacement and turnover, chemical washing, and phytoremediation, they are either too expensive and/or too slow. Various amendment materials, including lime, animal manures, and biochar, can be used to immobilize Cd in soils, but such fixation approach can only temporarily reduce Cd availability to rice uptake. Cultivation of alternative crops with low Cd accumulation in edible plant parts is impractical on large scales due to extensive contamination and food security concerns in southern China. Transgenic techniques can help develop rice cultivars with low Cd accumulation in grains, but little public acceptance is expected for such products. As an alternative, selection and development of low-Cd rice varieties and hybrids through plant biotechnology and breeding, particularly, by integration of marker-assisted selection (MAS) with traditional breeding, could be a practical and acceptable option that would allow continued rice production in soils with high bioavailability of Cd. Plant biotechnology and breeding can also help develop Cd-hyperaccumulating rice varieties, which can greatly facilitate phytoremediation of contaminated paddies. To eliminate the long-term risk of Cd entering the food chain, soils contaminated by Cd should be cleaned up when cost-effective remediation measures are available.

The PP-motif in luminal loop 2 of ZnT transporters plays a pivotal role in TNAP activation

Secretory and membrane-bound zinc-requiring enzymes are thought to be activated by binding zinc in the early secretory pathway. One such enzyme, tissue-non-specific alkaline phosphatase (TNAP), is activated through a two-step mechanism, via protein stabilization and subsequent enzyme activation through metalation, by ZnT5-ZnT6 heterodimers or ZnT7 homodimers. However, little is known about the molecular basis underlying the activation process. In the present study, we found that the di-proline motif (PP-motif) in luminal loop 2 of ZnT5 and ZnT7 is important for TNAP activation. TNAP activity was significantly reduced in cells lacking ZnT5-ZnT6 heterodimers and ZnT7 homodimers [triple knockout (TKO) cells]. The decreased TNAP activity was restored by expressing hZnT5 with hZnT6 or hZnT7, but significantly less so (almost 90% less) by expressing mutants thereof in which the PP-motif was mutated to alanine (PP-AA). In TKO cells, overexpressed hTNAP was not completely activated, and it was converted less efficiently into the holo form by expressing a PP-AA mutant of hZnT5 with hZnT6, whose defects were not restored by zinc supplementation. The zinc transport activity of hZnT7 was not significantly impaired by the PP-AA mutation, indicating that the PP-motif is involved in the TNAP maturation process, although it does not control zinc transport activity. The PP-motif is highly conserved in ZnT5 and ZnT7 orthologues, and its importance for TNAP activation is conserved in the Caenorhabditis elegans hZnT5 orthologue CDF5. These results provide novel molecular insights into the TNAP activation process in the early secretory pathway.

Vacuolar compartmentalization as indispensable component of heavy metal detoxification in plants

Plant cells orchestrate an array of molecular mechanisms for maintaining plasmatic concentrations of essential heavy metal (HM) ions, for example, iron, zinc and copper, within the optimal functional range. In parallel, concentrations of non-essential HMs and metalloids, for example, cadmium, mercury and arsenic, should be kept below their toxicity threshold levels. Vacuolar compartmentalization is central to HM homeostasis. It depends on two vacuolar pumps (V-ATPase and V-PPase) and a set of tonoplast transporters, which are directly driven by proton motive force, and primary ATP-dependent pumps. While HM non-hyperaccumulator plants largely sequester toxic HMs in root vacuoles, HM hyperaccumulators usually sequester them in leaf cell vacuoles following efficient long-distance translocation. The distinct strategies evolved as a consequence of organ-specific differences particularly in vacuolar transporters and in addition to distinct features in long-distance transport. Recent molecular and functional characterization of tonoplast HM transporters has advanced our understanding of their contribution to HM homeostasis, tolerance and hyperaccumulation. Another important part of the dynamic vacuolar sequestration syndrome involves enhanced vacuolation. It involves vesicular trafficking in HM detoxification. The present review provides an updated account of molecular aspects that contribute to the vacuolar compartmentalization of HMs.

Heavy Metal Tolerance in Plants: Role of Transcriptomics, Proteomics, Metabolomics, and Ionomics

Heavy metal contamination of soil and water causing toxicity/stress has become one important constraint to crop productivity and quality. This situation has further worsened by the increasing population growth and inherent food demand. It has been reported in several studies that counterbalancing toxicity due to heavy metal requires complex mechanisms at molecular, biochemical, physiological, cellular, tissue, and whole plant level, which might manifest in terms of improved crop productivity. Recent advances in various disciplines of biological sciences such as metabolomics, transcriptomics, proteomics, etc., have assisted in the characterization of metabolites, transcription factors, and stress-inducible proteins involved in heavy metal tolerance, which in turn can be utilized for generating heavy metal-tolerant crops. This review summarizes various tolerance strategies of plants under heavy metal toxicity covering the role of metabolites (metabolomics), trace elements (ionomics), transcription factors (transcriptomics), various stress-inducible proteins (proteomics) as well as the role of plant hormones. We also provide a glance of some strategies adopted by metal-accumulating plants, also known as "metallophytes."

Magnesium and iron deficiencies alter Cd accumulation in Salix viminalis L

Evidence exists that Cd and certain nutrient elements, such as Fe and Mg, could share similar mechanisms of plant uptake and accumulation. Here we report that Mg and Fe deficiency in mature plants of Salix viminalis, grown in hydroponic solutions containing 5 µg ml(-1) of Cd, caused a significant increase in Cd accumulation in roots, stems and leaves. Cd (µg g(-1) dry weight) was determined following three treatments: 1) Cd treatment in complete nutrient solution; 2) Cd treatment with Fe deficiency; and 3) Cd treatment with Mg deficiency, yielding, respectively: in young leaves (65.3, 76.1, and 92.2), mature leaves (51.5 to 76.3 and 87.1), upper stems (80.6, 116.8, and 130.6) lower stems (67.2, 119, and 102.3), roots (377.1, 744.8, and 442,5). Our results suggest that Cd utilizes the same uptake and transport pathways as Mg and Fe. Evidence exists that Mg and Fe uptake and translocation could be further facilitated by plants as an adaptive response to deficiency of these elements. Such physiological reaction could additionally stimulate Cd accumulation. Although Cd uptake was mostly confined in roots, high Cd content in aerial plant parts (51.5-130.6 µg g(-1)) indicates that the analysed Salix viminalis genotype is suitable for phytoextraction.

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.

Rice PCR1 influences grain weight and Zn accumulation in grains

Proteins containing a placenta-specific 8 domain (PLAC8) function as major organ size regulators in Solanum lycopersicum and Zea may, and putative metal ion transporters in Arabidopsis thaliana, Oryza sativa and Brassica juncea. However, it is unknown how PLAC8 domain-containing proteins fulfill such diverse roles. Here, we found that plant cadmium resistance 1 (PCR1) influences both zinc (Zn) accumulation and grain weight in rice. OsPCR1 knockout and knockdown lines produced lighter grains than the wild type, while OsPCR1 overexpression lines produced heavier grains. Furthermore, the grains of OsPCR1 knockdown lines exhibited substantially higher Zn and lower cadmium (Cd) concentrations than the control, as did yeast heterologously expressing OsPCR1. Through sequence analysis, we showed that the amino acid sequence of japonica-type PCR1 was distinct from that of indica-type and wild rice accessions. This difference was correlated with distinct Zn-related phenotypes. Japonica-type PCR1 had a shorter N-terminus than did PCR1 in the other rice types, and yeast heterologously expressing japonica-type PCR1 was more sensitive to Zn than was yeast expressing indica-type PCR1. Furthermore, japonica-type grains accumulated less Zn than did indica-type grains. Our study suggests that rice PCR1 maintains metal ion homeostasis and grain weight and might have been selected for during domestication.

Got to hide your Zn away: Molecular control of Zn accumulation and biotechnological applications

Zinc (Zn) is an essential micronutrient for all organisms, with key catalytic and structural functions. Zn deficiency in plants, common in alkaline soils, results in growth arrest and sterility. On the other hand, Zn can become toxic at elevated concentrations. Several studies revealed molecules involved with metal acquisition in roots, distribution within the plant and translocation to seeds. Transmembrane Zn transport proteins and Zn chelators are involved in avoiding its toxic effects. Plant species with the capacity to hyperaccumulate and hypertolerate Zn have been characterized. Plants that accumulate and tolerate high amounts of Zn and produce abundant biomass may be useful for phytoremediation, allowing cleaning of metal-contaminated soils. The study of Zn hyperaccumulators may provide indications of genes and processes useful for biofortification, for developing crops with high amounts of nutrients in edible tissues. Future research needs to focus on functional characterization of Zn transporters in planta, elucidation of Zn uptake and sensing mechanisms, and on understanding the cross-talk between Zn homeostasis and other physiological processes. For this, new research should use multidisciplinary approaches, combining traditional and emerging techniques, such as genome-encoded metal sensors and multi-element imaging, quantification and speciation using synchrotron-based methods.

Cation Diffusion Facilitator family: Structure and function

The Cation Diffusion Facilitators (CDFs) form a family of membrane-bound proteins capable of transporting zinc and other heavy metal ions. Involved in metal tolerance/resistance by efflux of ions, CDF proteins share a two-modular architecture consisting of a transmembrane domain (TMD) and C-terminal domain (CTD) that protrudes into the cytoplasm. Discovery of a Zn²⁺ and Cd²⁺ CDF transporter from a marine bacterium Maricaulis maris that does not possess the CTD questions current perceptions regarding this family of proteins. This article describes a new, CTD-lacking subfamily of CDFs and our current knowledge about this family of proteins in the view of these findings.

Comparative mapping combined with homology-based cloning of the rice genome reveals candidate genes for grain zinc and iron concentration in maize

BACKGROUND

Grain zinc and iron concentration is a complex trait that is controlled by quantitative trait loci (QTL) and is important for maintaining body health. Despite the substantial effort that has been put into identifying QTL for grain zinc and iron concentration, the integration of independent QTL is useful for understanding the genetic foundation of traits. The number of QTL for grain zinc and iron concentration is relatively low in a single species. Therefore, combined analysis of different genomes may help overcome this challenge.

RESULTS

As a continuation of our work on maize, meta-analysis of QTL for grain zinc and iron concentration in rice was performed to identify meta-QTL (MQTL). Based on MQTL in rice and maize, comparative mapping combined with homology-based cloning was performed to identify candidate genes for grain zinc and iron concentration in maize. In total, 22 MQTL in rice, 4 syntenic MQTL-related regions, and 3 MQTL-containing candidate genes in maize (ortho-mMQTL) were detected. Two maize orthologs of rice, GRMZM2G366919 and GRMZM2G178190, were characterized as natural resistance-associated macrophage protein (NRAMP) genes and considered to be candidate genes. Phylogenetic analysis of NRAMP genes among maize, rice, and Arabidopsis thaliana further demonstrated that they are likely responsible for the natural variation of maize grain zinc and iron concentration.

CONCLUSIONS

Syntenic MQTL-related regions and ortho-mMQTL are prime areas for future investigation as well as for marker-assisted selection breeding programs. Furthermore, the combined method using the rice genome that was used in this study can shed light on other species and help direct future quantitative trait research. In conclusion, these results help elucidate the molecular mechanism that underlies grain zinc and iron concentration in maize.

Transcriptional and physiological changes in relation to Fe uptake under conditions of Fe-deficiency and Cd-toxicity in roots of Vigna radiata L

We investigated transcriptional and physiological changes in relation to Fe transport and uptake under various conditions of iron (Fe)-deficiency and cadmium (Cd) toxicity. Responses to four such Fe/Cd conditions were evaluated, revealing that oxidative stress was generated in the presence of Cd, followed by a decrease in Fe and an increase in Cd concentrations in green gram (Vigna radiata) material, whereas supplementation with Fe had a protective effect against Cd toxicity. The involvement of enzymes in Fe-uptake for the formation of root-nodules was largely reduced in the presence of Cd toxicity, a condition recovered by Fe-supplementation. Insufficient ferric chelate reducing activity in Fe-deprived roots in the presence of Cd was also largely improved by Fe supplementation. The expression of Fe(2+) transporters (IRT1, IRT2, and IRT3), Fe(III) chelate reductase (FRO1-FRO8) and phytochelatin synthase (PCS1, PCS2 and PCS3) genes was up regulated for the first 5 days and decreased after 10 days in roots in the presence of Cd toxicity, but was sustained with Fe-supplementation. Additionally, root biomass was fully recovered in plants in the presence of Fe during Cd toxicity. Our results suggest that Fe-status plays a significant role in ameliorating the damage in Fe transport for chelation and its uptake caused by Cd toxicity. This supports the hypothesis that leguminous plants, particularly those that are sensitive to Fe such as green gram, can cope to some extent with Cd toxicity by improving the uptake and transport of Fe.

HMA4 expression in tobacco reduces Cd accumulation due to the induction of the apoplastic barrier

Ectopic expression in tobacco (Nicotiana tabacum v. Xanthi) of the export protein AtHMA4 (responsible in Arabidopsis for the control of Zn/Cd root to shoot translocation) resulted in decreased Cd uptake/accumulation in roots and shoots. This study contributes to understanding the mechanisms underlying this Cd-dependent phenotype to help predict the consequences of transgene expression for potential phytoremediation/biofortification-based strategies. Microarray analysis was performed to identify metal homeostasis genes that were differentially expressed in roots of Cd-exposed AtHMA4-expressing tobacco relative to the wild type. It was established that down-regulation of genes known to mediate Cd uptake was not responsible for reduced Cd uptake/accumulation in AtHMA4 transformants. The transcript levels of NtIRT1 and NtZIP1 were higher in transgenic plants, indicating an induction of the Fe and Zn deficiency status due to AtHMA4 expression. Interestingly, upon exposure to Cd, genes involved in cell wall lignification (NtHCT, NtOMET, and NtPrx11a) were up-regulated in transformants. Microscopic analysis of roots demonstrated that expression of AtHMA4 caused an induction of cell wall lignification in the external cell layers that was accompanied by enhanced H2O2 accumulation. Further study showed that the concentration of other elements (B, Co, Cu, Ni, Mo, and Zn) was reduced in AtHMA4 transformants in the presence of Cd. In conclusion, due to ectopic expression of 35S::AtHMA4, the physical apoplastic barrier within the external cell layer developed, which is likely to be responsible for the reduction of Cd uptake/accumulation.

Mn-euvering manganese: the role of transporter gene family members in manganese uptake and mobilization in plants

Manganese (Mn), an essential trace element, is important for plant health. In plants, Mn serves as a cofactor in essential processes such as photosynthesis, lipid biosynthesis and oxidative stress. Mn deficient plants exhibit decreased growth and yield and are more susceptible to pathogens and damage at freezing temperatures. Mn deficiency is most prominent on alkaline soils with approximately one third of the world's soils being too alkaline for optimal crop production. Despite the importance of Mn in plant development, relatively little is known about how it traffics between plant tissues and into and out of organelles. Several gene transporter families have been implicated in Mn transport in plants. These transporter families include NRAMP (natural resistance associated macrophage protein), YSL (yellow stripe-like), ZIP (zinc regulated transporter/iron-regulated transporter [ZRT/IRT1]-related protein), CAX (cation exchanger), CCX (calcium cation exchangers), CDF/MTP (cation diffusion facilitator/metal tolerance protein), P-type ATPases and VIT (vacuolar iron transporter). A combination of techniques including mutant analysis and Synchrotron X-ray Fluorescence Spectroscopy can assist in identifying essential transporters of Mn. Such knowledge would vastly improve our understanding of plant Mn homeostasis.

Roles of plant metal tolerance proteins (MTP) in metal storage and potential use in biofortification strategies

Zinc (Zn) is an essential micronutrient for plants, playing catalytic or structural roles in enzymes, transcription factors, ribosomes, and membranes. In humans, Zn deficiency is the second most common mineral nutritional disorder, affecting around 30% of the world's population. People living in poverty usually have diets based on milled cereals, which contain low Zn concentrations. Biofortification of crops is an attractive cost-effective solution for low mineral dietary intake. In order to increase the amounts of bioavailable Zn in crop edible portions, it is necessary to understand how plants take up, distribute, and store Zn within their tissues, as well as to characterize potential candidate genes for biotechnological manipulation. The metal tolerance proteins (MTP) were described as metal efflux transporters from the cytoplasm, transporting mainly Zn(2+) but also Mn(2+), Fe(2+), Cd(2+), Co(2+), and Ni(2+). Substrate specificity appears to be conserved in phylogenetically related proteins. MTPs characterized so far in plants have a role in general Zn homeostasis and tolerance to Zn excess; in tolerance to excess Mn and also in the response to iron (Fe) deficiency. More recently, the first MTPs in crop species have been functionally characterized. In Zn hyperaccumulator plants, the MTP1 protein is related to hypertolerance to elevated Zn concentrations. Here, we review the current knowledge on this protein family, as well as biochemical functions and physiological roles of MTP transporters in Zn hyperaccumulators and non-accumulators. The potential applications of MTP transporters in biofortification efforts are discussed.