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

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

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

A module centered on the transcription factor Msn2 from arbuscular mycorrhizal fungus Rhizophagus irregularis regulates drought stress tolerance in the host plant

Arbuscular mycorrhizal (AM) fungi can form mutualistic endosymbiosis with > 70% of land plants for obtaining fatty acids and sugars, in return, AM fungi promote plant nutrients and water acquisition to enhance plant fitness. However, how AM fungi orchestrate its own signaling components in response to drought stress remains elusive. Here, we identify a transcription factor containing C2H2 zinc finger domains, RiMsn2 from Rhizophagus irregularis. To characterize the RiMsn2, we combined heterologous expression, subcellular localization in yeasts, and biochemical and molecular studies with reverse genetics approaches during the in planta phase. The results indicate that RiMsn2 is highly conserved across AM fungal species and induced during the early stages of symbiosis. It is significantly upregulated in mycorrhizal roots under severe drought conditions. The nucleus-localized RiMsn2 regulates osmotic homeostasis and trehalose contents of yeasts. Importantly, gene silencing analyses indicate that RiMsn2 is essential for arbuscule formation and enhances plant tolerance to drought stress. Results from yeasts and biochemical experiments suggest that the RiHog1-RiMsn2-STREs module controls the drought stress-responsive genes in AM fungal symbiont. In conclusion, our findings reveal that a module centered on the transcriptional activator RiMsn2 from AM fungus regulates drought stress tolerance in host plant.

Identification and Functional Analysis of Two Mitoferrins, CsMIT1 and CsMIT2, Participating in Iron Homeostasis in Cucumber

Mitochondria are one of the major iron sinks in plant cells. Mitochondrial iron accumulation involves the action of ferric reductase oxidases (FRO) and carriers located in the inner mitochondrial membrane. It has been suggested that among these transporters, mitoferrins (mitochondrial iron transporters, MITs) belonging to the mitochondrial carrier family (MCF) function as mitochondrial iron importers. In this study, two cucumber proteins, CsMIT1 and CsMIT2, with high homology to Arabidopsis, rice and yeast MITs were identified and characterized. CsMIT1 and CsMIT2 were expressed in all organs of the two-week-old seedlings. Under Fe-limited conditions as well as Fe excess, the mRNA levels of CsMIT1 and CsMIT2 were altered, suggesting their regulation by iron availability. Analyses using Arabidopsis protoplasts confirmed the mitochondrial localization of cucumber mitoferrins. Expression of CsMIT1 and CsMIT2 restored the growth of the Δmrs3Δmrs4 mutant (defective in mitochondrial Fe transport), but not in mutants sensitive to other heavy metals. Moreover, the altered cytosolic and mitochondrial Fe concentrations, observed in the Δmrs3Δmrs4 strain, were recovered almost to the levels of WT yeast by expressing CsMIT1 or CsMIT2. These results indicate that cucumber proteins are involved in the iron transport from the cytoplasm to the mitochondria.

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

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

Zinc toxicity response in Ceratoides arborescens and identification of CaMTP, a novel zinc transporter

Zinc (Zn) is an essential micronutrient for several physiological and biochemical processes. Changes in soil Zn levels can negatively affect plant physiology. Although the mechanism of Zn nutrition has been studied extensively in crops and model plants, there has been little research on steppe plants, particularly live in alkaline soils of arid and semiarid regions. Ceratoides arborescens is used in arid and semiarid regions as forage and ecological restoration germplasm, which is studied can enrich the mechanism of Zn nutrition. The plants were exposed to three different Zn treatments, Zn-deficient (-Zn 0 mM L^-1), Zn-normal (Control, 0.015 mM L^-1), and Zn-excess (+Zn, 0.15 mM L^-1), for 3 weeks. Individual biomass, ion concentrations, photosynthetic system, and antioxidant characteristics were measured. High Zn supply significantly decreased plant biomass and induced chlorosis and growth defects and increased Zn concentration but decreased Fe and Ca concentrations, unlike in controls (p < 0.05). High Zn supply also reduced plant chlorophyll content, which consequently decreased the photosynthesis rate. Increased concentrations of malondialdehyde and soluble sugar and activities of peroxidase and superoxide dismutase could resist the high-level Zn stress. In contrast, low Zn supply did not affect plant growth performance. We also identified a novel protein through RNA transcriptome analysis, named CaMTP, that complemented the sensitivity of a yeast mutant to excessive Zn, which was found to be localized to the endoplasmic reticulum through transient gene expression in Nicotiana benthamiana. The gene CaMTP identified to be highly sensitive to Zn stress is a potential candidate for overcoming mineral stress in dicot crop plants.

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

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

Phosphorus Starvation- and Zinc Excess-Induced Astragalus sinicus AsZIP2 Zinc Transporter Is Suppressed by Arbuscular Mycorrhizal Symbiosis

Zinc (Zn) is one of the most essential micronutrients for plant growth and metabolism, but Zn excess can impair many basic metabolic processes in plant cells. In agriculture, crops often experience low phosphate (Pi) and high Zn double nutrient stresses because of inordinate agro-industrial activities, while the dual benefit of arbuscular mycorrhizal (AM) fungi protects plants from experiencing both deficient and toxic nutrient stresses. Although crosstalk between Pi and Zn nutrients in plants have been extensively studied at the physiological level, the molecular basis of how Pi starvation triggers Zn over-accumulation in plants and how AM plants coordinately modulate the Pi and Zn nutrient homeostasis remains to be elucidated. Here, we report that a novel AsZIP2 gene, a Chinese milk vetch (Astragalus sinicus) member of the ZIP gene family, participates in the interaction between Pi and Zn nutrient homeostasis in plants. Phylogenetic analysis revealed that this AsZIP2 protein was closely related to the orthologous Medicago MtZIP2 and Arabidopsis AtZIP2 transporters. Gene expression analysis indicated that AsZIP2 was highly induced in roots by Pi starvation or Zn excess yet attenuated by arbuscular mycorrhization in a Pi-dependent manner. Subcellular localization and heterologous expression experiments further showed that AsZIP2 encoded a functional plasma membrane-localized transporter that mediated Zn uptake in yeast. Moreover, overexpression of AsZIP2 in A. sinicus resulted in the over-accumulation of Zn concentration in roots at low Pi or excessive Zn concentrations, whereas AsZIP2 silencing lines displayed an even more reduced Zn concentration than control lines under such conditions. Our results reveal that the AsZIP2 transporter functioned in Zn over-accumulation in roots during Pi starvation or high Zn supply but was repressed by AM symbiosis in a Pi-dependent manner. These findings also provide new insights into the AsZIP2 gene acting in the regulation of Zn homeostasis in mycorrhizal plants through Pi signal.

Comparative and Systematic Omics Revealed Low Cd Accumulation of Potato StMTP9 in Yeast: Suggesting a New Mechanism for Heavy Metal Detoxification

The metal tolerance protein (MTP) family is a very old family with evolutionary conservation and less specific amplification. It seems to retain the original functions of the ancestral genes and plays an important role in maintaining metal homeostasis in plant cells. We identified the potato MTP family members for the first time, the specific and conservative StMPTs were discovered by using systematic and comparative omics. To be surprised, members of the StMTP family seem to have mutated before the evolution of dicotyledon and monocotyledon, and even the loss of the entire subfamily (subfamily G6, G7). Interestingly, StMTP9 represents the conserved structure of the entire subfamily involved in toxic metal regulation. However, the gene structure and transmembrane domain of StMTP8 have undergone specific evolution, showing that the transmembrane domain (Motif13) located at the NH₂ terminal has been replaced by the signal peptide domain, so it was selected as the control gene of StMTP9. Through real-time fluorescence quantitative analysis of StMTPs under Cd and Zn stress, a co-expression network was constructed, and it was found that StMTP9 responded significantly to Cd stress, while StMTP8 did the opposite. What excites us is that by introducing StMTPs 8/9 into the ∆ycf1 yeast cadmium-sensitive mutant strain, the functional complementation experiment proved that StMTPs 8/9 can restore Cd tolerance. In particular, StMTP9 can greatly reduce the cadmium content in yeast cells, while StMTP8 cannot. These findings provide a reference for further research on the molecular mechanism of potato toxic metal accumulation.

Genome-Wide Identification, Structure Characterization, Expression Pattern Profiling, and Substrate Specificity of the Metal Tolerance Protein Family in Canavalia rosea (Sw.) DC

Plant metal tolerance proteins (MTPs) play key roles in heavy metal absorption and homeostasis in plants. By using genome-wide and phylogenetic approaches, the origin and diversification of MTPs from Canavalia rosea (Sw.) DC. was explored. Canavalia rosea (bay bean) is an extremophile halophyte with strong adaptability to seawater and drought and thereby shows specific metal tolerance with a potential phytoremediation ability. However, MTP genes in leguminous plants remain poorly understood. In our study, a total of 12 MTP genes were identified in C. rosea. Multiple sequence alignments showed that all CrMTP proteins possessed the conserved transmembrane domains (TM1 to TM6) and could be classified into three subfamilies: Zn-CDFs (five members), Fe/Zn-CDFs (five members), and Mn-CDFs (two members). Promoter cis-acting element analyses revealed that a distinct number and composition of heavy metal regulated elements and other stress-responsive elements existed in different promoter regions of CrMTPs. Analysis of transcriptome data revealed organ-specific expression of CrMTP genes and the involvement of this family in heavy metal stress responses and adaptation of C. rosea to extreme coral reef environments. Furthermore, the metal-specific activity of several functionally unknown CrMTPs was investigated in yeast. These results will contribute to uncovering the potential functions and molecular mechanisms of heavy metal absorption, translocation, and accumulation in C. rosea plants.

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.

Current understanding of plant zinc homeostasis regulation mechanisms

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

A novel zinc transporter essential for Arabidopsis zinc and iron-dependent growth

Zinc (Zn), an essential micronutrient, is absorbed by plant roots and redistributed to leaves. This process must be finely regulated in order to avoid toxic Zn²⁺ overaccumulation, which can arise due to Zn²⁺ oversupply or Zn²⁺ hyperaccumulation induced by Fe²⁺ deficiency. Although several proteins in Arabidopsis thaliana are essential for retaining Zn in the root and partitioning it from roots to leaves, how Zn²⁺ homeostasis in leaves is maintained is largely unknown. In this study, we identified a novel Golgi-localized protein named ZINC NUTRIENT ESSENTIAL1 (AtZNE1,At3g08650) in Arabidopsis. AtZNE1 contains 14 putative transmembrane domains. AtZNE1 promoter has strong activity in the root and leaf. Its expression complemented the increased sensitivity of a yeast mutant to excess Zn²⁺. The disruption of AtZNE1 in the T-DNA insertion mutant atzne1 caused growth defect under excess-Zn or Fe deficit conditions, but had no effects on the total Zn and Fe contents. We propose that AtZNE1 plays a vital role in plant adaptation to excess Zn or Fe deficiency.

Genome-Wide Identification of Metal Tolerance Protein Genes in Populus trichocarpa and Their Roles in Response to Various Heavy Metal Stresses

Metal tolerance proteins (MTPs) are plant divalent cation transporters that play important roles in plant metal tolerance and homeostasis. Poplar is an ideal candidate for the phytoremediation of heavy metals because of its numerous beneficial attributes. However, the definitive phylogeny and heavy metal transport mechanisms of the MTP family in poplar remain unknown. Here, 22 MTP genes in P. trichocarpa were identified and classified into three major clusters and seven groups according to phylogenetic relationships. An evolutionary analysis suggested that PtrMTP genes had undergone gene expansion through tandem or segmental duplication events. Moreover, all PtrMTPs were predicted to localize in the vacuole and/or cell membrane, and contained typical structural features of the MTP family, cation efflux domain. The temporal and spatial expression pattern analysis results indicated the involvement of PtrMTP genes in poplar developmental control. Under heavy metal stress, most of PtrMTP genes were induced by at least two metal ions in roots, stems or leaves. In addition, PtrMTP8.1, PtrMTP9 and PtrMTP10.4 displayed the ability of Mn transport in yeast cells, and PtrMTP6 could transport Co, Fe and Mn. These findings will provide an important foundation to elucidate the biological functions of PtrMTP genes, and especially their role in regulating heavy metal tolerance in poplar.

Zinc homeostasis in the secretory pathway in yeast

It is estimated that up to 10% of proteins in eukaryotes require zinc for their function. Although the majority of these proteins are located in the nucleus and cytosol, a small subset is secreted from cells or is located within an intracellular compartment. As many of these compartmentalized metalloproteins fold to their native state and bind their zinc cofactor inside an organelle, cells require mechanisms to maintain supply of zinc to these compartments even under conditions of zinc deficiency. At the same time, intracellular compartments can also be the site for storing zinc ions, which then can be mobilized when needed. In this review, we highlight insight that has been obtained from yeast models about how zinc homeostasis is maintained in the secretory pathway and vacuole.

Genome-Wide Identification, Comprehensive Gene Feature, Evolution, and Expression Analysis of Plant Metal Tolerance Proteins in Tobacco Under Heavy Metal Toxicity

Plant metal tolerance proteins (MTPs) comprise a family of membrane divalent cation transporters that play essential roles in plant mineral nutrition maintenance and heavy metal stresses resistance. However, the evolutionary relationships and biological functions of MTP family in tobacco remain unclear. In the present study, 26, 13, and 12 MTPs in three main Nicotiana species (N. tabacum, N. sylvestris, and N. tomentosiformis) were identified and designated, respectively. The phylogenetic relationships, gene structures, chromosome distributions, conserved motifs, and domains of NtMTPs were systematic analyzed. According to the phylogenetic features, 26 NtMTPs were classified into three major substrate-specific groups that were Zn-cation diffusion facilitators (CDFs), Zn/Fe-CDFs, and Mn-CDFs, and seven primary groups (1, 5, 6, 7, 8, 9, and 12). All of the NtMTPs contained a modified signature sequence and the cation_efflux domain, whereas some of them also harbored the ZT_dimer. Evolutionary analysis showed that NtMTP family of N. tabacum originated from its parental genome of N. sylvestris and N. tomentosiformis, and further underwent gene loss and expanded via one segmental duplication event. Moreover, the prediction of cis-acting elements (CREs) and the microRNA target sites of NtMTP genes suggested the diverse and complex regulatory mechanisms that control NtMTPs gene expression. Expression profile analysis derived from transcriptome data and quantitative real-time reverse transcription-PCR (qRT-PCR) analysis showed that the tissue expression patterns of NtMTPs in the same group were similar but varied among groups. Besides, under heavy metal toxicity, NtMTP genes exhibited various responses in either tobacco leaves or roots. 19 and 15 NtMTPs were found to response to at least one metal ion treatment in leaves and roots, respectively. In addition, NtMTP8.1, NtMTP8.4, and NtMTP11.1 exhibited Mn transport abilities in yeast cells. These results provided a perspective on the evolution of MTP genes in tobacco and were helpful for further functional characterization of NtMTP genes.