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

Metal tolerance protein CsMTP4 has dual functions in maintaining zinc homeostasis in tea plant

Plants have evolved a series of zinc (Zn) homeostasis mechanisms to cope with the fluctuating Zn in the environment. How Zn is taken up, translocated and tolerate by tea plant remains unknown. In this study, on the basis of RNA-Sequencing, we isolated a plasma membrane-localized Metal Tolerance Protein (MTP) family member CsMTP4 from Zn-deficient tea plant roots and investigated its role in regulation of Zn homeostasis in tea plant. Heterologous expression of CsMTP4 specifically enhanced the tolerance of transgenic yeast to Zn excess. Moreover, overexpression of CsMTP4 in tea plant hairy roots stimulated Zn uptake under Zn deficiency. In addition, CsMTP4 promoted the growth of transgenic Arabidopsis plants by translocating Zn from roots to shoots under Zn deficiency and conferred the tolerance to Zn excess by enhancing the efflux of Zn from root cells. Transcriptome analysis of the CsMTP4 transgenic Arabidopsis found that the expression of Zn metabolism-related genes were differentially regulated compared with wild-type plants when exposed to Zn deficiency and excess conditions. This study provides a mechanistic understanding of Zn uptake and translocation in plants and a new strategy to improve phytoremediation efficiency.

Physiological response and molecular mechanism of Quercus variabilis under cadmium stress

Heavy metal pollution is a global environmental problem, and Quercus variabilis has a stronger tolerance to Cd stress than do other species. We aimed to explore the physiological response and molecular mechanisms of Q. variabilis to Cd stress. In this study, the antioxidant enzyme activities of leaves were determined, while the photosynthetic parameters of leaves were measured using Handy PEA, and ion fluxes and DEGs in the roots were investigated using noninvasive microtest technology (NMT) and RNA sequencing techniques, respectively. Cd stress at different concentrations and for different durations affected the uptake patterns of Cd2+ and H+ by Q. variabilis and affected the photosynthetic efficiency of leaves. Moreover, there was a positive relationship between antioxidant enzyme (CAT and POD) activity and Cd concentration. Transcriptome analysis revealed that many genes, including genes related to the cell wall, glutathione metabolism, ion uptake and transport, were significantly upregulated in response to cadmium stress in Q. variabilis roots. WGCNA showed that these DEGs could be divided into eight modules. The turquoise and blue modules exhibited the strongest correlations, and the most significantly enriched pathways were the phytohormone signaling pathway and the phenylpropanoid biosynthesis pathway, respectively. These findings suggest that Q. variabilis can bolster plant tolerance by modulating signal transduction and increasing the synthesis of compounds, such as lignin, under Cd stress. In summary, Q. variabilis can adapt to Cd stress by increasing the activity of antioxidant enzymes, and regulating the fluxes of Cd2+ and H+ ions and the expression of Cd stress-related genes.

Transcriptomics combined with physiological analysis reveals the mechanism of cadmium uptake and tolerance in Ligusticum chuanxiong Hort. under cadmium treatment

Introduction

Ligusticum chuanxiong Hort. is a widely used medicinal plant, but its growth and quality can be negatively affected by contamination with the heavy metal cadmium (Cd). Despite the importance of understanding how L. chuanxiong responds to Cd stress, but little is currently known about the underlying mechanisms.

Methods

To address this gap, we conducted physiological and transcriptomic analyses on L. chuanxiong plants treated with different concentrations of Cd2+ (0 mg·L-1, 5 mg·L-1, 10 mg·L-1, 20 mg·L-1, and 40 mg·L-1).

Results

Our findings revealed that Cd stress inhibited biomass accumulation and root development while activating the antioxidant system in L. chuanxiong. Root tissues were the primary accumulation site for Cd in this plant species, with Cd being predominantly distributed in the soluble fraction and cell wall. Transcriptomic analysis demonstrated the downregulation of differential genes involved in photosynthetic pathways under Cd stress. Conversely, the plant hormone signaling pathway and the antioxidant system exhibited positive responses to Cd regulation. Additionally, the expression of differential genes related to cell wall modification was upregulated, indicating potential enhancements in the root cell wall's ability to sequester Cd. Several differential genes associated with metal transport proteins were also affected by Cd stress, with ATPases, MSR2, and HAM3 playing significant roles in Cd passage from the apoplast to the cell membrane. Furthermore, ABC transport proteins were found to be key players in the intravesicular compartmentalization and efflux of Cd.

Discussion

In conclusion, our study provides preliminary insights into the mechanisms underlying Cd accumulation and tolerance in L. chuanxiong, leveraging both physiological and transcriptomic approaches. The decrease in photosynthetic capacity and the regulation of plant hormone levels appear to be major factors contributing to growth inhibition in response to Cd stress. Moreover, the upregulation of differential genes involved in cell wall modification suggests a potential mechanism for enhancing root cell wall capabilities in isolating and sequestering Cd. The involvement of specific metal transport proteins further highlights their importance in Cd movement within the plant.

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

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

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.

Physiological and molecular mechanisms of medicinal plants in response to cadmium stress: Current status and future perspective

Medicinal plants have a wide range of uses worldwide. However, the quality of medicinal plants is affected by severe cadmium pollution. Cadmium can reduce photosynthetic capacity, lead to plant growth retardation and oxidative stress, and affect secondary metabolism. Medicinal plants have complex mechanisms to cope with cadmium stress. On the one hand, an antioxidant system can effectively scavenge excess reactive oxygen species produced by cadmium stress. On the other hand, cadmium chelates are formed by chelating peptides and then sequestered through vacuolar compartmentalization. Cadmium has no specific transporter in plants and is generally transferred to plant tissues through competition for the transporters of divalent metal ions, such as zinc, iron, and manganese. In recent years, progress has been achieved in exploring the physiological mechanisms by which medicinal plants responding to cadmium stress. The exogenous regulation of cadmium accumulation in medicinal plants has been studied, and the aim is reducing the toxicity of cadmium. However, research into molecular mechanisms is still lagging. In this paper, we review the physiological and molecular mechanisms and regulatory networks of medicinal plants exposed to cadmium, providing a reference for the study on the responses of medicinal plants to cadmium stress.

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.

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.

The Effect of Cadmium on Plants in Terms of the Response of Gene Expression Level and Activity

Cadmium (Cd) is a heavy metal that can cause damage to living organisms at different levels. Even at low concentrations, Cd can be toxic to plants, causing harm at multiple levels. As they are unable to move away from areas contaminated by Cd, plants have developed various defence mechanisms to protect themselves. Hyperaccumulators, which can accumulate and detoxify heavy metals more efficiently, are highly valued by scientists studying plant accumulation and detoxification mechanisms, as they provide a promising source of genes for developing plants suitable for phytoremediation techniques. So far, several genes have been identified as being upregulated when plants are exposed to Cd. These genes include genes encoding transcription factors such as iron-regulated transporter-like protein (ZIP), natural resistance associated macrophage protein (NRAMP) gene family, genes encoding phytochelatin synthases (PCs), superoxide dismutase (SOD) genes, heavy metal ATPase (HMA), cation diffusion facilitator gene family (CDF), Cd resistance gene family (PCR), ATP-binding cassette transporter gene family (ABC), the precursor 1-aminocyclopropane-1-carboxylic acid synthase (ACS) and precursor 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) multigene family are also influenced. Thanks to advances in omics sciences and transcriptome analysis, we are gaining more insights into the genes involved in Cd stress response. Recent studies have also shown that Cd can affect the expression of genes related to antioxidant enzymes, hormonal pathways, and energy metabolism.

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.

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.

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

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

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.

Transcriptomic and proteomic analyses uncover the drought adaption landscape of Phoebe zhennan

BACKGROUND

Phoebe zhennan S.Lee (nanmu) is listed as a threatened tree species in China, whose growth and development, especially during the seedling stage, can be severely limited by drought. Previous studies on nanmu responses to drought stress involved physiological and biochemical analyses, while the molecular mechanisms remained unclear. Therefore, it is of great significance to carry out molecular biology research on the drought resistance of nanmu and reveal the genetic background and molecular regulation mechanism of nanmu drought resistance.

RESULTS

Drought stress enhanced the soluble sugar (SS), free proline(PRO), superoxide anion (O2·-), and hydrogen peroxide (H₂O₂) contents as well as the peroxidase (POD) and monodehydroascorbate reductase (MDHAR) activities of nanmu. However, glutathione S-transferase (GST) activity was sensitive to drought stress. Further transcriptomic and proteomic analyses revealed the abundant members of the differentially expressed genes(DEGs) and differentially expressed proteins(DEPs) that were related to phenylpropanoid and flavonoid biosynthesis, hormone biosynthesis and signal transduction, chlorophyll metabolism, photosynthesis, and oxidation-reduction reaction, which suggested their involvement in the drought response of nanmu. These enhanced the osmotic regulation, detoxification, and enzyme-induced and non-enzyme-induced antioxidant ability of nanmu. Moreover, 52% (447/867) of proteins that were up-regulated and 34% (307/892) down-regulated ones were attributed to the increase and decrease of transcription abundance. Transcript up (T^U) and protein up (P^U) groups had 447 overlaps, while transcript down (T^D) and protein down (P^D) groups had 307 overlaps, accounting for 54% of up and 35% of down-regulated proteins. The lack of overlap between DEGs and DEPs also suggested that post-transcriptional regulation has a critical role in nanmu response to drought.

CONCLUSIONS

Our research results provide significant insights into the regulatory mechanisms of drought stress in nanmu.

Identification and evolutionary analysis of the metal-tolerance protein family in eight Cucurbitaceae species

Metal-tolerance proteins (MTPs) are divalent cation transporters and play fundamental roles in plant metal tolerance and ion homeostasis. Despite that, a systematic investigation of MTPs in Cucurbitacea is still lacking. In this study, 142 MTPs were identified from 11 released genomes of 8 Cucurbitaceae species. They were phylogenetically separated into three clusters (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). Characterization analysis revealed that most MTPs were plasma membrane-located hydrophobic proteins. Motif and exon/intron analysis showed that members in the same group contained similar conserved motifs and gene structures. Moreover, 98 pairs of segmental-like duplication events were found. The nonsynonymous/synonymous substitution ratios between each pair were less than 1, implying that Cucurbitaceae MTPs were under purification selection. Expression profiling suggested that several MTP genes, such as CsCLMTP1, CmeMTP3, LsMTP3, and Cl97103MTP3, were constitutively expressed in corresponding Cucurbitaceae species, and their expression levels were not significantly altered by NaCl, drought, or pathogen infection. The expression patterns of cucumber MTP genes under Zn²⁺ , Cu²⁺ , Mn²⁺ , and Cd²⁺ stress were studied by quantitative real-time polymerase chain reaction and the results showed that these MTPs were induced by at least one metal ion, suggesting their involvement in metal tolerance or transportation. The identification and comprehensive investigation of MTP family members will provide a basis for the analysis of ion transport functions and ion tolerance mechanisms of Cucurbitaceae MTPs.

Involvement of several putative transporters of different families in β-cyclocitral-induced alleviation of cadmium toxicity in quinoa (Chenopodium quinoa) seedlings

Cadmium (Cd) has a serious negative impact on crop growth and human food security. This study investigated the alleviating effect of β-cyclocitral, a potential heavy metal barrier, on Cd stress in quinoa seedlings and the associated mechanisms. Our results showed that β-cyclocitral alleviated Cd stress-induced growth inhibition in quinoa seedlings and promoted quinoa seedling root development under Cd stress. Moreover, it maintained the antioxidant system of quinoa seedlings, including the enzymatic, i.e., superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), and nonenzymatic, i.e., reduced glutathione (GSH) and ascorbic acid (ASA), antioxidants, which eliminate the damage from excessive reactive oxygen species (ROS). Our results showed that β-cyclocitral could reduce the amount of Cd absorbed by roots. Furthermore, we systematically identified five transporter families from the quinoa genome, and the RT-qPCR results showed that ZIP, Nramp and YSL gene families were downregulated by β-cyclocitral to reduce Cd uptake by roots. Thus, β-cyclocitral promoted the growth, photosynthetic capacity and antioxidant capacity of the aboveground parts of quinoa seedlings. Taken together, these results suggested that the β-cyclocitral-induced decrease in Cd uptake may be caused by the downregulation of several selected transporter genes.

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.

Dose-Dependent Physiological and Transcriptomic Responses of Lettuce (Lactuca sativa L.) to Copper Oxide Nanoparticles-Insights into the Phytotoxicity Mechanisms

Understanding the complex mechanisms involved in plant response to nanoparticles (NPs) is indispensable in assessing the environmental impact of nano-pollutants. Plant leaves can directly intercept or absorb NPs deposited on their surface; however, the toxicity mechanisms of NPs to plant leaves are unclear. In this study, lettuce leaves were exposed to copper oxide nanoparticles (CuO-NPs, 0, 100, and 1000 mg/L) for 15 days, then physiological tests and transcriptomic analyses were conducted to evaluate the negative impacts of CuO-NPs. Both physiological and transcriptomic results demonstrated that CuO-NPs adversely affected plant growth, photosynthesis, and enhanced reactive oxygen species (ROS) accumulation and antioxidant system activity. The comparative transcriptome analysis showed that 2270 and 4264 genes were differentially expressed upon exposure to 100 and 1000 mg/L CuO-NPs. Gene expression analysis suggested the ATP-binding cassette (ABC) transporter family, heavy metal-associated isoprenylated plant proteins (HIPPs), endocytosis, and other metal ion binding proteins or channels play significant roles in CuO-NP accumulation by plant leaves. Furthermore, the variation in antioxidant enzyme transcript levels (POD1, MDAR4, APX2, FSDs), flavonoid content, cell wall structure and components, and hormone (auxin) could be essential in regulating CuO-NPs-induced stress. These findings could help understand the toxicity mechanisms of metal NPs on crops, especially NPs resulting from foliar exposure.

Comparative transcriptome profile analysis of rice varieties with different tolerance to zinc deficiency

Zinc (Zn) is an indispensable element for rice growth. Zn deficiency results in brown blotches and streaks 2-3 weeks after transplanting, as well as stunting, reduced tillering, and low productivity of rice plants. These processes are controlled by different families of expressed genes. A comparative transcriptome profile analysis was conducted using the roots of two Zn deficiency tolerant varieties (UCP122 and KALIBORO26) and two sensitive varieties (IR26 and IR64) by merging data from untreated control (CK) and Zn deficiency treated samples. Results revealed a total of 4,688 differentially expressed genes (DEGs) between the normal Zn and deficient conditions, with 2,702 and 1,489 unique DEGs upregulated and downregulated, respectively. Functional enrichment analysis identified transcription factors (TFs), such as WRKY, MYB, ERF, and bHLH which are important in the regulation of the Zn deficiency response. Furthermore, chitinases, jasmonic acid, and phenylpropanoid pathways were found to be important in the Zn deficiency response. The metal tolerance protein (MTP) genes also appeared to play an important role in conferring tolerance to Zn deficiency. A heavy metal-associated domain-containing protein 7 was associated with tolerance to Zn deficiency and negatively regulated downstream genes. Collectively, our findings provide valuable expression patterns and candidate genes for the study of molecular mechanisms underlying the response to Zn deficiency and for improvements in breeding for tolerance to Zn deficiency in rice.

Genome-wide analysis of the NRAMP gene family in potato (Solanum tuberosum): Identification, expression analysis and response to five heavy metals stress

NRAMP family genes participate in the absorption and transport of heavy metals such as cadmium (Cd), zinc (Zn), copper (Cu), lead (Pb), iron (Fe) and manganese (Mn) and play an important role in the response to heavy metal stress. There is an abundance of research on these genes in bacteria, plants and fungi, although not in S. tuberosum. A total of 48 members(potato(5), Arabidopsis(7), Tomato(9), pepper(9), rice(12) and tobacco(6)) were identified from 6 species (potato (Solanum tuberosum), Arabidopsis thaliana, Tomato (Solanum lycopersicum), pepper (Capsicum annuum), rice (Oryza sativa) and tobacco (Nicotiana attenuate)) and were classified into four subgroups. Across NRAMP gene family members, there are 15 highly conserved motifs that have similar genetic structures and characteristics. In addition, a total of 16 pairs of colinear genes were found in eight species. Analysis of cis-elements indicated that, in response to abiotic stress, NRAMPs are mainly regulated by phytohormones and transcription factors. In addition, analysis of expression profiles indicated that StNRAMP4 is mainly expressed in the roots. According to a qRT-PCR-based analysis of the StNRAMP family, with the exception of Pb²⁺ stress, StNRAMPs positively responded to stress from Cu²⁺, Cd²⁺, Zn²⁺ and Ni²⁺ and The expression patterns is similar of StNRAMP2, under Pb²⁺, and Cu²⁺ treatment, the relative expression peaked at 24 h. the relative expression peaked at 12 h and was upregulated 428-fold in the roots under Ni²⁺ stress. Under Cd²⁺ stress, StNRAMP3 was upregulated 28-fold in the leaves. StNRAMP1, StNRAMP4 and StNRAMP5 showed significant upregulation under Cu²⁺, Cd²⁺ and Zn²⁺ stress, respectively. Expression of StNRAMPs could be specifically induced by heavy metals, implying their possible role in the transport and absorption of heavy metals. This research explains the colinear characteristics of NRAMPs in several food crop species, which is useful for providing important genetic resources for cultivating food crop that accumulate low amounts of heavy metals and for explaining the biological functions of NRAMPs in plants.

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.

Heavy Metal Stress-Associated Proteins in Rice and Arabidopsis: Genome-Wide Identification, Phylogenetics, Duplication, and Expression Profiles Analysis

Heavy metal exposure is a serious environmental stress in plants. However, plants have evolved several strategies to improve their heavy metal tolerance. Heavy metal-associated proteins (HMPs) participate in heavy metal detoxification. Here, we identified 46 and 55 HMPs in rice and Arabidopsis, respectively, and named them OsHMP 1–46 and AtHMP 1–55 according to their chromosomal locations. The HMPs from both plants were divided into six clades based on the characteristics of their heavy metal-associated domains (HMA). The HMP gene structures and motifs varied greatly among the different classifications. The HMPs had high collinearity and were segmentally duplicated. A cis-element analysis revealed that the HMPs may be regulated by different transcription factors. An expression profile analysis disclosed that only eight OsHMPs were constitutive in rice tissues. Of these, the expression of OsHMP37 was far higher than that of the other seven genes while OsHMP28 was expressed exclusively in the roots. For Arabidopsis, nine AtHMPs presented with very high transcript levels in all organs. Most of the selected OsHMPs were differentially expressed in various tissues under different heavy metal stresses. Only OsHMP09, OsHMP18, and OsHMP22 showed higher expression levels in all tissues under different heavy metal stresses. In contrast, most of the selected AtHMPs had nearly constant expression levels in different tissues under various heavy metal stresses. The AtHMP20, AtHMP23, AtHMP25, AtHMP31, AtHMP35, AtHMP46 expression levels under different heavy metal stresses were higher in the leaves and roots. The foregoing discoveries elucidated HMP evolution in monocotyledonous and dicotyledonous plants and may helpful functionally characterize HMPs in the future.

Comprehensive Analysis of Autophagy-Related Genes in Sweet Orange (Citrus sinensis) Highlights Their Roles in Response to Abiotic Stresses

Autophagy is a highly conserved intracellular degradation pathway that breaks down damaged macromolecules and/or organelles. It is involved in plant development and senescence, as well as in biotic and abiotic stresses. However, the autophagy process and related genes are largely unknown in citrus. In this study, we identified 35 autophagy-related genes (CsATGs—autophagy-related genes (ATGs) of Citrus sinensis, Cs) in a genome-wide manner from sweet orange (Citrus sinensis). Bioinformatic analysis showed that these CsATGs were highly similar to Arabidopsis ATGs in both sequence and phylogeny. All the CsATGs were randomly distributed on nine known (28 genes) and one unknown (7 genes) chromosomes. Ten CsATGs were predicted to be segmental duplications. Expression patterns suggested that most of the CsATG were significantly up- or down-regulated in response to drought; cold; heat; salt; mannitol; and excess manganese, copper, and cadmium stresses. In addition, two ATG18 members, CsATG18a and CsATG18b, were cloned from sweet orange and ectopically expressed in Arabidopsis. The CsATG18a and CsATG18b transgenic plants showed enhanced tolerance to osmotic stress, salt, as well as drought (CsATG18a) or cold (CsATG18b), compared to wild-type plants. These results highlight the essential roles of CsATG genes in abiotic stresses.

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.

Role of lncRNA-ATB in ovarian cancer and its mechanisms of action

This study aimed to elucidate the role of long non-coding RNA activated by transforming growth factor-β (lncRNA-ATB) in ovarian cancer and its underlying mechanisms of action. Expression levels of lncRNA-ATB in ovarian cancer cell line SKOV3 and in a healthy human ovarian cell line were compared using reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The results indicated that lncRNA-ATB was expressed at significantly higher levels in SKOV3 cells compared with the healthy cell line. After downregulation of lncRNA-ATB expression in SKOV3 cells using lncRNA-ATB-short hairpin RNA, cell proliferation, apoptosis, invasion and migration were assessed using Cell counting kit-8, Live Dead staining, Transwell assay and wound healing assay, respectively. RT-qPCR and western blotting were used to quantify the expression of signal transducer and activator of transcription 3 (STAT3), phosphorylated (p)-STAT3, and the additional epithelial to mesenchymal transition (EMT)-related proteins E-cadherin and vimentin in SKOV3 cells. LncRNA-ATB downregulation significantly reduced SKOV3 cell proliferation, invasion and migration, promoted apoptosis, decreased the expression of p-STAT3 and vimentin, and increased E-cadherin expression. Taken together, these results suggest that lncRNA-ATB downregulation can inhibit ovarian cancer cell proliferation, invasion and migration, and promote cell apoptosis. Lnc-RNA-ATB may therefore be a new target for ovarian cancer treatment.

Involvement of glutamate receptors in regulating calcium influx in rice seedlings under Cr exposure

Glutamate receptors (GLRs) are ligand-gated Ca²⁺-permeable channels that govern and modulate the dynamic influx of cytosolic Ca²⁺ in plants. The present study investigated the interaction of OsGLR3 gene expression with subcellular Ca distribution in rice seedlings exposed to chromium (Cr) solution containing Cr(III) or Cr(VI). The results displayed that the accumulation of Ca was evaluated or higher in shoots compared to roots under Cr exposure, and a similar pattern of subcellular Ca distribution was observed between rice tissues exposed to Cr(III) and Cr(VI). Real-time quantitative polymerase chain reaction (qRT-PCR) analysis revealed that eight OsGLR3 isogenes were distinctly expressed in different rice tissues at different levels of Cr exposures. This differential expressions could possible be due to the uptake variations, subcellular distribution and chemical speciation of the two Cr species. Notably, distinct expression patterns of OsGLR3 genes were found between Cr(III) and Cr(VI) exposures, suggesting that different regulation strategies are used to mediate Ca influx in rice materials under different Cr exposures. These results demonstrated a full picture of Cr-induced transcriptional alterations in OsGLR3 expression levels in rice seedlings, and provided suggestive evidence for further investigation on specific OsGLR3 genes participated in the regulation of cytosolic Ca²⁺ concentrations under Cr exposure.

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.

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.

Metallothioneins enhance chromium detoxification through scavenging ROS and stimulating metal chelation in Oryza sativa

Metallothioneins (MTs) is a metal ion binding protein to detoxify heavy metal stress in plant cells. This study examines involvement of MTs in metal chelation and ROS scavenging in rice seedling under Cr induction either Cr(VI) or Cr(III) at three different effective concentrations using Agilent 44K rice microarray and real-time PCR technology. Results showed that the concentration of Cr was higher in roots than in shoots in both Cr treatments. Accumulation of both H₂O₂ and O₂^- in rice tissues was evident, but the fluctuation of H₂O₂ was more remarkable than O₂^-. Both Cr exposures resulted in enhancement of MTs in plant tissues. Results from PCR analysis confirmed that ten specific OsMT genes responsible for regulating ROS removal were expressed differentially in plant tissues as well as in Cr variants, suggesting that their different regulation and responsiveness strategies. Expression patterns of metal chelation-related OsMT genes, after Cr exposure were also inconsistent in rice tissues. Longer exposure periods caused more transcriptional changes in both Cr treatments. We also noticed that OsMT1b might carry more weight during Cr chelation in roots rather than in shoots, while OsMT2c had more important role in eliminating H₂O₂ accumulation in shoots than roots. These results suggest that different speciation of Cr in rice tissues resulted in inconsistent transcriptional changes of OsMT genes, which functioned in different regulation and responsiveness pathways responsible for metal ions chelating and ROS scavenging during Cr detoxification.

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.