Metallothioneins 2 and 3 contribute to the metal-adapted phenotype but are not directly linked to Zn accumulation in the metal hyperaccumulator, Thlaspi caerulescens

The Contribution of Trichoderma viride and Metallothioneins in Enhancing the Seed Quality of Avena sativa L. in Cd-Contaminated Soil

Pollution of arable land with heavy metals is a worldwide problem. Cadmium (Cd) is a toxic metal that poses a severe threat to humans' and animals' health and lives. Plants can easily absorb Cd from the soil, and plant-based food is the main means of exposure to this hazardous element for humans and animals. Phytoremediation is a promising plant-based approach to removing heavy metals from the soil, and plant growth-promoting micro-organisms such as the fungi Trichoderma can enhance the ability of plants to accumulate metals. Inoculation of Avena sativa L. (oat) with Trichoderma viride enhances germination and seedling growth in the presence of Cd and, in this study, the growth of 6-month-old oat plants in Cd-contaminated soil was not increased by inoculation with T. viride, but a 1.7-fold increase in yield was observed. The content of Cd in oat shoots depended on the Cd content in the soil. Still, it was unaffected by the inoculation with T. viride. A. sativa metallothioneins (AsMTs) participate in plant-fungi interaction, however, their role in this study depended on MT type and Cd concentration. The inoculation of A. sativa with T. viride could be a promising approach to obtaining a high yield in Cd-contaminated soil without increasing the Cd content in the plant.

A novel metallothionein gene HcMT from halophyte shrub Halostachys caspica respond to cadmium and sodium stress

Cadmium (Cd) and sodium (Na) are two of the most phytotoxic metallic elements causing environmental and agricultural problems. Metallothioneins (MTs) play an important role in the adaptation to abiotic stress. We previously isolated a novel type 2 MT gene from Halostachys caspica (H. caspica), named HcMT, which responded to metal and salt stress. To understand the regulatory mechanisms controlling HcMT expression, we cloned the HcMT promoter and characterized its tissue-specific and spatiotemporal expression patterns. β-Glucuronidase (GUS) activity analysis showed that the HcMT promoter was responsive to CdCl2, CuSO4, ZnSO4 and NaCl stress. Therefore, we further investigated the function of HcMT under abiotic stress in yeast and Arabidopsis thaliana (Arabidopsis). In CdCl2, CuSO4 or ZnSO4 stress, HcMT significantly enhanced the metal ions tolerance and accumulation in yeast through function as a metal chelator. Moreover, the HcMT protein also protected yeast cells from NaCl, PEG and hydrogen peroxide (H2O2) toxicity with less effectiveness. However, transgenic Arabidopsis carrying HcMT gene only displayed tolerance to CdCl2 and NaCl, accompanying by higher content of Cd2+ or Na+ and lower H2O2, compared to wild-type (WT) plants. Next, we demonstrated that the recombinant HcMT protein has the ability to bind Cd2+ and the potential of scavenging ROS (reactive oxygen species) in vitro. This result further confirmed that the role of HcMT to influence plants to CdCl2 and NaCl stress may bind metal ions and scavenge ROS. Overall, we described the biological functions of HcMT and developed a metal- and salt-inducible promoter system for using in genetic engineering.

Heterologous expression of the Leymus chinensis metallothionein gene LcMT3 confers enhanced tolerance to salt stress in Escherichia coli, yeast, and Arabidopsis thaliana

Salinity is poisonous to various plant physiological processes and poses an increasingly severe threat to agricultural productivity worldwide. As a tactic to mitigate this issue, the hunt for salt-tolerance genes and pathways is intensifying. The low-molecular-weight proteins known as metallothioneins (MTs) can effectively reduce salt toxicity in plants. In seeking concrete evidence of its function under salt stress conditions, a unique salt-responsive metallothionein gene, LcMT3, was isolated from the extremely salt-enduring Leymus chinensis and heterologously characterized in Escherichia coli (E. coli), yeast (Saccharomyces cerevisiae), as well as Arabidopsis thaliana. Overexpression of LcMT3 imparted resistance to salt in E. coli cells and yeast, while the development of control cells was completely inhibited. Besides, transgenic plants expressing LcMT3 exhibited significantly enhanced salinity tolerance. They had higher germination rates and longer roots than their nontransgenic counterparts during NaCl tolerance. For several physiological indices of salt tolerance, transgenic lines reduced the accumulation of malondialdehyde (MDA), relative conductivity, and reactive oxygen species (ROS) in comparison to nontransgenic Arabidopsis. They also possessed increased concentrations of proline (Pro), relative water content, chlorophyll content, coupled with three more active antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)). Transgenic plants also accumulated less Na+ and maintained a lower Na+/K+ ratio than control, which can be attributable to the transgene's regulatory effect on transporter proteins such as salt overly sensitive (SOS) and Na+/H+ antiporter (NHX1), as demonstrated by qPCR experiments. Collectively, LcMT3 could have a vital function in salinity resistance and be an essential candidate protein for abiotic stress.

Genome-wide analysis of the metallothionein gene family in cassava reveals its role in response to physiological stress through the regulation of reactive oxygen species

BACKGROUND

Cassava (Manihot esculenta Crantz) is widely planted in tropical and several subtropical regions in which drought, high temperatures, and other abiotic stresses occur. Metallothionein (MT) is a group of conjugated proteins with small molecular weight and rich in cysteine. These proteins play a substantial role in response to physiological stress through the regulation of reactive oxygen species (ROS). However, the biological functions of MT genes in cassava are unknown.

RESULTS

A total of 10 MeMT genes were identified in the cassava genome. The MeMTs were divided into 3 groups (Types 2-4) based on the contents and distribution of Cys residues. The MeMTs exhibited tissue-specific expression and located on 7 chromosomes. The MeMT promoters contain some hormones regulatory and stresses responsiveness elements. MeMTs were upregulated under hydrogen peroxide (H₂O₂) treatment and in respond to post-harvest physiological deterioration (PPD). The results were consistent with defense-responsive cis-acting elements in the MeMT promoters. Further, four of MeMTs were selected and silenced by using the virus-induced gene silencing (VIGS) method to evaluate their functional characterization. The results of gene-silenced cassava suggest that MeMTs are involved in oxidative stress resistance, as ROS scavengers.

CONCLUSION

We identified the 10 MeMT genes, and explore their evolutionary relationship, conserved motif, and tissue-specific expression. The expression profiles of MeMTs under three kinds of abiotic stresses (wounding, low-temperature, and H₂O₂) and during PPD were analyzed. The tissue-specific expression and the response to abiotic stresses revealed the role of MT in plant growth and development. Furthermore, silenced expression of MeMTs in cassava leaves decreased its tolerance to ROS, consistent with its predicted role as ROS scavengers. In summary, our results suggest an important role of MeMTs in response to physiological stress as well as species adaptation via the regulation of ROS homeostasis.

Exogenous hydrogen sulfide and methylglyoxal alleviate cadmium-induced oxidative stress in Salix matsudana Koidz by regulating glutathione metabolism

BACKGROUND

Cadmium (Cd) is a highly toxic element for plant growth. In plants, hydrogen sulfide (H₂S) and methylglyoxal (MG) have emerged as vital signaling molecules that regulate plant growth processes under Cd stress. However, the effects of sodium hydrosulfide (NaHS, a donor of H₂S) and MG on Cd uptake, physiological responses, and gene expression patterns of Salix to Cd toxicity have been poorly understood. Here, Salix matsudana Koidz. seedlings were planted in plastic pot with applications of MG (108 mg kg^- 1) and NaHS (50 mg kg^- 1) under Cd (150 mg kg^- 1) stress.

RESULTS

Cd treatment significantly increased the reactive oxygen species (ROS) levels and malondialdehyde (MDA) content, but decreased the growth parameters in S. matsudana. However, NaHS and MG supplementation significantly decreased Cd concentration, ROS levels, and MDA content, and finally enhanced the growth parameters. Cd stress accelerated the activities of antioxidative enzymes and the relative expression levels of stress-related genes, which were further improved by NaHS and MG supplementation. However, the activities of monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR) were sharply decreased under Cd stress. Conversely, NaHS and MG applications restored the MDHAR and DHAR activities compared with Cd-treated seedlings. Furthermore, Cd stress decreased the ratios of GSH/GSSG and AsA/DHA but considerably increased the H₂S and MG levels and glyoxalase I-II system in S. matsudana, while the applications of MG and NaHS restored the redox status of AsA and GSH and further improved glyoxalase II activity. In addition, compared with AsA, GSH showed a more sensitive response to exogenous applications of MG and NaHS and plays more important role in the detoxification of Cd.

CONCLUSIONS

The present study illustrated the crucial roles of H₂S and MG in reducing ROS-mediated oxidative damage to S. matsudana and revealed the vital role of GSH metabolism in regulating Cd-induced stress.

Phytoremediation of Potentially Toxic Elements: Role, Status and Concerns

Environmental contamination with a myriad of potentially toxic elements (PTEs) is triggered by various natural and anthropogenic activities. However, the industrial revolution has increased the intensity of these hazardous elements and their concentration in the environment, which, in turn, could provoke potential ecological risks. Additionally, most PTEs pose a considerable nuisance to human beings and affect soil, aquatic organisms, and even nematodes and microbes. This comprehensive review aims to: (i) introduce potentially toxic elements; (ii) overview the major sources of PTEs in the major environmental compartments; (iii) briefly highlight the major impacts of PTEs on humans, plants, aquatic life, and the health of soil; (iv) appraise the major methods for tackling PTE-caused pollution; (v) discuss the concept and applications of the major eco-technological/green approaches (comprising phytoextraction, rhizofiltration, phytostabilization, phytovolatilization, and phytorestoration); (vi) highlight the role of microbes in phytoremediation under PTE stress; and (vii) enlighten the major role of genetic engineering in advancing the phytoremediation of varied PTEs. Overall, appropriate strategies must be developed in order to stop gene flow into wild species, and biosafety issues must be properly addressed. Additionally, consistent efforts should be undertaken to tackle the major issues (e.g., risk estimation, understanding, acceptance and feasibility) in order to guarantee the successful implementation of phytoremediation programs, raise awareness of this green technology among laymen, and to strengthen networking among scientists, stakeholders, industrialists, governments and non-government organizations.

SaMT3 in Sedum alfredii drives Cd detoxification by chelation and ROS-scavenging via Cys residues

Metallothioneins (MTs), a group of cysteine-rich proteins, are effective chelators of cadmium (Cd) and play a key role in plant Cd detoxification. However, little is known about the role of single cysteine (Cys) residues in the MTs involved in the adaptation of plants to Cd stress, especially, in hyperaccumulators. In the present study, we functionally characterised SaMT3 in S. alfredii, a Cd/Zn hyperaccumulator native to China. Our results showed that the C- and N- terminal regions of SaMT3 had differential functional natures in S. alfredii and determined its Cd hypertolerance and detoxification. Two CXC motifs within the C-terminal region were revealed to play a crucial role in Cd sensing and binding, whereas the four Cys-residues within the N-terminal region were involved in scavenging reactive oxygen species (ROS). An S. alfredii transgenic system based on callus transformation was developed to further investigate the in-planta gene function. The SaMT3-overexpressing transgenic plant roots were more tolerant to Cd than those of wild-type plants. Knockout of SaMT3 resulted in significantly decreased Cd concentrations and increased ROS levels after exposure to Cd stress. We demonstrated the SaMT3-mediated adaptation strategy in S. alfredii, which uses metal chelation and ROS scavenging in response to Cd stress. Our results further reveal the molecular mechanisms underlying Cd detoxification in hyperaccumulating plants, as well as the relation between Cys-related motifs and the metal binding properties of MTs. This research provides valuable insights into the functions of SaMT3 in S. alfredii, and improves our understanding of Cd hyperaccumulation in plants.

Genome-wide analysis of metallothionein gene family in maize to reveal its role in development and stress resistance to heavy metal

BACKGROUND

Maize (Zea mays L.) is a widely cultivated cereal and has been used as an optimum heavy metal phytoremediation crop. Metallothionein (MT) proteins are small, cysteine-rich, proteins that play important roles in plant growth and development, and the regulation of stress response to heavy metals. However, the MT genes for maize have not been fully analyzed so far.

METHODS

The putative ZmMT genes were identified by HMMER.The heat map of ZmMT genes spatial expression analysis was generated by using R with the log2 (FPKM + 1).The expression profiles of ZmMT genes under three kinds of heavy metal stresses were quantified by using qRT-PCR. The metallothionein proteins was aligned using MAFFT and phylogenetic analysis were constructed by ClustalX 2.1. The protein theoretical molecular weight and pI, subcellular localization, TFs binding sites, were predicted using ProtParam, PSORT, PlantTFDB, respectively.

RESULTS

A total of 9 ZmMT genes were identified in the whole genome of maize. The results showed that eight of the nine ZmMT proteins contained one highly conserved metallothio_2 domain, while ZmMT4 contained a Metallothio_PEC domain. All the ZmMT proteins could be classified into three major groups and located on five chromosomes. The ZmMT promoters contain a large number of hormone regulatory elements and hormone-related transcription factor binding sites. The ZmMT genes exhibited spatiotemporal specific expression patterns in 23 tissues of maize development stages and showed the different expression patterns in response to Cu, Cd, and Pb heavy metal stresses.

CONCLUSIONS

We identified the 9 ZmMT genes, and explored their conserved motif, tissue expression patterns, evolutionary relationship. The expression profiles of ZmMT genes under three kinds of heavy metal stresses (Cu, Cd, Pb) were analyzed. In summary, the expression of ZmMTs have poteintial to be regulated by hormones. The specific expression of ZmMTs in different tissues of maize and the response to different heavy metal stresses are revealed that the role of MT in plant growth and development, and stress resistance to heavy metals.

Comparative transcriptome combined with biochemical and physiological analyses provide new insights toward cadmium accumulation with two contrasting Nicotiana species

Cadmium (Cd) is known as one of the most hazardous elements in the environment and a persistent soil constraint toxic to all flora and fauna. In this study, we conducted physiological, biochemical, and transcriptomic analyses of Nicotiana rustica (N. rustica) and Nicotiana tabacum (N. tabacum) treated with CdCl2 to know the underlying molecular mechanisms of Cd accumulation. As a result, N. rustica had more dry weight than N. tabacum. Additionally, N. rustica accumulated higher Cd concentration (69.65 times), Cd2+ influx (1.32-fold), glutathione S-transferases (GST) enzyme activity (2.54 times), GSH/GSSG (oxidized form of GSH) ratio, increase of superoxide dismutase and CAT and a lower H2 O2 and superoxide (O2 •- ) accumulation in their roots than N. tabacum. Cd mainly distributed in the cytoplasm of both species and N. rustica had a significant proportion in the cell wall. Furthermore, the transcriptomic analysis revealed 173 and 710 differentially expressed genes (DEGs) between control and Cd-stressed plants in the leaves and roots of N. rustica, while 576 and 1543 DEGs were found in the leaves and roots of N. tabacum, respectively. In N. rustica, phenylpropanoid biosynthesis and phenylalanine metabolism were the most enriched pathways, while GSH metabolism, ATP-binding cassette transporters and phenylpropanoid biosynthesis were the most enriched in N. tabacum. Finally, we found that DEGs related to metal influx, sequestration, remobilization, and chelation were responsible for Cd accumulation. These results indicated that N. rustica accumulated higher Cd content than N. tabacum, suggesting that each species utilized different response mechanism under the same Cd stress conditions. The DEGs identified in this study might lead to the identification of genes or pathways related to Cd regulation. This study identifies important regulators related to Cd accumulation.

Screening and Identification of Host Proteins Interacting with Iris lactea var. chinensis Metallothionein IlMT2a by Yeast Two-Hybrid Assay

Iris lactea var. chinensis (Fisch.) (I. lactea var. chinensis) is a well-known cadmium (Cd)-tolerant plant and we have previously shown that the metallothionein gene, IlMT2a, of the plant may be playing a key role in conferring the Cd tolerance. In this study, we have identified several proteins interacting with the IlMT2a by screening yeast two-hybrid library constructed from cDNAs isolated from Cd-treated I. lacteal var. chinensis plants. Putative functions of these proteins include those involved in photosynthesis, ROS scavenge, nutrient transport, and transcriptional regulation, to name a few. In particular, another metallothionein, which we assigned the name of IlMT3, was identified as an interacting partner of the IlMT2a. Unlike IlMT2a, it did not provide any significant protection against Cd toxicity in transgenic Arabidopsis thaliana L. (A. thaliana). To our knowledge, this is the first time ever reporting the interaction of two metallothionein proteins in plants. Learning the biological significance of the interaction between IlMT2a and IlMT3 would be the focus of future study and would be able to provide valuable insights into the understanding plant metallothionein’s diverse and complex roles in coordinating many important cellular physiologies including stress responses, gene regulations, and energy metabolisms.

A metallothionein type 2 from Avicennia marina binds to iron and mediates hydrogen peroxide balance by activation of enzyme catalase

Metallothioneins (MTs) are low molecular weight, cysteine-rich, metal-binding proteins that are important for essential metal homeostasis, protection against oxidative stress, and buffering against toxic heavy metals. In this work the gene encoding an MT type 2 from Avicennia marina (Forssk.) Vierh. (AmMT2) was cloned into pET41a and transformed into the Escherichia coli strain Rosetta (DE3). Following the induction with isopropyl β-_D-1-thiogalactopyranoside, AmMT2 was expressed as glutathione-S-transferase (GST)-tagged fusion protein. The accumulation of Zn²⁺, Cu²⁺, Fe²⁺, Ni²⁺ and Cd²⁺ for strain R-AmMT2 was 4, 8, 5.4, 2 and 1.6 fold of control strain suggesting the role of AmMT2 in accumulation of metals. Particularly the strain R-AmMT2 was able to accumulate 30.7 mg per g dry weight. The cells expressing AmMT2 was more tolerant to hydrogen peroxide and had higher catalase (CAT) activity. To understand the mechanistic action of AmMT2 hydrogen peroxide tolerance, the activity of CAT in the E. coli protein extract was assayed after addition of pure Fe²⁺/GST-AmMT complex and Apo/GST-AmMT2 in vitro. Whereas, the activity of CAT did not change by the addition of Apo/GST-AmMT2, the activity of CAT significantly increased after addition of Fe²⁺/GST-AmMT2. These results show that AmMT2 activates CAT through Fe²⁺ transfer which subsequently causes the oxidative stress tolerance.

Differences of Cd uptake and expression of MT family genes and NRAMP2 in two varieties of ryegrasses

In order to understand the mechanism of the difference of Cd absorption and Cd enrichment in different ryegrass varieties, pot experiment was conducted to study on the response of two varieties of ryegrass (Bond and Abbott) to Cd stress as well as the differences of Cd uptake and expression of MT family genes and NRAMP2. Results showed that root dry weights of two varieties and shoot dry weights of Abbott increased first and then decreased with the increase of Cd level in soil. When exposed to 75 mg kg^-1 Cd, shoot dry weight and plant dry weight of Abbott both reached maximum values (10.92 and 12.03 g pot^-1), which increased by 11.09 and 10.67% compared with the control, respectively. Shoot dry weight and plant dry weight of Bond decreased with the increase of Cd level in soil. When the Cd level in soil was 75 mg kg^-1, shoot Cd concentrations of the two varieties were 111.19 mg kg^-1 (Bond) and 133.69 mg kg^-1 (Abbott), respectively, both of which exceeded the critical value of Cd hyperaccumulator (100 mg kg^-1). The expression of MT gene family and NRAMP2 in the leaf of Bond variety significantly increased at the Cd level of 75 mg kg^-1 and reached maximum value (except MT2C) at Cd level of 150 mg kg^-1. The expression of MT gene family in the stem of Bond variety showed a double-peak pattern, while the expression of NRAMP2 was a single-peak pattern. The expression of MT gene family and NRAMP2 in Abbott variety was consistent with single-peak pattern. The expression of MT gene family and NRAMP2 in leaf both significantly increased at Cd level of 150 mg kg^-1, while that in stem and root significantly increased at Cd level of 75 mg kg^-1. For both varieties of ryegrass, the expression amount of MT family genes and Nramp2 in leaf was higher than that in root and stem, indicating the Cd tolerance of ryegrass can be improved by increasing the expression levels of MT family genes and Nramp2 in stem and root. There was significant genotypic difference in the expression of MT gene family and NRAMP2 between the two varieties of ryegrass, and the expression of MT gene family and NRAMP2 in leaves and stems of Bond variety was higher than that in Abbott variety, while the expression of MT gene family and NRAMP2 in roots of Abbott variety was higher than that in Bond variety. The two gene families investigated in this study may be closely related to Cd uptake, but not related to Cd transport from root to leaf and Cd enrichment in shoot.

Necrotrophic fungus Macrophomina phaseolina tolerates chromium stress through regulating antioxidant enzymes and genes expression (MSN1 and MT)

Cr(VI) tolerance level of phytopathogenic fungus viz., Macrophomina phaseolina (Tassi) Goid was assessed through growth, morphological, physiological, and metal accumulation assays. Initially, the fungus growth assays indicated that the fungus can grow over concentration range of 20-3000 ppm and exhibited high tolerance index (0.88-1.00) and minimum inhibitory concentration at 3500 ppm of Cr. Observations under compound and scanning electron microscope un-revealed the structural features of hyphae under Cr stress as thick-walled, aggregated, branched, short and broken, along with attachment of irregular objects on them. Metal accumulation analysis revealed reduction in Cr(VI) accumulation by the fungus with increase in metal concentration in the growth medium (500-3000 ppm). Cr stress induced upregulation of antioxidant enzyme activities (catalase, peroxidase and polyphenol oxidase), expression of genes (MSN1 and metallothionein) and appearnace of new protein bands suggesting the possible role in protection and survival of M. phaseolina against Cr(VI)-induced oxidative stress. This study concludes that interference of Cr with growth and physiological process of M. phaseolina could affect its infection level on its host plant, therefore, synergistic action of two factors needs to be addressed, which may aid to guide future research efforts in understanding impact of plant-pathogen-heavy metal interaction.

Characterization analysis and heavy metal-binding properties of CsMTL3 in Escherichia coli

Members of the metallothionein (MT) superfamily are involved in coordinating transition metal ions. In plants, MT family members are characterized by their arrangement of Cys residues. In this study, one member of the MT superfamily, CsMTL3, was characterized from a complementary DNA (cDNA) library from young cucumber fruit; CsMTL3 is predicted to encode a 64 amino acid protein with a predicted molecular mass of 6.751 kDa. Phylogenetic analysis identified it as a type 3 family member as the arrangement of N-terminal Cys residues was different from that of MT-like 2. Heterologous expression of CsMTL3 in Escherichia coli improved their heavy metal tolerance, particularly to Cd2+ and Cu2+, and led to increased uptake of Cd2+ and Cu2+; increased uptake was also observed for cells expressing Arabidopsis thaliana metallothionein 3 (AtMT3) and phytochelatin-like (PCL), with greatest uptake in PCL-expressing cells. These findings demonstrate that CsMTL3 can improve metal tolerance, especially for Cd2+ ions, when heterologously expressed in E. coli, and suggest that the composition and arrangement of N-terminal Cys residues are associated with binding capacity and preference for different metal ions.

Iris lactea var. chinensis (Fisch.) cysteine-rich gene llCDT1 enhances cadmium tolerance in yeast cells and Arabidopsis thaliana

IlCDT1, a cysteine-rich protein, was isolated from Iris lactea var. chinensis (Fisch.) (I. lactea var. chinensis). Its transcription was up-regulated by the exogenous application of Cd. The truncated IlCDT1 (25-54) containing 14 Cys residues confers Cd tolerance to yeast as the intact IlCDT1, indicating that Cys residues are required for Cd tolerance presumably by chelating Cd. When the gene was constitutively expressed in A. thaliana, root length of transgenic lines was longer than that of wild-type under 100 μM or 200 μM Cd stress. However, Cd absorption in wild-type was more than in two trangenic lines under 100 μM Cd exposure. IlCDT1 may directly bind Cd, through chelating Cd and avoiding the Cd uptake into the cells. Together, IlCDT1 may be a promising gene for the Cd tolerance improvement.

SUMMARY

Cysteine-rich gene llCDT1 enhances cadmium tolerance in yeast cells and Arabidopsis thaliana.

Genome-Wide Characterization and Analysis of Metallothionein Family Genes That Function in Metal Stress Tolerance in Brassica napus L

Brassica plants exhibit both high biomass productivity and high rates of heavy metal absorption. Metallothionein (MT) proteins are low molecular weight, cysteine-rich, metal-binding proteins that play crucial roles in protecting plants from heavy metal toxicity. However, to date, MT proteins have not been systematically characterized in Brassica. In this study, we identified 60 MTs from Arabidopsis thaliana and five Brassica species. All the MT family genes from Brassica are closely related to Arabidopsis MTs, encoding putative proteins that share similar functions within the same clades. Genome mapping analysis revealed high levels of synteny throughout the genome due to whole genome duplication and segmental duplication events. We analyzed the expression levels of 16 Brassica napus MTs (BnaMTs) by RNA-sequencing and real-time RT-PCR (RT-qPCR) analysis in plants under As3+ stress. These genes exhibited different expression patterns in various tissues. Our results suggest that BnaMT3C plays a key role in the response to As3+ stress in B. napus. This study provides insight into the phylogeny, origin, and evolution of MT family members in Brassica, laying the foundation for further studies of the roles of MT proteins in these important crops.

Comparative expression analysis of genes encoding metallothioneins in response to heavy metals and abiotic stresses in rice (Oryza sativa) and Arabidopsis thaliana

To get insights into the functions of metallothionein (MT) in plant response to multiple stresses, expressions of 10 rice MT genes (OsMTs) and 7 Arabidopsis MT genes (AtMTs) were comprehensively analyzed under combined heavy metal and salt stress. OsMT1a, OsMT1b, OsMT1c, OsMT1g, and OsMT2a were increased by different heavy metals. Notably, ABA remarkably increased OsMT4 up to 80-fold. Combined salt and heavy metals (Cd, Pb, Cu) synergistically increased OsMT1a, OsMT1c, and OsMT1g, whereas combined salt and H₂O₂ or ABA synergistically increased OsMT1a and OsMT4. Heavy metals decreased AtMT1c, AtMT2b, and AtMT3 but cold or ABA increased AtMT1a, AtMT1c, and AtMT2a. AtMT4a was markedly increased by salt stress. Combined salt and other stresses (Pb, Cd, H₂O₂) synergistically increased AtMT4a. Taken together, these findings suggest that MTs in monocot and dicot respond differently to combined stresses, which provides a valuable basis to further determine the roles of MTs in broad stress tolerance.

Characterization of the rubber tree metallothionein family reveals a role in mitigating the effects of reactive oxygen species associated with physiological stress

Metallothioneins (MTs) as reactive oxygen species (ROS) scavengers play important roles in stress response and heavy metal homeostasis. In Hevea brasiliensis (the para rubber tree that is the source of commercial natural rubber) and in other trees, the functions of MTs are not well understood. Latex exudes when the rubber tree is tapped. The flow of latex and its regeneration can be enhanced by tapping, wounding and ethylene treatment, all of which produce ROS as a by-product. Here, we show the presence of four MT genes in H. brasiliensis, comprising three Type 2 (HbMT2, -2a and -2b) and one Type 3 (HbMT3L) isoforms, representing one of the smallest MT gene families among angiosperms. The four HbMTs exhibited distinct tissue expression patterns: HbMT2 and HbMT3L mainly in leaves, HbMT2a specifically in flowers and HbMT2b in diverse tissues. The expression of HbMT2b, an isoform present in latex, decreased significantly in the latex following the stress-inducing treatments of tapping, wounding and ethephon (an ethylene generator). The expressions of the leaf-abundant isoforms, HbMT2 and -3L were up-regulated following pathogenic fungus infection and high-temperature stress, but down-regulated by low-temperature stress. These reactions were consistent with multiple defense- and hormone-responsive cis-acting elements in the HbMT promoters. Nine transcription factors were shown to implicate in the high-temperature responsiveness of HbMT2 and -3L in leaves. Overexpression of HbMT2 in Escherichia coli enhanced the bacterium's tolerance to heavy metals and ROS, consistent with its predicted role as an ROS scavenger. Taken together, our results, along with other relevant studies, suggest an important role of HbMTs in latex regeneration as well as species adaptation via the regulation of ROS homeostasis.

Functional characterization of a type 2 metallothionein gene, SsMT2, from alkaline-tolerant Suaeda salsa

A type 2 metallothionein gene, SsMT2, was cloned from Suaeda salsa, a salt- and alkali-tolerant plant, which is dominant species on the saline/alkali soil of northeast China. The SsMT2 gene was expressed in all organs except the flower and its expression was induced by various stresses such as CdCl₂, NaCl, NaHCO₃, and H₂O₂ treatments. SsMT2-transgenic yeast (Saccharomyces cerevisiae) and plants (Arabidopsis thaliana) showed significantly increased resistance to metal, salt and oxidant stresses. These transgenics accumulated more Cd²⁺, but less Na⁺ than their wild type counterparts. SsMT2 transgenic Arabidopsis maintained lower level of H₂O₂ than wild type plants did in response to the stress treatments. These results demonstrated that the SsMT2 gene plays an important role in reactive oxygen species scavenging and confers enhanced metal and oxidant tolerance to plants.

Enhanced metal tolerance correlates with heterotypic variation in SpMTL, a metallothionein-like protein from the hyperaccumulator Sedum plumbizincicola

Mechanistic insight into metal hyperaccumulation is largely restricted to Brassicaceae plants; therefore, it is of great importance to obtain corresponding knowledge from system outside the Brassicaceae. Here, we constructed and screened a cDNA library of the Cd/Zn hyperaccumulator Sedum plumbizincicola and identified a novel metallothionein-like protein encoding gene SpMTL. SpMTL showed functional similarity to other known MT proteins and also to its orthologues from non-hyperaccumulators. However, three additional cysteine residues were observed in SpMTL and appeared to be hyperaccumulator specific. Removal of these three residues significantly decreased its ability to tolerate Cd and the stoichiometry of Cd against SpMTL (molar ratio of Cd/SpMTL) to a level comparable to those of Cd/SaMTL and Cd/SeMTL in the corresponding non-hyperaccumulating relatives. SpMTL expressed in S. plumbizincicola roots at a much higher level than those of its orthologues in the non-hyperaccumulator roots. Interestingly, a positive correlation was observed between transcript levels of SpMTL in roots and Cd accumulation in leaves. Taking these results together, we propose that elevated transcript levels and heterotypic variation in protein sequences of SpMTL might contribute to the trait of Cd hyperaccumulation and hypertolerance in S. plumbizincicola.

Cucumber Metallothionein-Like 2 (CsMTL2) Exhibits Metal-Binding Properties

We identified a novel member of the metallothionein (MT) family, Cucumis sativus metallothionein-like 2 (CsMTL2), by screening a young cucumber fruit complementary DNA (cDNA) library. The CsMTL2 encodes a putative 77-amino acid Class II MT protein that contains two cysteine (Cys)-rich domains separated by a Cys-free spacer region. We found that CsMTL2 expression was regulated by metal stress and was specifically induced by Cd2+ treatment. We investigated the metal-binding characteristics of CsMTL2 and its possible role in the homeostasis and/or detoxification of metals by heterologous overexpression in Escherichia coli cells. Furthermore, we produced a deletion mutant form of the protein, CsMTL2m, that contained the two Cys-rich clusters but lacked the spacer region, in E. coli. We compared the metal-binding properties of CsMTL2 with those of CsMTL2m, the β domain of human metallothionein-like protein 1 (HsMTXb), and phytochelatin-like (PCL) heterologously expressed in E. coli using metal-binding assays. We found that E. coli cells expressing CsMTL2 accumulated the highest levels of Zn2+ and Cd2+ of the four transformed cell types, with levels being significantly higher than those of control cells containing empty vector. E. coli cells expressing CsMTL2 had a higher tolerance for cadmium than for zinc ions. These findings show that CsMTL2 improves metal tolerance when heterologously expressed in E. coli. Future studies should examine whether CsMTL2 improves metal tolerance in planta.

Gain-of-function in Arabidopsis (GAINA) for identifying functional genes in Hevea brasiliensis

BACKGROUND

Forward genetics approaches are not popularly applied in non-model plants due to their complex genomes, long life cycles, backward genetic studies etc. Researchers have to adopt reverse genetic methods to characterize gene functions in non-model plants individually, the efficiency of which is usually low.

RESULTS

In this study, we report a gain-of-function in Arabidopsis (GAINA) strategy which can be used for batch identification of functional genes in a plant species. This strategy aims to obtain the gain-of-function of rubber tree genes through overexpressing transformation ready full-length cDNA libraries in Arabidopsis. An initial transformation test produced about two thousand independent transgenic Arabidopsis lines, in which multiple obvious aberrant phenotypes were observed, suggesting the gain-of-function of rubber tree genes. The transferred genes were further isolated and identified. One gene identified to be metallothionein-like protein type 3 gene was further transferred into Arabidopsis and reproduced a similar aberrant phenotype.

CONCLUSION

The GAINA system proves to be an efficient tool for batch identification of functional genes in Hevea brasiliensis, and also applicable in other non-model plants.

Elemental imaging of leaves from the metal hyperaccumulating plant Noccaea caerulescens shows different spatial distribution of Ni, Zn and Cd

Elemental imaging using laser ablation inductively coupled plasma mass spectrometry was performed on whole leaves of the hyperaccumulating plantNoccaea caerulescensafter treatments with either Ni, Zn or Cd.

Expression response of duplicated metallothionein 3 gene to copper stress in Silene vulgaris ecotypes

Metallothioneins (MTs) were identified as important players in metal metabolism. MT3 gene presents a key metallothionein controlling copper homeostasis in plants. We have selected one cupricolous and one non-cupricolous ecotype to isolate and analyse the MT3 gene in Silene vulgaris. For expression data comparison, we have also included other metal-tolerant ecotypes. Based on a S. vulgaris BAC library screening, we have identified and sequenced a genomic clone containing MT3 gene (SvMT3). We found that SvMT3 gene has been locally duplicated in a tandem arrangement. Expression analysis and complementation studies using yeast mutants showed that both copies of the SvMT3 gene were functional. Moreover, we examined the expression of MT3 gene(s) in selected ecotypes under different copper treatments to show the tissue-specific expression response to copper stress. We demonstrated that higher copper concentrations specifically affected MT3 expression among ecotypes. Our analysis shows that MT3a has similar expression pattern in cupricolous ecotypes while MT3b has common expression features shared by all metallophyte S. vulgaris ecotypes. Our data indicate that down-regulation of MT3b root expression in higher copper concentrations is associated with copper stress. We propose that there might be a specific regulation of SvMT3s transcription depending on the type of heavy metal tolerance.

The SbMT-2 gene from a halophyte confers abiotic stress tolerance and modulates ROS scavenging in transgenic tobacco

Heavy metals are common pollutants of the coastal saline area and Salicornia brachiata an extreme halophyte is frequently exposed to various abiotic stresses including heavy metals. The SbMT-2 gene was cloned and transformed to tobacco for the functional validation. Transgenic tobacco lines (L2, L4, L6 and L13) showed significantly enhanced salt (NaCl), osmotic (PEG) and metals (Zn++, Cu++ and Cd++) tolerance compared to WT plants. Transgenic lines did not show any morphological variation and had enhanced growth parameters viz. shoot length, root length, fresh weight and dry weight. High seed germination percentage, chlorophyll content, relative water content, electrolytic leakage and membrane stability index confirmed that transgenic lines performed better under salt (NaCl), osmotic (PEG) and metals (Zn++, Cu++ and Cd++) stress conditions compared to WT plants. Proline, H2O2 and lipid peroxidation (MDA) analyses suggested the role of SbMT-2 in cellular homeostasis and H2O2 detoxification. Furthermore in vivo localization of H2O2 and O2-; and elevated expression of key antioxidant enzyme encoding genes, SOD, POD and APX evident the possible role of SbMT-2 in ROS scavenging/detoxification mechanism. Transgenic lines showed accumulation of Cu++ and Cd++ in root while Zn++ in stem under stress condition. Under control (unstressed) condition, Zn++ was accumulated more in root but accumulation of Zn++ in stem under stress condition suggested that SbMT-2 may involve in the selective translocation of Zn++ from root to stem. This observation was further supported by the up-regulation of zinc transporter encoding genes NtZIP1 and NtHMA-A under metal ion stress condition. The study suggested that SbMT-2 modulates ROS scavenging and is a potential candidate to be used for phytoremediation and imparting stress tolerance.

Differential responses of three sweetpotato metallothionein genes to abiotic stress and heavy metals

Metallothioneins (MTs) are cysteine-rich, low molecular weight, metal-binding proteins that are widely distributed in living organisms. Plants produce metal-chelating proteins such as MTs to overcome the toxic effects of heavy metals. We cloned three MT genes from sweetpotato leaves [Ipomoea batatas (L.) Lam.]. The three IbMT genes were classified according to their cysteine residue alignment into type 1 (IbMT1), type 2 (IbMT2), and type 3 (IbMT3). IbMT1 was the most abundantly transcribed MT. It was predominantly expressed in leaves, roots, and callus. IbMT2 transcript was detected only in stems and fibrous roots, whereas IbMT3 was strongly expressed in leaves and stems. The IbMT expression profiles were investigated in plants exposed to heavy metals and abiotic stresses. The levels of IbMT1 expression were strongly elevated in response to Cd and Fe, and moderately higher in response to Cu. The IbMT3 expression pattern in response to heavy metals was similar to that of IbMT1. Exposure to abiotic stresses such as methyl viologen (MV; paraquat), NaCl, polyethylene glycol (PEG), and H2O2 up-regulated IbMT expression; IbMT1 responded strongly to MV and NaCl, whereas IbMT3 was induced by low temperature and PEG. Transgenic Escherichia coli overexpressing IbMT1 protein exhibited results suggest that IbMT could be a useful tool for engineering plants with enhanced tolerance to environmental stresses and heavy metals.

Metallothionein 2 (SaMT2) from Sedum alfredii Hance confers increased Cd tolerance and accumulation in yeast and tobacco

Metallothioneins are cysteine-rich metal-binding proteins. In the present study, SaMT2, a type 2 metallothionein gene, was isolated from Cd/Zn co-hyperaccumulator Sedum alfredii Hance. SaMT2 encodes a putative peptide of 79 amino acid residues including two cysteine-rich domains. The transcript level of SaMT2 was higher in shoots than in roots of S. alfredii, and was significantly induced by Cd and Zn treatments. Yeast expression assay showed SaMT2 significantly enhanced Cd tolerance and accumulation in yeast. Ectopic expression of SaMT2 in tobacco enhanced Cd and Zn tolerance and accumulation in both shoots and roots of the transgenic plants. The transgenic plants had higher antioxidant enzyme activities and accumulated less H2O2 than wild-type plants under Cd and Zn treatment. Thus, SaMT2 could significantly enhance Cd and Zn tolerance and accumulation in transgenic tobacco plants by chelating metals and improving antioxidant system.

Overexpression of Iris. lactea var. chinensis metallothionein llMT2a enhances cadmium tolerance in Arabidopsis thaliana

Metallothioneins (MTs) are cysteine-rich, low molecular weight, heavy metal-binding protein molecules. Here, a full-length cDNA homologue of MT2a (type 2 metallothionein) was isolated from the cadmium-tolerant species Iris. lactea var. chinensis (I. lactea var. chinensis). Expression of IlMT2a in I. lactea var. chinensis roots and leaves was up-regulated in response to cadmium stress. When the gene was constitutively expressed in Arabidopsis thaliana (A. thaliana), root length of transgenic lines was longer than that of wild-type under 50μM or 100μM cadmium stress. However, there was no difference of cadmium absorption between wild-type and trangenic lines. Histochemical staining by 3,3-diaminobenzidine (DAB) and nitroblue tetrazoliu (NBT) clearly demonstrated that transgenic lines accumulated remarkably less H2O2 and O2(-) than wild-type. Together, IlMT2a may be a promising gene for the cadmium tolerance improvement.

Abiotic stress response in yeast and metal-binding ability of a type 2 metallothionein-like protein (PutMT2) from Puccinellia tenuiflora

Metallothioneins are low-molecular weight and cysteine-rich metal-binding proteins that play predominant cellular roles in the scavenging of reactive oxygen species and in mediating metal metabolism. To evaluate the role of a type-2 metallothionein-like gene from Puccinellia tenuiflora (PutMT2), the gene was over-expressed in yeast, and growth was assessed under a variety of abiotic stress conditions including peroxide (H2O2), salinity (NaCl and NaHCO3), and metal stress. PutMT2 overexpression in yeast improved the tolerance of cells to H2O2, NaCl, NaHCO3, Zn(2+), Fe(2+), Fe(3+), Cd(2+), Cr(6+), and Ag(+), but increased the sensitivity of cells to Mn(2+), Co(2+), Cu(2+), and Ni(2+) compared with control cells. PutMT2 was then expressed in Escherichia coli BL21as a glutathione S-transferase (GST) fusion protein (GST-PutMT2), and the metal-binding ability of GST-PutMT2 was analyzed and compared with GST alone using inductively coupled plasma atomic emission spectroscopy. Results showed that PutMT2 could bind to Cr, Cd, Co, Ag, Ba, Pb, Mn, Zn, Fe, Cu, P, Al, and Mg, but not Ni and As. There was no evidence to suggest that PutMT2 exhibited a specific or selective binding tendency to any individual metal ion. PutMT2 protein bound to Zn, Na, and Cu in vivo, perhaps with the highest affinity for Cu. Taken together, our results suggest that PutMT2 protein could play an important role in improving metal tolerance by metal binding in yeast. However, additional studies are required to confirm these results and to clarify the function of PutMT2 in transgenic plants.

Expression of metallothionein of freshwater crab (Sinopotamon henanense) in Escherichia coli enhances tolerance and accumulation of zinc, copper and cadmium

Metallothioneins (MTs) are ubiquitous metal-binding, cysteine-rich, small proteins and play a major role in metal homeostasis and/or detoxification in all organisms. In a previous study, a novel full length MT gene was isolated from the freshwater crab (Sinopotamon henanense), a species widely distributed in Shanxi and Henan Provinces, China. In this report, the gene for the crab MT was inserted into a PET-28a-6His-SUMO vector and recombinant soluble MT was over-expressed as fusions with SUMO in Escherichia coli. The recombinant fusion protein was purified by affinity chromatography and its biochemical properties were analyzed. In addition, on the basis of constructing SUMO-MT, two mutants, namely SUMO-MTt1 and SUMO-MTt2, were constructed to change the primary structure of SUMO-MT using site-directed mutagenesis techniques with the amino acid substitutions D3C and S37C in order to increase metal-binding capacity of MT. E. coli cells expressing SUMO-MT and these single-mutant proteins exhibited enhanced metal tolerance and higher accumulation of metal ions than control cells. The results showed that the bioaccumulation and tolerance of Zn(2+), Cu(2+) and Cd(2+) in these strains followed the decreasing order of SUMO-MTt1 > SUMO-MTt2 > SUMO-MT. E. coli cells have low tolerance and high accumulation towards cadmium compared to zinc and copper. These results show that the MT of S. henanense could enhance tolerance and accumulation of metal ions. Moreover, we were able to create a novel protein based on the crab MT to bind metal ions at high density and with high affinity. Therefore, SUMO-MT and its mutants can provide potential candidates for heavy metal bioremediation. This study could help further elucidate the mechanism of how the crab detoxifies heavy metals and provide a scientific basis for environment bioremediation of heavy metal pollution using the over-expression of the crab MT and mutant proteins.

Recent advances in the analysis of metal hyperaccumulation and hypertolerance in plants using proteomics

Hyperaccumulator/hypertolerant plant species have evolved strategies allowing them to grow in metal-contaminated soils, where they accumulate high concentrations of heavy metals in their shoots without signs of toxicity. The mechanisms that allow enhanced metal uptake, root-to-shoot translocation and detoxification in these species are not fully understood. Complementary approaches such as transcriptomic-based DNA microarrays and proteomics have recently been used to gain insight into the molecular pathways evolved by metal hyperaccumulator/hypertolerant species. Proteomics has the advantage of focusing on the translated portion of the genome and it allows to analyze complex networks of proteins. This review discusses the recent analysis of metal hyperaccumulator/hypertolerant plant species using proteomics. Changes in photosynthetic proteins, sulfur, and glutathione metabolism, transport, biotic and xenobiotic defenses as well as the differential regulation of proteins involved in signaling and secondary metabolism are discussed in relation to metal hyperaccumulation. We also consider the potential contribution of several proteins to the hyperaccumulation phenotype.

Expression pattern of a type-2 metallothionein gene in a wild population of the psammophyte Silene nicaeensis

Silene nicaeensis is a wild Mediterranean grass often restricted to sandy sea shore and exhibiting an excellent tolerance to drought and salinity. Within Silene genus, several heavy metal-tolerant ecotypes have been identified, but information on molecular basis of such metal tolerance is still limited. Conceivably, salt-tolerant plants may represent a powerful tool for the remediation of heavy metal contaminated sites in saline environment. Here, a gene encoding a metallothionein protein was isolated from S. nicaeensis. Sequence analysis identified the motifs characteristic of type II metallothionein and designated as SnMT2. SnMT2 expression was investigated in plants collected from two sites differing in Metal Pollution Index (MPI). SnMT2 expression by polymerase chain reaction-based semi-quantitative transcript analysis showed a high accumulation in the leaves; in situ hybridization showed a steady localization of SnMT2 mRNA in the vascular bundle and in proliferating tissues. Moreover, an increase of SnMT2 was observed in the root of plants collected from area with higher MPI. The putative role of SnMT2 in metal tolerance is discussed.

Zn tolerance of novel Colocasia esculenta metallothionein and its domains in Escherichia coli and tobacco

Contrary to extensive researches on the roles of metallothioneins (MTs) in metal tolerance of animals, the roles of plant MTs in metal tolerance are largely under investigation. In this study, we evaluated the functional role of type 2 MT from Colocasia esculenta (CeMT2b) in Zn tolerance of tobacco and E. coli cells. Under Zn-stress conditions, transgenic tobacco overexpressing CeMT2b displayed much better seedling growth, a significant decrease in the levels of H(2)O(2) and an increase in Zn accumulation compared with the wild type. Overexpression of CeMT2b in E. coli greatly enhanced Zn tolerance and Zn accumulation under Zn stresses compared with control cells. CeMT2b bound 5.38 ± 0.29 atoms of Zn per protein. To identify a structural domain of CeMT2b for Zn binding, we investigated the growth of E. coli expressing each of the N-terminal, C-terminal, and central linker domains or a CNC motif deletion from the C-terminus of full-length CeMT2b. The results showed that the CNC motif is required for Zn tolerance, and the N-terminal domain is more effective in Zn tolerance than the C-terminal domain. Taken together, our results provide direct evidence for functional contributions of CeMT2b in Zn tolerance of tobacco and E. coli cells.

The molecular mechanism of zinc and cadmium stress response in plants

When plants are subjected to high metal exposure, different plant species take different strategies in response to metal-induced stress. Largely, plants can be distinguished in four groups: metal-sensitive species, metal-resistant excluder species, metal-tolerant non-hyperaccumulator species, and metal-hypertolerant hyperaccumulator species, each having different molecular mechanisms to accomplish their resistance/tolerance to metal stress or reduce the negative consequences of metal toxicity. Plant responses to heavy metals are molecularly regulated in a process called metal homeostasis, which also includes regulation of the metal-induced reactive oxygen species (ROS) signaling pathway. ROS generation and signaling plays an important duel role in heavy metal detoxification and tolerance. In this review, we will compare the different molecular mechanisms of nutritional (Zn) and non-nutritional (Cd) metal homeostasis between metal-sensitive and metal-adapted species. We will also include the role of metal-induced ROS signal transduction in this comparison, with the aim to provide a comprehensive overview on how plants cope with Zn/Cd stress at the molecular level.

Tolerance to high Zn in the metallophyte Erica andevalensis Cabezudo & Rivera

The tolerance to high Zn was studied in the metallophyte Erica andevalensis Cabezudo & Rivera grown in nutrient solutions at different Zn concentrations (5, 500, 1,000, 1,500 and 2,000 μM Zn). Plant growth and nutrient uptake were determined. Metabolic changes were assessed by the analysis of peroxidase activity, organic metabolites related to metal chelation (amino acids, organic acids (malate, citrate) or protection (polyamines). While plants tolerated up to 1,500 μM Zn, despite presenting of low growth rates, the concentration of 2,000 μM Zn was toxic producing high mortality rates. Roots accumulated high Zn concentration (11,971 mg/kg) at 1,500 μM external Zn) apparently avoiding metal transfer into shoots. After 30 days of treatment with high Zn (1,000 and 1,500 μM Zn), the leaves accumulated high levels of glutamine. Short-term treatment with 500 μM Zn, significantly increased the concentration of asparagine and glutamine in roots. Citrate concentration was also considerably increased when exposing roots to Zn excess. Metal immobilization in the root system, low interference with the uptake of nutrients and an increased production of putative organic ligands (amino acids, citrate) might have provided the Zn tolerance displayed by Erica andevalensis.

The zinc homeostasis network of land plants

The use of the essential element zinc (Zn) in the biochemistry of land plants is widespread, and thus comparable to that in other eukaryotes. Plants have evolved the ability to adjust to vast fluctuations in external Zn supply, and they can store considerable amounts of Zn inside cell vacuoles. Moreover, among plants there is overwhelming, but yet little explored, natural genetic diversity that phenotypically affects Zn homeostasis. This results in the ability of specific races or species to thrive in different soils ranging from extremely Zn-deficient to highly Zn-polluted. Zn homeostasis is maintained by a tightly regulated network of low-molecular-weight ligands, membrane transport and Zn-binding proteins, as well as regulators. Here we review Zn homeostasis of land plants largely based on the model plant Arabidopsis thaliana, for which our molecular understanding is most developed at present. There is some evidence for substantial conservation of Zn homeostasis networks among land pants, and this review can serve as a reference for future comparisons. Major progress has recently been made in our understanding of the regulation of transcriptional Zn deficiency responses and the role of the low-molecular-weight chelator nicotianamine in plant Zn homeostasis. Moreover, we have begun to understand how iron (Fe) and Zn homeostasis interact as a consequence of the chemical similarity between their divalent cations and the lack of specificity of the major root iron uptake transporter IRT1. The molecular analysis of Zn-hyperaccumulating plants reveals how metal homeostasis networks can be effectively modified. These insights are important for sustainable bio-fortification approaches. This article is part of a Special Issue entitled: Cell Biology of Metals.

The chloroplast permease PIC1 regulates plant growth and development by directing homeostasis and transport of iron

The membrane-spanning protein PIC1 (for permease in chloroplasts 1) in Arabidopsis (Arabidopsis thaliana) was previously described to mediate iron transport across the inner envelope membrane of chloroplasts. The albino phenotype of pic1 knockout mutants was reminiscent of iron-deficiency symptoms and characterized by severely impaired plastid development and plant growth. In addition, plants lacking PIC1 showed a striking increase in chloroplast ferritin clusters, which function in protection from oxidative stress by sequestering highly reactive free iron in their spherical protein shell. In contrast, PIC1-overexpressing lines (PIC1ox) in this study rather resembled ferritin loss-of-function plants. PIC1ox plants suffered from oxidative stress and leaf chlorosis, most likely originating from iron overload in chloroplasts. Later during growth, plants were characterized by reduced biomass as well as severely defective flower and seed development. As a result of PIC1 protein increase in the inner envelope membrane of plastids, flower tissue showed elevated levels of iron, while the content of other transition metals (copper, zinc, manganese) remained unchanged. Seeds, however, specifically revealed iron deficiency, suggesting that PIC1 overexpression sequestered iron in flower plastids, thereby becoming unavailable for seed iron loading. In addition, expression of genes associated with metal transport and homeostasis as well as photosynthesis was deregulated in PIC1ox plants. Thus, PIC1 function in plastid iron transport is closely linked to ferritin and plastid iron homeostasis. In consequence, PIC1 is crucial for balancing plant iron metabolism in general, thereby regulating plant growth and in particular fruit development.

The chloroplast permease PIC1 regulates plant growth and development by directing homeostasis and transport of iron

The membrane-spanning protein PIC1 (for permease in chloroplasts 1) in Arabidopsis (Arabidopsis thaliana) was previously described to mediate iron transport across the inner envelope membrane of chloroplasts. The albino phenotype of pic1 knockout mutants was reminiscent of iron-deficiency symptoms and characterized by severely impaired plastid development and plant growth. In addition, plants lacking PIC1 showed a striking increase in chloroplast ferritin clusters, which function in protection from oxidative stress by sequestering highly reactive free iron in their spherical protein shell. In contrast, PIC1-overexpressing lines (PIC1ox) in this study rather resembled ferritin loss-of-function plants. PIC1ox plants suffered from oxidative stress and leaf chlorosis, most likely originating from iron overload in chloroplasts. Later during growth, plants were characterized by reduced biomass as well as severely defective flower and seed development. As a result of PIC1 protein increase in the inner envelope membrane of plastids, flower tissue showed elevated levels of iron, while the content of other transition metals (copper, zinc, manganese) remained unchanged. Seeds, however, specifically revealed iron deficiency, suggesting that PIC1 overexpression sequestered iron in flower plastids, thereby becoming unavailable for seed iron loading. In addition, expression of genes associated with metal transport and homeostasis as well as photosynthesis was deregulated in PIC1ox plants. Thus, PIC1 function in plastid iron transport is closely linked to ferritin and plastid iron homeostasis. In consequence, PIC1 is crucial for balancing plant iron metabolism in general, thereby regulating plant growth and in particular fruit development.

Plant metallothioneins--metal chelators with ROS scavenging activity?

Metallothioneins (MTs) are ubiquitous cysteine-rich proteins present in plants, animals, fungi and cyanobacteria. In plants, MTs are suggested to be involved in metal tolerance or homeostasis, as they are able to bind metal ions through the thiol groups of their cysteine residues. Recent reports show that MTs are also involved in the scavenging of reactive oxygen species (ROS). The interplay between these roles is not entirely clear. Plants have many MT isoforms with overlapping expression patterns, and no specific role for any of them has been assigned. This review is focused on recent findings on plant MTs.

Tamarix hispida metallothionein-like ThMT3, a reactive oxygen species scavenger, increases tolerance against Cd(2+), Zn(2+), Cu(2+), and NaCl in transgenic yeast

A metallothionein-like gene, ThMT3, encoding a type 3 metallothionein, was isolated from a Tamarix hispida leaf cDNA library. Expression analysis revealed that mRNA of ThMT3 was upregulated by high salinity as well as by heavy metal ions, and that ThMT3 was predominantly expressed in the leaf. Transgenic yeast (Saccharomyces cerevisiae) expressing ThMT3 showed increased tolerance to Cd(2+), Zn(2+), Cu(2+), and NaCl stress. Transgenic yeast also accumulated more Cd(2+), Zn(2+), and NaCl, but not Cu(2+). Analysis of the expression of four genes (GLR1, GTT2, GSH1, and YCF1) that aid in transporting heavy metal (Cd(2+)) from the cytoplasm to the vacuole demonstrated that none of these genes were induced under Cd(2+), Zn(2+), Cu(2+), and NaCl stress in ThMT3-transgenic yeast. H(2)O(2) levels in transgenic yeast under such stress conditions were less than half those in control yeast under the same conditions. Three antioxidant genes (SOD1, CAT1, and GPX1) were specifically expressed under Cd(2+), Zn(2+), Cu(2+), and NaCl stress in the transgenic yeast. Cd(2+), Zn(2+), and Cu(2+) increased the expression levels of SOD1, CAT1, and GPX1, respectively, whereas NaCl induced the expression of SOD1 and GPX1.

HbMT2, an ethephon-induced metallothionein gene from Hevea brasiliensis responds to H(2)O(2) stress

Metallothioneins (MTs) are the cysteine-rich proteins with low molecular weight, which play important roles in maintaining intracellular ion homeostasis, detoxification of heavy metal ions and protecting against intracellular oxidative damages. In this study a novel ethephon-induced metallothionein gene, designated as HbMT2, was isolated and characterized from Hevea brasiliensis. The HbMT2 cDNA contained a 237 bp open reading frame encoding 78 amino acids and the deduced protein showed high similarity to the type 2 MTs from other plant species. Expression analysis revealed more significant accumulation of HbMT2 transcripts in leaves and latex than in roots and barks. The transcription of HbMT2 in latex was strongly induced by ethephon and hydrogen peroxide (H(2)O(2)) stress. Overproduction of recombinant HbMT2 protein gave the Escherichia coli cells more tolerance on Cu(2+) and Zn(2+), and the recombinant HbMT2 could scavenge the reactive oxidant species (ROS) in vitro. All these results indicated that HbMT2 could respond to ethephon stimulation and H(2)O(2) stress as a ROS scavenger in H. brasiliensis. It is also suggested that HbMT2 function in improving the tolerance of rubber trees to heavy metal ions, and repressing the ethephon-induced senilism and tapping panel dryness (TPD) development by ROS scavenge system in H. brasiliensis.

Buckwheat (Fagopyrum esculentum Moench) FeMT3 gene in heavy metal stress: protective role of the protein and inducibility of the promoter region under Cu(2+) and Cd(2+) treatments

The protective role in vivo of buckwheat metallothionein type 3 (FeMT3) during metal stress and the responsiveness of its promoter to metal ions were examined. Increased tolerance to heavy metals of FeMT3 producing Escherichia coli and cup1(Delta) yeast cells was detected. The defensive ability of buckwheat MT3 during Cd and Cu stresses was also demonstrated in Nicotiana debneyii leaves transiently expressing FeMT3. In contrast to phytochelatins, the cytoplasmatic localization of FeMT3 was not altered under heavy metal stress. Functional analysis of the corresponding promoter region revealed extremely high inducibility upon Cu(2+) and Cd(2+) treatments. The confirmed defense ability of FeMT3 protein in vivo and the great responsiveness of its promoter during heavy metal exposure make this gene a suitable candidate for biotechnological applications.

Comparison of two ecotypes of the metal hyperaccumulator Thlaspi caerulescens (J. & C. PRESL) at the transcriptional level

This paper investigates differences in gene expression among the two Thlaspi caerulescens ecotypes La Calamine (LC) and Lellingen (LE) that have been shown to differ in metal tolerance and metal uptake. LC originates from a metalliferous soil and tolerates higher metal concentrations than LE which originates from a non-metalliferous soil. The two ecotypes were treated with different levels of zinc in solution culture, and differences in gene expression were assessed through application of a cDNA microarray consisting of 1,700 root and 2,700 shoot cDNAs. Hybridisation of root and shoot cDNA from the two ecotypes revealed a total of 257 differentially expressed genes. The regulation of selected genes was verified by quantitative reverse transcriptase polymerase chain reaction. Comparison of the expression profiles of the two ecotypes suggests that LC has a higher capacity to cope with reactive oxygen species and to avoid the formation of peroxynitrite. Furthermore, increased transcripts for the genes encoding for water channel proteins could explain the higher Zn tolerance of LC compared to LE. The higher Zn tolerance of LC was reflected by a lower expression of the genes involved in disease and defence mechanisms. The results of this study provide a valuable set of data that may help to improve our understanding of the mechanisms employed by plants to tolerate toxic concentrations of metal in the soil.