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 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.

Enhancement of β-Caryophyllene Biosynthesis in Saccharomyces cerevisiae via Synergistic Evolution of β-Caryophyllene Synthase and Engineering the Chassis

β-Caryophyllene is a plant-derived bicyclic sesquiterpene with multiple biological functions. β-Caryophyllene production by engineered Saccharomyces cerevisiae represents a promising technological route. However, the low catalytic activity of β-caryophyllene synthase (CPS) is one of the main restrictive factors for β-caryophyllene production. Here, directed evolution of the Artemisia annua CPS was performed, and variants of CPS enhancing the β-caryophyllene biosynthesis in S. cerevisiae were obtained, in which an E353D mutant enzyme presented large improvements in Vmax and Kcat. The Kcat/Km of the E353D mutant was 35.5% higher than that of wild-type CPS. Moreover, the E353D variant exhibited higher catalytic activity in much wider pH and temperature ranges. Thus, both the higher catalytic activity and the robustness of the E353D variant contribute to the 73.3% increase in β-caryophyllene production. Furthermore, the S. cerevisiae chassis was engineered by overexpressing genes related to β-alanine metabolism and MVA pathway to enhance the synthesis of the precursor, and ATP-binding cassette transporter gene variant STE6T1025N to improve the transmembrane transport of β-caryophyllene. The combined engineering of CPS and chassis resulted in 70.45 mg/L of β-caryophyllene after 48 h of cultivation in a test tube, which was 2.93-fold of that of the original strain. Finally, a β-caryophyllene yield of 594.05 mg/L was obtained by fed-batch fermentation, indicating the potential of β-caryophyllene production by yeast.

Overexpression of SmMYC2 enhances salt resistance in Arabidopsis thaliana and Salvia miltiorrhiza hairy roots

Soil salinity significantly affects both Salvia miltiorrhiza growth and development as well as seed germination throughout field cultivation and production. The basic helix-loop-helix (bHLH) transcription factor (TF) MYC2 contributes significantly to plant stress resistance as a key regulator of the jasmonic acid signaling pathway. In transgenic S. miltiorrhiza hairy roots, SmMYC2 has been shown to promote the accumulation of tanshinone and salvianolic acid, but its role in S. miltiorrhiza of resistance to abiotic stress is unclear. Herein, we found methyl jasmonate (MeJA), NaCl, and PEG treatment all significantly increased SmMYC2 expression. In response to salt stress, SmMYC2 overexpression in yeast increased its rate of growth. Additionally, overexpression of SmMYC2 transgenic Arabidopsis thaliana and S. miltiorrhiza hairy root showed that it might improve salt resistance in transgenic plant. In particular, compared to WT, overexpression of SmMYC2 transgenic Arabidopsis had higher levels of three antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)), proline (Pro) content, and ABA-dependent and ABA-independent genes expression. They also had lower levels of malondialdehyde (MDA) and reactive oxygen species (ROS) accumulation. What's more, overexpression of SmMYC2 increases the expression of flavonoid synthesis genes and the accumulation of related components in Arabidopsis. These findings imply that SmMYC2 functions as a positive regulator that regulates plant tolerance to salt through ABA-dependent and independent signaling pathways.

Transcriptomic Analysis of Cadmium Stressed Tamarix hispida Revealed Novel Transcripts and the Importance of Abscisic Acid Network

Cadmium (Cd) pollution is widely detected in soil and has been recognized as a major environmental problem. Tamarix hispida is a woody halophyte, which can form natural forest on the desert and soil with 0.5 to 1% salt content, making it an ideal plant for the research on response to abiotic stresses. However, no systematic study has investigated the molecular mechanism of Cd tolerance in T. hispida. In the study, RNA-seq technique was applied to analyze the transcriptomic changes in T. hispida treated with 150 μmol L^-1 CdCl₂ for 24, 48, and 72 h compared with control. In total, 72,764 unigenes exhibited similar sequences in the Non-redundant nucleic acid database (NR database), while 36.3% of all these unigenes may be new transcripts. In addition, 6,778, 8,282, and 8,601 DEGs were detected at 24, 48, and 72 h, respectively. Functional annotation analysis indicated that many genes may be involved in Cd stress response, including ion bonding, signal transduction, stress sensing, hormone responses and ROS metabolism. A ThUGT gene from the abscisic acid (ABA) signaling pathway can enhance Cd resistance ability of T. hispida by regulating the production of ROS under Cd stress and inhibit absorption of Cd. The new transcriptome resources and data that we present in this study for T. hispida may facilitate investigation of molecular mechanisms governing Cd resistance.

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.

Recent progress in understanding salinity tolerance in plants: Story of Na+/K+ balance and beyond

High salt concentrations in the growing medium can severely affect the growth and development of plants. It is imperative to understand the different components of salt-tolerant network in plants in order to produce the salt-tolerant cultivars. High-affinity potassium transporter- and myelocytomatosis proteins have been shown to play a critical role for salinity tolerance through exclusion of sodium (Na⁺) ions from sensitive shoot tissues in plants. Numerous genes, that limit the uptake of salts from soil and their transport throughout the plant body, adjust the ionic and osmotic balance of cells in roots and shoots. In the present review, we have tried to provide a comprehensive report of major research advances on different mechanisms regulating plant tolerance to salinity stress at proteomics, metabolomics, genomics and transcriptomics levels. Along with the role of ionic homeostasis, a major focus was given on other salinity tolerance mechanisms in plants including osmoregulation and osmo-protection, cell wall remodeling and integrity, and plant antioxidative defense. Major proteins and genes expressed under salt-stressed conditions and their role in enhancing salinity tolerance in plants are discussed as well. Moreover, this manuscript identifies and highlights the key questions on plant salinity tolerance that remain to be discussed in the future.

Transcription Factor GmWRKY142 Confers Cadmium Resistance by Up-Regulating the Cadmium Tolerance 1-Like Genes

Cadmium (Cd) is a widespread pollutant that is toxic to living organisms. Previous studies have identified certain WRKY transcription factors, which confer Cd tolerance in different plant species. In the present study, we have identified 29 Cd-responsive WRKY genes in Soybean [Glycine max (L.) Merr.], and confirmed that 26 of those GmWRKY genes were up-regulated, while 3 were down-regulated. We have also cloned the novel, positively regulated GmWRKY142 gene from soybean and investigated its regulatory mechanism in Cd tolerance. GmWRKY142 was highly expressed in the root, drastically up-regulated by Cd, localized in the nucleus, and displayed transcriptional activity. The overexpression of GmWRKY142 in Arabidopsis thaliana and soybean hairy roots significantly enhanced Cd tolerance and lead to extensive transcriptional reprogramming of stress-responsive genes. ATCDT1, GmCDT1-1, and GmCDT1-2 encoding cadmium tolerance 1 were induced in overexpression lines. Further analysis showed that GmWRKY142 activated the transcription of ATCDT1, GmCDT1-1, and GmCDT1-2 by directly binding to the W-box element in their promoters. In addition, the functions of GmCDT1-1 and GmCDT1-2, responsible for decreasing Cd uptake, were validated by heterologous expression in A. thaliana. Our combined results have determined GmWRKYs to be newly discovered participants in response to Cd stress, and have confirmed that GmWRKY142 directly targets ATCDT1, GmCDT1-1, and GmCDT1-2 to decrease Cd uptake and positively regulate Cd tolerance. The GmWRKY142-GmCDT1-1/2 cascade module provides a potential strategy to lower Cd accumulation in soybean.

Identification of the metallothionein gene family from cucumber and functional characterization of CsMT4 in Escherichia coli under salinity and osmotic stress

Metallothionein (MT) proteins are low-molecular-weight, cysteine-rich and metal-binding proteins that play important roles in the maintenance of metal homeostasis and detoxification, but their roles in abiotic stress tolerance remain largely unknown. In this study, three MT family genes (CsMT2, CsMT3 and CsMT4) were identified in the cucumber genome. CsMT2, CsMT3 and CsMT4 possessed 14, 10, and 18 Cys residues, which were clustered into 2, 2, and 3 Cys-rich regions, respectively. Phylogenetic analysis of MTs from cucumber, Arabidopsis and soybean revealed that these MTs were clustered into four groups in accordance with the MT types (types 1-4). An analysis of the cis-acting regulatory elements revealed that a series of hormone-, stress-, and development-related cis-elements were present in the promoter regions of CsMT genes. Expression pattern analysis by RT-PCR showed that the CsMT genes exhibited different tissue expression patterns. CsMT2 showed relatively higher expression in stem, leaf, and flower; CsMT3 was mainly expressed in leaf, flower, and fruit, while CsMT4 was highly expressed in fruit and leaf. The qRT-PCR results showed that the CsMT genes were induced by various stress treatments including NaCl, PEG, and ABA, while CsMT4 displayed much higher expression levels in response to these stresses than CsMT2 and CsMT3. Escherichia coli cells expressing CsMT4 exhibited higher salinity and osmotic tolerance compared with control cells, indicating the significant function of CsMT4 to confer tolerance to these stresses. These results lay a foundation for further research on the function of MT family genes in plant stress responses.

Recent strategies of increasing metal tolerance and phytoremediation potential using genetic transformation of plants

Avoidance and reduction of soil contamination with heavy metals is one of the most serious global challenges. Nowadays, science offers us new opportunities of utilizing plants to extract toxic elements from the soil by means of phytoremediation. Plant abilities to uptake, translocate, and transform heavy metals, as well as to limit their toxicity, may be significantly enhanced via genetic engineering. This paper provides a comprehensive review of recent strategies aimed at the improvement of plant phytoremediation potential using plant transformation and employing current achievements in nuclear and cytoplasmic genome transformation. Strategies for obtaining plants suitable for effective soil clean-up and tolerant to excessive concentrations of heavy metals are critically assessed. Promising directions in genetic manipulations, such as gene silencing and cis- and intragenesis, are also discussed. Moreover, the ways of overcoming disadvantages of phytoremediation using genetic transformation approachare proposed. The knowledge gathered here could be useful for designing new research aimed at biotechnological improvement of phytoremediation efficiency.

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.

H₂O₂ Is Involved in the Metallothionein-Mediated Rice Tolerance to Copper and Cadmium Toxicity

Cadmium (Cd) and excess copper (Cu) are toxic to plants, causing a wide range of deleterious effects including the formation of reactive oxygen species. Metallothioneins (MTs) may protect plant cells from heavy metal toxicity by chelating heavy metals via cysteine thiol groups. They may also function as antioxidants. The study investigated the relationship of H₂O₂ production and ricMT expression in rice radicles and rice suspension cells under Cu or Cd stress. The results showed that H₂O₂ production in the rice radicles increased before Cu-induced ricMT expression, and after Cd-induced ricMT expression. Rice suspension cells of sense- and antisense-ricMT transgenic lines were obtained by an Agrobacterium-mediated transformation. Overexpression of ricMT significantly decreased the death rate of rice cells, which was accompanied by blocked H₂O₂ accumulation in rice suspension cells subject to Cu and Cd stress. Our findings confirm that H₂O₂ is involved in the MT-mediated tolerance of Cu and Cd toxicity in rice.

Responses of Plant Proteins to Heavy Metal Stress-A Review

Plants respond to environmental pollutants such as heavy metal(s) by triggering the expression of genes that encode proteins involved in stress response. Toxic metal ions profoundly affect the cellular protein homeostasis by interfering with the folding process and aggregation of nascent or non-native proteins leading to decreased cell viability. However, plants possess a range of ubiquitous cellular surveillance systems that enable them to efficiently detoxify heavy metals toward enhanced tolerance to metal stress. As proteins constitute the major workhorses of living cells, the chelation of metal ions in cytosol with phytochelatins and metallothioneins followed by compartmentalization of metals in the vacuoles as well as the repair of stress-damaged proteins or removal and degradation of proteins that fail to achieve their native conformations are critical for plant tolerance to heavy metal stress. In this review, we provide a broad overview of recent advances in cellular protein research with regards to heavy metal tolerance in plants. We also discuss how plants maintain functional and healthy proteomes for survival under such capricious surroundings.

Heterologous expression of a rice metallothionein isoform (OsMTI-1b) in Saccharomyces cerevisiae enhances cadmium, hydrogen peroxide and ethanol tolerance

Metallothioneins are a superfamily of low-molecular-weight, cysteine (Cys)-rich proteins that are believed to play important roles in protection against metal toxicity and oxidative stress. The main purpose of this study was to investigate the effect of heterologous expression of a rice metallothionein isoform (OsMTI-1b) on the tolerance of Saccharomyces cerevisiae to Cd²⁺, H₂O₂ and ethanol stress. The gene encoding OsMTI-1b was cloned into p426GPD as a yeast expression vector. The new construct was transformed to competent cells of S. cerevisiae. After verification of heterologous expression of OsMTI-1b, the new strain and control were grown under stress conditions. In comparison to control strain, the transformed S. cerevisiae cells expressing OsMTI-1b showed more tolerance to Cd²⁺ and accumulated more Cd²⁺ ions when they were grown in the medium containing CdCl₂. In addition, the heterologous expression of GST-OsMTI-1b conferred H₂O₂ and ethanol tolerance to S. cerevisiae cells. The results indicate that heterologous expression of plant MT isoforms can enhance the tolerance of S. cerevisiae to multiple stresses.

Copper-induced hydrogen peroxide upregulation of a metallothionein gene, OsMT2c, from Oryza sativa L. confers copper tolerance in Arabidopsis thaliana

Metallothioneins (MTs) are low-molecular-weight, cysteine-rich metal-binding proteins found in numerous genera and species, but their functions in abiotic stress tolerance remain unclear. Here, a MT gene from Oryza sativa, OsMT2c, was isolated and characterized, encoding a type 2 MT, and observed expression in the roots, leaf sheathes, and leaves, but only weak expression in seeds. OsMT2c was upregulated by copper (Cu) and hydrogen peroxide (H2O2) treatments. Excessive Cu elicited a rapid and sustained production and release of H2O2 in rice, and exogenous H2O2 scavengers N,N'-dimethylthiourea (DMTU) and ascorbic acid (Asc) decreased H2O2 production and OsMT2c expression. Furthermore, the expression of OsMT2c increased in the osapx2 mutant in which the H2O2 levels were higher than in wild-type (WT) plants. These results showed that Cu increased MT2c expression through the production and accumulation of Cu-induced H2O2 in O. sativa. In addition, the transgenic OsMT2c-overexpressing Arabidopsis displayed improved tolerance to Cu stress and exhibited increased reactive oxygen species (ROS) scavenging ability compared to WT and empty-vector (Ev) seedlings.

Two metallothionein genes in Oxya chinensis: molecular characteristics, expression patterns and roles in heavy metal stress

Metallothioneins (MTs) are small, cysteine-rich, heavy metal-binding proteins involved in metal homeostasis and detoxification in living organisms. In the present study, we cloned two MT genes (OcMT1 and OcMT2) from Oxya chinensis, analyzed the expression patterns of the OcMT transcripts in different tissues and at varying developmental stages using real-time quantitative PCR (RT-qPCR), evaluated the functions of these two MTs using RNAi and recombinant proteins in an E. coli expression system. The full-length cDNAs of OcMT1 and OcMT2 encoded 40 and 64 amino acid residues, respectively. We found Cys-Cys, Cys-X-Cys and Cys-X-Y-Z-Cys motifs in OcMT1 and OcMT2. These motifs might serve as primary chelating sites, as in other organisms. These characteristics suggest that OcMT1 and OcMT2 may be involved in heavy metal detoxification by capturing the metals. Two OcMT were expressed at all developmental stages, and the highest levels were found in the eggs. Both transcripts were expressed in all eleven tissues examined, with the highest levels observed in the brain and optic lobes, followed by the fat body. The expression of OcMT2 was also relatively high in the ovaries. The functions of OcMT1 and OcMT2 were explored using RNA interference (RNAi) and different concentrations and treatment times for the three heavy metals. Our results indicated that mortality increased significantly from 8.5% to 16.7%, and this increase was both time- and dose-dependent. To evaluate the abilities of these two MT proteins to confer heavy metal tolerance to E. coli, the bacterial cells were transformed with pET-28a plasmids containing the OcMT genes. The optical densities of both the MT-expressing and control cells decreased with increasing concentrations of CdCl2. Nevertheless, the survival rates of the MT-overexpressing cells were higher than those of the controls. Our results suggest that these two genes play important roles in heavy metal detoxification in O. chinensis.

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.

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.

Discrimination between two rice metallothionein isoforms belonging to type 1 and type 4 in metal-binding ability

Metallothioneins (MTs) are a superfamily of low-molecular-weight, cysteine (Cys)-rich proteins that are believed to play important roles in protection against metal toxicity and oxidative stress. Plants have several MT isoforms, which are classified into four types based on the arrangement of Cys residues. In this study, two rice (Oryza sativa) MT isoforms, OsMTI-1b and OsMTII-1a from type 1 and type 4, respectively, were heterologously expressed in Escherichia coli as carboxy-terminal extensions of glutathione-S-transferase (GST). Transformed cells expressing GST-OsMTI-1b showed increased tolerance to Ni(2+) , Cd(2+) , and Zn(2+) and accumulated more metal ions compared with cells expressing GST alone. However, heterologous expression of GST-OsMTII-1a had no significant effects on metal tolerance or ion accumulation. The UV absorption spectra and competitive reactions of in vitro Cd-incubated proteins with 5-5'-dithiobis(2-nitrobenzoic) acid revealed that GST-OsMTI-1b, but not GST-OsMTII-1a, is able to form Cd-thiolate clusters. Furthermore, heterologous expression of both GST-OsMTI-1b and GST-OsMTII-1a conferred H2 O2 tolerance to E. coli cells. Taken together, the results presented here show that two different rice MT isoforms belonging to type 1 and type 4 differ in Ni(2+) , Cd(2+) , and Zn(2+) binding abilities, but they may have overlapping function in protection of cells against oxidative stress.

Heterologous expression and metal-binding characterization of a type 1 metallothionein isoform (OsMTI-1b) from rice (Oryza sativa)

Metallothioneins (MTs) are ubiquitous, low molecular mass and cysteine-rich proteins that play important roles in maintaining intracellular metal homeostasis, eliminating metal toxification and protecting the cells against oxidative damages. MTs are able to bind metal ions through the thiol groups of their cysteine residues. Plants have several MT isoforms which are classified into four types based on the arrangement of cysteine residues. In the present study, a rice (Oryza sativa) gene encoding type 1 MT isoform, OsMTI-1b, was inserted in vector pET41a and overexpressed in Escherichia coli as carboxy-terminal extensions of glutathione-S-transferase (GST). The recombinant protein GST-OsMTI-1b was purified using affinity chromatography and its ability to bind with Ni(2+), Cd(2+), Zn(2+) and Cu(2+) ions was analyzed. The results demonstrated that this isoform has ability to bind Ni(2+), Cd(2+) and Zn(2+) ions in vitro, whereas it has no substantial ability to bind Cu(2+) ions. From competitive reaction with 5,5'-dithiobis(2-nitrobenzoic acid), DTNB, the affinity of metal ions for recombinant form of GST-OsMTI-1b was as follows: Ni(2+)/Cd(2+) > Zn(2+) > Cu(2+).

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.

Ycf1p attenuates basal level oxidative stress response in Saccharomyces cerevisiae

Ycf1p function is regulated by casein kinase 2α, Cka1p, via phosphorylation of Ser251. Cka1p-mediated phosphorylation of Ycf1p is attenuated in response to high salt stress. Previous results from our lab suggest a role for Ycf1p in cellular resistance to salt stress. Here, we show that Ycf1p plays an important role in cellular resistance to salt stress by maintaining the cellular redox balance via glutathione recycling. Our results suggest that during acute salt stress increased Sod1p, Sod2p and Ctt1p activity is the main compensatory for the loss in Ycf1p function that results from reduced Ycf1p-dependent recycling of cellular GSH levels.

Overexpression of a heat shock protein (ThHSP18.3) from Tamarix hispida confers stress tolerance to yeast

It is well known that plant heat shock proteins (HSPs) play important roles both in response to adverse environmental conditions and in various developmental processes. However, among plant HSPs, the functions of tree plant HSPs are poorly characterized. To improve our understanding of tree HSPs, we cloned and characterized an HSP gene (ThHSP18.3) from Tamarix hispida. Sequence alignment reveals that ThHSP18.3 belongs to the class I small heat shock protein family. A transient expression assay showed that ThHSP18.3 protein was targeted to the cell nucleus. Treatment of Tamarix hispida with cold and heat shock highly induced ThHSP18.3 expression in all studied leaves, roots and stems, whereas, treatment of T. hispida with NaCl, NaHCO(3), and PEG induced ThHSP18.3 expression in leaves and decreased its expression in roots and stems. Further, to study the role of ThHSP18.3 in stress tolerance under different stress conditions, we cloned ThHSP18.3 into the pYES2 vector, transformed and expressed the vector in yeast Saccharomyces cerevisiae. Yeast cells transformed with an empty pYES2 vector were employed as a control. Compared to the control, yeast cells expressing ThHSP18.3 showed greater tolerance to salt, drought, heavy metals, and both low and high temperatures, indicating that ThHSP18.3 confers tolerance to these stress conditions. These results suggested that ThHSP18.3 is involved in tolerance to a variety of stress conditions in T. hispida.