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

Involvement of the Metallothionein gene OsMT2b in Drought and Cadmium Ions Stress in Rice

Abiotic stress is one of the major factors restricting the production of rice (Oryza sativa L.). Developing rice varieties with dual abiotic stress tolerance is essential to ensure sustained rice production, which is necessary to illustrate the regulation mechanisms underlying dual stress tolerance. At present, only a few genes that regulate dual abiotic stress tolerance have been reported. In this study, we determined that the expression of OsMT2b was induced by both drought and Cd2+ stress. After stress treatment, OsMT2b-overexpression lines exhibited enhanced drought tolerance and better physiological performance in terms of relative water content and electrolyte leakage compared with wild-type (WT). Further analysis indicated that ROS levels were lower in OsMT2b-overexpression lines than in WT following stress treatment, suggesting that OsMT2b-overexpression lines had a stronger ability to scavenge ROS under stress. Reverse transcription-quantitative PCR (RT-qPCR) results demonstrated that under drought stress, OsMT2b influenced the expression of genes involved in ROS scavenging to enhance drought tolerance in rice. In addition, OsMT2b-overexpression plants displayed increased tolerance to Cd2+ stress, and physiological assessment results were consistent with the observed phenotypic improvements. Thus, enhancing ROS scavenging ability through OsMT2b overexpression is a novel strategy to boost rice tolerance to both drought and Cd2+ stress, offering a promising approach for developing rice germplasm with enhanced resistance to the abiotic stressors.

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.

Methods for metal chelation in plant homeostasis: Review

Metal uptake, transport and storage in plants depend on specialized ligands with closely related functions. Individual studies differing by species, nutrient availability, tissue type, etc. are not comprehensive enough to understand plant metal homeostasis in its entirety. A thorough review is required that distinguishes the role of ligands directly involved in chelation from the myriad of plant responses to general stress. Distinguishing between the functions of metal chelating compounds is the primary focus of this review; reactive oxygen species mediation and other aspects of metal homeostasis are also discussed. High molecular weight ligands (polysaccharides, phytochelatin, metallothionein), low molecular weight ligands (nicotianamine, histidine, secondary metabolites) and select studies which demonstrate the complex nature of plant metal homeostasis are explored.

Identification of a new function of metallothionein-like gene OsMT1e for cadmium detoxification and potential phytoremediation

Cadmium (Cd) is a biologically non-essential and toxic heavy metal leaking to the environment via natural emission or anthropogenic activities, thereby contaminating crops and threatening human health. Metallothioneins (MTs) are a group of metal-binding proteins playing critical roles in metal allocation and homeostasis. In this study, we identified a novel function of OsMT1e from rice plants. OsMT1e was dominantly expressed in roots at all developmental stages and, to less extent, expressed in leaves at vegetative and seed filling stages. OsMT1e was mainly targeted to the nucleus and substantially induced by Cd exposure. Expression of OsMT1e in a yeast Cd-sensitive strain ycf1 conferred cellular tolerance to Cd, even though the ycf1 + OsMT1e cells accumulated more Cd than the control cells (ycf1 + pYES2). Both transgenic rice overexpressing (OX) and repressing OsMT1e by RNA interference (RNAi) were developed. Phenotypic analysis revealed that OsMT1e overexpression enhanced the rice growth concerning the increased shoot or root elongation, dry weight and chlorophyll contents, whereas the RNAi lines displayed a sensitive growth phenotype compared to wild-type. Assessment with 0.5, 2 and 10 μM Cd for two weeks revealed that the RNAi lines accumulated less Cd, while the OX lines had an increased Cd accumulation in root and shoot tissues. The contrasting Cd accumulation phenotypes between the OX and RNAi lines were further confirmed by a long-term study with 0.5 μM Cd for one month. Overall, the study unveiled a new function of OsMT1e in rice, which can be potentially used for engineering genotypes for phytoremediation or minimizing Cd in rice crops.

Two metallothionein genes highly expressed in rice nodes are involved in distribution of Zn to the grain

A rice node is a hub for distribution of mineral elements; however, most genes highly expressed in the node have not been functionally characterized. Transcriptomic analysis of a rice node revealed that two metallothionein genes, OsMT2b and OsMT2c, were highly expressed in the node I. We functionally characterized these genes in terms of gene expression pattern, cellular and subcellular localization, phenotypic analysis of the single and double knockout mutants and metal-binding ability. Both OsMT2b and OsMT2c were mainly and constitutively expressed in the phloem region of enlarged and diffuse vascular bundles in the nodes and of the anther. Knockout of either OsMT2b or OsMT2c increased zinc (Zn) accumulation in the nodes, but decreased Zn distribution to the panicle, resulting in decreased grain yield. A double mutant, osmt2bmt2c, showed further negative effects on the Zn distribution and grain yield. By contrast, knockout of OsMT2b had a small effect on copper (Cu) accumulation. Both OsMT2b and OsMT2c showed binding ability with Zn, whereas only OsMT2b showed binding ability with Cu in yeast. Our results suggest that both OsMT2b and OsMT2c play an important role mainly in the distribution of Zn to grain through chelation and subsequent transport of Zn in the phloem in rice.

Metalation of a rice type 1 metallothionein isoform (OsMTI-1b)

The simultaneously functions of Metallothioneins (MTs) are relied on their metalation mechanisms that can be divided into non-cooperative, weakly cooperative and strongly cooperative mechanisms. In this study, we recombinantly synthesized OsMTI-1b, N- and C-terminal Cys-rich regions as glutathione-S-transferase (GST)-fusion proteins in E. coli. In comparison with control strains (The E. coli cells containing pET41a without gene), transgenic E. coli cells showed more tolerance against Cd²⁺ and Zn²⁺. The recombinant GST-proteins were purified using affinity chromatography. According to in vitro assays, the recombinant proteins showed a higher binding ability to Cd²⁺ and Zn²⁺. However, the affinity of apo-proteins to Cu²⁺ ions were very low. The coordination of Cd²⁺ ions in OsMTI-1b demonstrates a strongly cooperative mechanism with a priority for the C-terminal Cys-rich region that indicates the detoxifying of heavy metals as main role of P1 subfamily of MTs. While the metalation with Zn²⁺ conformed to a weakly cooperative mechanism with a specificity to N-terminal Cys-rich region. It implies the specific function of OsMTI-1b is involved in zinc homeostasis. Nevertheless, a non-cooperative metalation mechanism was perceived for Cu²⁺ that suggests the fully metalation does not occur and OsMTI-1b cannot play a significant role in dealing with Cu²⁺ ions.

Expression of Rice Metallothionein Isoforms in Escherichia coli Enhances the Accumulation of Trivalent and Hexavalent Chromium

INTRODUCTION

Metallothioneins (MTs) are members of a family of low molecular weight and cysteine-rich proteins that are involved in heavy metal homeostasis and detoxification in living organisms. Plants have multiple MT types that are generally divided into four subgroups according to the arrangement of Cys residues.

METHODS

In the present study the E. coli cells which heterologously express four different rice MT (OsMT) isoforms were analyzed for the accumulation of two forms of chromium, Cr3+ and Cr6+.

RESULTS

The results show that the transgenic bacteria were more tolerant than control cells when they were grown up in the medium comprising Cr(NO3)3.9H2O or Na2CrO4. The cells expressing OsMT1, OsMT2, OsMT3 and OsMT4 give rise to 6.5-, 2.7-, 5.5- and 2.1-fold improvements on the accumulation capacity for Cr3+ and 9-, 3-, 5- and 3- fold Cr6+ respectively compared with comparison to the control strain. Furthermore, the purified recombinant GST-OsMTs were tested for their binding ability to Cr+3 and Cr+6 in vitro.

DISCUSSION

The data show that the recombinant GST-OsMT1 and GST-OsMT2 were able to bind both Cr3+ and Cr6+, in vitro. However, their binding strength was low with respect to previous tested divalent ions like Cd2+.

Secretory expression, immunoaffinity purification and metal-binding ability of recombinant metallothionein (ShMT) from freshwater crab Sinopotamon henanense

Metallothioneins (MTs) are a super-family of ubiquitous, low-molecular-weight, cysteine-rich and metal-binding proteins. They are thought to play a predominant role in mediating metal metabolism and antioxidation. However, the accurate functions of MTs remain unclear in the physiological processes due to native proteins deficiency and little information of their metal-binding character. Freshwater crab Sinopotamon henanense is a decapod crustacean widely distributed in northern China, in which only one MT isoform (ShMT) has been reported so far. In order to shed light on the accurate role of ShMT, a novel recombinant ShMT in native form was over-expressed by phoA secreted expression system in Escherichia coli (E. coli). Then the ShMT proteins were purified using a one-step gentle immunoaffinity chromatography with a polyol-responsive mAb (PR-mAb) to ShMT, which was generated by conventional hybridoma technology followed by ELISA-elution. The Zn-, Cu-, and Cd-ShMT complexes were prepared by recombinant synthesis in metal-enriched media and reconstitution with metal ions, respectively. Further analysis about metal-binding capacity showed recombinant ShMT has high ability to bind Zn, Cu and Cd metals, although the recombinantly expressed and reconstituted metal-ShMT complexes have different metal-to-protein stoichiometry. Moreover, the affinity of recombinant protein for metal ions has been analyzed using competitive reaction with 5, 5-dithiobis (2-nitrobenzoic acid) (DTNB). The results demonstrated the affinity of recombinant ShMT for metals was as follows: Cu＞Cd＞Zn. In summary, the experimental procedure we have developed facilitates production of recombinant ShMT with native characteristics for further research and the study of metal-binding ability could help further clarify the accurate functions of ShMT.

Cyamopsis tetragonoloba type 1 metallothionein (CtMT1) gene is upregulated under drought stress and its protein product has an additional C-X-C motif and unique metal binding pattern

Metallothioneins (MTs) are involved in cellular homeostasis of essential metal ions and detoxification of nonessential metal ions. We report here the identification of four MT genes, CtMT1, CtMT2, CtMT3 and CtMT4, encoding CtMT1, CtMT2, CtMT3 and CtMT4 proteins, respectively, from the industrial guar crop. The primary structures of last three proteins were similar to those of respective MT proteins of other plants but the CtMT1 protein primary structure was different from the other plant MT1 proteins in having an additional C-X-C motif. The four MT genes showed tissue specific expression patterns suggesting their specific roles in different tissues. High expression of CtMT1 gene was observed in roots and nodules whereas CtMT2 and CtMT3 genes showed high expression in leaves. The expression of CtMT4 gene was high in seeds. The qRT-PCR studies revealed upregulation in expression of CtMT1 gene under drought stress. Recombinant CtMT1 protein was produced in E. coli Rosetta cells and purified by metal affinity chromatography. The purified protein showed antioxidant property and the order of its metal ion binding affinities was Cu²⁺ > Zn²⁺ > Fe²⁺ > Cd²⁺. This information about CtMT1 protein is expected to be useful in understanding its role in drought tolerance and other physiological processes of guar.

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.

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.

Hydrogen sulfide alleviates mercury toxicity by sequestering it in roots or regulating reactive oxygen species productions in rice seedlings

Soil mercury (Hg) contamination is a major factor that affects agricultural yield and food security. Hydrogen sulfide (H₂S) plays multifunctional roles in mediating a variety of responses to abiotic stresses. The effects of exogenous H₂S on rice (Oryza sativa var 'Nipponbare') growth and metabolism under mercuric chloride (HgCl₂) stress were investigated in this study. Either 100 or 200 μM sodium hydrosulfide (NaHS, a donor of H₂S) pretreatment improved the transcription of bZIP60, a membrane-associated transcription factor, and then enhanced the expressions of non-protein thiols (NPT) and metallothioneins (OsMT-1) to sequester Hg in roots and thus inhibit Hg transport to shoots. Meanwhile, H₂S promoted seedlings growth significantly even in the presences of Hg and superoxide dismutase (SOD, EC 1.15.1.1) or catalase (CAT, EC 1.11.1.6) inhibitors, diethyldithiocarbamate (DDC) or 3-amino-1,2,4-triazole (AT). H₂S might act as an antioxidant to inhibit or scavenge reactive oxygen species (ROS) productions for maintaining the lower MDA and H₂O₂ levels, and thereby preventing oxidative damages. All these results indicated H₂S effectively alleviated Hg toxicity by sequestering it in roots or by regulating ROS in seedlings and then thus significantly promoted rice growth.

Independent metal-thiolate cluster formation in C-terminal Cys-rich region of a rice type 1 metallothionein isoform

In this study we examined the independent self assembly of metal-binding in C-terminal Cys- rich region of a type 1 metallothionein (MT) isoform from rice (OsMTI-1b). To this end the N-terminal of OsMTI-1b (C-OsMTI-1b) was heterologously expressed in Escherichia coli as fusion protein with glutathione-S-transferase (GST). As compared with control (The E. coli cells containing pET41a without gene), transgenic E. coli cells expressing GST-C-OsMTI-1b accumulated more Ni²⁺, Cd²⁺, and Zn²⁺ from culture medium and showed increased tolerance against these metals. The recombinant GST-C-OsMTI-1b was purified using affinity chromatography. According to in vitro assays the protein GST-C-OsMTI-1b was able to form complexes with Ni²⁺, Cd²⁺ and Zn²⁺. These results demonstrate the formation of independent metal-thiolate cluster at C-terminal Cys-rich region of OsMTI-1b without participation of N-terminal Cys-rich region.

Metallothionein-immobilized silica-coated magnetic particles as a novel nanobiohybrid adsorbent for highly efficient removal of cadmium from aqueous solutions

In this contribution, magnetic nanocomposite particles of Fe₃O₄ cluster@SiO₂ (MNPs) were functionalized with N-(2-aminoethyl)-3-aminopropyltrimethoxy-silane (EDS) to obtain amine-functionalized magnetic nanocomposite particles (AF-MNPs).

Expression of the rgMT gene, encoding for a rice metallothionein-like protein in Saccharomyces cerevisiae and Arabidopsis thaliana

Metallothioneins (MTs) are cysteine-rich proteins of low molecular weight with many attributed functions, such as providing protection against metal toxicity, being involved in regulation of metal ions uptake that can impact plant physiology and providing protection against oxidative stress. However, the precise function of the metallothionein-like proteins such as the one coded for rgMT gene isolated from rice (Oryza sativa L.) is not completely understood. The whole genome analysis of rice (O. sativa) showed that the rgMT gene is homologue to the Os11g47809 on chromosome 11 of O. sativa sp. japonica genome. This study used the rgMT coding sequence to create transgenic lines to investigate the subcellular localization of the protein, as well as the impact of gene expression in yeast (Saccharomyces cerevisiae) and Arabidopsis thaliana under heavy metal ion, salt and oxidative stresses. The results indicate that the rgMT gene was expressed in the cytoplasm of transgenic cells. Yeast cells transgenic for rgMT showed vigorous growth compared to the nontransgenic controls when exposed to 7 mM CuCl2, 10 mM FeCl2, 1 M NaCl, 24 mM NaHCO3 and 3.2 mM H2O2, but there was no significant difference for other stresses tested. Similarly, Arabidopsis transgenic for rgMT displayed significantly improved seed germination rates over that of the control when the seeds were stressed with 100 μM CuCl2 or 1 mM H2O2. Increased biomass was observed in the presence of 100 μM CuCl2, 220 μM FeCl2, 3 mM Na2CO3, 5 mM NaHCO3 or 1 mM H2O2. These results indicate that the expression of the rice rgMT gene in transgenic yeast and Arabidopsis is implicated in improving their tolerance for certain salt and peroxide stressors.

Sunflower metallothionein family characterisation. Study of the Zn(II)- and Cd(II)-binding abilities of the HaMT1 and HaMT2 isoforms

Plant metallothioneins (MTs) constitute a family of small Cys-rich proteins capable of coordinating metal ions, significantly differing from microbial and animal MTs. They are divided into four subfamilies depending on the Cys pattern in their sequence. In this work, the MT system of the sunflower plant (Helianthus annuus) has been defined, with ten genes coding for MTs (HaMT) belonging to the four plant MT subfamilies; three HaMT1, four HaMT2, one HaMT3 and two HaMT4 isoforms. The gene expression pattern and capacity to confer metal resistance to yeast cells have been analysed for at least one member of each subfamily. The divalent metal ion-binding abilities of HaMT1-2 and HaMT2-1 (the isoforms encoded by the most abundantly expressed HaMT1 and HaMT2 isogenes) have been characterised, as HaMT3 and HaMT4 were previously studied. Those isoforms constitute an optimum material to study the effect of Cys number variability on their coordination abilities, as they exhibit additional Cys residues regarding the canonical Cys pattern of each subfamily. Our results show that the variation in the number of Cys does not drastically modify their M(II)-binding abilities, but instead modulates the degree of heterogeneity of the corresponding recombinant syntheses. Significantly, the Zn(II)-HaMT1 complexes were highly susceptible to proteolytic cleavage. The recombinant Cd-MT preparations of both isoforms exhibit significant acid-labile sulphide content-Cd6S8 or Cd7S7 species. Overall results suggest that HaMT2-1 is probably associated with Cd(II) detoxification, in contrast to HaMT1-2, which may be more related to physiological functions, such as metal ion transport and delivery.

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.

Dehydroascorbate reductase and monodehydroascorbate reductase activities of two metallothionein-like proteins from sweet potato (Ipomoea batatas [L.] Lam. 'Tainong 57') storage roots

BACKGROUND

Metallothionein (MT) is a group of proteins with low molecular masses and high cysteine contents, and it is classified into different types, which generally contains two domains with typical amino acid sequences.

RESULTS

In this report, two full-length cDNAs (MT-1 and MT-II) encoding MT-like proteins were isolated from the roots of sweet potato (Ipomoea batatas [L.] Lam. 'Tainong 57'). Their open reading frames contained 642 and 519 nucleotides (66 and 81 amino acids) for MT-1 and MT-II, respectively, and exhibited a relatively low amino acid sequence similarity. On the basis of the amino acid sequence similarity and conserved residues, it is suggested that MT-I is a member of the plant MT Type-I family, and MT-II is a member of the plant MT Type-II family. The corresponding mRNA levels of MT-1 and MT-II were the highest found in the storage roots. Recombinant MT-1 and MT-II protein overproduced in E. coli (M15) was purified by Ni²⁺-chelated affinity chromatography. MT-1 and MT-II reduced dehydroascorbate (DHA) in the presence of glutathione (GSH) to regenerate L-ascorbic acid (AsA). However, without GSH, MT-1 and MT-II has very low DHA reductase activity. And AsA was oxidized by AsA oxidase to generate monodehydroascorbate (MDA) free radical. MDA was also reduced by MT-1 and MT-II to AsA in the presence of NADH mimicking the MDA reductase catalyzed reaction.

CONCLUSIONS

These data suggest that MT-1 and MT-II have both DHA reductase and MDA reductase activities. MT-1 and MT-II are apparently the first reported plant MTs exhibiting both DHA and MDA activities in vitro.