Cloning and characterization of HbMT2a, a metallothionein gene from Hevea brasiliensis Muell. Arg differently responds to abiotic stress and heavy metals

Abscisic-Acid-Regulated Responses to Alleviate Cadmium Toxicity in Plants

High levels of cadmium (Cd) in soil can cause crop yield reduction or death. Cadmium accumulation in crops affects human and animal health as it passes through the food chain. Therefore, a strategy is needed to enhance the tolerance of crops to this heavy metal or reduce its accumulation in crops. Abscisic acid (ABA) plays an active role in plants' response to abiotic stress. The application of exogenous ABA can reduce Cd accumulation in shoots of some plants and enhance the tolerance of plants to Cd; therefore, ABA may have good application prospects. In this paper, we reviewed the synthesis and decomposition of ABA, ABA-mediated signal transduction, and ABA-mediated regulation of Cd-responsive genes in plants. We also introduced physiological mechanism underlying Cd tolerance because of ABA. Specifically, ABA affects metal ion uptake and transport by influencing transpiration and antioxidant systems, as well as by affecting the expression of metal transporter and metal chelator protein genes. This study may provide a reference for further research on the physiological mechanism of heavy metal tolerance in plants.

Roles of Canavalia rosea metallothioneins in metal tolerance and extreme environmental adaptation to tropical coral reefs

Canavalia rosea (Sw.) DC is a perennial twining herb distributed in the semi-arid and saline-alkali areas of coastal regions and has evolved halotolerance. In this study, we present the first comprehensive survey of the metallothionein (MT) gene family in C. rosea. MT proteins belong to a family of low-molecular-weight polypeptides with a high content of cysteine residues, which have an affinity to bind with heavy metal ions. MTs also play important roles in stress responses as reactive oxygen species (ROS) scavengers. A total of six CrMTs were identified in the C. rosea genome and classified into four subgroups by phylogenetic analysis. An analysis of the cis-acting elements revealed that a series of hormone-, stress-, and development-related cis-acting elements were present in the promoter regions of CrMTs. The expression of CrMTs also showed habitat- and environmental stress-regulated patterns in C. rosea. CrMT overexpression in yeast enhanced tolerance to heavy metals and ROS, as well as high osmotic and alkalinity stress, which is consistent with their predicted roles as metal-chelating proteins and ROS scavengers. Our results indicate that the CrMT genes might contribute to the detoxification of plants to metals and provide marked tolerance against abiotic stress. The expression patterns of CrMTs in C. rosea also indicate that CrMTs play important roles in this species' response to extreme environments on tropical islands and reefs, probably by improving the thermotolerance of C. rosea plants.

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.

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.

Overexpression of a Metallothionein 2A Gene from Date Palm Confers Abiotic Stress Tolerance to Yeast and Arabidopsis thaliana

Although the date palm tree is an extremophile with tolerance to drought and certain levels of salinity, the damage caused by extreme salt concentrations in the soil, has created a need to explore stress-responsive traits and decode their mechanisms. Metallothioneins (MTs) are low-molecular-weight cysteine-rich proteins that are known to play a role in decreasing oxidative damage during abiotic stress conditions. Our previous study identified date palm metallothionein 2A (PdMT2A) as a salt-responsive gene, which has been functionally characterized in yeast and Arabidopsis in this study. The recombinant PdMT2A protein produced in Escherichia coli showed high reactivity against the substrate 5′-dithiobis-2-nitrobenzoic acid (DTNB), implying that the protein has the property of scavenging reactive oxygen species (ROS). Heterologous overexpression of PdMT2A in yeast (Saccharomyces cerevisiae) conferred tolerance to drought, salinity and oxidative stresses. The PdMT2A gene was also overexpressed in Arabidopsis, to assess its stress protective function in planta. Compared to the wild-type control, the transgenic plants accumulated less Na⁺ and maintained a high K⁺/Na⁺ ratio, which could be attributed to the regulatory role of the transgene on transporters such as HKT, as demonstrated by qPCR assay. In addition, transgenic lines exhibited higher chlorophyll content, higher superoxide dismutase (SOD) activity and improved scavenging ability for reactive oxygen species (ROS), coupled with a better survival rate during salt stress conditions. Similarly, the transgenic plants also displayed better drought and oxidative stress tolerance. Collectively, both in vitro and in planta studies revealed a role for PdMT2A in salt, drought, and oxidative stress tolerance.

Using tree leaves and barks collected from contaminated and uncontaminated areas as indicators of air metallic pollution

The aim of this study was to show the usefulness of woody species in assessing air pollution by cadmium, copper, lead, and zinc. For this, washed leaves, unwashed leaves, and barks of a coniferous species (Cupressus sempervirens var. fastigiata) and a broadleaved one (Eucalyptus cladocalyx F. Muell) were analyzed for cadmium, copper, lead and zinc contents. A transect sampling approach was carried out during spring 2016. Fifty stations were selected along an intensive traffic road, and three in a control area. The results showed that the highest concentrations were recorded in barks for copper (21.86 µg/g, 23.33 µg/g) and lead (14.53 µg/g, 63.33 µg/g), and in unwashed leaves for cadmium (0.57 µg/g, 1.19 µg/g) and zinc (48.94 µg/g, 47.6 µg/g) for E. cladocalyx F. Muell, and C. sempervirens var. fastigiata, respectively. Lead and zinc are the most accumulated metals compared to cadmium and copper in all samples. The studied metal contents in urban area were significantly higher than that of the control one. This represents that traffic road has influenced the metals contamination of the surrounding area. Results of the bioconcentration factor (BCF) for both species indicate that the studied species could be used as biomonitors to identify ecological problems and to predict effect on wildlife habitats. The highest values of metal accumulation index (MAI) indicate the effectiveness of these trees for controlling the air metals in the polluted areas. Result shows too that the studied species could be used for phytoextraction of heavy metals from the polluted soils and/or air.

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.

Hydrogen sulfide - cysteine cycle system enhances cadmium tolerance through alleviating cadmium-induced oxidative stress and ion toxicity in Arabidopsis roots

Cadmium (Cd²⁺) is a common toxic heavy metal ion. We investigated the roles of hydrogen sulfide (H₂S) and cysteine (Cys) in plant responses to Cd²⁺ stress. The expression of H₂S synthetic genes LCD and DES1 were induced by Cd²⁺ within 3 h, and endogenous H₂S was then rapidly released. H₂S promoted the expression of Cys synthesis-related genes SAT1 and OASA1, which led to endogenous Cys accumulation. The H₂S and Cys cycle system was stimulated by Cd²⁺ stress, and it maintained high levels in plant cells. H₂S inhibited the ROS burst by inducing alternative respiration capacity (AP) and antioxidase activity. H₂S weakened Cd²⁺ toxicity by inducing the metallothionein (MTs) genes expression. Cys promoted GSH accumulation and inhibited the ROS burst, and GSH induced the expression of phytochelatin (PCs) genes, counteracting Cd²⁺ toxicity. In summary, the H₂S and Cys cycle system played a key role in plant responses to Cd²⁺ stress. The Cd²⁺ tolerance was weakened when the cycle system was blocked in lcddes1-1 and oasa1 mutants. This paper is the first to describe the role of the H₂S and Cys cycle system in Cd²⁺ stress and to explore the relevant and specificity mechanisms of H₂S and Cys in mediating Cd²⁺ stress.

Cloning, heterologous expression and characterization of ascorbate peroxidase (APX) gene in laticifer cells of rubber tree (Hevea brasiliensis Muell. Arg.)

Ascorbate peroxidases (APXs) are a kind of crucial enzymes for removing reactive oxygen species (ROS) in plant cell. In the present study, a full-length cDNA encoding an APX, designated HbAPX, was isolated from Hevea brasiliensis by the rapid amplification of cDNA ends (RACE) method. HbAPX was 1174-bp in length and contained a 912-bp open reading frame (ORF) encoding a putative protein of 304 amino acids. The predicted molecular mass of HbAPX was 27.6 kDa (kDa) with an isoelectric point (pI) of 6.73. The phylogenetic analysis showed that HbAPX belonged to the cytosolic subgroup and was more relative to PtAPX and MdAPX2. By using PlantCare online analysis, such cis-acting elements as W-box and MRE were detected in the promoter region of HbAPX. Overproduction of recombinant HbAPX protein either in Escherichia coli or yeast enhanced their tolerance to such abiotic stresses as Cu(2+), Zn(2+), Na(2+) and hydrogen peroxide (H2O2). Ethrel application significantly down-regulated the expression of HbAPX and inhibited the activity of HbAPX in vivo. The ethrel-caused down-regulation of HbAPX may disturb the redox homeostasis in laticifer cells of rubber tree.

Characterization of a Type 1 Metallothionein Gene from the Stresses-Tolerant Plant Ziziphus jujuba

Plant metallothioneins (MTs) are a family of low molecular weight, cysteine-rich, and metal-binding proteins, which play an important role in the detoxification of heavy metal ions, osmotic stresses, and hormone treatment. Sequence analysis revealed that the open-reading frame (ORF) of ZjMT was 225 bp, which encodes a protein composed of 75 amino acid residues with a calculated molecular mass of 7.376 kDa and a predicated isoelectric point (pI) of 4.83. ZjMT belongs to the type I MT, which consists of two highly conserved cysteine-rich terminal domains linked by a cysteine free region. Our studies showed that ZjMT was primarily localized in the cytoplasm and the nucleus of cells and ZjMT expression was up-regulated by NaCl, CdCl2 and polyethylene glycol (PEG) treatments. Constitutive expression of ZjMT in wild type Arabidopsis plants enhanced their tolerance to NaCl stress during the germination stage. Compared with the wild type, transgenic plants accumulate more Cd2+ in root, but less in leaf, suggesting that ZjMT may have a function in Cd2+ retension in roots and, therefore, decrease the toxicity of Cd2+.