Barley metallothioneins differ in ontogenetic pattern and response to metals

Trichoderma viride Colonizes the Roots of Brassica napus L., Alters the Expression of Stress-Responsive Genes, and Increases the Yield of Canola under Field Conditions during Drought

In this work, we present the results of the inoculation of canola seeds (Brassica napus L.) with Trichoderma viride strains that promote the growth of plants. Seven morphologically different strains of T. viride (TvI-VII) were shown to be capable of synthesizing auxins and exhibited cellulolytic and pectinolytic activities. To gain a deeper insight into the molecular mechanisms underlying canola-T. viride interactions, we analyzed the canola stress genes metallothioneins (BnMT1-3) and stringent response genes (BnRSH1-3 and BnCRSH). We demonstrated the presence of cis-regulatory elements responsive to fungal elicitors in the promoter regions of B. napus MT and RSH genes and observed changes in the levels of the transcripts of the above-mentioned genes in response to root colonization by the tested fungal strains. Of the seven tested strains, under laboratory conditions, T. viride VII stimulated the formation of roots and the growth of canola seedlings to the greatest extent. An experiment conducted under field conditions during drought showed that the inoculation of canola seeds with a suspension of T. viride VII spores increased yield by 16.7%. There was also a positive effect of the fungus on the height and branching of the plants, the number of siliques, and the mass of a thousand seeds. We suggest that the T. viride strain TvVII can be used in modern sustainable agriculture as a bioinoculant and seed coating to protect B. napus from drought.

Functional characterization of rice metallothionein OsMT-I-Id: Insights into metal binding and heavy metal tolerance mechanisms

Metallothioneins (MTs) are cysteine-rich proteins known for their strong metal-binding capabilities, making them effective in detoxifying heavy metals (HMs). This study focuses on characterizing the functional properties of OsMT-I-Id, a type-I Metallothionein found in rice. Using a HM-responsive yeast cup1Δ (DTY4), ycf1∆ (for cadmium), and acr3∆ mutants (for trivalent arsenic), we assessed the impact of OsMT-I-Id on metal accumulation and cellular resilience. Our results demonstrated that yeast cells expressing OsMT-I-Id showed increased tolerance and accumulated higher levels of copper (Cu), arsenic (As), and cadmium (Cd), compared to control cells. This can be attributed to the protein's ability to chelate and bind HMs. Site-directed mutagenesis was employed to investigate the specific contributions of cysteine residues. The study revealed that yeast cells with a mutated C-domain displayed heightened HM sensitivity, while cells with a mutated N-domain exhibited reduced sensitivity. This underscores the critical role of C-cysteine-rich domains in metal binding and tolerance of type-I rice MTs. Furthermore, the study identified the significance of the 12th cysteine position at the N-domain and the 68th and 72nd cysteine positions at the C-domain in influencing OsMT-I-Id metal-binding capacity. This research provides novel insights into the structure-function relationship and metal binding properties of type-I plant MTs.

Barley metallothionein isoforms, MT2b2 and MT4, differentially respond to photohormones in barley aleurone layer and their recombinant forms show different affinity for binding to zinc and cadmium

Metallothioneins (MTs) are metal-binding proteins that have important roles in the homeostasis of heavy metals. In this study, the two MT genes was studied in response to phytohormones using the barley aleurone layer as a kind of model system. The aleurone layer was isolated from barley embryo-less half grains and was incubated for 24 h with different phytohormones. Based on the results the genes encoding HvMT2b2 and HvMT4 were down-regulated through gibberellic acid (GA), while they were and up-regulated through salicylic acid (SA). Despite this, these two genes were differentially expressed to other hormones. Furthermore, the proteins HvMT2b2 and HvMT4 were heterologous expressed as GST-fusion proteins in E. coli. The HvMT4 and HvMT2b2 heterologous expression in E. coli gives rise to 10- and 3-fold improvements in the accumulation capacity for Zn2+, respectively. Whereas the transgenic E. coli strain that expresses HvMT2b2 could accumulate Cd2+ three-fold higher than control. The expression of HvMT4 did not affect the accumulation of Cd2+. HvMT4 which is known as seed-specific isoform seems to be able to bind to Zn2+ with good affinity and cannot bind Cd2+. In comparison, HvMT2b2 was able to bind both Zn2+ and Cd2+. Therefore HvMT4 could serve a noteworthy role in zinc storage in barley seeds. The expression of HvMT4 is induced by SA 30-fold, concerning the untreated aleurone layer. Such results could provide good insights for the assessment of the effects of phytohormones in the molecular mechanism involved in essential metal storage in cereal seeds.

Examination of the Metallothionein Gene Family in Greater Duckweed Spirodela polyrhiza

Duckweeds are aquatic plants that proliferate rapidly in a wide range of freshwaters, and they are regarded as a potential source of sustainable biomass for various applications and the cost-effective bioremediation of heavy metal pollutants. To understand the cellular and molecular basis that underlies the high metal tolerance and accumulation capacity of duckweeds, we examined the forms and transcript profiles of the metallothionein (MT) gene family in the model duckweed Spirodela polyrhiza, whose genome has been completely sequenced. Four S. polyrhiza MT-like genes were identified and annotated as SpMT2a, SpMT2b, SpMT3, and SpMT4. All except SpMT2b showed high sequence homology including the conserved cysteine residues with the previously described MTs from flowering plants. The S. polyrhiza genome appears to lack the root-specific Type 1 MT. The transcripts of SpMT2a, SpMT2b, and SpMT3 could be detected in the vegetative whole-plant tissues. The transcript abundance of SpMT2a was upregulated several-fold in response to cadmium stress, and the heterologous expression of SpMT2a conferred copper and cadmium tolerance to the metal-sensitive ∆cup1 strain of Saccharomyces cerevisiae. Based on these results, we proposed that SpMT2a may play an important role in the metal detoxification mechanism of duckweed.

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.

Combined effect of nano-CuO and nano-ZnO in plant-related system: From bioavailability in soil to transcriptional regulation of metal homeostasis in barley

The co-existence of engineered nanoparticles (ENPs) in the environment is an emerging issue remaining poorly investigated. The present study aimed at analyzing the fate of binary mixtures of CuO and ZnO ENPs in a soil-plant system. The ENPs were singly or jointly dosed into soil at 300 mg kg^-1 and aged for 7 and 30 days. To evaluate nano-specific effects, individual and combined treatments of metal salts were also applied. Interactions between ENPs and soil-grown barley Hordeum vulgare were determined in terms of biomass, plant mineral composition as well as expression of genes regulating metal homeostasis (ZIP1,3,6,8,10,14, RAN1, PAA1,2, MTP1, COPT5) and detoxification (MT1-3). The bioavailability of Zn and Cu in bulk soil and in the rooting zone was determined using the 0.01 mol L^-1 CaCl₂ extraction. After combined treatment of ENPs, the extractable concentrations of Cu and Zn were lower than upon individual exposure in bulk soil. The opposite tendency was noted for metal salts. Genes related to metal uptake (ZIP) and cellular compartment (PAA2, RAN1) were mostly up-regulated by single rather than combined application of ENPs. The single and joint exposure to metals salts induced the down-regulation of these genes.

Hyperaccumulation of Cu, Zn, Ni, and Cd in Azolla species inducing expression of methallothionein and phytochelatin synthase genes

Azolla is a floating aquatic fern, having amazing capacity for concentrating toxic heavy metals. Metallothioneins (MTs) and phytochelatins (PCs) are well-defined heavy metal-binding ligands in plants. Bioaccumulation potential of different Azolla species varies according to their heavy metal ions. Therefore, the accumulation of Ni, Zn, Cu, and Cd was studied in A. pinnata, A. filiculoides, and a sample taken from Anzali wetland. Moreover, the expression of metallothionein and phytochelatin synthase encoding genes was examined at different metal concentrations. The highest level of Cu and Cd absorption was detected in A. pinnata, while the maximum amount of Ni and Zn absorption was observed in A. filiculoides and the sample taken from Anzali, respectively. The MT2 and PCS1 gene expression patterns were significantly induced by the heavy metal treatments, confirming their roles in phytoremediation potential of Azolla. However, as the results concerning heavy metal accumulation and gene expression vary in different species, only specific species of Azolla can be used for special purposes. It can be concluded that the Azolla is a good candidate for phytoremediation purposes, and the formation of phytochelatin-heavy metal complexes and their sequestration in vacuole are the main processes influencing susceptibility of Azolla to heavy metals.

Functional characterization of metallothionein-like genes from Physcomitrella patens: expression profiling, yeast heterologous expression, and disruption of PpMT1.2a gene

Physcomitrella patens contains four metallothionein-like genes. Three were shown to confer metal tolerance in yeast. Transcript profiling suggests their roles in senescence and reproductive development or cadmium and oxidative stress. Metallothioneins (MTs) have been suggested to play various roles including metal detoxification, nutrient remobilization, ROS scavenging, stress tolerance, and plant development. However, little is known about the forms and functions of MTs in bryophytes. The moss Physcomitrella patens genome was found to contain four MT-like genes. Amino acid sequence composition showed that the P. patens MTs (PpMTs) were clustered with Type 1 plant MTs, and could be further classified into two sub-types, herein referred to as sub-type 1: PpMT1.1a and PpMT1.1b and sub-type 2: PpMT1.2a and PpMT1.2b. Transcript abundance of PpMT1.1b and PpMT1.2b was upregulated in the gametophore compared to protonema, and all, except PpMT1.2a, were highly induced in senescing gametophytes. PpMT1.1a and PpMT1.1b transcripts were upregulated in protonema treated with cadmium and hydrogen peroxide. Unlike many higher plant MTs, the PpMT transcript abundance was not strongly induced in response to copper and zinc. These results suggest that PpMTs may play a role in protecting P. patens from cadmium and oxidative stress and may be involved in tissues senescence and reproductive development. The PpMTs, except PpMT1.2b, were also able to confer metal tolerance and accumulation when heterologously expressed in the ∆cup1 yeast. A P. patens mutant lacking PpMT1.2a through targeted gene disruption was generated. However, it did not show any alteration in growth phenotypes under senescence-induced conditions or hypersensitivity to cadmium, copper, zinc, H₂O₂, and NaCl stresses. Further characterization of additional P. patens mutants lacking single or multiple PpMTs may provide insight into the physiological roles of bryophytic MTs.

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.

Preventive and Therapeutic Role of Functional Ingredients of Barley Grass for Chronic Diseases in Human Beings

Barley grass powder is the best functional food that provides nutrition and eliminates toxins from cells in human beings; however, its functional ingredients have played an important role as health benefit. In order to better cognize the preventive and therapeutic role of barley grass for chronic diseases, we carried out the systematic strategies for functional ingredients of barley grass, based on the comprehensive databases, especially the PubMed, Baidu, ISI Web of Science, and CNKI, between 2008 and 2017. Barley grass is rich in functional ingredients, such as gamma-aminobutyric acid (GABA), flavonoids, saponarin, lutonarin, superoxide dismutase (SOD), K, Ca, Se, tryptophan, chlorophyll, vitamins (A, B1, C, and E), dietary fiber, polysaccharide, alkaloid, metallothioneins, and polyphenols. Barley grass promotes sleep; has antidiabetic effect; regulates blood pressure; enhances immunity; protects liver; has anti-acne/detoxifying and antidepressant effects; improves gastrointestinal function; has anticancer, anti-inflammatory, antioxidant, hypolipidemic, and antigout effects; reduces hyperuricemia; prevents hypoxia, cardiovascular diseases, fatigue, and constipation; alleviates atopic dermatitis; is a calcium supplement; improves cognition; and so on. These results support that barley grass may be one of the best functional foods for preventive chronic diseases and the best raw material of modern diet structure in promoting the development of large health industry and further reveal that GABA, flavonoids, SOD, K-Ca, vitamins, and tryptophan mechanism of barley grass have preventive and therapeutic role for chronic diseases. This paper can be used as a scientific evidence for developing functional foods and novel drugs for barley grass for preventive chronic diseases.

Enrichment and Identification of the Most Abundant Zinc Binding Proteins in Developing Barley Grains by Zinc-IMAC Capture and Nano LC-MS/MS

Background: Zinc accumulates in the embryo, aleurone, and subaleurone layers at different amounts in cereal grains. Our hypothesis is that zinc could be stored bound, not only to low MW metabolites/proteins, but also to high MW proteins as well. Methods: In order to identify the most abundant zinc binding proteins in different grain tissues, we microdissected barley grains into (1) seed coats; (2) aleurone/subaleurone; (3) embryo; and (4) endosperm. Initial screening for putative zinc binding proteins from the different tissue types was performed by fractionating proteins according to solubility (Osborne fractionation), and resolving those via Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis (SDS-PAGE) followed by polyvinylidene fluoride (PVDF) membrane blotting and dithizone staining. Selected protein fractions were subjected to Zn²⁺-immobilized metal ion affinity chromatography, and the captured proteins were identified using nanoscale liquid chromatography coupled to tandem mass spectrometry (nanoLC-MS/MS). Results: In the endosperm, the most abundant zinc binding proteins were the storage protein B-hordeins, gamma-, and D-hordeins, while in the embryo, 7S globulins storage proteins exhibited zinc binding. In the aleurone/subaleurone, zinc affinity captured proteins were late abundant embryogenesis proteins, dehydrins, many isoforms of non-specific lipid transfer proteins, and alpha amylase trypsin inhibitor. Conclusions: We have shown evidence that abundant barley grain proteins have been captured by Zn-IMAC, and their zinc binding properties in relationship to the possibility of zinc storage is discussed.

Differential reactivity of closely related zinc(II)-binding metallothioneins from the plant Arabidopsis thaliana

The dynamics of metal binding to and transfer from metalloproteins involved in metal homeostasis are important for understanding cellular distribution of metal ions. The dicotyledonous plant Arabidopsis thaliana has two type 4 seed-specific metallothionein homologues, MT4a and MT4b, with likely roles in zinc(II) homeostasis. These two metallothioneins are 84% identical, with full conservation of all metal-binding cysteine and histidine residues. Yet, differences in their spatial and temporal expression patterns suggested divergence in their biological roles. To investigate whether biological functions are reflected in molecular properties, we compare aspects of zinc(II)-binding dynamics of full-length MT4a and MT4b, namely the pH dependence of zinc(II) binding and protein folding, and zinc(II) transfer to the chelator EDTA. UV-Vis and NMR spectroscopies as well as native electrospray ionisation mass spectrometry consistently showed that transfer from Zn6MT4a is considerably faster than from Zn6MT4b, with pseudo-first-order rate constants for the fastest observed step of k obs = 2.8 × 10-4 s-1 (MT4b) and k obs = 7.5 × 10-4 s-1 (MT4a) (5 µM protein, 500 µM EDTA, 25 mM Tris buffer, pH 7.33, 298 K). 2D heteronuclear NMR experiments allowed locating the most labile zinc(II) ions in domain II for both proteins. 3D homology models suggest that reactivity of this domain is governed by the local environment around the mononuclear Cys2His2 site that is unique to type 4 MTs. Non-conservative amino acid substitutions in this region affect local electrostatics as well as whole-domain dynamics, with both effects rendering zinc(II) ions bound to MT4a more reactive in metal transfer reactions. Therefore, domain II of MT4a is well suited to rapidly release its bound zinc(II) ions, in broad agreement with a previously suggested role of MT4a in zinc(II) transport and delivery to other proteins.

The type 4 metallothionein from Brassica napus seeds folds in a metal-dependent fashion and favours zinc over other metals

Rapeseed MT4 only folds properly in the presence of Zn²⁺ and thus may serve as a selectivity filter for metal accumulation in plant embryos.

Diversity in cadmium accumulation and resistance associated with various metallothionein genes (type III) in Phytolacca americana L

Metallothioneins (MTs) are known for their heavy metal deoxidation during phytoremediation. To estimate their roles in the cadmium (Cd) hyperaccumulator Phytolacca americana L., three MT genes, PaMT3-1, PaMT3-2 and PaMT3-3, belonging to the MT3 subfamily were cloned. They separately encoded 63, 65 and 65 amino acids, containing12, 10 and 11 cysteines (Cys), respectively. Each gene was individually transformed and expressed in Escherichia coli cells. A Cd-resistance assay showed that the recombinant strains had enhanced survival rates, especially those containing PaMT3-1 and PaMT3-3. Additionally, the recombinant strains were high Cd accumulators, with the recombinant PaMT3-1's maximum accumulation being 2.16 times that of the empty vector strains. The numbers of cysteines and the structures of MT proteins were associated with the Cd enrichment and resistance capabilities. PaMT3-1 could be an effective gene resource in future plant Cd remediation-related breeding programs.

Interactive effects of sulfur and chromium on antioxidative defense systems and BnMP1 gene expression in canola (Brassica napus L.) cultivars differing in Cr(VI) tolerance

Plants suffer with combined stress of sulfur (S) deficiency and hexavalent chromium [Cr(VI)] in soils. There are a few reports on the interactive effects of S-deficiency and Cr(VI) stress. Therefore, the interactions between S nutrition and Cr(VI) stress were investigated in hydroponically grown canola (Brassica napus L.) cultivars differing in Cr(VI) tolerance. The relatively Cr(VI)-tolerant (NK Petrol) and Cr(VI)-susceptible (Sary) cultivars were grown in S-sufficient nutrient solution and then exposed to variable S concentrations [deficient (0 mM S, -S) and sufficient (1 mM S, +S)]. The seedlings were then exposed to 100 μM Cr(VI) for 3 days. S-deficiency (-S/-Cr) and combined stress (-S/+Cr) caused a significant decrease in growth parameters of Sary than NK Petrol (P < 0.05). In -S/+Cr treatment, Cr accumulation in Sary was significantly higher than NK Petrol. The higher level of Cr in Sary increased lipid peroxidation and decreased chlorophyll content. The activities of antioxidant enzymes and cysteine content were significantly higher in NK Petrol than in Sary under combined stress. The levels of ascorbate (AsA) and glutathione (GSH) were significantly decreased by S deficiency. The expression level of metallothionein gene (BnMP1) in the tolerant NK Petrol was increased by -S/+Cr treatment. However, expression level of BnMP1 gene in the susceptible Sary was enhanced by +S/+Cr treatment. This result suggests metallothionein (MT) may be involved in Cr(VI) tolerance under S-deficient condition. In conclusion, S nutrition influenced Cr accumulation and enhanced tolerance caused by a positive effect on defense systems and gene expression.

Breeding crops for improved mineral nutrition under climate change conditions

Improvements in understanding how climate change may influence chemical and physical processes in soils, how this may affect nutrient availability, and how plants may respond to changed availability of nutrients will influence crop breeding programmes. The effects of increased atmospheric CO2 and warmer temperatures, both individually and combined, on soil microbial activity, including mycorrhizas and N-fixing organisms, are evaluated, together with their implications for nutrient availability. Potential changes to plant growth, and the combined effects of soil and plant changes on nutrient uptake, are discussed. The organization of research on the efficient use of macro- and micronutrients by crops under climate change conditions is outlined, including analysis of QTLs for nutrient efficiency. Suggestions for how the information gained can be used in plant breeding programmes are given.

Phosphate and zinc transport and signalling in plants: toward a better understanding of their homeostasis interaction

Inorganic phosphate (Pi) and zinc (Zn) are two essential nutrients for plant growth. In soils, these two minerals are either present in low amounts or are poorly available to plants. Consequently, worldwide agriculture has become dependent on external sources of Pi and Zn fertilizers to increase crop yields. However, this strategy is neither economically nor ecologically sustainable in the long term, particularly for Pi, which is a non-renewable resource. To date, research has emphasized the analysis of mineral nutrition considering each nutrient individually, and showed that Pi and Zn homeostasis is highly regulated in a complex process. Interestingly, numerous observations point to an unexpected interconnection between the homeostasis of the two nutrients. Nevertheless, despite their fundamental importance, the molecular bases and biological significance of these interactions remain largely unknown. Such interconnections can account for shortcomings of current agronomic models that typically focus on improving the assimilation of individual elements. Here, current knowledge on the regulation of the transport and signalling of Pi and Zn individually is reviewed, and then insights are provided on the recent progress made towards a better understanding of the Zn-Pi homeostasis interaction in plants.