Protection of tobacco cells from oxidative copper toxicity by catalytically active metal-binding DNA oligomers

microRNAs: Key Players in Plant Response to Metal Toxicity

Environmental metal pollution is a common problem threatening sustainable and safe crop production. Heavy metals (HMs) cause toxicity by targeting key molecules and life processes in plant cells. Plants counteract excess metals in the environment by enhancing defense responses, such as metal chelation, isolation to vacuoles, regulating metal intake through transporters, and strengthening antioxidant mechanisms. In recent years, microRNAs (miRNAs), as a small non-coding RNA, have become the central regulator of a variety of abiotic stresses, including HMs. With the introduction of the latest technologies such as next-generation sequencing (NGS), more and more miRNAs have been widely recognized in several plants due to their diverse roles. Metal-regulated miRNAs and their target genes are part of a complex regulatory network. Known miRNAs coordinate plant responses to metal stress through antioxidant functions, root growth, hormone signals, transcription factors (TF), and metal transporters. This article reviews the research progress of miRNAs in the stress response of plants to the accumulation of HMs, such as Cu, Cd, Hg, Cr, and Al, and the toxicity of heavy metal ions.

CG Demethylation Leads to Sequence Mutations in an Anther Culture of Barley Due to the Presence of Cu, Ag Ions in the Medium and Culture Time

During plant tissue cultures the changes affecting regenerants have a broad range of genetic and epigenetic implications. These changes can be seen at the DNA methylation and sequence variation levels. In light of the latest studies, DNA methylation change plays an essential role in determining doubled haploid (DH) regenerants. The present study focuses on exploring the relationship between DNA methylation in CG and CHG contexts, and sequence variation, mediated by microelements (CuSO4 and AgNO3) supplemented during barley anther incubation on induction medium. To estimate such a relationship, a mediation analysis was used based on the results previously obtained through metAFLP method. Here, an interaction was observed between DNA demethylation in the context of CG and the time of culture. It was also noted that the reduction in DNA methylation was associated with a total decrease in the amount of Cu and Ag ions in the induction medium. Moreover, the total increase in Cu and Ag ions increased sequence variation. The importance of the time of tissue culture in the light of the observed changes resulted from the grouping of regenerants obtained after incubation on the induction medium for 28 days. The present study demonstrated that under a relatively short time of tissue culture (28 days), the multiplication of the Cu2+ and Ag+ ion concentrations ('Cu*Ag') acts as a mediator of demethylation in CG context. Change (increase) in the demethylation in CG sequence results in the decrease of 'Cu*Ag', and that change induces sequence variation equal to the value of the indirect effect. Thus, Cu and Ag ions mediate sequence variation. It seems that the observed changes at the level of methylation and DNA sequence may accompany the transition from direct to indirect embryogenesis.

Extracellular ATP released by copper stress could act as diffusible signal in alleviating the copper stress-induced cell death

In the present work, by using tobacco cell suspension and wheat seedlings, we studied that eATP (extracellular ATP) released by copper (Cu) stress could act as diffusible signal in alleviating the Cu stress-induced cell death. A semipermeable membrane was fixed in the middle of a plastic box to divide the box into two equal compartments (A and B, respectively). This semipermeable membrane can prevent direct cell-to-cell (or seedling-to-seedling) contact and the diffusion of the macromolecules [such as ATPase (adenosine 5'-triphosphatase)] between these two compartments. The cell suspension directly stressed with CuCl₂ was placed in compartment A and was incubated with the untreated cell suspension in compartment B. Such treatment significantly increased the levels of cell death and eATP content of the cell suspension in these two compartments. In contrast, addition of ATPase into the cell suspension directly stressed with CuCl₂ decreased the eATP level in these two compartments but further increased the level of cell death in compartment B, compared to no addition of ATPase. Similar results were obtained when tobacco cell suspension was replaced by wheat seedlings. These observations indicate that when Cu stress from compartment A induced the plant cell death in compartment B, ATP transferred from compartment A could play a role in alleviating this cell death. Thus, it is suggested that eATP released by copper stress could act as diffusible signal in alleviating the Cu stress-induced cell death.

Melatonin Protects Cultured Tobacco Cells against Lead-Induced Cell Death via Inhibition of Cytochrome c Translocation

Melatonin was discovered in plants more than two decades ago and, especially in the last decade, it has captured the interests of plant biologists. Beyond its possible participation in photoperiod processes and its role as a direct free radical scavenger as well as an indirect antioxidant, melatonin is also involved in plant defense strategies/reactions. However, the mechanisms that this indoleamine activates to improve plant stress tolerance still require identification and clarification. In the present report, the ability of exogenous melatonin to protect Nicotiana tabacum L. line Bright Yellow 2 (BY-2) suspension cells against the toxic exposure to lead was examined. Studies related to cell proliferation and viability, DNA fragmentation, possible translocation of cytochrome c from mitochondria to cytosol, cell morphology after fluorescence staining and also the in situ accumulation of superoxide radicals measured via the nitro blue tetrazolium reducing test, were conducted. This work establishes a novel finding by correcting the inhibition of release of mitochondrial ctytocrome c in to the cytoplasm with the high accumulation of superoxide radicals. The results show that pretreatment with 200 nm of melatonin protected tobacco cells from DNA damage caused by lead. Melatonin, as an efficacious antioxidant, limited superoxide radical accumulation as well as cytochrome c release thereby, it likely prevents the activation of the cascade of processes leading to cell death. Fluorescence staining with acridine orange and ethidium bromide documented that lead-stressed cells additionally treated with melatonin displayed intact nuclei. The results revealed that melatonin at proper dosage could significantly increase BY-2 cell proliferation and protected them against death. It was proved that melatonin could function as an effective priming agent to promote survival of tobacco cells under harmful lead-induced stress conditions.

Fluorescent monitoring of copper-occupancy in His-ended catalytic oligo-peptides

Controlled generation of reactive oxygen species (ROS) is widely beneficial to various medical, environmental, and agricultural studies. As inspired by the functional motifs in natural proteins, our group has been engaged in development of catalytically active oligo-peptides as minimum-sized metalloenzymes for generation of superoxide anion, an active member of ROS. In such candidate molecules, catalytically active metal-binding minimal motif was determined to be X-X-H, where X can be most amino acids followed by His. Based on above knowledge, we have designed a series of minimal copper-binding peptides designated as G_nH series peptides, which are composed of oligo-glycyl chains ended with C-terminal His residue such as GGGGGH sequence (G₅H). In order to further study the role of copper binding to the peptidic catalysts sharing the X-X-H motif such as G₅H-conjugated peptides, we should be able to score the occupancy of the peptide population by copper ion in the reaction mixture. Here, model peptides with Cu-binding affinity which show intrinsic fluorescence due to tyrosyl residue (Y) in the UV region (excitation at ca. 230 and 280 nm, and emission at ca. 320 nm) were synthesized to score the effect of copper occupancy. Synthesized peptides include GFP-derived fluorophore sequence, TFSYGVQ (designated as Gfp), and Gfp sequence fused to C-terminal G₅H (Gfp-G₅H). In addition, two Y-containing tri-peptides derived from natural GFP fluorophores, namely, TYG and SYG were fused to the G₅H (TYG-G₅H and SYG-G₅H). Conjugation of metal-binding G₅H sequence to GFP-fluorophore peptide enhanced the action of Cu²⁺ on quenching of intrinsic fluorescence due to Y residue. Two other Y-containing peptides, TYG-G₅H and SYG-G₅H, also showed intrinsic fluorescence which is sensitive to addition of Cu²⁺. There was linear relationship between the loading of Cu²⁺ and the quenching of fluorescence in these peptide, suggesting that Cu²⁺-dependent quenching of Y-reside-derived fluorescence could be a measure of copper occupancy in the peptides. Lastly, the fate of Y residue in the Cu-loaded peptides under oxidative condition in the presence of H₂O₂ was discussed based on the Cu/H₂O₂-dependent changes in fluorescence spectra.

Anti-oxidative feedback and biomarkers in the intertidal seagrass Zostera japonica induced by exposure to copper, lead and cadmium

To investigate the potential influences of anthropogenic pollutants, we evaluated the responses of the intertidal seagrass Zostera japonica to three heavy metals: copper (Cu), lead (Pb), and cadmium (Cd). Z. japonica was exposed to various concentrations of Cu, Pb, and Cd (0, 0.5, 5, 50μM) over seven days. The effects were then analyzed using the antioxidant enzyme catalase (CAT), superoxide dismutase (SOD), peroxidase (POD), glutathione peroxidase (GPX), and lipid peroxidation measured using malondialdehyde (MDA) as proxy. Metal accumulation in the above-ground tissues and phenotypic changes were also investigated. Our results revealed that heavy metal concentration increased in seagrass exposed to high levels of metals. Z. japonica has great potential for metal accumulation and a suitable candidate for the decontamination of moderately Cu contaminated bodies of water and can also potentially enhanced efforts of environmental decontamination, either through phytoextraction abilities or by functioning as an indicator for monitoring programs that use SOD, CAT, GPX, POD and MDA as biomarkers.

Prevention of copper-induced cell death by GC-rich DNA oligomers in murine macrophage-like RAW264.7 cells

Impact of redox active transition metals on activation of cell death signaling in plant cells have been documented to date. We have recently reported that GC-rich DNA oligomers with high affinity for binding of copper and catalytic activity for removal of ROS as novel plant cell-protecting agents. Here, we show that similar DNA oligomers protect the mouse macrophage-like RAW264.7 cells from copper-induced cell death, suggesting that the phenomenon firstly observed in plant model can be expanded to a wider range of cells and/or organisms including mammalian cells.

Production and removal of superoxide anion radical by artificial metalloenzymes and redox-active metals

Generation of reactive oxygen species is useful for various medical, engineering and agricultural purposes. These include clinical modulation of immunological mechanism, enhanced degradation of organic compounds released to the environments, removal of microorganisms for the hygienic purpose, and agricultural pest control; both directly acting against pathogenic microorganisms and indirectly via stimulation of plant defense mechanism represented by systemic acquired resistance and hypersensitive response. By aiming to develop a novel classes of artificial redox-active biocatalysts involved in production and/or removal of superoxide anion radicals, recent attempts for understanding and modification of natural catalytic proteins and functional DNA sequences of mammalian and plant origins are covered in this review article.

Mitigation of copper toxicity by DNA oligomers in green paramecia

Impact of transition metals which catalyze the generation of reactive oxygen species (ROS), on activation of cell death signaling in plant cells have been documented to date. Similarly in green paramecia (Paramecium bursaria), an aquatic protozoan species harboring symbiotic green algae in the cytoplasm, toxicities of various metallic ions have been documented. We have recently examined the effects of double-stranded GC-rich DNA fragments with copper-binding nature and ROS removal catalytic activity as novel plant cell-protecting agents, using the suspension-cultured tobacco cells. Here, we show that above DNA oligomers protect the cells of green paramecia from copper-induced cell death, suggesting that the phenomenon firstly observed in tobacco cells is not limited only within higher plants but it could be universally observable in wider range of organisms.