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.

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

The impact of copper ions on the oxidative and calcium signal transductions, leading to cell death in plant cells, have been documented. Copper induces a series of biological and chemical reactions in plant cells including the oxidative burst reflecting the production of reactive oxygen species and the stimulation of calcium channel opening allowing a transient increase in cytosolic calcium concentrations. These early events, completed within a few minutes after the contact with copper, are known to trigger the development of cell death. The effects of DNA fragments with copper-binding motifs as novel plant cell-protecting agents were assessed using cell suspension cultures of transgenic tobacco (Nicotiana tabacum L., cell line BY-2) expressing the aequorin gene. The addition of GC-rich double-stranded DNA fragments, prior to the addition of copper ions, effectively blocked both the copper-induced calcium influx and cell death. In addition, the DNA-Cu complex examined was shown to possess superoxide-scavenging catalytic activity, suggesting that DNA-mediated protection of the cells from copper toxicity is due to the removal of superoxide. Lastly, a possible mechanism of DNA-Cu interaction and future applications of these DNA fragments in the protection of plant roots from metal toxicity or in aid of phyto-remediation processes are discussed.

Peroxyacetyl nitrate-induced oxidative and calcium signaling events leading to cell death in ozone-sensitive tobacco cell-line.

It has long been concerned that some secondary air pollutants such as smog components, ozone (O3) and peroxyacetyl nitrate (PAN), are highly phytotoxic even at low concentrations. Compared with the biology of O3, we largely lack the information on the toxicity model for PAN at the cellular signaling levels. Here, we studied the cell-damaging impact of PAN using suspension culture of smog-sensitive tobacco variety (Bel-W3). The cells were exposed to freshly synthesized PAN and the induced cell death was assessed under microscope after staining with Evans blue. Involvement of reactive oxygen species (ROS) in PAN toxicity was suggested by PAN-dependently increased intracellular H2O2 and also by the cell-protective effects of ROS scavengers and related inhibitors. Calcium chelator also lowered the level of PAN-induced cell death, indicating that Ca2+ is also involved. Using a transgenic cell line expressing aequorin, an increase in cytosolic Ca2+ concentration responsive to the pulse of PAN, but sensitive to Ca2+ channel blockers, was recorded, indicating that Ca2+ channels are activated by PAN or PAN-derived signals. Above data show some similarity between the signaling mechanisms responsive to O3 and PAN.

Lethal impacts of cigarette smoke in cultured tobacco cells.

BACKGROUND: In order to understand and generalize the toxic mechanism of cigarette smoke in living cells, comparison of the data between animal systems and other biological system such as microbial and plant systems is highly beneficial. OBJECTIVE: By employing the tobacco cells as model materials for cigarette smoke toxicity assay, the impacts of the combustion by-products such as nitrogen oxides could be highlighted as the toxic impacts of the plant-derived endogenous chemicals could be excluded in the plant cells. METHODS: Cigarette smoke-induced cell death was assessed in tobacco cell suspension cultures in the presence and absence of pharmacological inhibitors. RESULTS: Cigarette smoke was effective in induction of cell death. The smoke-induced cell death could be partially prevented by addition of nitric oxide (NO) scavenger, suggesting the role for NO as the cell death mediator. Addition of NO donor to tobacco cells also resulted in development of partial cell death further confirming the role of NO as cell death mediator. Members of reactive oxygen species and calcium ion were shown to be protecting the cells from the toxic action of smoke-derived NO.

Involvement of salicylic acid signal transduction in aluminum-responsive oxidative burst in Arabidopsis thaliana cell suspension culture.

To date, a number of studies have documented the toxic impacts of Al ions in plant cells. One of the key factors required for Al cytotoxicity is the generation of reactive oxygen species (ROS). Here we observed that Al treatments of suspension-cultured Arabidopsis thaliana cells resulted in biphasic superoxide generation monitored with chemiluminescence. Among six respiratory burst oxidase homologs (Atrbohs) coding for plant NADPH oxidase, AtrbohD was shown to be the only gene responsive to Al. As the expression of AtrbohD was rapid and long-lasting (1 min to 24 h). Al-induced superoxide generation, AtrbohD expression and cell death were all inhibited by NADPH oxidase inhibitor and superoxide dismutase. Interestingly, Al-induced AtrbohD expression and cell death were inhibited in the mutant and transgenic cell lines lacking salicylic acid biosyhthesis and accumulation (sid2 and NahG). Involvements of salicylic acid signaling in Al-induced AtrbohD expression and cell death development were also confirmed by the use of npr1 mutant cells and NPR1-overexpressing cells. Taken together, there would be a loop of SA signaling and SA-dependent expression of AtrbohD gene leading to prolonged ROS production and cell death development in the Al-exposed Arabidopsis cells.

Increased anion channel activity is an unavoidable event in ozone-induced programmed cell death.

BACKGROUND: Ozone is a major secondary air pollutant often reaching high concentrations in urban areas under strong daylight, high temperature and stagnant high-pressure systems. Ozone in the troposphere is a pollutant that is harmful to the plant. PRINCIPAL FINDINGS: By exposing cells to a strong pulse of ozonized air, an acute cell death was observed in suspension cells of Arabidopsis thaliana used as a model. We demonstrated that O(3) treatment induced the activation of a plasma membrane anion channel that is an early prerequisite of O(3)-induced cell death in A. thaliana. Our data further suggest interplay of anion channel activation with well known plant responses to O(3), Ca(2+) influx and NADPH-oxidase generated reactive oxygen species (ROS) in mediating the oxidative cell death. This interplay might be fuelled by several mechanisms in addition to the direct ROS generation by O(3); namely, H(2)O(2) generation by salicylic and abscisic acids. Anion channel activation was also shown to promote the accumulation of transcripts encoding vacuolar processing enzymes, a family of proteases previously reported to contribute to the disruption of vacuole integrity observed during programmed cell death. SIGNIFICANCE: Collectively, our data indicate that anion efflux is an early key component of morphological and biochemical events leading to O(3)-induced programmed cell death. Because ion channels and more specifically anion channels assume a crucial position in cells, an understanding about the underlying role(s) for ion channels in the signalling pathway leading to programmed cell death is a subject that warrants future investigation.