Effects of scavengers for active oxygen species on cell death by cryptogein

Crucial roles of 3D-MoO2-PBC cocatalytic electrodes in the enhanced degradation of imidacloprid in heterogeneous electro-Fenton system: Degradation mechanisms and toxicity attenuation

Imidacloprid (IMI), as the most-consumed pesticide, has posed a severe threat to the water ecosystem due to its recalcitrance and inefficient elimination in the traditional wastewater treatment. Herein, a heterogeneous electro-Fenton (EF) system coupled with 3D-MoO₂-porous biochar (PBC) cocatalytic electrodes, abbreviated as 3D-MPE-EF, is initially applied to promote the elimination of IMI in the agrochemical wastewater from pesticide production. The elimination rate of IMI by 3D-MPE-EF system is 18.15 times higher than that by traditional EF system at pH 7.0. The utilization of 3D-MoO₂-PBC electrodes sufficiently compensates for inherent deficiencies of traditional EF system. The circular utilization of Fe is also addressed by 3D-MoO₂-PBC cocatalytic electrodes to reduce the consumption of Fe²⁺ and the deposition of iron mud. Through comparison, MoO₂ is considered the most appropriate cocatalyst in terms of the reutilization of Fe and degradation of IMI. Eight mechanisms are identified in the degradation pathways of IMI by UPLC-Q-TOF-MS. The ecotoxicities of IMI are remarkably attenuated in the 3D-MPE-EF system. This study provides insights into the roles of 3D-MoO₂-PBC cocatalytic electrodes in the enhanced elimination of IMI in heterogeneous EF system.

Isolation and characterization of a cryptogein-like gene from drought and salt-treated Alternanthera philoxeroides roots

Alligator weed, Alternanthera philoxeroides (Mart.) Grisb, is an amphibious plant with long thick fleshy roots that develop from adventitious roots under drought conditions. To clone differentially-expressed genes from the roots of A. philoxeroides we applied both mRNA differential display and rapid amplification of cDNA ends techniques. A cryptogein-like gene of A. philoxeroides, designated as ApCL, was cloned. On the basis of semi-quantitative RT-PCR analysis results, we demonstrated that the ApCL gene was upregulated under drought and salt stress conditions. After ApCL was transferred to Pichia pastoris GS115 and its resistance to drought and salt then increased by >100 %. Therefore, the ApCL gene of A. philoxeroides was likely involved in drought and salt tolerance responses.

Type-2 histone deacetylases as new regulators of elicitor-induced cell death in plants

• Plant resistance to pathogen attack is often associated with a localized programmed cell death called hypersensitive response (HR). How this cell death is controlled remains largely unknown. • Upon treatment with cryptogein, an elicitor of tobacco defence and cell death, we identified NtHD2a and NtHD2b, two redundant isoforms of type-2 nuclear histone deacetylases (HDACs). These HDACs are phosphorylated after a few minutes' treatment, and their rate of mRNAs are rapidly and strongly reduced, leading to a 40-fold decrease after 10 h of treatment. • By using HDAC inhibitors, RNAi- and overexpression-based approaches, we showed that HDACs, and especially NtHD2a/b, act as inhibitors of cryptogein-induced cell death. Moreover, in NtHD2a/b-silenced plants, infiltration with cryptogein led to HR-like symptoms in distal leaves. • Taken together, these results show for the first time that type-2 HDACs, which are specific to plants, act as negative regulators of elicitor-induced cell death in tobacco (Nicotiana tabacum), suggesting that the HR is controlled by post-translational modifications including (de)acetylation of nuclear proteins.

The antagonistic effect of hydroxyl radical on the development of a hypersensitive response in tobacco

Reactive oxygen species (ROS) are important signalling molecules in living cells. It is believed that ROS molecules are the main triggers of the hypersensitive response (HR) in plants. In the present study of the effect of riboflavin, which is excited to generate ROS in light, on the development of the HR induced by the elicitin protein ParA1 in tobacco (Nicotiana tabacum), we found that the extent of the ParA1-induced HR was diminished by hydroxyl radical (OH•), a type of ROS. As compared with the zones treated with ParA1 only, the HR symptom in the zones that were infiltrated with ParA1 plus riboflavin was significantly diminished when the treated plants were placed in the light. However, this did not occur when the plants were maintained in the dark. Trypan blue staining and the ion leakage measurements confirmed HR suppression in the light. Further experiments proved that HR suppression is attributed to the involvement of the photoexcited riboflavin, and that the suppression can be eliminated with the addition of hydrogen peroxide scavengers or OH• scavengers. Fenton reagent treatment and EPR measurements demonstrated that it is OH• rather than hydrogen peroxide that contributes to HR suppression. Accompanying the endogenous OH• formation, suppression of the ParA1-induced HR occurred in the tobacco leaves that had been treated with high-level abscisic acid, and that suppression was also removed by OH• scavengers. These results offer evidence that OH•, an understudied and little appreciated ROS, participates in and modulates biologically relevant signalling in plant cells.

The cell death factor, cell wall elicitor of rice blast fungus (Magnaporthe grisea) causes metabolic alterations including GABA shunt in rice cultured cells

An elicitor derived from the cell wall of rice blast fungus (Magnaporthe grisea) causes cell death in suspension cultured cells of rice (Oryza sativa L.). To elucidate the role of M. grisea elicitor on metabolic pathway of rice cells, we performed metabolite profiling using capillary electrophoresis-mass spectrometry (CE/MS). Treatment with M. grisea elicitor increased the amounts of antioxidants and free amino acids and decreased the amount of metabolites in the tricarboxylic acid (TCA) cycle. Lower ATP concentration caused aberrant energy charge, concurrently with reduced amount of NAD(P)H in elicitor treated cells. Among free amino acids detected in this study, the level of gamma-aminobutyric acid (GABA) increased. GABA is metabolized through a bypass pathway of the TCA cycle called GABA shunt, which is composed of glutamate decarboxylase (GAD), GABA transaminase (GABA-T) and succinic semialdehyde dehydrogenase (SSADH). While M. grisea elicitor negligibly affected GAD and SSADH, GABA-T activity significantly decreased. The decrease in GABA-T activity was recovered by NADPH oxidase inhibitor, which prevents cell death induced by M. grisea elicitor. Thus, GABA accumulation observed in rice cells under elicitor stress is partly associated with GABA-T activity.

PCNA damage caused by antineoplastic drugs

Structurally diverse chemotherapeutic and chemopreventive drugs, including camptothecin, doxorubicin, sanguinarine, and others, were found to cause covalent crosslinking of proliferating cell nuclear antigen (PCNA) trimers in mammalian cells exposed to fluorescent light. This PCNA damage was caused by both nuclear and cytoplasmically localizing drugs. For some drugs, the PCNA crosslinking was evident even with very brief exposures to laboratory room lighting. In the absence of drugs, there was no detectable covalent crosslinking of PCNA trimers. Other proteins were photo-crosslinked to PCNA at much lower levels, including crosslinking of additional PCNA to the PCNA trimer. The proteins photo-crosslinked to PCNA did not vary with cell type or drug. PCNA was not crosslinked to itself or to other proteins by superoxide, hydrogen peroxide or hydroxyl radicals, but hydrogen peroxide caused monoubiquitination of PCNA. Quenching of PCNA photo-crosslinking by histidine, and enhancement by deuterium oxide, suggest a role for singlet oxygen in the crosslinking. SV40 large T antigen hexamers were also efficiently covalently photo-crosslinked by drugs and light. Photodynamic crosslinking of nuclear proteins by cytoplasmically localizing drugs, together with other evidence, argues that these drugs may reach the nucleoplasm in amounts sufficient to photodamage important chromosomal enzymes. The covalent crosslinking of PCNA trimers provides an extremely sensitive biomarker for photodynamic damage. The damage to PCNA and large T antigen raises the possibility that DNA damage signaling and repair mechanisms may be compromised when cells treated with antineoplastic drugs are exposed to visible light.

Cryptogein-induced anion effluxes: electrophysiological properties and analysis of the mechanisms through which they contribute to the elicitor-triggered cell death

Anion effluxes are amongst the earliest reactions of plant cells to elicitors of defence responses. However, their properties and their role in disease resistance remain almost unknown. We previously demonstrated that cryptogein, an elicitor of tobacco defence responses, induces a nitrate (NO(3) (-)) efflux. This efflux is an early prerequisite to the cryptogein-triggered hypersensitive response (HR). Here, we analyzed the electrophysiological properties of the elicitor-mediated NO(3) (-) efflux and clarified the mechanisms through which it contributes to cell death. Application of the discontinuous single electrode voltage-clamp technique in tobacco cells elicited with cryptogein enabled us to record the activation of slow-type deactivating anion channel currents. Cryptogein-induced plasma membrane depolarization and Ca(2+) influx, an essential component of elicitor signalling for HR cell death, were prevented by inhibiting the NO(3) (-) efflux. Similarly, pharmacological blocking of the anion efflux suppressed vacuolar collapse, a hallmark of cell death. The role of NO(3) (-) efflux in mediating proteases activation was further assessed. It is shown that cryptogein induced the activation of three proteases with apparent molecular masses of 95, 190 and 240 kDa. Their activation occurred independently on the anion efflux and, together with cell death, was strongly reduced by cycloheximide and the protease inhibitor PMSF. In contrast, the NO(3) (-) efflux was 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 the HR. Collectively, our data indicate that anion efflux is an early prerequisite to morphological and biochemical events participating to cell death.

Photon emissions from rice cells elicited by N-acetylchitooligosaccharide are generated through phospholipid signaling in close association with the production of reactive oxygen species

Biophotons are ultraweak light emissions from biochemical reactions in a living body. They increase in suspension-cultured rice (Oryza sativa L.) cells when elicited by N-acetylchitooligosaccharide. Biochemical analyses were undertaken to investigate the relationship between disease response and biophotons in order to clarify the emission mechanism of biophotons caused by this elicitor. Photon emissions induced by N-acetylchitohexaose were suppressed when cells were pretreated with the reactive oxygen species (ROS)-generating inhibitors: pyrocatechol-3,5-disulfonic acid disodium salt (Tiron); diphenylene iodonium (DPI); and salicylhydroxamic acid (SHAM). Conversely, exogenously applied ROS (superoxide and hydrogen peroxide) were able to induce photon emissions. The effects of protein phosphorylation (K-252a) and the Ca(2+) signaling inhibitors, ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) and LaCl(3), caused photon emissions to decrease. It is clear that photon emissions from rice cells elicited by N-acetylchitohexaose are closely associated with the ROS-generating system, and are regulated by Ca(2+) signaling and protein phosphorylation. Exogenously applied phosphatidic acid (PA), the second messenger in the signal transduction of disease response, raised photon emissions in rice cells. Comparisons of photon emissions from PA and N-acetylchitohexaose regarding time courses, spectral compositions, and the inhibition ratios of several inhibitors, as well as a loss- and gain-of-function assay using the protein synthesis inhibitor cycloheximide (CHX) and PA, showed the possibility that photon emissions from rice cells elicited by N-acetylchitooligosaccharide were generated through PA, an intermediate of phospholipid signaling.

Thaxtomin A induces programmed cell death in Arabidopsis thaliana suspension-cultured cells

Thaxtomin A is the main phytotoxin produced by Streptomyces scabiei, the causative agent of common scab disease of potato. Pathogenicity of S. scabiei is dependent on the production of thaxtomin A which is required for the development of disease symptoms, such as growth inhibition and cell death. We investigated whether thaxtomin A-induced cell death was similar to the hypersensitive cell death that often occurs in response to specific pathogens or phytotoxins during the so-called hypersensitive response (HR). We demonstrated that thaxtomin A induced in Arabidopsis thaliana suspension-cultured cells a genetically controlled cell death that required active gene expression and de novo protein synthesis, and which involved fragmentation of nuclear DNA, a characteristic hallmark of apoptosis. The thaxtomin A-induced form of programmed cell death (PCD) was not a typical HR, since defence responses generally preceding or associated with the HR, such as rapid medium alkalization, oxidative burst and expression of defence-related genes PR1 and PDF1.2, were not observed in plant cells following addition of thaxtomin A. Thaxtomin A has been shown to inhibit cellulose biosynthesis (Scheible et al. in Plant Cell 15:1781, 2003). We showed that isoxaben, a specific inhibitor of cellulose biosynthesis, also induced in Arabidopsis cell suspensions a PCD similar to that induced by thaxtomin A. These data suggested that rapid changes in the plant cell wall composition and organization can induce PCD in plant cells. We discuss how rapid inhibition of cellulose biosynthesis may trigger this process.