Cycloheximide and buthionine sulfoximine prevent induction of genotoxic adaptation by cadmium salt against methyl mercuric chloride in embryonic shoot cells of Hordum vulgare L

Investigating the underlying mechanism of cadmium-induced plant adaptive response to genotoxic stress

Plants as sessile organisms have developed some unique strategies to withstand environmental stress and adaptive response (AR) is one of them. In the present study Cadmium (Cd)-induced AR was evaluated to ameliorate the genotoxicity of a known chemical mutagen ethyl methanesulphonate (EMS) based on cytotoxicity, genotoxicity and oxidative stress in two model plant systems Allium cepa L. and Vicia faba L. Priming the plants with cadmium chloride (CdCl₂, 25 and 50 μM) reduced the genotoxicity of EMS (0.25 mM). Cd-induced AR was evident by the magnitude of adaptive response (MAR) values calculated for cytotoxicity, genotoxicity and biochemical parameters. In addition the involvement of some major metabolic pathways and epigenetic modifications in AR was investigated. Metabolic blockers of protein kinase cascades, DNA repair, oxidative stress and de novo translation interfered with the adaptive response implying their role in AR whereas, inhibitors involved in post-replication repair and autophagy were ineffective implicating that they probably have no role in the AR studied. Moreover to find the role of DNA methylation in AR, methylation-sensitive comet assay was carried out. Simultaneously 5-methyl- 2'-deoxycytidine (5mdC) levels were quantified by HPLC (high performance liquid chromatography). AR was eliminated in cells treated with a demethylating agent, 5-aza- 2'deoxycytidine (AZA). Results implied a contribution of DNA hypermethylation. To the best of our knowledge this is a first report correlating DNA methylation to Cd-induced adaptive response in plants undergoing genotoxic stress.

Response to copper bromide exposure in Vicia sativa L. seeds: analysis of genotoxicity, nucleolar activity and mineral profile

Copper bromide (CuBr2) effects on seed germination and plantlet development of Vicia sativa L. are evaluated through mitotic index, chromosome aberrations, nucleolar activity and mineral profile. CuBr2 induces a significant presence of micronuclei, sticky and c-metaphases, anaphase bridges and chromosome breaks. Increased number of nucleoli and scattering of AgNOR proteins from the nucleolus in the nuclear surface at CuBr2 1mM and in the cytoplasm at CuBr2 5mM, goes along with the decrease of root growth. In V. sativa embryo the content of many macro and micronutrients increases up to copper 1mM in agreement with reserve mobilization while at CuBr2 5mM some elements are present in lower amount. We hypothesize that inhibitory effects observed at 5mM are due either to a nutrient shortage or to a direct influence of copper on root cell division, evidenced by low mitotic index, high occurrence of chromosome aberrations and loss of material from the nucleolus.

Reduced clastogenic activity of maleic hydrazide in Vicia faba seedlings grown in a situation of overcrowding stress

A pre-treatment stress situation of overcrowding of Vicia faba seedlings in the phase of germination and growth influenced their subsequent sensibility to treatment with the mutagenic herbicide maleic hydrazide. The seedlings showed a significant reduction in the frequency of micronucleated cells when they grew in a strongly crowded manner compared with scattered and uniformly distributed seedlings (3.83% versus 11.46%). The findings do not provide evidence for the involvement of phytochelatins in response to stress conditions in this process: pre-treatment with buthionine sulfoximine, a specific inhibitor of phytochelatin synthesis, did not modify the response of the seedlings to maleic hydrazide under conditions of overcrowding or under normal conditions. Scanning electron microscopy (SEM) was used to observe the root tip of V. faba grown in conditions of normal growth or overcrowding. SEM micrographs revealed differences between the tips with regards to root hair density and root surface morphology. Finally, we found a positive correlation between the frequency of micronucleated cells and the length of the primary root, for every time of growth considered (1, 3, 4 and 5 days).

Carboxymethylcellulose improves the antimutagenic activity of sodium selenite against maleic hydrazide in Vicia faba seedlings

In this study the genotoxic effects induced by a treatment with different doses of sodium selenite in Vicia faba seedlings were evaluated with or without the addition of carboxymethylcellulose. A further objective of this study was to verify whether the adduct selenite-carboxymethylcellulose was also able to reduce the genotoxic damages induced by the herbicide maleic hydrazide, a strong mutagenic agent in plants, at a higher extent than selenite alone. The results obtained showed a genotoxic activity of sodium selenite at concentrations up to 8.6 mg L-1. In the treatments with selenite-carboxymethylcellulose, the genotoxicity induced by the complex was significantly lower in comparison to how much was observed in the treatment with selenite only. When sodium selenite's protective activity against the genotoxic effects induced by the herbicide maleic hydrazide was tested, a reduction of mutagenic damages was observed at the highest application doses of selenite (from 86 mg L-1). The treatments with selenite-carboxymethylcellulose resulted in a further increase of selenium protective activity, which was observable for all doses used. These findings suggest a possible role played by carboxymethylcellulose in the regulation of the genotoxic activity of selenium.

Buthionine sulfoximine prevents the reduction of the genotoxic activity of maleic hydrazide by soil humic substances in Vicia faba seedlings

A significant reduction of the genotoxic effects caused by herbicide maleic hydrazide (MH) in Vicia faba seedlings was observed to be induced by a growth step in an organic soil as well as by a pretreatment with highly purified humic substances. In addition, such protective activity was resulted quite similar to that observed when the conditioning pretreatment was carried out with metal salts, so suggesting the involvement of the GSH biosynthesis in determining the protective activity observed. In agreement with this hypothesis, a previous exposure to buthionine sulfoximine (BSO), an inhibitor of the phytochelatins production, through the inhibition of GSH synthesis, prevented the reduction of the genotoxic activity of MH. The findings provide evidence for the involvement of the GSH biosynthesis pathway in determining the antigenotoxic activity revealed and suggest a possible involvement of the phytochelatins in this process. However, yet to be clarified is whether the stimulation of GSH production results as a consequence of a nonspecific influence on the protein synthesis by humic substances or of its direct activation due to the presence, as contaminants, of some heavy metals in both organic soil and humic acids extracts.

A comparison of biochemical responses to oxidative and metal stress in seedlings of barley, Hordeum vulgare L

Biochemical responses on the bases of activities of antioxidant enzymes; peroxidase, catalase, superoxide dismutase and glutathione reductase as well as estimations of total protein, lipid peroxidation and thiols in the form of protein, non-protein, glutathione and phytochelatin measured in growing seedlings of barley, Hordeum vulgare L., from Day 2 through 8 were compared following treatment of seeds for 2 h with oxidative agents, paraquat 5 x 10(-5), 10(-4), 10(-3) M, H2O2 10(-3), 5 x 10(-3), 10(-2) M and a metal salt, CdSO4 10(-5), 10(-4), 10(-3) M. A significant induction of all antioxidant enzymes along with an increase in the levels of protein, lipid peroxidation and glutathione was noted in response to oxidative stress, CdSO4 induced significant peroxidase and catalase activities but not superoxide dismutase. In a marked contrast from oxidative stress, CdSO4 decreased glutathione reductase activity as well as glutathione levels but increased phytochelatin level. The differential biochemical responses thus underlined the crucial involvement of glutathione and phytochelatin in the oxidative and metal-induced adaptive responses, respectively.

Differential induction of adaptive responses by paraquat and hydrogen peroxide against the genotoxicity of methyl mercuric chloride, maleic hydrazide and ethyl methane sulfonate in plant cells in vivo

Induction of adaptive response by conditioning doses of paraquat (PQ) and hydrogen peroxide (H2O2) in embryonic shoot cells of Hordeum vulgare and root meristem cells of Allium cepa was tested against the genotoxicity of challenge doses of methyl mercuric chloride (MMCl), maleic hydrazide (MH) or ethylmethane sulfonate (EMS). Plant tissue fixed at different recovery hours following the challenge treatments was analysed for cells with genotoxicity markers that include spindle or chromosome aberrations and micronuclei. The results provided clear-cut evidence that whereas H2O2 induced adaptive response for the chromosome damage caused by MMCl and MH, PQ induced the same for MMCl and EMS, but not for damage caused by MH. The findings pointed to the differences in the underlying mechanisms of oxidative responses induced by H2O2 and O2-.

Persistence of cadmium-induced adaptive response to genotoxicity of maleic hydrazide and methyl mercuric chloride in root meristem cells of Allium cepa L.: differential inhibition by cycloheximide and buthionine sulfoximine

With an objective to determine the period of persistence of the metal-induced adaptive response to chemical mutagens and heavy metals, growing root meristems of Allium cepa were conditioned by cadmium sulfate (CdSO4), 4 x 10(-7) and 4 x 10(-6) M for 1 h and subsequently challenged by maleic hydrazide (MH), 5 x 10(-3) M or methyl mercuric chloride (MMCl), 1.26 x 10(-6) M for 3 h at different time intervals ranging from a few minutes to several hours following the conditioning dose. Root meristems, fixed at regular intervals during recovery from 6 to 48 h, were cytologically analysed for cells with micronuclei (MNC). The adaptive responses to MH and MMCl were observed as early as 5 min after the Cd-conditioning that persisted for at least 48 h. Metabolic inhibitors, cycloheximide (CH). 10(-7) M and buthionine sulfoximine (BSO), 10(-4) M administered either prior to or simultaneous with Cd-conditioning effectively prevented the adaptive response to MH. Whereas BSO, an inhibitor of phytochelatin synthesis, prevented the adaptive responses from 15 min to 8 h after the conditioning dose, CH an inhibitor of cytoplasmic protein synthesis prevented the same from 6 to 48 h. The findings underscored the differential roles of phytochelatins and proteins underlying the foregone metallo-adaptive response.