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Cadmium-induced transcriptional and enzymatic alterations related to oxidative stress
No full textThe early antioxidative defence mechanisms were studied in Arabidopsis thaliana by applying a range of realistic Cd concentrations. Our data suggest that a 24 h exposure to 20 mu M CdSO4 is already too toxic to study moderate toxicity, whereas a highly coordinated oxidative stress-related defence response could be observed after Cd application of 5 and 10 mu M. Significant differences in transcript abundance of several genes involved in antioxidative defence were observed. The generation of superoxide seems the main cause of oxidative stress in the roots, whereas in the leaves hydrogen peroxide appears to be an important player. Furthermore, an increased transcript level of lipoxygenase, a potential inducer of oxidative stress, suggests a central role of this gene in causing the Cd-related redox imbalance. Our results show that Cd as a non-redox-active metal induces oxidative stress and indicate that the antioxidative defence system is moderated by the activation of different genes in different organs and cellular compartments.This work was supported by a PhD grant for Karen Smeets
from Hasselt University (tUl-impulsfinanciering-toxicology) and two BOF (Bijzonder onderzoeksfonds) projects of Hasselt University, BOF02G01 and BOF04G01
Role of Ascorbate and Glutathione in Cellular Defence Against Cadmium Exposure in Arabidopsis thaliana
No full textCadmium (Cd) is a trace element found naturally in soils and water. Cadmium pollution is often a severe environmental problem in industrial areas; this metal is easily taken up by plants through the root system and translocated to the aerial parts. Exposure to increased Cd concentrations can lead to visible symptoms of toxicity and cause growth inhibition, root damage or chlorosis. This study focuses on three aspects of plant defence mechanisms induced by Cd exposure: (1) detoxification processes (2) oxidative stress related mechanisms and (3) the effect of Cd at the protein level in the test plant Arabidopsis thaliana. The tripeptide glutathione (GSH) is an important metabolite in Cd detoxification processes in two ways (i) GSH is the precursor for phytochelatin (PC) synthesis that detoxifies Cd through metal complexation and vacuolar sequestration and (ii) GSH is a major antioxidant metabolite and in coordination with ascorbate (AsA), both are involved in the AsA-GSH cycle to cope with oxidative stress by scavenging reactive oxygen species (ROS). The first experimental part of this work evaluates the effects of one-week exposure of three weeks old plants two Cd concentrations (1 and 10 µM in rockwool) at the leaf level. Assessment of the different objectives brought us to the following conclusions (1) a dose-response was imposed by Cd; (2) plants actively detoxified Cd by PC complexation; (3) antioxidant metabolites were depleted and (4) Cd caused a disequilibrium in the antioxidative defence mechanism leading to an oxidative cellular environment (Chapter 3: Semane et al. 2007). Moreover, using a proteomic approach, it was clear that Cd stress affected lots of proteins involved in different biological pathways (Chapter 4). Coordination of carbon, nitrogen and sulphur metabolism providing energy and precursors eliciting plant defence were highlighted. In the second experimental part, plants were grown for three weeks in hydroponic culture to allow us to study also the effects of Cd at the root level. Plants were exposed for three days to 1 and 5 µM Cd. Asa and/or GSH deficient Arabidopsis mutants containing about 30-50 % of AsA (vtc1-1), GSH (cad2-1) and AsA-GSH (vtc1-1–cad2-1) as compared to the wild type (col0) were used. The results obtained, show differential responses between roots and leaves. Roots are in direct contact with the metal and actively defend themselves by induction of PCs, and gene expression levels related to GSH metabolism. Although GSH biosynthesis genes were induced, the GSH content decreased. The latter result emphasised both (i) the importance of GSH for the synthesis of PCs and (ii) PCs are not involved in Cd translocation to the shoot as i.e. Cd-PC complex (Chapter 5). As observed in the first experimental part, the respective redox ratio GSH/GSSG dramatically decreased leading to a higher oxidative environment. A similar decrease was observed for AsA and its relative redox ratio AsA/DHA (Chapter 5). However, in leaves of all genotypes, AsA and GSH concentrations and their respective ratios increased upon Cd exposure. Therefore, we hypothesised that H2O2 might be involved in both oxidative stress induced by Cd in the roots and as a signalling molecule leading to an acclimation of the leaves by an increment of antioxidant metabolites. Performing proteomic comparisons of the different genotypes using principal component analysis (PCA), we were able to separate the mutant cad from the other genotypes. This was mainly due to either the expression or no expression at all of several proteins in both roots and leaves. Root proteomic profiling revealed that lots of proteins related to metabolism, stress responses and protein metabolism were involved (Chapter 6). Moreover, the highly induced expression of two proteins observed only in the mutants treated with Cd was identified as isoforms of cytosolic glyceraldehyde dehydrogenase (cGAPDH) (Chapter 6). Recent studies have shown this enzyme to be one of the major targets of H2O2. Although this strongly suggests that Cd induces H2O2 production, a role for the associated couple H2O2-cGAPDH as a signalling molecule or as a probable ROS scavenger needs to be further investigated. Studying the leaf proteome of the different genotypes under Cd stress, we clearly observed that the genotype effect was prominent on the Cd effect (Chapter 7). In conclusion, taken together the results provided by this work emphasise the complex relations of Cd with both plants organs and at the different cellular levels. Undoubtedly, GSH is a key component of the plants’ defence against Cd and its relation with plant physiology in adaptation to its environment is crucial. The signalling network mediated by both antioxidants and their redox couple are extensive and need to be further explored. Additional experiments integrating transcriptome and ROS-targeted proteins should be conducted to explore further plants’ adaptation to environmental stress (Chapter 8)
Role of Ascorbate and Glutathione in Cellular Defence Against Cadmium Exposure in Arabidopsis thaliana
No full textCadmium (Cd) is a trace element found naturally in soils and water. Cadmium pollution is often a severe environmental problem in industrial areas; this metal is easily taken up by plants through the root system and translocated to the aerial parts. Exposure to increased Cd concentrations can lead to visible symptoms of toxicity and cause growth inhibition, root damage or chlorosis. This study focuses on three aspects of plant defence mechanisms induced by Cd exposure: (1) detoxification processes (2) oxidative stress related mechanisms and (3) the effect of Cd at the protein level in the test plant Arabidopsis thaliana. The tripeptide glutathione (GSH) is an important metabolite in Cd detoxification processes in two ways (i) GSH is the precursor for phytochelatin (PC) synthesis that detoxifies Cd through metal complexation and vacuolar sequestration and (ii) GSH is a major antioxidant metabolite and in coordination with ascorbate (AsA), both are involved in the AsA-GSH cycle to cope with oxidative stress by scavenging reactive oxygen species (ROS). The first experimental part of this work evaluates the effects of one-week exposure of three weeks old plants two Cd concentrations (1 and 10 µM in rockwool) at the leaf level. Assessment of the different objectives brought us to the following conclusions (1) a dose-response was imposed by Cd; (2) plants actively detoxified Cd by PC complexation; (3) antioxidant metabolites were depleted and (4) Cd caused a disequilibrium in the antioxidative defence mechanism leading to an oxidative cellular environment (Chapter 3: Semane et al. 2007). Moreover, using a proteomic approach, it was clear that Cd stress affected lots of proteins involved in different biological pathways (Chapter 4). Coordination of carbon, nitrogen and sulphur metabolism providing energy and precursors eliciting plant defence were highlighted. In the second experimental part, plants were grown for three weeks in hydroponic culture to allow us to study also the effects of Cd at the root level. Plants were exposed for three days to 1 and 5 µM Cd. Asa and/or GSH deficient Arabidopsis mutants containing about 30-50 % of AsA (vtc1-1), GSH (cad2-1) and AsA-GSH (vtc1-1–cad2-1) as compared to the wild type (col0) were used. The results obtained, show differential responses between roots and leaves. Roots are in direct contact with the metal and actively defend themselves by induction of PCs, and gene expression levels related to GSH metabolism. Although GSH biosynthesis genes were induced, the GSH content decreased. The latter result emphasised both (i) the importance of GSH for the synthesis of PCs and (ii) PCs are not involved in Cd translocation to the shoot as i.e. Cd-PC complex (Chapter 5). As observed in the first experimental part, the respective redox ratio GSH/GSSG dramatically decreased leading to a higher oxidative environment. A similar decrease was observed for AsA and its relative redox ratio AsA/DHA (Chapter 5). However, in leaves of all genotypes, AsA and GSH concentrations and their respective ratios increased upon Cd exposure. Therefore, we hypothesised that H2O2 might be involved in both oxidative stress induced by Cd in the roots and as a signalling molecule leading to an acclimation of the leaves by an increment of antioxidant metabolites. Performing proteomic comparisons of the different genotypes using principal component analysis (PCA), we were able to separate the mutant cad from the other genotypes. This was mainly due to either the expression or no expression at all of several proteins in both roots and leaves. Root proteomic profiling revealed that lots of proteins related to metabolism, stress responses and protein metabolism were involved (Chapter 6). Moreover, the highly induced expression of two proteins observed only in the mutants treated with Cd was identified as isoforms of cytosolic glyceraldehyde dehydrogenase (cGAPDH) (Chapter 6). Recent studies have shown this enzyme to be one of the major targets of H2O2. Although this strongly suggests that Cd induces H2O2 production, a role for the associated couple H2O2-cGAPDH as a signalling molecule or as a probable ROS scavenger needs to be further investigated. Studying the leaf proteome of the different genotypes under Cd stress, we clearly observed that the genotype effect was prominent on the Cd effect (Chapter 7). In conclusion, taken together the results provided by this work emphasise the complex relations of Cd with both plants organs and at the different cellular levels. Undoubtedly, GSH is a key component of the plants’ defence against Cd and its relation with plant physiology in adaptation to its environment is crucial. The signalling network mediated by both antioxidants and their redox couple are extensive and need to be further explored. Additional experiments integrating transcriptome and ROS-targeted proteins should be conducted to explore further plants’ adaptation to environmental stress (Chapter 8)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Oxidative stress-related responses at transcriptional and enzymatic levels after exposure to Cd or Cu in a multipollution context
No full textThe physiological effects of Cd and Cu have been highlighted in several studies over the last years. At the cellular level, oxidative stress has been reported as a common mechanism in both stress situations. Nevertheless, because of differences in their redox-related properties, the origin of the stress and regulation of these effects can be very different. Our results show a specific Cd-related induction of NADPH oxidases, whereas both metals induced lipid peroxidation via the activation of lipoxygenases. With respect to the antioxidative defense system, metal-specific patterns of superoxide dismutases (SODs) were detected, whereas gene expression levels of the H2O2-quenching enzymes were equally induced by both metals. Because monometallic exposure is very unusual in real-world situations, the metal-specific effects were compared with the mechanisms induced when the plants are exposed to both metals simultaneously. Combined exposure to Cd and Cu enhanced some of the effects that were induced when only one metal was applied to the medium. Other specific monometallically induced effects, such as a copper zinc superoxide dismutase (CSD2) downregulation due to Cd, were also sustained in a multipollution context, irrespective of the other monometallic effects. Furthermore, specific multipollution effects were unravelled, as iron superoxide dismutase 1 (FSD1) upregulation in the leaves was significant only when both Cu and Cd were applied. Additional relationships between these treatments and the common and specific stress induction mechanisms are discussed. (C) 2009 Elsevier GmbH. All rights reserved.This work was supported by a PhD grant awarded to Karen Smeets from Hasselt University (tUl-impulsfinanciering-toxicology) and by the Fund for Scientific Research - Flanders (FWO Belgium; Project G.0436.06). Additional funding came from two BOF (Bijzonder onderzoeksfonds) projects at Hasselt University, BOF02G01 and BOF04G01. T Remans is a post-doctoral research fellow of the FWO (Research Foundation - Flanders). The authors gratefully acknowledge the advice of Prof. Dr. R. Carleer for the element determinations and support on statistics from Prof. Dr. T. Burzykowski. They wish to thank Carine Put and Ann Wijgaerts for skilful technical assistance
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