1,720,959 research outputs found
Loss of nuclear gene expression during the phytochrome A-mediated far-red block of greening response
Full text linkWe have examined the expression of the HEMA1 gene, which encodes the key chlorophyll synthesis enzyme glutamyl-tRNA reductase, during the phytochrome A-mediated far-red light (FR) block of greening response in Arabidopsis. Our results demonstrate that the FR block of greening comprises two separate responses: a white light (WL) intensity-independent response that requires 3 d of FR and is associated with a loss of expression of the nuclear genes HEMA1 and Lhcb following the transfer to WL (transcriptionally coupled response) and a WL intensity-dependent response that is induced by 1 d of FR and is transcriptionally uncoupled. Both responses required phytochrome A. The transcriptionally uncoupled response correlated with a deregulation of tetrapyrrole synthesis and potential photooxidative damage and was inhibited by cytokinin. The transcriptionally coupled FR response was additive with the loss of expression following Norflurazon-induced photobleaching and was absent in the presence of sucrose or after lower fluence rate (1 µmol m2 s1) FR treatments. Both pathways leading to the loss of nuclear gene expression were inhibited by overexpression of NADPH:protochlorophyllide oxidoreductase, indicating a role for plastid signaling in the FR-mediated pathway. The significance of identifying a distinct phytochrome A-mediated plastid signaling pathway is discussed
The nuclear genes Lhcb and HEMA1 are differentially sensitive to plastid signals and suggest distinct roles for the GUN1 and GUN5 plastid-signalling pathways during de-etiolation
No full textFeedback mechanisms are critical to the regulation of chloroplast development and signals from functional plastids are required to maintain nuclear gene expression of chloroplast proteins. To understand the role of these signals in de-etiolating Arabidopsis thaliana L. seedlings, we followed the expression of three nuclear genes, Lhcb, HEMA1 and GSA, under a variety of treatments (Norflurazon, lincomycin and a far-red light pretreatment) leading to plastid damage in white light and in a range of genetic backgrounds known to modulate plastid signalling: the genomes uncoupled mutants, gun1, gun4, gun5 and the gun1,5 double mutant, and in a transgenic line over-expressing NADPH:protochlorophyllide oxidoreductase. The three nuclear genes were differentially sensitive to changes in plastid signalling, with Lhcb the most strongly repressed and GSA insensitive to all but the most severe treatments. Analysis of plastid morphology in seedlings grown under identical conditions demonstrated that these responses corresponded closely to the degree of plastid damage. Furthermore, although Lhcb and HEMA1 were responsive to both GUN1 and GUN5 signals, the relative inputs from these pathways differed for each transcript with GUN1 being dominant for HEMA1 regulation. Further analysis of HEMA1 expression in gun1 seedlings under non-photobleaching conditions indicates that GUN1 is an important suppressor of HEMA1 expression in the dark and under saturating white light. These results are consistent with plastid signals functioning in a feedback regulatory mechanism during chloroplast biogenesis, and suggest a key role for GUN1 during the early stages of chloroplast development
Light-signalling pathways leading to the co-ordinated expression of HEMA1 and Lhcb during chloroplast development in Arabidopsis thaliana
No full textDuring de-etiolation, the co-ordinated synthesis of chlorophyll and the chlorophyll a/b-binding proteins is critical to the development of functional light-harvesting complexes. To understand how this co-ordination is achieved, we have made a detailed study of the light-regulated signalling pathways mediating the expression of the HEMA1 and Lhcb genes encoding glutamyl-tRNA reductase, the first committed enzyme of 5-aminolaevulinic acid formation, and chlorophyll a/b-binding proteins, respectively. To do this, we have screened 7 photoreceptor and 12 light-signalling mutants of Arabidopsis thaliana L. for induction of HEMA1 and Lhcb expression in continuous red, far-red and blue light and following a red pulse. We have categorised these mutants into two groups. The phyA, phyB, phyAphyB, cry1, cry2, cop1, det1, poc1, eid1, and far1 mutations lead to diverse effects on the light regulation of HEMA1, but affect Lhcb expression to a similar degree. The hy1, hy2, hy5, fin219, fhy1, fhy3, spa1, ndpk2, and pat1 mutants also affect light regulation of both HEMA1 and Lhcb expression, but with differences in the relative magnitude of the two responses. The fhy1 and fhy3 mutants show the most significant differences in light regulation between the two genes, with both showing a strong inhibition of HEMA1 expression under continuous red light. These results demonstrate that co-ordinated regulation of HEMA1 and Lhcb is largely achieved through parallel light regulation mediated by shared phytochrome- and crytochrome-signalling pathways. However, glutamyl-tRNA reductase is also required for the synthesis of other tetrapyrroles and this dual role may account for the observed differences in these light-signalling pathways
Tetrapyrrole profiling in <em>Arabidopsis</em> seedlings reveals that retrograde plastid nuclear signaling is not due to Mg-protoporphyrin IX accumulation.
No full textChloroplast biogenesis involves careful coordination of both plastid and nuclear gene expression, which is achieved in part by retrograde signaling from the chloroplast to the nucleus. This can be demonstrated by the fact that the herbicide, Norflurazon (NF), which causes bleaching of chloroplasts, prevents the light induction of photosynthesis-related genes in the nucleus. It has been proposed that the tetrapyrrole pathway intermediate Mg-protoporphyrin IX acts as the signaling molecule in this pathway and accumulates in the chloroplasts and cytosol of the cell after NF treatment. Here we present data that demonstrate that this model is too simplistic. We have developed a sensitive liquid chromatography-mass spectrometry (LC/MS) method to measure tetrapyrrole intermediates and have shown that no Mg-protoporphyrin IX, nor indeed any other chlorophyll-biosynthesis intermediate, can be detected in NF-treated plants under conditions in which nuclear gene expression is repressed. Conversely when endogenous Mg-protoporphyrin IX levels are artificially increased by supplementation with the tetrapyrrole precursor, 5-aminolevulinic acid, the expression of nuclear-encoded photosynthetic genes is induced, not repressed. We also demonstrate that NF-treatment leads to a strong down-regulation of tetrapyrrole biosynthesis genes, consistent with the absence of an accumulation of tetrapyrrole intermediates. Finally, there is no correlation between nuclear-gene expression and any of the chlorophyll biosynthetic intermediates over a range of growth conditions and treatments. Instead, it is possible that a perturbation of tetrapyrrole synthesis may lead to localized ROS production or an altered redox state of the plastid, which could mediate retrograde signaling
Singlet oxygen initiates a plastid signal controlling photosynthetic gene expression
Full text linkRetrograde signals from the plastid regulate photosynthesis-associated nuclear genes and are essential to successful chloroplast biogenesis. One model is that a positive haem-related signal promotes photosynthetic gene expression in a pathway that is abolished by the herbicide norflurazon. Far-red light (FR) pretreatment and transfer to white light also results in plastid damage and loss of photosynthetic gene expression. Here, we investigated whether norflurazon and FR pretreatment affect the same retrograde signal.We used transcriptome analysis and real-time reverse transcription polymerase chain reaction (RT-PCR) to analyse the effects of these treatments on nuclear gene expression in various Arabidopsis (Arabidopsis thaliana) retrograde signalling mutants.Results showed that the two treatments inhibited largely different nuclear gene sets, suggesting that they affected different retrograde signals. Moreover, FR pretreatment resulted in singlet oxygen (1O2) production and a rapid inhibition of photosynthetic gene expression. This inhibition was partially blocked in the executer1executer2 mutant, which is impaired in 1O2 signalling.Our data support a new model in which a 1O2 retrograde signal, generated by chlorophyll precursors, inhibits expression of key photosynthetic and chlorophyll synthesis genes to prevent photo-oxidative damage during de-etiolation. Such a signal would provide a counterbalance to the positive haem-related signal to fine tune regulation of chloroplast biogenesis
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
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