1,720,975 research outputs found
Downregulation of ethylene production and biosynthetic gene expression is associated to changes in putrescine metabolism in shoot-forming tobacco thin layers
No full textThe effect of aminoethoxyvinylglycine (AVG), an inhibitor of 1-aminocyclopropane-1-carboxylate synthase (ACS) activity, on ethylene emission and biosynthetic gene expression, on gene expression and/or activity of polyamine (putrescine, spermidine and spermine) biosynthetic enzymes, and on diamine oxidase (DAO, EC 1.4.3.6) activity was evaluated in tobacco (Nicotiana tabacum L. cv. Samsun) thin layers cultured on a shoot-forming medium (1 muM indol-3-acetic acid (IAA) plus 10 muM benzyladenine (BA)). Northern analyses showed that ACS and 1-aminocyclopropane-1-carboxylate oxidase (ACO) transcripts were present throughout culture with a maximum accumulation on day 7. Besides ethylene emission, AVG (0.5 muM) increasingly reduced ACS and ACO messages. The time course of labelled methionine incorporation into spermidine and spermine, which share with ethylene the common precursor S-adenosylmethionine (SAM), as well as SAM decarboxylase (SAMDC, EC 4.1.1.21) activity and gene expression. were not affected by AVG treatment. On the contrary, labelled putrescine incorporation into the higher polyamines (spermidine and spermine) and into trichloroacetic acid (TCA)-soluble polyamine conjugates was enhanced early in culture (day 2) by the drug. Putrescine biosynthetic enzyme activities, arginine decarboxylase (ADC, EC 4.1.1.19) and ornithine decarboxylase (ODC, EC 4.1.1.17), were also increased in AVG-treated explants. Moreover, inhibition of ethylene synthesis by AVG led to a strong reduction in diamine oxidising activity, especially the one associated with a cell wall-enriched fraction. Changes in putrescine biosynthesis. oxidation and flux into higher polyamines are discussed in the light of the rejuvenating effect of AVG
FREE AND CONJUGATED POLYAMINES DURING DENOVO FLORAL AND VEGETATIVE BUD FORMATION IN THIN CELL-LAYERS OF TOBACCO
No full textThe concentrations of three classes of polyamines, trichloroacetic acid-soluble (free), TCA-soluble conjugated (to small molecules) and TCA-insoluble conjugated (to macromolecules), was examined during de novo floral and vegetative bud formation in thin cell layers of Nicotiana tabacum L. cv. Samsun. Explants (consisting of 5–6 layers of epidermal, subepidermal and parenchyma cells) were excised either from floral pedicels or from stem internodes at the unripe fruit stage and cultured on the same medium. In the former, the first de novo formed flower buds appeared on day 8 of culture, while in the latter the first vegetative domes appeared on day 10. In both cases the number of floral and vegetative buds increased up to day 12 and 15, respectively. Changes in dry weight were determined throughout the culture period. Free and conjugated putrescine titer increased 5–60 times in both types of culture and in the three classes of polyamines examined; spermidine content also increased, while spermine, when present, did not show significant changes. TCA-soluble conjugated polyamines were most abundant, being about 2-fold the TCA-insoluble conjugated ones and 10-fold the free ones. The major increment in putrescine and spermidine content occurred in stem internode explants developing vegetative buds. In pedicel explants the maximum putrescine level was reached before or on day 8 in culture (emergence of the first flower buds with calyx initials), while in stem internode explants the maximum level was reached on day 12, at the emergence of the first vegetative buds with leaf primordia. While spermidine prevailed on day 0, putrescine was the most abundant polyamine during both differentiation processes. The putrescine content rapidly increased immediately after the onset of culture. Thus conjugated polyamines, especially putrescine, and not only the free ones, seem to be involved in both the reproductive and vegetative phases of tobacco growth and development
REGULATION OF RHIZOGENESIS BY POLYAMINES IN TOBACCO THIN-LAYERS
No full textTobacco thin layer explants were cultured on a rhizogenic medium for 21 days in the presence or absence of methylglyoxal-bis(guanylhydrazone) (MGBG) or cyclohexylamine (CHA), competitive inhibitors of spermidine synthesis through S-adenosylmethionine decarboxylase and spermidine synthase, respectively. On day 21, explants were transferred to hormone-free medium of the same composition with or without MGBG or CHA, alone or in combination with labelled and unlabelled spermidine. The effects of these treatments on free and bound polyamine levels, on the putrescine biosynthetic activity (ornithine decarboxylase, ODC, and arginine decarboxylase, ADC), on labelled spermidine incorporation and on the rhizogenic response were studied. In MGBG-treated explants rhizogenesis was strongly inhibited while with CHA it was less affected. In the former, the addition of spermidine caused a significant reversion of the rooting inhibition (considering both rooting percentage and the mean number of roots per explant), while not in the latter. MGBG induced strong putrescine and spermidine depletion both in the free and bound forms; CHA induced strong spermidine depletion (especially on day 21), but putrescine accumulation. In both treatments, in the presence of labelled spermidine, labelled free and bound putrescine were also detected. A general enhancement in ADC activity was observed in inhibitor-treated explants; the addition of spermidine to the inhibitors caused a further strong increase in ADC activity. These results are discussed in relation to the different metabolic pathways affected by the inhibitors
Methyl jasmonate and ethylene interfere with polyamine metabolism and organogenesis in tobacco thin cell layers
No full textStudying the interrelationships between the various plant growth regulators is a necessary step for the understanding of the physiological and morphogenic responses in vivo and in vitro. Polyamines (PAs) occupy a particular niche amongst the plant growth regulators in that, although they share some of the features of traditional phytohormones, such as gradients of synthesis/accumulation associated with juvenility, and growth and differentiation processes (Bagni and Torrigiani 1992), other characteristics, namely order of magnitude and lack of specific receptors, differentiate them and suggest similarities with second messengers. Further research on PA-binding proteins (Tassoni et al 1998) may help to clarify this issue
Morphogenesis in cultured thin layers and pith explants of tobacco. I. Effect of putrescine on cell size, xylogenesis and meristemoid organization
No full textSuperficial thin layers and pith explants from vegetative tobacco plants were cultured on a caulogenic medium (1 mu mol/L IAA + 10 mu mol/L BA), or with IAA alone (1 mu mol/L), BA alone (10 mu mol/L) or on a hormone-free medium, in the presence or absence of 100 mu mol/L putrescine. The aim was to investigate histologically possible, and explant type independent, putrescine effects on cytological events specifically induced by the hormone treatments during caulogenesis and callogenesis in vitro. In both types of explants the earliest cytological events (48 h) observed were fragmented or multilobed nuclei and newly formed cell walls, both occurring in IAA and IAA+BA treatments. Effects of putrescine on specific cytological events, induced by IAA or BA in both model systems, were from day 4 to the end of culture. Putrescine strongly counteracted the nuclear fragmentation, cell hypertrophy and xylogenesis specifically induced by IAA during callus growth. Furthermore, the polyamine strongly enhanced BA-induced formation of vegetative meristemoids. However, the extent of putrescine effects declined when the explants were treated with the two hormones in combination, or cultured under hormone-free conditions
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
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