1,720,972 research outputs found
Twenty Years of Transgenic Plants Resistant to Cucumber mosaic virus
No full textPlant genetic engineering has promised researchers improved speed and flexibility with regard to the introduction
of new traits into cultivated crops. A variety of approaches have been applied to produce virus-resistant transgenic plants, some of which have proven to be remarkably
successful. Studies on transgenic resistance to Cucumber mosaic virus probably have been the most intense of any plant virus. Several effective strategies based on pathogenderived
resistance have been identified; namely, resistance mediated by the viral coat protein, the viral replicase, and
post-transcriptional gene silencing. Techniques using nonpathogen-derived resistance strategies, some of which could
offer broader resistance, generally have proven to be much less effective. Not only do the results obtained so far provide
a useful guide to help focus on future strategies, but they also suggest that there are a number of possible mechanisms
involved in conferring these resistances. Further detailed studies on the interplay between viral transgenederived
molecules and their host are needed in order to elucidate the mechanisms of resistance and pathogenicity
Evaluation of the Potential Role of Recombination in Virus-Resistant Transgenic Plants in the Emergence of Novel Viruses
No full textInfections with two or more viruses occur frequently in nature, and when two related viruses replicate simultaneously in the same cells, genetic recombination between them can take place. Recombination can occur also between cellular and viral RNAs. Thus, virus-resistant transgenic plants (VRTPs) that express viral sequences could be a source of novel recombinant viral genomes. From a biosafety point of view, it is therefore important to understand if the recombinants that are generated in VRTPs are novel, and if they could contribute to an increased risk of emergence of recombinant viruses.
To enhance the detection of host-messenger/viral RNA recombinants we analyzed them under conditions of high selection pressure in their favor. To do this, we studied transgenic tobacco plants expressing the coat protein and 3’ non-coding region (3’NCR) of R-CMV (Cucumber mosaic virus (CMV) subgroup II). Forty-four plants were inoculated with in vitro transcripts of a mutant I17F-CMV (CMV subgroup I) that had been attenuated by a 6-nt deletion in the 3'NCR of RNA3. Samples were taken from symptomatic plants 8, 10, and 15 days post-inoculation. The plants were screened for recombinant RNA3 by RT-PCR, and the amplicons were analyzed by cloning and sequencing. Many of the recombinants were different from those observed in the same plant line when inoculated with wild-type I17F-CMV under conditions of minimal selection pressure
Analysis of recombination between viral RNAs and transgene mRNA under conditions of high selection pressure in favour of recombinants
No full textOne possible environmental risk related to the utilization of virus-resistant transgenic plants expressing viral sequences is the emergence of new viruses generated by recombination between the viral transgene mRNA and the RNA of an infecting virus. This hypothesis has been tested recently for cucumber mosaic virus (CMV) by comparing the recombinant populations in transgenic and non-transgenic plants under conditions of minimal selection pressure in favour of
the recombinants. Equivalent populations were observed in transgenic and non-transgenic plants but, in both, there was a strongly dominant hotspot recombinant which was shown recently to be nonviable alone in planta, suggesting that its predominance could be reduced by applying an increased selection pressure in favour of viable recombinants. Partially disabled I17F-CMV
mutants were created by engineering 6 nt deletions in five sites in the RNA3 39-non-coding region (39-NCR). One mutant was used to inoculate transgenic tobacco plants expressing the coat protein and 39-NCR of R-CMV. A total of 22 different recombinant types were identified, of which 12 were, as expected, between the transgene mRNA and the mutated I17F-CMV RNA3, while 10 resulted from recombination between the mutated RNA3 and I17F-CMV RNA1. Twenty recombinants were of the aberrant type, while two, including the dominant one detected previously under conditions of minimal selection pressure, were homologous recombinants. All
recombinants detected were very similar to ones observed in nature, suggesting that the
deployment of transgenic lines similar to the one studied here would not lead to the emergence of new viruses
Analysis of recombination between RNA3 of Cucumber mosaic virus and transgene mRNAs under conditions of high selection pressure
No full textIt has been shown that mixed infections (with two or more viruses) occur frequently in nature, and that when two related viruses replicate simultaneously in the same cells, genetic recombination between them can take place. There is also evidence that recombination can occur between cellular and viral RNAs. Thus, virus-resistant transgenic plants (VRTPs) that express viral sequences could be a source of novel recombinant viral genomes. From a biosafety point of view, it is therefore important to understand if the recombinants that are generated in VRTPs are novel, and if they could contribute to an increased risk of emergence of recombinant viruses. Since it is impossible to test all possible recombinants, the best way to evaluate this risk is to compare the recombinants that appear in infected transgenic plants with those that appear in doubly infected non-transgenic plants under conditions of low selection pressure, i.e. in the presence of wild-type virus that can replicate recombinants in trans. Using this approach, previous results under conditions of low selection pressure showed that the populations of recombinant viral RNAs found in tobacco plants infected with two strains of Cucumber mosaic virus (CMV), a subgroup I strain (I17F-CMV) and subgroup II strain (R-CMV), were comparable to those found in tobacco transgenic for part of the R-CMV genome infected with I17F-CMV (Turturo et al. 2008). This suggests that in this case novel viral recombinants are not expected to appear. However, the recombinant viral RNAs were quite rare, and the populations of recombinants were strongly dominated by a single hot spot, which severely limited the number of recombination sites observed. In addition, when inoculated to plants in the absence of wild-type virus, the predominant recombinant was not viable (Pierrugues et al. 2007). In order to obtain a broader sample of the types of recombinant molecules generated in transgenic plants, the same system was studied under conditions of high selection pressure, using partially disabled viruses that contain a six-nucleotide deletion of in the 3’ untranslated region of RNA3. Under these conditions, additional recombination sites between RNA3 and either the transgene mRNA or CMV genomic RNA1 were observed
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
Studies of populations of recombinant cucumoviruses that appear within an experimental time frame
No full text- …
