1,720,963 research outputs found
Production of mycosporine-alanine, acetic acid and uridine by cultures of Colletotrichum graminicola: Their role in conidial maturation and development
No full textBioassays demonstrated the existence of two mechanisms of self-inhibition in Colletotrichum graminicola cultures. The first was caused by a low-molecular-weight compound in the mucilage that surrounds conidia. The second was the result of the production of a volatile component. The low molecular weight self-inhibitor was isolated and analyzed by mass spectrometry. The molecular mass obtained was consistent with the structure of mycosporine-alanine. Mycosporine-alanine, the production of which was shown to be light dependent, was easily detected as part of the mucilage due to its specific peak of absorption at 310 nm. The presence of high concentrations (4 mM) of this inhibitory component in the mucilage of acervuli was consistent with the necessity of a self-inhibitor where conidium concentration is the greatest. It is surprising that the self-inhibition caused by mycosporine-alanine is dependent on conidium age. Conidia from 14-day-old cultures were consistently inhibited under the proper conditions. However, it was observed that, as conidia age, the effect of the self-inhibitor was progressively decreased. HPLC analysis also demonstrated the presence of uridine in the mucilage but its significance was not determined. The volatile inhibitor was demonstrated to be acetic acid which was detected by gas chromatography-mass spectrometry (GC/MS). Acetic acid is known to be a strong sporostatic factor. The volatile acetic acid from a parent culture was shown to prevent conidium germination but not to kill conidia. This effect was reproduced in experiments that showed that the volatile material prevented conidia from causing disease on sorghum seedlings. However, long-term inhibition without conidial death could not be completely reproduced in vitro in experiments with a commercial source of acetic acid at various concentrations. I assume that there are many components in the phenomenon of inhibition by the volatile acetic acid that are not understood. Both types of inhibitory activity could be reversed by addition of exogenous nutrients. In addition, conidium viability remains unchanged throughout the inhibition period. Therefore, both inhibitors represent natural mechanisms which prevent conidia from germinating under unfavorable conditions
Characterization of 4-hydroxycinnamic acid: CoA ligase, anthocyanins and pathogen ingress in resistant and susceptible maize cultivars
No full textPhenylpropanoid metabolism in maize cultivars B73\sb{Ht} (susceptible) and B73\sb{Ht\ rhm} (resistant) is conspicuously affected following inoculation with the fungus Bipolaris maydis race O. The purpose of this research was to examine the role of 4-hydroxycinnamic acid: CoA ligase (4CL) in the disease interaction from infected host tissues and to establish whether isozymes are induced. Anion exchange and hydrophobic interaction chromatography of protein preparations extracted in bulk from maize leaves infected with B. maydis resulted in over a 600-fold purification of 4CL. The native enzyme had an estimated molecular weight of 51.6 kDa. Kinetic analysis of enzyme activities with various phenylpropanoid substrates demonstrated there is little preferential substrate-specificity among phenylpropanoids tested. The enzyme exhibited no activity when assayed with sinapic acid. No evidence for multiple forms was found in the present investigation, suggesting that the maize 4CL is not responsible for metabolic channeling of phenylpropanoids and their distribution within the cell which occurs in response to infection. The second goal of this research was to chemically characterize anthocyanin pigments which often accumulate in uninfected cells adjacent to restricted lesions in the resistant B73\sb{Ht\ rhm} cultivar. Results showed that anthocyanins begin to accumulate by 32 hours post-inoculation and continue to accumulate through 72 hours. Plasma Desorption Mass Spectrometry (PDMS) of anthocyanins that accumulate in the healthy leaf sheath tissues revealed a major ion with a molecular mass of 621. A search of the literature identified this 621 ion as a cyanidin 3-dimalonylglucoside. Importantly, PDMS of anthocyanins isolated from infected tissues showed the identical 621 ion. The final goal of this research was to use fluorescent staining and microscopic examination of B. maydis hyphae at various times after infection to delineate the time when fungal ingress is arrested. Computer analysis revealed that the growth of B. maydis in the resistant cultivar is restricted prior to 18 hours post-inoculation. However, analysis of the susceptible tissues showed an almost linear growth of fungal hyphae. Importantly, this investigation shows that the cessation of pathogen ingress in the resistant cultivar occurs much earlier than the previously reported accumulation of phenylpropanoids
Characterization of a laccase present in the conidial mucilage of Colletotrichum graminicola and factors influencing the production of phenol oxidizing enzymes by the fungus
No full textThe maize and sorghum hosts of Collectotricham graminicola produce phenylpropanoids and anthocyanidins that are toxic to the fungus. The purpose of this research was to determine if the conidial mucilage of C. graminicola, isolate CgM2, conmins an enzyme capable of oxidizing these compounds. A enzyme identified as a laccase based on substrate specificity was found in the conidial mucilage. The fungus also produced a laccase in liquid cultures which appears to be identical to the laccase found in the conidial mucilage. Anion exchange and molecular sieve column chromatography were used to partially purify laccase produced by the fungus in liquid cultures. This resulted in at least a 39 fold purification of the activity present in the culture medium, or a 820 fold increase in specific activity over that contained in the conidial mucilage. The enzyme had an estimated molecular weight of 85 kDa based on SDS PAGE results. This enzyme has a copper absorbance band at 593 nm characteristic of laccase. Kinetic analysis of enzyme activity with syringaldazine as the substrate gave a V\sb{\rm max} of 1.01 mM liter\sp{-1} min\sp{-1} and a K\sb{\rm m} of 2.14 \times 10\sp{-4} M. Phenolic compounds with hydroxyl groups para or ortho to each other were oxidized more readily by this laccase than compounds with meta hydroxyl groups. This laccase can readily oxidize two of the phenylpropanoids, caffeic and ferulic acids, produced by maize that had been previously shown to be toxic to the fungus. It can also oxidize p-coumaric acid, the third toxic phenylpropanoid, produced by corn but at a much slower rate. This enzyme can also oxidize luteolinidin and apigeninidin, the two phytoalexins produced by sorghum. The fungus produces conidial mucilage containing laccase when grown on corn leaves. Another goal of this research was to identify factors influencing the production of phenol-oxidizing enzymes by the fungus. Three bands of phenol oxidizing enzymes were detected when the intracellular proteins produced in the fungal hypha were separated on native polyacrylamide gels. One of these bands corresponded to the laccase present in conidial mucilage. A second band containing laccase was found, while the third band had activity consistent with polyphenol oxidase or tyrosinase. Light was required for the production of all three of these enzymes. Assays of material harvested from cultures of different ages indicated that the production of these enzymes is developmentally controlled. Light was also required for melanization of hyphae and the production of conidia by isolate CgM2. CgM2-N5, a photoinsensitive strain C. graminicola, produced melanized hyphae, conidia, and the three bands of enzyme activity when grown in the dark. Laccase activity was also found inside the conidia
The infection process of Colletotrichum graminicola (Ces.) Wils. on Sorghum bicolor L. and analysis of an extracellular DNase produced by the fungus
No full textThe interactions that occur between host and pathogen during the disease process involve a complex series of structural and biochemical reactions initiated by both organisms. Defense mechanisms of Sorghum bicolor L. produced in response to attempted penetration by Colletotrichum graminicola were examined as were several extracellular enzymes found within the conidial mucilage and the culture fluid of the pathogen. One of the enzymes, a DNase, was characterized and purified from the culture fluid. The 3-deoxyanthocyanidin flavonoid phytoalexins produced by sorghum in response to infection are visible pigments. The subcellular site of accumulation of phytoalexins was monitored in vivo without disruption to the processes involved in the plant\u27s defense against a pathogen. Sorghum responded to infection by producing red spherical bodies within epidermal cells immediately under fungal attack. These bodies originated as small colorless inclusions which enlarged, moved toward the site of penetration and became pigmented as the infection process proceeded. HPLC analysis of extracts from infection sites showed accumulation of the red phytoalexin pigments corresponding to the appearance of the spherical, red pigmented inclusions within infected cells. This research suggests that these bodies are the site of accumulation of the sorghum phytoalexins. Analysis of the conidial mucilage of C. graminicola identified enzymes which could hydrolyze DNA, RNA, protein and phosphomonoesters. One of those enzymes, the DNase, is produced by vegetative hyphae and by sporulating mycelia. Isolates of Colletotrichum spp were screened for the presence of the enzyme within the conidial mucilage. The DNase was characterized according to pH and temperature optima, cation effects, substrate hydrolysis and inhibitors of the enzyme. C. graminicola secretes a DNase into culture medium when grown on potato sucrose broth. Maximal DNase activity occurred on this medium after 11 days of growth. The extracellular DNase was purified almost 1,600-fold by separation on DEAE-Sephadex, affinity chromatography on DNA-cellulose and by passage through a Sephadex G-100 column. Polyacrylamide gel electrophoresis of the iodinated DNase from the Sephadex G-100 column revealed a single band of radioactivity. The purified DNase exhibited optimum activity at pH 8.2 against single-stranded DNA and was free from RNase activity
Physiological and molecular basis of phytoalexin biosynthesis in sorghum
No full text3-Deoxyanthocyanidins are the principal phytoalexins that accumulate in sorghum. The phytoalexin response was induced in response to attempted infection by Cochliobolus heterostrophus, a non-pathogen of sorghum or Colletotrichum sublineolum, the causal agent of the sorghum anthracnose. In the resistant responses (non-host or host), fungal development in the hosts was contained during early stages of pathogenesis. Accumulation of phytoalexins was localized in the area of infection and was associated with the restriction of fungal development in the host. When compared to the compatible interaction, defense responses in the incompatible interaction with C. sublineolum could be characterized by a greater and faster accumulation of phytoalexins and an earlier induction of defense-related genes. The pathogen was able to colonize the susceptible host with the proliferation of primary and secondary hyphae. In addition to the quantitative and timing differences, there were qualitative differences between the incompatible and compatible interactions in their phytoalexin response. Thus, the resistant cultivar accumulated a complex phytoalexin mixture including luteolinidin and 5-methoxyluteolinidin, neither of which was produced by the susceptible cultivar. These compounds were also shown to exhibit higher fungitoxicity than other sorghum phytoalexin components. Activation of the biosynthesis of 3-deoxyanthocyanidin phytoalexins was found to be under precise metabolic control. Light-induced anthocyanin accumulation was repressed in plants which were exposed to stimuli of light and pathogen induction simultaneously. Anthocyanidin structural genes encoding flavanone 3-hydroxylase, dihydroflavone 4-reductase, and anthocyanidin synthase were all down-regulated following inoculation. This would increase the availability of flavonoid substrates for the synthesis of the structurally related 3-deoxyanthocyanidin phytoalexins. Chalcone synthase (CHS) is a key branchpoint enzyme in the phenylpropanoid pathway from which flavonoid compounds, including 3-deoxy-anthocyanidins and anthocyanidins, originate. Accumulation of CHS mRNA was stimulated by light and pathogen induction. A family of seven chalcone synthase genes was isolated from a sorghum genomic bacterial artificial chromosome library. Expression analysis by reverse transcription-polymerase chain reactions indicated that there was a subset of CHS genes responsive to pathogen induction only
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
- …
