Browsing by Author "Bester, Rachelle"

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    Citrus tristeza virus genotype detection using high-throughput sequencing
    (MDPI, 2021-01-23) Bester, Rachelle; Cook, Glynnis; Maree, Hans J.
    The application of high-throughput sequencing (HTS) has successfully been used for virus discovery to resolve disease etiology in many agricultural crops. The greatest advantage of HTS is that it can provide a complete viral status of a plant, including information on mixed infections of viral species or virus variants. This provides insight into the virus population structure, ecology, or evolution and can be used to differentiate among virus variants that may contribute differently toward disease etiology. In this study, the use of HTS for citrus tristeza virus (CTV) genotype detection was evaluated. A bioinformatic pipeline for CTV genotype detection was constructed and evaluated using simulated and real data sets to determine the parameters to discriminate between false positive read mappings and true genotype-specific genome coverage. A 50% genome coverage cut-off was identified for non-target read mappings. HTS with the associated bioinformatic pipeline was validated and proposed as a CTV genotyping assay.
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    Next-generation sequencing for virus detection : covering all the bases
    (BioMed Central, 2016-06) Visser, Marike; Bester, Rachelle; Burger, Johan T.; Maree, Hans J.
    Background: The use of next-generation sequencing has become an established method for virus detection. Efficient study design for accurate detection relies on the optimal amount of data representing a significant portion of a virus genome. Findings: In this study, genome coverage at different sequencing depths was determined for a number of viruses, viroids, hosts and sequencing library types, using both read-mapping and de novo assembly-based approaches. The results highlighted the strength of ribo-depleted RNA and sRNA in obtaining saturated genome coverage with the least amount of data, while even though the poly(A)-selected RNA yielded virus-derived reads, it was insufficient to cover the complete genome of a non-polyadenylated virus. The ribo-depleted RNA data also outperformed the sRNA data in terms of the percentage of coverage that could be obtained particularly with the de novo assembled contigs. Conclusion: Our results suggest the use of ribo-depleted RNA in a de novo assembly-based approach for the detection of single-stranded RNA viruses. Furthermore, we suggest that sequencing one million reads will provide sufficient genome coverage specifically for closterovirus detection.
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    Phylogenomic analysis reveals deep divergence and recombination in an economically important grapevine virus.
    (Public Library of Science, 2015) Maree, Hans J.; Pirie, Michael D.; Oosthuizen, Kristin; Bester, Rachelle; Rees, D. Jasper G.; Burger, Johan T.
    The evolutionary history of the exclusively grapevine (Vitis spp.) infecting, grapevine leafroll-associated virus 3 (GLRaV-3) has not been studied extensively, partly due to limited available sequence data. In this study we trace the evolutionary history of GLRaV-3, focussing on isolate GH24, a newly discovered variant. GH24 was discovered through the use of next-generation sequencing (NGS) and the whole genome sequence determined and validated with Sanger sequencing. We assembled an alignment of all 13 available whole genomes of GLRaV-3 isolates and all other publicly available GLRaV-3 sequence data. Using multiple recombination detection methods we identified a clear signal for recombination in one whole genome sequence and further evidence for recombination in two more, including GH24. We inferred phylogenetic trees and networks and estimated the ages of common ancestors of GLRaV-3 clades by means of relaxed clock models calibrated with asynchronous sampling dates. Our results generally confirm previously identified variant groups as well as two new groups (VII and VIII). Higher order groups were defined as supergroups designated A to D. Supergroup A includes variant groups I-V and supergroup B group VI and its related unclassified isolates. Supergroups C and D are less well known, including the newly identified groups VII (including isolate GH24) and VIII respectively. The inferred node ages suggest that the origins of the major groups of GLRaV-3, including isolate GH24, may have occurred prior to worldwide cultivation of grapevines, whilst the current diversity represents closely related isolates that diverged from common ancestors within the last century.
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    Real-time RT-PCR high resolution melting curve analysis and multiplex RT-PCR to detect and differentiate grapevine leafroll-associated associated virus 3 variant groups I, II, III and VI
    (BioMed Central, 2012-09) Bester, Rachelle; Jooste, Anna E. C.; Maree, Hans J.; Burger, Johan T.
    Abstract Background Grapevine leafroll-associated virus 3 (GLRaV-3) is the main contributing agent of leafroll disease worldwide. Four of the six GLRaV-3 variant groups known have been found in South Africa, but their individual contribution to leafroll disease is unknown. In order to study the pathogenesis of leafroll disease, a sensitive and accurate diagnostic assay is required that can detect different variant groups of GLRaV-3. Methods In this study, a one-step real-time RT-PCR, followed by high-resolution melting (HRM) curve analysis for the simultaneous detection and identification of GLRaV-3 variants of groups I, II, III and VI, was developed. A melting point confidence interval for each variant group was calculated to include at least 90% of all melting points observed. A multiplex RT-PCR protocol was developed to these four variant groups in order to assess the efficacy of the real-time RT-PCR HRM assay. Results A universal primer set for GLRaV-3 targeting the heat shock protein 70 homologue (Hsp70h) gene of GLRaV-3 was designed that is able to detect GLRaV-3 variant groups I, II, III and VI and differentiate between them with high-resolution melting curve analysis. The real-time RT-PCR HRM and the multiplex RT-PCR were optimized using 121 GLRaV-3 positive samples. Due to a considerable variation in melting profile observed within each GLRaV-3 group, a confidence interval of above 90% was calculated for each variant group, based on the range and distribution of melting points. The intervals of groups I and II could not be distinguished and a 95% joint confidence interval was calculated for simultaneous detection of group I and II variants. An additional primer pair targeting GLRaV-3 ORF1a was developed that can be used in a subsequent real-time RT-PCR HRM to differentiate between variants of groups I and II. Additionally, the multiplex RT-PCR successfully validated 94.64% of the infections detected with the real-time RT-PCR HRM. Conclusion The real-time RT-PCR HRM provides a sensitive, automated and rapid tool to detect and differentiate different variant groups in order to study the epidemiology of leafroll disease.
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    Sequencing and detection of a new strain of grapevine leafroll-associated virus 3 in South Africa
    (Stellenbosch : Stellenbosch University, 2012-12) Bester, Rachelle; Burger, J. T.; Maree, H. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Genetics.
    ENGLISH ABSTRACT: Grapevine leafroll-associated virus 3 (GLRaV-3) is the type member of the genus Ampelovirus in the family Closteroviridae and is considered to be the main contributing agent of grapevine leafroll disease (GLD) worldwide. A metagenomic sequencing study of a grapevine leafroll-diseased vineyard led to the discovery of a new variant of GLRaV-3 in South Africa. This new variant was most related to a New Zealand isolate, NZ-1. In this study, we sequenced two isolates, GH11 and GH30, of the new variant group of GLRaV-3. These isolates have less than 70% nucleotide (nt) identity to other known GLRaV-3 variants, indicating that they should be considered variants of a different strain of GLRaV-3. We propose that the GLRaV-3-like virus identified in this study be grouped together with NZ-1 and some Napa Valley isolates as Group VI of GLRaV-3. This study also provided further evidence that next-generation sequencing is an invaluable approach to identify novel viruses and variants, in that the draft sequence generated with bioinformatic tools in this study was 98% identical to the GH11 sequence generated using Sanger sequencing. The study further confirmed that the industry standard ELISA is still an effective GLRaV-3 diagnostic method and that it is able to detect all known variant groups of GLRaV-3. However, this assay is not able to differentiate between GLRaV-3 variant groups. In the current study therefore, a real-time RT-PCR was designed that is able to detect GLRaV-3 variant groups I, II, III and VI, using a single primer pair targeting the Hsp70h gene of GLRaV-3. If high-resolution melting (HRM) curve analysis is added to the real-time RT-PCR, it is possible to differentiate between variant groups based on three melting point intervals. The RT-PCR HRM assay provides a more sensitive and rapid tool to detect and differentiate between different GLRaV-3 variant groups. Finally, a multiplex RT-PCR was designed to differentiate between the variant groups present in South Africa. This multiplex RT-PCR offers a validation method for the RT-PCR HRM and provides an end-point PCR alternative for variant identification. In order to investigate the spread and impact of different GLRaV-3 variants in vineyards, sensitive diagnostic techniques are a necessity. The abovementioned tools will contribute to the understanding of the pathogenesis of GLD and aid epidemiological studies to investigate how these different GLRaV-3 variant groups are spreading, the association of specific GLRaV-3 variants to disease symptoms and the mealybug vector transmission efficiency for each GLRaV-3 variant.
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    Small RNA profiling of grapevine leafroll-associated virus 3 infected grapevine plants
    (Stellenbosch : Stellenbosch University, 2016-12) Bester, Rachelle; Maree, H. J.; Burger, J. T.; Stellenbosch University. Faculty of AgriSciences. Dept. of Genetics.
    ENGLISH ABSTRACT: One of the most important viral diseases of grapevine worldwide is grapevine leafroll disease (GLD). A number of viruses from the family Closteroviridae have been associated with this disease, though Grapevine leafroll-associated virus 3 is considered the leading causative agent due to its consistent association with GLD. To better understand the disease and develop effective control strategies, it is necessary to characterise the molecular interactions between the virus and the plant. Small RNA (sRNA) molecules have been shown to play an important role in gene regulation of normal development and defence responses to biotic and abiotic stresses in plants. Therefore, the aim of this study was to characterise the sRNA species in healthy and infected grapevine to contribute to the growing database of sRNAs present in Vitis vinifera. Microarray analysis and next-generation sequencing was used to identify sRNA species in Chardonnay, Chenin blanc, Cabernet Sauvignon and own-rooted Cabernet Sauvignon plants. Differential expression of sRNAs was evaluated to identify sRNAs associated with GLRaV-3 infection. The modulation of the differentially expressed microRNAs (miRNAs) was validated with stemloop RT-qPCR assays. Transcriptome NGS was also performed to validate the differential expression of the predicted miRNA targets, and to identify metabolic pathways modulated in response to GLRaV-3 independently from sRNA regulation. The transcriptome NGS transcripts that were differentially expressed in all cultivar groups, and transcripts that anti-correlated with miRNA expression, were validated with RT-qPCR assays. These highthroughput approaches identified several differentially expressed sRNAs and (target) genes in infected plants. The anti-correlation of miRNA expression and putative target expression were shown for two miRNAs. Cultivar specificity was identified in the sRNA and gene expression analyses, and both approaches identified Chenin blanc-specific responses. This comparison of symptomatic and asymptomatic GLRaV-3-infected plants provides the first insight into the disease symptom inhibition observed in certain cultivars. The differentially expressed genes identified in all cultivar groups, using the NGS transcriptome data, provides a collection of genes displaying a potentially universal molecular response against GLRaV-3. These genes showed strong associations with cell wall biosynthesis and signalling during pathogen recognition. This study has contributed significantly to the knowledge of sRNAs produced in grapevine and significantly extended the existing sRNA reference database for grapevine. The knowledge generated in this study can be utilised as potential targets for grapevine functional studies, and be translated into potential management strategies to control the disease. A better understanding of both the host defence and viral counter-defence strategies can lead to the prevention of virus replication or the impaired ability of the virus to induce pathogenesis in plants.
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    Towards the validation of high-throughput sequencing (HTS) for routine plant virus diagnostics: measurement of variation linked to HTS detection of citrus viruses and viroids
    (BioMed Central, 2021-03-22) Bester, Rachelle; Cook, Glynnis; Breytenbach, Johannes H. J.; Steyn, Chanel; De Bruyn, Rochelle; Maree, Hans J.
    Background: High-throughput sequencing (HTS) has been applied successfully for virus and viroid discovery in many agricultural crops leading to the current drive to apply this technology in routine pathogen detection. The validation of HTS-based pathogen detection is therefore paramount. Methods: Plant infections were established by graft inoculating a suite of viruses and viroids from established sources for further study. Four plants (one healthy plant and three infected) were sampled in triplicate and total RNA was extracted using two different methods (CTAB extraction protocol and the Zymo Research Quick-RNA Plant Miniprep Kit) and sent for Illumina HTS. One replicate sample of each plant for each RNA extraction method was also sent for HTS on an Ion Torrent platform. The data were evaluated for biological and technical variation focussing on RNA extraction method, platform used and bioinformatic analysis. Results: The study evaluated the influence of different HTS protocols on the sensitivity, specificity and repeatability of HTS as a detection tool. Both extraction methods and sequencing platforms resulted in significant differences between the data sets. Using a de novo assembly approach, complemented with read mapping, the Illumina data allowed a greater proportion of the expected pathogen scaffolds to be inferred, and an accurate virome profile was constructed. The complete virome profile was also constructed using the Ion Torrent data but analyses showed that more sequencing depth is required to be comparative to the Illumina protocol and produce consistent results. The CTAB extraction protocol lowered the proportion of viroid sequences recovered with HTS, and the Zymo Research kit resulted in more variation in the read counts obtained per pathogen sequence. The expression profiles of reference genes were also investigated to assess the suitability of these genes as internal controls to allow for the comparison between samples across different protocols. Conclusions: This study highlights the need to measure the level of variation that can arise from the different variables of an HTS protocol, from sample preparation to data analysis. HTS is more comprehensive than any assay previously used, but with the necessary validations and standard operating procedures, the implementation of HTS as part of routine pathogen screening practices is possible.