Thesis (Ph. D.)--University of Wisconsin--Madison, 1962. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 114-117).

Reciprocal interference experiments between Potato virus Y (PVY) and Pepper mottle virus (PeMV) in Capsicum annuum L. 'Tabasco' and 'Special pepper' (a selection of Anaheim chilli peppers), showed suppression of local lesion production in both directions but incomplete suppression of challenge virus replication (incomplete cross protection). However, suppression was reduced by increasing the concentration of the challenge virus. The source of inoculum of the viruses did not have a significant effect on the subsequent interference between the viruses. A direct relationship was established between counts of local lesions and virus particles counted using the electron microscope. Tobacco etch virus (TEV) was transmitted from one Capsicum annuum L. 'Tabasco' plant to another in the same pot within 4 days after infection of the test plant. Transmission probably took place via root grafts, as the necrotic roots of the test plants intertwined with the uninoculated Tabasco plants. Mixed infections of PVY and PeMV resulted in the production of PVY-N, a new strain of PVY. The new strain, which could be recognized by changes in biological and serological properties, was produced only in mixed infections and was stable after six serial transfers in several hosts. Evidence suggests that the development of the new strain is host dependent. Potato Virus Y in mixed infections with PeMV or TEV in C. annuum L. 'Anaheim' did not induce local lesions, systemic necrosis nor death of Special pepper, while PeMV alone induced the death of Tabasco. The behavior of TEV in Tabasco in a mixed infection was temperature dependent, while TEV alone induced wilt and death of Tabasco irrespective of the greenhouse temperature and season of the year.

No description available.

A thesis presented to The Faculty of Science, University of the Witwatersrand, Johannesburg in fulfillment of the requirements for the degree of Doctor of Philosophy in Molecular and Cell Biology 2015 / Cassava brown streak disease (CBSD) ranks among the top seven biological threats to global food security and is considered to be a major risk to food security in tropical Africa. In Uganda, overall CBSD incidence has increased by c. 20% since 2004, and persistently reduces cassava yields and storage root quality. Presently the disease negatively impacts the livelihoods of over 80% of the farming families who rely on cassava as a staple food and source of income. Two distinct ipomoviruses, Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV) cause CBSD. The viruses systemically infect primary host plants and accumulate, and cause severe disease symptoms as the plant matures, reducing yields through the induction of necrotic lesions in the storage roots and suppressing utility of cassava stems for subsequent vegetative propagation. Effective control strategies require screening of available germplasm for sources of natural resistance in combination with improved understanding of host-virus interaction to facilitate targeted breeding. Due to a lack of known sources of resistance to CBSD in the cassava germplasm, incorporating new virus resistance into existing cassava genotypes through transgenic RNA interference (RNAi) approaches offers an additional, relevant avenue to reduce the increasing impact of CBSD. The research presented in this thesis provides insights into the complex mechanisms of virus-host interactions linking genotype to phenotype in CBSV- and UCBSV-cassava pathosystems and provides proof of principle for CBSD control by RNAi-mediated technology. Both are contributions to progress towards potential control of the CBSD epidemic in East Africa. To correlate CBSD symptoms with virus titer, within-host CBSV and UCBSV accumulation was studied in leaf, stem and storage root samples collected from 10 genotypes of field-grown cassava with varied levels of resistance to CBSD. CBSV was found to be present in 100% of CBSD samples collected from symptomatic plants. Presence of both CBSV and UCBSV was seen in 45.3% of the samples. Quantitative PCR (RT-qPCR) analysis showed that tolerant genotypes were infected with CBSV alone and accumulated lower virus titer compared to susceptible genotypes, which were co-infected with CBSV and UCBSV. To further comprehend the molecular interaction between CBSD viruses and cassava, deep sequencing was performed to compare profiles of virus-derived small RNAs (vsRNAs) in CBSV- and UCBSV-infected cassava genotypes of NASE 3 (CBSD tolerant), TME 204 and 60444 (CBSD susceptible). The results showed an abundance of 21-24 nt sized vsRNAs which when mapped were shown to cover the entire CBSV and UCBSV genomes. The 21- and 22-nt sizes were predominant compared to the 23- and 24-nt size classes. CBSV-infected plants accumulated higher populations of vsRNAs across the genotypes compared to UCBSV-infected plants, which accumulated moderate amounts of UCBSV-derived sRNAs in TME 204 and 60444, and insignificant amounts in UCBSV-challenged NASE 3, respectively. Quantitative RT-PCR analysis was performed to determine transcript levels of cassava homologues of Dicer (DCL) proteins, particularly DCL4 and DCL2, which are involved in the biogenesis of 21- and 22-nt small RNAs, and to correlate to the abundance of 21- and 22-nt vsRNAs in CBSV- and UCBSV-infected cassava. Similarly, RT-qPCR was performed to determine the expression of Argonaute (AGO) proteins, specifically AGO2 which preferentially sort and bind sRNAs with 5’ adenine (A) or uracil (U) to effector complexes to target mRNAs repression or cleavage, since in this study a major proportion of the vsRNAs were found to have A or U at the first 5’-end. Expression levels of cassava homologues of AGO2, DCL2 and DCL4, which are core components of the gene-silencing pathway, were found to be affected in virus-infected plants across all three genotypes. The levels of viral RNA and vsRNAs correlated with disease phenotype in infected plants. CBSV-infected plants showed more severe CBSD symptoms compared with UCBSV-infected plants of the same genetic background. These results showed that CBSV is more aggressive compared to UCBSV and supports the hypothesis of occurrence of genotype-specific resistance to CBSD viruses. The abundance of 21- and 22-nt vsRNAs in CBSV- and UCBSV-infected plants signifies the viruses activated the RNA-silencing mechanism, referred to as transcriptional or post-transcriptional gene silencing (TGS or PTGS). To test efficacy of RNAi-mediated resistance to control CBSD under field conditions, 14 lines of cassava plants transgenically modified to express, as inverted repeats, two RNAi constructs p718 and p719 targeting near full-length (894 bp) and N-terminal (402 bp) portions of UCBSV coat protein sequence were tested under confined field trial conditions at Namulonge, Uganda. Transgenic plants expressing p718 showed a 3-month delay in CBSD symptom development, while 100% of non-transgenic plants (n = 60) developed CBSD shoot symptoms. Over the 11-month trial duration, 98% of clonal replicates within line 718-001 were found to remain free of CBSD symptoms. RT-PCR analysis detected UCBSV within leaves of 57% of non-transgenic plants compared to only 0.5% across the 14 transgenic lines. Presence of the non-homologous CBSV was detected in all transgenic plants that developed CBSD symptoms. However, 93% of plants of line 718-001 were free of CBSV and UCBSV. At harvest, 90% of storage roots of non-transgenic plants showed severe necrosis, whereas plants of lines 718-001 and 718-005 showed significant suppression of CBSD. Line 718-001 had 95% of roots free from necrosis and was RT-PCR negative for presence of both viral pathogens. To determine durability of RNAi-mediated resistance to CBSD, stem cuttings were obtained from mature plants of lines p718-001, p718-002 and p718-005, replanted and monitored for 11 more months. CBSV but not UCBSV was detected in tissues of plants of lines p718-002 and p718-005, whereas all leaves and roots of p718-001 plants were free of CBSV and UCBSV. Thus, RNAi constructs conferred durable CBSD resistance across the vegetative cropping cycle, providing proof of concept for application of RNAi technology to control CBSD in farmers’ fields. The findings presented in this thesis contribute to understanding the complex interconnected mechanisms involved in CBSV- and UCBSV-host interactions and will contribute to the long-term goals of devising new methods of CBSD control.

Geminiviruses ( family Geminiviridae ) have circular single - stranded genomes encapsidated in twinned quasi - isometric particles and are responsible for major crop losses worldwide. The largest genus, Begomovirus, comprises viruses transmitted by the whitefly Bemisia tabaci. Most begomoviruses have bipartite genomes, termed DNA A and DNA B. The DNA A component encodes proteins required for viral DNA replication and encapsidation whereas the DNA B encodes two proteins that are essential for systemic movement. A small number of begomoviruses have a monopartite DNA genome that resembles the DNA A of bipartite begomoviruses. This DNA carries all gene functions for replication and pathogenesis. Specific small circular single - stranded DNA satellites containing a single open reading frame ( ORF ), termed DNA β, have recently been found in association with certain monopartite begomovirus infections. They comprise about 1350 nucleotides and require a helper begomovirus for replication and encapsidation. DNA β contributes to the production of symptoms and enhanced helper virus accumulation in certain hosts. This study examines the role of DNA β satellite in viral pathogenicity and movement in the host plant. Infectivity analysis of Tomato leaf curl virus and DNA β having mutation in the C1 and V1 ORF indicated that the complementary - sense ORF, βC1, is responsible for inducing disease symptoms in Nicotiana tabacum. An ORF present on the plus strand, βV1, appeared to have no role in pathogenesis. Tobacco plants transformed with the βC1 ORF under the control of the Cauliflower mosaic virus 35S promoter, or with a dimeric DNA β exhibited severe disease - like phenotypes, while plants transformed with a mutated version of βC1 appeared normal. Northern blot analysis of RNA from the transgenic plants using strand - specific probes identified a single complementary - sense transcript. The transcript carried the full βC1 ORF encoding a 118 amino acids product. It mapped to the DNA β nucleotide ( nt ) position 186 - 563 and contained a polyadenylation signal 18 nt upstream of the stop codon. A TATA box was located 43 nt upstream of the start codon. These results indicate that βC1 protein is responsible for DNA β induced disease symptoms. Tomato leaf curl New Delhi virus ( ToLCNDV ) is a bipartite begomovirus in which both DNA A and DNA B are required for systemic infection. Inoculation of tomato plants with ToLCNDV DNA A alone induced local but not systemic infection whereas co - inoculation with DNA A and the DNA β resulted in systemic infection. The presence of both DNA A and the DNA β in systemically infected tissues and the absence of DNA B was confirmed by probe hybridization. DNA β containing a disrupted βC1 ORF did not mobilize the DNA A for systemic infection. Co - inoculation of plants with DNA A and a construct of βC1 ORF, under the control of the Cauliflower mosaic virus 35S promoter, resulted in the systemic movement of the DNA A. βC1 fused to GFP accumulated around and inside the nucleus, at the periphery of tobacco and onion epidermis cells and co - localized with the endoplasmic reticulum. This distribution would be consistent with βC1 mediating intra cellular transport from the nucleus to the plasma membrane. These results showed that the βC1 protein can replace the functions of DNA B in allowing the systemic movement of a bipartite geminivirus DNA A. / Thesis (Ph.D.)--School of Agriculture, Food and Wine, 2006.

Bibliography: leaves 78-90. Designed to localize viroids at the histological and subcellular level and to determine with which cellular compartments the different viroids are associated. The majority of the work, in both the viroid and the phytoplasma studies involved the development of different methods and techniques.

Graduation date: 2000

The Feldmannia sp. virus is a large icosahedral virus that persistently infects marine brown alga Feldmannia sp.. So far, there is no information available about viral genome replication, gene structure and gene expression in this unique viral-host system. The purpose of this study was to characterize the general features of viral transcripts in the virus producing sporophyte plants. Northern analysis, using four cosmid clones that cover the entire viral genome, showed that there were six major transcripts and at least eighteen minor transcripts in the virus producing sporophyte plants. These transcripts are not evenly distributed in the viral genome. A 5.7 kb BamHI fragment-R was found to encode a 1.5 kb and a 0.9 kb major transcript, and those two major transcripts were chosen for detailed sequence analysis. The 1.5 kb transcript was identified as the putative major capsid protein (MCP) gene. The FsV MCP has significant similarity with the major capsid protein of Chlorella virus-PBCV-1 and with iridoviruses, fish lymphocystis disease virus, frog virus 3, and with African swine fever virus. / Graduation date: 1996

No abstract available. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1976.

Includes bibliography / 108 leaves, [15] leaves of plates : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Biochemistry, 1986