Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references. Also available on the World Wide Web.

Thesis (Ph. D.)--Michigan State University. Dept. of Plant Biology, 2008. / Title from PDF t.p. (viewed on Mar. 27, 2009). Includes bibliographical references (p. 120-135). Also issued in print.

Tomato spotted wilt virus (TSWV), a Tospovirus, is one of the ten most economically destructive plant viruses worldwide, causing losses exceeding one billion U.S. dollars annually on several crops. In South Africa (SA), TSWV has become an important virus in many economically important crops. The main objective of this research project was to isolate, identify and characterise TSWV isolates occurring in SA. A review of current literature assembled background information on TSWV molecular biology, epidemiology, transmission, detection and control. A TSWV isolate infecting pepper (Capsicum sp.) occurring in KZN was isolated and partially characterised. The virus was positively identified as TSWV using the enzyme-linked immunosorbent assay (ELISA) and the presence of typical necrotic TSWV symptoms on Nicotinia rustica L. Symptomatic leaves were harvested and the virus was partially purified using standard procedures. Under the transmission electron microscope (TEM), typical quasi-spherical and dumbbell-shaped particles of 80-100nm in diameter were observed in negatively stained preparations of both crude and purified virus samples. In negatively stained ultra-thin virus infected leaf sections, an abundance of mature viral particles (100nm) housed in the cisternae of the endoplasmic reticulum (ER) were observed among typical viroplasm inclusions (30nm) and hollow tubules (200-300nm). A viral protein migrating as a 29kDa band, which corresponds to the TSWV nucleocapsid (N) protein, was observed after sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) analysis. Total plant RNA, isolated from N. rustica displaying typical symptoms was subjected to reverse-transcription polymerase chain reaction (RT-PCR) using .primers specific to the nucleocapsid (N) gene. An expected 760bp product was amplified. The results obtained in this study confirm the presence of TSWV in infected pepper plants from KZN. The genetic diversity of TSWV isolates occurring in SA was examined. The nucleocapsid (N) gene sequences of six SA TSWV isolates originating from Gauteng, KwaZulu-Natal, North West, Limpopo and Mpumulanga provinces were determined and used in a phylogenetic tree comparison with TSWV isolates occurring in different geographical locations in the world. Nucleotide sequence comparisons of the N gene revealed high levels of similarity between the SA isolates and TSWV isolates from Asia and Europe. SA isolates showed a high degree of sequence similarity (99-100%) which was reflected in their distinct clustering pattern. The resistance of tomato (Lycopersicon escuJentum Mill.) plants with natural and transgenic resistance against mechanical inoculation with TSWV isolates occurring in SA was evaluated. The Stevens cultivar which has natural resistance conferred by the Sw-5 gene and the transgenic 13-1 line, which expresses the nucleocapsid (N) protein gene of the TSWV-BL isolate, was used as test cultivars. Plants were assessed for TSWV resistance using a disease severity rating scale and measurements of virion accumulation levels (A405nm). There were no significant differences among the reactions produced by the six TSWV isolates on the test plants. Although both plants were susceptible to the SA TSWV isolates by exhibiting similarly high viral accumulation levels, the transgenic tomato line showed milder disease severity compared to the natural resistant cultivar. Results suggest that transgenic resistance is a more effective approach in the control of TSWV in SA. The information generated in this study will be useful in formulating effective control measures using genetic engineering approaches for this economically important virus. Such approaches will be used as a tool to make strategic decisions in an integrated control programme for ISWV. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2006.

Sweet potato (Ipomoea batatas Lam.) is ranked as the seventh most important food crop in the world and its large biomass and nutrient production give it a unique role in famine relief. However, multiple virus infection is the main disease limiting factor in sweet potato production worldwide. The main objective of this research project was to develop a transgenic sweet potato cultivar with broad virus resistance in South Africa (SA). A review of current literature assembled background information pertaining to the origin, distribution and importance of the sweet potato crop; viruses and complexes infecting sweet potato; and the strategies used in sweet potato virus detection and control. A survey to determine the occurrence and distribution of viruses infecting sweet potato (Ipomoea batatas Lam.) was conducted in major sweet potato-growing areas in KwaZulu-Natal (KZN). A total of 84 symptomatic vine samples were collected and graft inoculated onto universal indicator plants, Ipomoea setosa Ker. and Ipomoea nil Lam. Six weeks post inoculation, typical sweet potato virus-like symptoms of chlorotic flecking, severe leaf deformation, stunting, chlorotic mosaic, and distinct interveinal chlorotic patterns were observed on indicator plants. Under the transmission electron microscope (TEM), negatively stained preparations of crude leaf sap and ultra-thin sections from symptomatic grafted I.setosa plants revealed the presence of elongated flexuous particles and pinwheel type inclusions bodies‟ that are characteristic to the cytopathology of Potyviruses. Symptomatic leaf samples from graft-inoculated I. setosa and I. nil were assayed for Sweet potato feathery mottle virus (SPFMV), Sweet potato mild mottle virus (SPMMV), Sweet potato chlorotic stunt virus (SPCSV), Sweet potato chlorotic fleck virus (SPCFV), Sweet potato virus G (SPVG), Sweet potato mild speckling virus (SPMSV), Sweet potato caulimo-like virus (SPCaLV), Sweet potato latent virus (SPLV), Cucumber mosaic virus (CMV), and Sweet potato C-6 virus (C-6) using the nitrocellulose membrane enzyme-linked immunosorbent assay (NCM-ELISA). The majority of leaf samples (52%) tested positive for virus disease and showed the occurrence of SPFMV, SPMMV, SPCSV, SPCFV, SPVG, SPMSV, and SPCaLV. Of these 7 viruses, the most frequently detected were SPFMV (39%), SPVG (30%), followed by SPCSV (13%) and SPMMV (12%). SPCaLV and SPCFV at 10% and SPMSV at 7% were found exclusively in samples collected from one area. SPFMV, SPVG, SPCSV, and SPMMV were identified as the most prevalent viruses infecting sweet potato in KZN. The genetic variability of the three major viruses infecting sweet potato (Ipomoea batatas Lam.) in KZN was determined in this study. A total of 16 virus isolates originating from three different locations (Umbumbulu, Umfume and Umphambanyomi River) in KZN were analyzed. These comprised of 10 isolates of Sweet potato feathery mottle virus (SPFMV), five isolates of Sweet potato virus G (SPVG) and one isolate of Sweet potato chlorotic stunt virus (SPCSV). The phylogenetic relationships of the SPFMV, SPVG and SPCSV isolates from KZN relative to isolates occurring in SA and different parts of the world were assessed. The division of SPFMV into four genetic groups (strains) according to the phylogenetic analysis of coat protein encoding sequences revealed mixed infections of the O (ordinary) and C (common) strains in sweet potato crops from KZN. All SPFMV isolates showed close lineage with isolates from South America, East Asia and Africa. The SPVG isolates showed high relatedness to each other and close lineage with other isolates, especially those from China and Egypt. Analysis of the partial sequence of the Heat shock protein 70 homologue (Hsp70h) gene indicated that the SPCSV isolate from KZN belongs to the West African (WA) strain group of SPCSV and showed close relatedness to an isolate from Argentina. The knowledge of specific viral diversity is essential in developing effective control measures against sweet potato viruses in KZN. Multiple virus infections of Sweet potato feathery mottle virus (SPFMV), Sweet potato chlorotic stunt virus (SPCSV), Sweet potato virus G (SPVG) and Sweet potato mild mottle virus (SPMMV) cause a devastating synergistic disease complex of sweet potato (Ipomoea batatas Lam.) in KZN. In order to address the problem of the multiplicity and synergism of sweet potato viruses in KZN, this study aimed to develop transgenic sweet potato cv. Blesbok with broad virus resistance. An efficient and reproducible plant regeneration protocol for sweet potato (Ipomoea batatas Lam.) cultivar Blesbok was also developed in this study. The effect of different hormone combinations and type of explants on shoot regeneration was evaluated in order to optimize the regeneration protocol. Coat protein (CP) gene segments of SPFMV, SPCSV, SPVG and SPMMV were fused to a silencer DNA, the middle half of the nucleocapsid (N) gene of Tomato spotted wilt virus (TSWV) and used as a chimeric transgene in a sense orientation to induce gene silencing in the transgenic sweet potato. Transformation of apical tips of sweet potato cv. Blesbok was achieved by using Agrobacterium tumefaciens strain LBA4404 harboring a modified binary vector pGA482G carrying the plant expressible neomycin phosphotransferase ll gene (nptll), the bacterial gentamycin-(3)-N-acetyl-transferase gene and the expression cassette. A total of 24 putative transgenic plants were produced from the transformed apical tips via de novo organogenesis and regeneration into plants under 50mg/L kanamycin and 200 mg/L carbenicillin selection. Polymerase chain reaction (PCR) and Southern blot analyses showed that six of the 24 putative transgenic plants were transgenic with two insertion loci and that all plants were derived from the same transgenic event. The six transgenic sweet potato plants were challenged by graft inoculation with SPFMV, SPCSV, SPVG and SPMMV- infected Ipomoea setosa Ker. Although virus presence was detected using NCM-ELISA, all transgenic plants displayed delayed and milder symptoms, of chlorosis and mottle of lower leaves when compared to the untransformed control plants. These results warrant further investigation under field conditions. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.