The published articles from Appendix III (p. 133 to end) are not available in the electronic version of this thesis due to copyright restrictions. The full version can be consulted at the University of Leicester Library.Narcissus latent virus (NLV) and Maclura mosaic virus (MacMV) are serologically related. However, they have poor serological relationships with other plant viruses with which they have shared characteristics. Coat protein size, particle shape and structure, mode of vector transmission, cytology and serology proved insufficient to classify them. Molecular techniques were employed in order to create tools for the rapid and accurate classification of plant viruses. A carlavirus-specific PCR primer test failed to amplify NLV or MacMV but confirmed that several other viruses belonged to the carlavirus genus.\ud The nucleotide sequences of part of the nuclear inclusion body (NIb) gene, the complete coat protein gene and the 3' untranslated regions of narcissus latent virus (NLV) and Maclura mosaic virus (MacMV) were determined. Deduced amino acid sequences for the NIb and coat protein genes revealed that NLV and MacMV are closely related.\ud Comparison of the NIb sequences with other viruses showed that NLV and MacMV have closer affinities with viruses of the Potyviridae than to those of the carlavirus genus with which they were initially classified. It is proposed that NLV and MacMV may form a new genus within the Potyviridae, the Macluraviruses. The viruses associated with narcissus yellow stripe disease were re-evaluated.\ud In order to identify further members for the new genus a second PCR primer was designed to amplify a region shared by the bymoviruses and macluraviruses. Sequence data obtained for the 3'-terminal region of rice necrosis mosaic virus (RNMV) using a fragment amplified by this primer confirmed that RNMV was a member of the bymovirus genus. MacMV and barley mild mosaic bymovirus replicase sequences were used to transform tobacco plants in an attempt to create transgenic resistance
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