Studies on structure, immunochemistry and taxonomy of plant potyviruses

Abstract

INTRODUCTIONThe potyvirus group is the largest and economically most important of the 34 plant virus groups and families currently recognised. It contains over 180 distinct members (or 34% of all known plant viruses) and causes significant disease in agricultural, pasture and horticultural crops. In 1974 members of the group were reported to infect 1,112 species of 369 genera in 53 plant families. Since then many more species, genera and families have been added as hosts of potyviruses (18, 23, 30). It was pointed out repeatedly by taxonomists and reviewers, until 1988 that the taxonomy of the potyvirus group is very unsatisfactory and that successful resolution of potyvirus detection and identification is a major challenge for plant virologists. This unsatisfactory state of potyvirus taxonomy has been due to the large size of the group, the apparent vast variation among the members and the lack of suitable taxonomic parameters to assign viruses to the group and to differentiate between distinct members and strains. In the past, the use of conventional approaches such as host range, symptomatology, cross-protection, cytoplasmic inclusion morphology, serology and molecular hybridization using randomly primed complementary DNA suggested the presence of a 'continuum' between strains of two or more distinct potyviruses, implying that 'species' and 'strain' concepts can not be applied to potyviruses. According to the 'continuum' hypothesis, a distant strain of one virus could appear more closely related to a distant strain of a second virus than either are to their homologous viruses (see references cited in paper's no 18, 23, 30). However, results based on the use of nucleic acid and coat protein amino acid sequences presented in this thesis cast doubt on the 'continuum' hypothesis (16) and clearly demonstrate that potyviruses can be divided into distinct members and strains (16, 18, 23, 30). Tlie sequence data (9, 10, 11, 14, 15, 16, 20, 22, 28, 32, 33, 41), In combination with information on the structural organisation of potyvirus particle (13) and immunochemlcal analyses of native virus particles, trypsin-treated virus particles, dissociated core proteins (13) and overlapping, synthetic octapeptides that account for entire coat protein (24) have established the molecular basis of potyvirus serology, explained many of the problems currently associated with the application of conventional approaches (23) and provided several novel methods for the accurate identification and classification of potyviruses (12, 13, 16, 17, 21,22,38). -------------------------------------------------------------------------------------------------Numbers in parenthesis refer to the publication's number mentioned in Section VI (List of Publications)