The Phytopathogenic Spiroplasmas
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The Phytopathogenic Spiroplasmas JACQUELINE FLETCHER, ULRICH MELCHER AND ASTRI WAYADANDE
Introduction Spiroplasmas (family Spiroplasmataceae, order Mycoplasmatales) comprise a group of helical prokaryotes whose external morphology, unlike that of the spirochetes, is defined by an internal skeletal framework rather than by an external peptidoglycan-containing cell wall. The loss of a cell wall in mollicutes (spiroplasmas, mycoplasmas, phytoplasmas, acholeplasmas and others) is due to degenerative evolution from their walled precursors (see also The Phytopathogenic Spiroplasma in this Volume, which provides extensive coverage of this taxon). Most of the more than 50 spiroplasma species that have been described are associated with insects and other animals in relationships that range from pathogenic to benign. For example, Spiroplasma melliferum infects honeybees (Whitcomb and Williamson, 1979), other spiroplasmas cause disease in mosquitoes (Humphery-Smith et al., 1991), and the “sex ratio” spiroplasma kills the male progeny of fruit flies (Williamson et al., 1989). Many spiroplasmas, however, colonize the gut lumen and/or hemolymph of their insect hosts without causing detrimental effects (Whitcomb and Williamson, 1979). Only three species of spiroplasmas (S. citri, S. kunkelii and S. phoeniceum) are known to cause disease in plants. These microbes are important for two reasons. First, they cause significant losses in economically important crops such as maize (corn stunt disease) and citrus (citrus stubborn disease), and in specialty crops (brittle root of horseradish). Second, and perhaps just as important, the plant pathogenic spiroplasmas, which are cultivable in artificial media, serve as models for the more numerous, widespread, and noncultivable phytoplasmas, a group of related mollicutes that cause disease in over 300 plant species globally. In this chapter, the focus is on the unique features and issues related to the small, but economically important, group of spiroplasmas that cause diseases in plant hosts.
Historical Background For many years, the diseases citrus stubborn and corn stunt were attributed to viral etiology, as their agents were graft transmissible and could not be cultivated. Remission of symptoms in stubborn diseased citrus trees following application of tetracyclines, but not penicillin (Igwegbe and Calavan, 1970), suggested that a nonviral agent might be involved. But a novel microbial taxon was not recognized until the late 1960s when thin sections of maize displaying strong symptoms of a long-known, economically serious stunting and yellowing disease called “corn stunt” were examined by electron microscopy (Davis and Worley, 1973). The unusual prokaryotes visualized in those sections (later designated “spiroplasmas”) were smaller than most bacteria and had a distinctive helical morphology and lacked the peptidoglycan-rich outer wall that defines most prokaryotes.
Morphology The first descriptions of the morphology of spiroplasmas we
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