Viroids and the nature of viroid diseases*
In its methodology, the unexpected discovery of the viroid in 1971 resembles that of the virus by Beijerinck some 70 years earlier. In either case, a novel type of plant pathogen was recognized by its ability to penetrate through a medium with pores small
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Summary. In its methodology, the unexpected discovery of the viroid in 1971 resembles that of the virus by Beijerinck some 70 years earlier. In either case, a novel type of plant pathogen was recognized by its ability to penetrate through a medium with pores small enough to exclude even the smallest previously known pathogen: bacteria as compared with the tobacco mosaic agent; viruses as compared with the potato spindle tuber agent. Interestingly, one of the two methods used by Beijerinck, diffusion of the tobacco mosaic agent into agar gels, is conceptually similar to one method used to establish the size of the potato spindle tuber agent, namely polyacrylamide gel electrophoresis. Further work demonstrated that neither agent is an unusually small conventional pathogen (a microbe in the case of the tobacco mosaic agent; a virus in the case of the potato spindle tuber agent), but that either agent represents the prototype of a fundamentally distinct class of pathogen, the viruses and the viroids, respectively. With the viroids, this distinction became evident once their unique molecular structure, lack of mRNA activity, and autonomous replication had become elucidated. Functionally, viroids rely to a far greater extent than viruses on their host's biosynthetic systems: Whereas translation of viral genetic information is essential for virus replication, viroids are totally dependent on their hosts' transcriptional system and, in contrast to viruses, no viroid-coded proteins are involved. Because of the viroids' simplicity and extremely small size they approach more closely even than viruses Beijerinck's concept of a contagium vivumfiuidum. Introduction Recognition of the fundamental disparity between viruses and viroids became possible only after certain basic principles of 'virology and molecular biology had been established. These principles helped create an intellectual climate in *Portions of this presentation are expanded and modified versions of a paper entitled "Portraits of Viruses: the Viroid" [Intervirology 22: 1-16 (1984)], published by S. Karger A. G., Basel, Switzerland
C. H. Calisher et al. (eds.), 100 Years of Virology © Springer-Verlag Wien 1999
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Fig. 1. Size comparison between a bacterium, several viruses, and the viroid. Adapted from Scientific American 244: 66-73 (1981)
which the existence of free nucleic acid pathogens could not a priori be ruled out. Aside from general biological principles, at least four important prerequisites can be identified: (i) Foremost was the realization that the genetic information of viruses resides in their nucleic acid component, a fact that, in the case of plant viruses, was most dramatically established with the demonstration that RNA isolated from tobacco mosaic virus is infecti
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