At beginning of '70 T.O. Diener discovered that the causal agent of potato spindle tuber disease is an infectious agent with very surprising features. It is a tiny RNA (246-401 nucleotides, nt) with a size much smaller than the genome of the smallest known virus. In the following years, scientists isolated other RNAs with similar properties from several plants, and the term "viroid" was generally accepted to refer to these biological entities which, from a structural point of view, are the simplest infectious pathogens. Further studies soon revealed other surprising properties of viroids. Their genome is circular and unable to code for any protein, a distinctive feature of viroids with respect to viruses which code at least one, and generally more than one, proteins that they need for completing their infectious cycle. By contrast, the catalytic activities needed for viroid survival are supplied by host proteins or by the same viroidal RNAs, some of which "encode" in their strands ribozymes (enzymes of RNA). The discovery of a particular class of ribozymes (hammerhead structures) in viroids led to to propose that they may be considered molecular fossils of a pre-cellullar world based on RNA. It is assumed that in such an ancient world the RNA molecules, as still happens today in viroids, should have combined hereditary information with catalytic activity. Notwithstanding their ancient origin, viroids are extremely modern not only because of the economic impact of some of the plant diseases they induce, but also because of progress made in biological sciences by studying this pathogens. In addition to ribozymes, the discovery of RNA-mediated DNA methylation and of RNA-dependent RNA polymerase, both involved in gene silencing mechanisms, are illustrative examples. Moreover, since viroids must exploit molecular machineries pre-existing in the host, they are useful tools to unveil the mechanisms of cellular RNA localization and movement, as well as the pathogenic processes induced by non-coding RNAs.
One of the most intriguing unanswered questions about viroids is how they induce diseases. Direct interaction of viroid RNAs with host factors should be the starting event. However, identification of the viroid structural elements directly involved in symptom induction is essential to dissect the molecular mechanism involved. Recently, a very interesting finding has been obtained in this direction: a pathogenic determinant responsible for an extreme chlorosis in peach has been identified in some Peach latent mosaic viroid (PLMVd) variants. The origin of this determinant, composed by a fragment of 12-13 nt that fold into a stem-loop conformation, is unknown, but most likely it is inserted in the viroid molecule during replication. Therefore, a system of this kind is useful not only for further dissecting the molecular mechanisms involved in pathogenesis, but also for studying recombination and molecular evolution of non-coding RNAs.
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