A proposed scheme for viroid classification and nomenclature
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Virology Division News Arch Virol 143/3 (1998)
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Virology Division News
A proposed scheme for viroid classification and nomenclature R. Flores1, J. W. Randles2, M. Bar-Joseph3, and T. O. Diener4 1
Instituto de Biología Molecular y Celular de Plantas (UPV-CSIC), Valencia, Spain Department of Crop Protection, Waite Campus, University of Adelaide, Glen Osmond, Australia 3 The S. Tolkowsky Laboratory, Agricultural Research Organization, Bet Dagan, Israel 4 Center for Agricultural Biotechnology, University of Maryland, College Park, Maryland, U.S.A.
2
Introduction Viroids are the only class of autonomously replicating subviral pathogens whose molecular structure is well defined. Their structural and functional properties, as well as their evolutionary origin differ fundamentally from those of viruses (see below), posing specific problems for the classification of these subviral pathogens. For example, some of the criteria suggested by the Executive Committee of the International Committee on Taxonomy of Viruses (ICTV) for the definition of species, which include those regarding morphological characteristics, protein characteristics and antigenic properties, are not applicable to viroids. The purpose of this note is to present a consensus taxonomic proposal (see Fig. 1) that has emerged from consultations among the members of the Viroid Study Group of the Plant Virus Subcommittee of the ICTV. This proposal updates and expands the previous classification, which appeared in the Sixth Report of the ICTV [9], and is open to discussion and suggestions. Functional properties of viroids Viroids have significant agricultural implications [10]. They have been identified as the aetiologic agents of a variety of maladies affecting mono- and dicotyledonous crops of economic importance, including potato, tomato, hop, coconut, grapevine, several subtropical and temperate fruit trees such as citrus, avocado, peach, apple, pear and plum, and ornamental plants such as chrysanthemum and coleus. All the available data support the contention that, in contrast to viruses, viroids do not code for any peptide or protein. Therefore, viroids must be replicated by pre-existing host enzymes which are activated or sequestered by the viroid genome, and the pathogenic effects that viroids induce must result from direct interaction of their RNA with one or more cellular targets. Viroids can be regarded as parasites of the transcription machinery, whereas viruses are essentially parasites of the translation machinery. Viroid replication occurs by a rolling circle mechanism, using either a symmetric or asymmetric pathway, in three main steps: RNA transcription, RNA processing and RNA
Virology Division News
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ligation, catalyzed respectively by enzymes with RNA polymerase, RNase or RNA ligase activities [4]. One remarkable aspect of this process is that, at least in the replication of three viroids, avocado sunblotch viroid (ASBVd) [16], peach latent mosaic viroid (PLMVd) [15], and chrysanthemum chlorotic mottle viroid (CChMVd) [28], a ribo
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