Current status of viroid taxonomy

Abstract

Viroids are the smallest autonomous infectious nucleic acids known so far. With a small circular RNA genome of about 250-400 nt, which apparently does not code for any protein, viroids replicate and move systemically in host plants. Since the discovery of the first viroid almost forty-five years ago, many different viroids have been isolated, characterized and, frequently, identified as the causal agents of plant diseases. The first viroid classification scheme was proposed in the early 1990s and adopted by the International Committee on Taxonomy of Viruses (ICTV) a few years later. Here, the current viroid taxonomy scheme and the criteria for viroid species demarcation are discussed, highlighting the main taxonomic questions currently under consideration by the ICTV Viroid Study Group. The impact of correct taxonomic annotation of viroid sequence variants is also addressed, taking into consideration the increasing application of nextgeneration sequencing and bioinformatics for known and previously unrecognized viroids.

Small RNA Derived from the Virulence Modulating Region of the Potato spindle tuber viroid Silences callose synthase Genes of Tomato Plants

“…The current viroid classification scheme consists of 32 species, broadly divided into two families (Di Serio et al, 2014). The Avsunviroidae family, whose type species is the Avocado sunblotch viroid, includes four members that replicate in chloroplasts through a symmetric rolling-circle mechanism and exhibit selfcleavage activity.…”

Modelling the three-dimensional structure of the right-terminal domain of pospiviroids

“…This step proceeds either by a viroid-internal ribozyme or by proteinaceous nucleases of the host. Based on this feature, viroids are classified into two families 4 : members of Pospiviroidae possess a thermodynamically stable rod-like secondary structure (Fig. 1a), replicate in the nucleus, and do not self-cleave; members of Avsunviroidae replicate in the chloroplast and self-cleave via a hammerhead ribozyme.…”

Infectious long non-coding RNAs

“…They were reported for the first time in 1971 in the spindle tuber disease of potato [8]. Since then, thirty-two different viroid species have been identified and separated into two families (Pospiviroidae and Avsunviroidae) and eight genera (Avsunviroid, Pelamoviroid, Elaviroid for Avsunviroidae and Pospiviroid, Hostuviroid, Cocadviroid, Apscaviroid, Coleviroid for Pospiviroidae) [15,16]. However, new viroids have been identified (ex.…”