The Enigma of Viroid Pathogenesis

Aside from their obvious agricultural interest [1], viroids are fascinating molecules in their own right. They are the smallest known agents of infectious disease, consisting solely of short strands of covalently closed circular, single-stranded RNA (240–380 nucleotides) [22]. Because of extensive intramolecular complementarity, viroids can assume rodlike, quasi double-stranded configurations, in which short, base-paired regions alternate with mismatched, single-stranded loops [3] (Figure 25.1). Despite their small size and severely limited genetic information content, viroids replicate (or, more accurately, are replicated) in susceptible cells without the assistance of recognizable helper viruses [4]. Their autonomous replication clearly distinguishes viroids from plant viral satellite RNAs, which may be of similar size but which can replicate only in the presence of a specific helper virus in the same cell [2].