LAUSR

Simple Summary Viroids are the smallest infectious agents currently known. Despite consisting of a relatively small RNA molecule that does not code for any protein, viroids manage to reproduce their genomes and completely invade a host plant when they successfully enter into an initial single cell, frequently inducing a disease. This article recalls viroid discovery about 50 years ago and reviews our current knowledge about most aspects of viroid biology, including the structure of viroid molecules and taxonomic classification, the mechanisms of viroid genome replication and movement, how viroids transmit from plan to plant and how they induce disease in the host plants. Finally, the article also reviews recent efforts to transform these infectious agents into tools useful in biotechnology. Abstract Viroids are a unique type of infectious agent, exclusively composed of a relatively small (246–430 nt), highly base-paired, circular, non-coding RNA. Despite the small size and non-coding nature, the more-than-thirty currently known viroid species infectious of higher plants are able to autonomously replicate and move systemically through the host, thereby inducing disease in some plants. After recalling viroid discovery back in the late 60s and early 70s of last century and discussing current hypotheses about their evolutionary origin, this article reviews our current knowledge about these peculiar infectious agents. We describe the highly base-paired viroid molecules that fold in rod-like or branched structures and viroid taxonomic classification in two families, Pospiviroidae and Avsunviroidae, likely gathering nuclear and chloroplastic viroids, respectively. We review current knowledge about viroid replication through RNA-to-RNA rolling-circle mechanisms in which host factors, notably RNA transporters, RNA polymerases, RNases, and RNA ligases, are involved. Systemic movement through the infected plant, plant-to-plant transmission and host range are also discussed. Finally, we focus on the mechanisms of viroid pathogenesis, in which RNA silencing has acquired remarkable importance, and also for the initiation of potential biotechnological applications of viroid molecules.