Viroids

Article By:

Horst, R. K. Department of Plant Pathology, Cornell University, Ithaca, New York.

Last reviewed:January 2021

DOI:https://doi.org/10.1036/1097-8542.733200

Small infectious nucleic acid particles containing independently replicating single-stranded circular ribonucleic acids (RNAs) of 246–399 nucleotides, which are known to infect certain monocot and dicot plants. Viroids are the smallest known disease agents, with an estimated molecular weight (sum of the atomic weights of a molecule's constituent atoms) of as little as 1.0–1.3 × 10⁴, in marked contrast to the conventional plant virus genomes, which have molecular weights of approximately 2 × 10⁶. Conventional viruses are made up of nucleic acid encapsulated in protein (capsid), whereas viroids are uniquely characterized by the absence of a capsid. In spite of their small size, viroid RNAs can replicate and produce characteristic disease syndromes when introduced into cells. Although the viroids thus far identified are associated with plants, based on the unusual properties of the infectious agents they may also be found to affect other forms of life, including humans. It is possible that in instances where a viral etiology of disease has been assumed but where no causal virus has been identified, viroid infectious agents are involved.

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