Who discovered viroids?

In 1971, T.O. Diener identified a new, virus-smaller infectious agent responsible for potato spindle tuber disease. This agent—named a viroid—consists solely of free RNA and lacks the protein coat characteristic of viruses. Viroid RNA is circular, single-stranded, and of very low molecular weight.

Key features of viroids

• Smallest known infectious pathogens.
• Composed only of a short, circular ssRNA; no protein coat (capsid).
• Infect higher plants; many cause economically important diseases.
• Do not code for proteins.
• Replicate in host cells using RNA polymerase II, which, instead of making mRNA from DNA, catalyzes a rolling-circle synthesis using the viroid RNA as template.

Viroids vs. Viruses

• Protein coat: Present in viruses; absent in viroids.
• Genetic material: Viruses carry either DNA or RNA (not both at once); viroids have only RNA.
• Host range: Many viruses infect plants and animals; viroids are known to infect plants.