Viroids as disease agents.

Viroids are nucleic acid species of low molecular weight and unique structure that cause several important diseases of cultivated plants. Viroids are the smallest known agents of infectious disease. Unlike viral nucleic acids, viroids are not encapsidated. Despite their small size, viroids replicate autonomously in cells of susceptible plant species. Known viroids are single-stranded, covalently closed, circular, as well as linear, RNA molecules with extensive regions of intramolecular complementarity; they exist in their native state as highly base-paired rods. The biological properties of viroids are determined by their primary structures; viroids thus constitute genetic systems of minimal complexity. So far, viroids have been identified only as pathogens of higher plants, but it is likely that certain animal (including human) diseases are caused by similar agents.