Progress in 50 years of viroid research—Molecular structure, pathogenicity, and host adaptation

Viroids are non-encapsidated, single-stranded, circular RNAs consisting of 246–434 nucleotides. Despite their non-protein-encoding RNA nature, viroids replicate autonomously in host cells. To date, more than 25 diseases in more than 15 crops, including vegetables, fruit trees, and flowers, have been reported. Some are pathogenic but others replicate without eliciting disease. Viroids were shown to have one of the fundamental attributes of life to adapt to environments according to Darwinian selection, and they are likely to be living fossils that have survived from the pre-cellular RNA world. In 50 years of research since their discovery, it was revealed that viroids invade host cells, replicate in nuclei or chloroplasts, and undergo nucleotide mutation in the process of adapting to new host environments. It was also demonstrated that structural motifs in viroid RNAs exert different levels of pathogenicity by interacting with various host factors. Despite their small size, the molecular mechanism of viroid pathogenicity turned out to be more complex than first thought.