Taxonomy of Family: Viroids and Viroids like RNAs

Viroids are the smallest known autonomously replicating infectious agents, consisting solely of short, circular, single-stranded RNA molecules that lack protein-coding capacity. Despite their minimal genome size, they cause significant diseases in numerous economically important crops. This chapter provides a comprehensive overview of viroid taxonomy, structural organization, replication strategies, and biological diversity, emphasizing recent insights from molecular and phylogenetic analyses. Viroids are currently classified into two families—Pospiviroidae and Avsunviroidae—based on sequence and secondary-structure features, replication mechanisms, subcellular localization, and the presence or absence of hammerhead ribozymes. Members of Pospiviroidae replicate in the nucleus via an asymmetric rolling-circle mechanism and adopt quasi-rod or rod-like conformations, whereas Avsunviroidae replicate symmetrically in the chloroplast and contain self-cleaving ribozymes. Within Pospiviroidae, conserved structural domains such as the CCR, TCR, and TCH are critical for replication, pathogenicity, and host adaptation. Sequence heterogeneity exceeding 10% among isolates defines criteria for distinct viroid species along with other considerations such as host range, transmission, etc. Advances in high-throughput sequencing have revealed numerous tentative or unclassified viroids, expanding our understanding of their evolutionary relationships and ecological niches. The chapter also highlights the formation of complex secondary structures, including hairpin and loop motifs that mediate replication and host interactions. Overall, understanding the structural and molecular determinants governing viroid diversity provides a framework for refined taxonomy and improved disease management strategies.