With an amazingly small genome size (250–400 nt) and simple structure, viroids are giants in terms of functional versatility in the world of RNA. Without encoding proteins to provide specific functions and without being protected in a protein or membrane shell, these single-stranded, circular RNAs can triumph over the cellular RNA degradation machineries and further enlist host-encoded factors for replicating themselves in specific subcellular compartments and trafficking throughout a plant. They can also cause devastating plant diseases.
Viroids are single-stranded, circular, and noncoding RNAs that infect plants. They replicate in the nucleus or chloroplast and then traffic cell-to-cell through plasmodesmata and long distance through the phloem to establish systemic infection. They also cause diseases in certain hosts. All functions are mediated directly by the viroid RNA genome or genome-derived RNAs. This review summarizes recent advances in the understanding of viroid structures and cellular factors enabling these functions, emphasizing conceptual developments, major knowledge gaps, and future directions. Newly emerging experimental systems and research tools are discussed that are expected to enable significant progress in a number of key areas. It highlights examples of groundbreaking contributions of viroid research to the development of new biological principles and offer perspectives on using viroid models to continue advancing some frontiers of life science.