Viroids

Viroids are a unique class of infectious agents that lack a protein envelope, and are composed only of a closed circular single-stranded RNA molecule. Viroids have been found to infect plants and cause diseases such as Potato spindle tuber viroid, Citrus exocortis viroid and Hop stunt viroid. They have a small genome size (240-400 nucleotides) and their circular structure allows them to replicate autonomously using host plant enzymes. Viroids are not viable in a laboratory context; instead they must be transmitted through infected plants which have the appropriate environment to sustain their replication. Thus, the study of viroids is of major importance for understanding the factors that allow for the maintenance of infectious agents in nature. The study of viroids has provided valuable insights into the structure and function of nucleic acids as well as plant disease epidemiology. Furthermore, the recent development of techniques allowing for the engineering of viroids has generated great potential for their use as biological tools for molecular biology research, gene transfer, gene expression control and disease diagnosis.