Viroid

Viroids are the smallest known infectious agents and consist solely of a short, circular RNA molecule. They lack a protein coat and do not encode any proteins. Viroids infect plants and can cause a range of symptoms, from mild to severe, but many infections are latent. Their compact genomes and reliance on host cellular machinery distinguish them from viruses.

The viroid genome is a single circular RNA, typically about 246 to 400 nucleotides in length, with a high degree of secondary structure. Viroid RNA is obligately dependent on plant hosts for replication and movement. Some viroids produce functional RNA motifs, including self-cleaving hammerhead ribozymes in the Avsunviroidae lineage, which facilitate processing during replication.

Taxonomically, viroids are grouped into two families. Pospiviroidae members replicate in the nucleus and generally rely on host enzymes for processing of RNA transcripts. Avsunviroidae members replicate in chloroplasts and often contain hammerhead ribozymes that enable self-cleavage during the rolling-circle replication cycle. Both groups spread within the plant and between plants through mechanical contact, contaminated tools, vegetative propagation, seeds, and pollen in certain species.

Pathogenesis is variable; viroids can alter growth, cause leaf deformation, rosette formation, stunting, or yield loss, but symptoms are influenced by host species and environmental conditions. Detection relies on molecular methods such as RT-PCR, RT-qPCR, and RNA hybridization assays. Control measures emphasize using viroid-free planting material, strict sanitation, and meristem culture to produce clean stock. Notable examples include Potato spindle tuber viroid, Citrus exocortis viroid, Hop stunt viroid, and Avocado sunblotch viroid.