Viroid

Viroid is a type of pathogen that is smaller and simpler than a virus. Viroids are single-stranded RNA molecules that do not code for any protein. Despite their simplicity, viroids can cause severe diseases in plants.

Structure and Composition

Viroids are composed of a short stretch of circular, single-stranded RNA without the protein coat (capsid) that is found in most other viruses. The RNA strand of a viroid can be as short as 246 nucleotides, making it one of the smallest known pathogens. The RNA does not code for any proteins and is not surrounded by any protein coat. Instead, the RNA molecule folds onto itself to form a highly stable, rod-like structure.

Replication

Viroids replicate in the host plant's nucleus or chloroplasts, where they use the host's own RNA polymerase to synthesize new copies of their RNA. The replication process is not fully understood, but it is believed to involve a rolling-circle mechanism.

Pathogenicity

Despite their simplicity, viroids can cause severe diseases in plants. The exact mechanism of pathogenicity is not fully understood, but it is believed to involve the interaction of the viroid's RNA with the host plant's gene expression machinery. This interaction can lead to the silencing of certain plant genes, resulting in disease symptoms.

Classification

Viroids are classified into two families: Pospiviroidae and Avsunviroidae. The Pospiviroidae family includes the Potato spindle tuber viroid (PSTVd), the first viroid to be discovered. The Avsunviroidae family includes the Avocado sunblotch viroid (ASBVd), which causes a disease in avocado trees.

History

The concept of viroids was first postulated by Theodor O. Diener in 1971. Diener discovered the Potato spindle tuber viroid (PSTVd), a pathogen that was smaller and simpler than any known virus. This discovery led to the recognition of a new class of pathogens, which Diener named "viroids".

See also

• Virus
• Prion
• Satellite