Viroids

Viroids are the smallest infectious pathogens known, consisting solely of a short strand of circular, single-stranded RNA without a protein coat. Unlike viruses, viroids do not encode proteins and rely entirely on the host cell's machinery for replication. They are known to cause diseases in plants, leading to significant agricultural losses.

Structure

Viroids are composed of a short, circular RNA molecule, typically 246 to 375 nucleotides in length. The RNA is highly structured, forming a rod-like shape due to extensive internal base pairing. This structure is crucial for their stability and function within host cells. Unlike viruses, viroids lack a protective protein coat, which makes them unique among infectious agents.

Replication

Viroids replicate through a "rolling circle" mechanism within the host cell's nucleus or chloroplasts. The host's RNA polymerase II, which normally transcribes DNA into mRNA, is hijacked by the viroid RNA to synthesize new viroid RNA strands. This process involves the synthesis of a complementary RNA strand, which serves as a template for the production of new circular viroid RNA molecules.

Pathogenicity

Viroids are known to cause a variety of plant diseases, often leading to stunted growth, deformation, and reduced yield. The exact mechanism of pathogenicity is not fully understood, but it is believed that viroids interfere with the host's gene expression and RNA silencing pathways. Some well-known viroid diseases include Potato spindle tuber disease, Citrus exocortis, and Chrysanthemum stunt disease.

Transmission

Viroids can be transmitted through mechanical means, such as contaminated tools, or through vegetative propagation. They can also be spread by insects, although this is less common. Seed transmission is possible in some cases, contributing to the spread of viroid diseases across regions.

Detection and Control

Detection of viroids is typically performed using molecular techniques such as RT-PCR and nucleic acid hybridization. Control measures include the use of certified disease-free planting material, sanitation of tools, and the removal of infected plants. Breeding for resistant plant varieties is also an important strategy in managing viroid diseases.

Also see

• Virus
• RNA silencing
• Plant pathology
• Infectious disease