viroiders

Viroiders is an informal term sometimes used to refer to viroids, the smallest known infectious agents that cause disease in plants. In formal literature, the pathogens are simply called viroids, while researchers who study them are viroid researchers or plant virologists. Viroiders are noncoding RNA molecules and do not produce proteins or have a protective coat.

Viroids are circular, single-stranded RNA molecules typically about 246 to 401 nucleotides long. They lack genes for proteins and depend entirely on host plant factors for replication, movement, and transmission. Viroids are classified into two principal families: Avsunviroidae and Pospiviroidae. The two groups differ in replication locale and biochemistry; their genomes are structurally simple but capable of orchestrating complex disease symptoms through interactions with host pathways.

Replication and intracellular movement vary by family. Avsunviroidae replication occurs in chloroplasts and involves a symmetric rolling-circle mechanism, with hammerhead ribozymes that aid self-cleavage. Pospiviroidae replicate in the nucleus via host RNA polymerase II, using asymmetric rolling-circle replication and host enzymes for processing. Movement within the plant typically occurs through plasmodesmata and, over long distances, the phloem.

Hosts and transmission are broad within the plant kingdom, especially among crops and ornamentals. Viroids spread primarily by mechanical means, grafting, contaminated tools, and the use of infected planting material. Seed- or pollen-transmission occurs for some viroids. They are not spread by most insect vectors due to their noncoding nature and lack of a protective coat.

Detection and management rely on molecular methods such as RT-PCR and sequencing, plus field practices like using certified clean material, sanitation, quarantines, and removing infected plants. Notable viroids include Potato spindle tuber viroid, Citrus exocortis viroid, Hop stunt viroid, and Avocado sunblotch viroid.