viroid

vi·roid   (vī'roid')    n.  An infectious particle, similar to but smaller than a virus, that consists solely of a strand of RNA and is capable of causing disease in plants.

Main Entry: ¹vi·roid
Pronunciation: 'vI-"roid
Function: noun
1 : a hypothetical symbiont resembling a virus that was formerly heldto be present in cells, to be favorable to the host, and to tend to mutate to a virus
2 : any of several causative agents of plant disease that consist solely of a single-strandedRNA of low molecular weight arranged in a closed loop or a linear chain

viroid vi·roid (vī'roid')
n.
An infectious particle, smaller than a virus, that consists solely of a strand of RNA and is capable of causing disease in plants.

viroid   (vī'roid')  Pronunciation Key  An infectious agent that consists solely of a single strand of RNA and causes disease in certain plants. Viroids lack the protein coat (known as a capsid) of viruses and are the smallest known infectious agents. Containing only about 250 to 375 base pairs, they are much smaller than the smallest genomes of viruses and have no genes for encoding proteins. After invading a host cell, viroids are thought to mimic the cell's DNA, so that the cell's RNA polymerase replicates them in the nucleus. Viroids are believed to cause disease by interfering with the host cell's gene regulation. They are destructive to many important commercial plants, including potatoes, tomatoes, cucumbers, coconuts, and chrysanthemums.