Viroids
Viroids are tiny fragments of nucleic acid known to cause several diseases in plants and thought to be involved in human and animal diseases.
The discovery of viroids resulted from studies conducted in 1960s at the U.S. Department of Agriculture's Beltsville, Maryland, research center near Washington, D.C. Scientis led by Theodore O.Diener were investigating a suspected viral disease, potato spindle tuber (PST), which results in small, cracked potatoes shaped like spindles. Nothing would destroy the disease agent except an RNA- dissolving enzyme, and in 1971 , the group postulated that a rfagment of RNA was involved. Diener called the agent a viroid, meaning viruslike. The next year, a team led by Joseph Semancik at the University of California found a similar agent in a disease of citrus trees.R
Currently at least a dozen plant diseases have been related to viroids. The largest of these particles is about one-twentieh of the size of the smallest virus. The RNA chain of the PST viroid has a known molecular sequence, but it contains so few genetic codes that the replication cycle is not understood. Diener has speculated that the viroids originated as introns, the sections of RNA spliced out of messenger RNA molecues before the messengers are able to function. The similarity in size between introns and viroids and the ring shape for both has feuled the speculation. Semancik has offered the theory that viroids may be regulatory genes because viroid diseases are characterized by interference with plant growth.
An interesting twist in viroid research occured in 1981 when it was found that viroids could infect animals. Researchers from Knoxville, Tennessee, reported DNA viroids in hamster colonies afflicted with lymphatic cancer. Currently, some microbiologists believe that viroids may cause the mysteries slow virus diseases of humans.
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