Viroids are small RNA particles that infect plants, lacking a protein coat and encoding no protein products. They take over RNA polymerase II to make copies of themselves and inhibit gene expression. Their discovery challenged established wisdom and is now being investigated for agricultural applications.
Viroids are small RNA particles that infect plants. Viroids are notable for their extremely small size (only a few hundred bases of nucleic acids, the smallest having only 220) compared to the largest viruses, which contain at least 2,000 bases. Viroids were discovered and given their name by Theodor Diener, a plant pathologist at the Agricultural Research Service in Maryland, in 1971. Viroids are a subviral agent about 80 times smaller than typical viruses. Their discovery was considered a major breakthrough in 20th-century biology.
Unlike viruses, viroids lack a protein coat and also do not encode any protein products in their structure. Traditional viruses hijack the cell’s core genetic machinery and cause it to pump out copies of the virus. Viroids instead take over RNA polymerase II, an enzyme that synthesizes messenger RNA within the cell and uses it to make copies of itself. Before the discovery of viroids, it was thought that protein-free agents could never infect any living thing. Their discovery proved this wrong.
Most viruses must wear a protein coat to protect themselves from being digested by the powerful enzymes found inside the cells of other organisms. Viroids avoid these enzymes by masquerading as components of the host organism’s cells.
Viroids were discovered in the process of investigating a potato disease called potato spindle disease. This caused the tubers to come out all spindly and twisted. It was largely harmless, but it made potatoes look bad. Since its effect was only cosmetic, it took some time for researchers to study the disease closely. They found it difficult to isolate an agent responsible for the disease. After six years of painstaking work, the tiny viroids have been discovered, overturning the accepted dogma about the size of pathogens.
The first viroids were in fact isolated from tomato plants, which take only two weeks to become infected, while potatoes take two years. After high-speed centrifugation, the traditional method of virus extraction, failed to isolate appreciable amounts of virus, scientists turned to other methods. They combined samples of infected plant matter with enzymes that selectively dissolve RNA, DNA and proteins. After exposure to DNA and protein-dissolving enzymes, the mixture was still capable of infecting plants. Only after using an RNA-dissolving enzyme did his virulent power stop.
Unlike normal viruses that afflict organisms by inserting themselves into chromosomes and modifying the proteins that are produced, the viroid attacks organisms simply by selectively inhibiting the expression of certain genes, similar to RNA interference, a medical technique avant-garde. After a couple of weeks or a couple of years, depending on the species, the plants grow stunted.
The discovery of viroids is an indication that established wisdom in science should be challenged when appropriate. Starting out as a threat, viroids are now being investigated for agricultural applications, such as shrinking citrus trees.
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