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Viroids are infectious agents that consist of single-stranded RNA. They are
much smaller and simpler than viruses and lack the protein cover that is typical for viruses. Viroids use higher plants (such as potatoes, tomatoes and cucumbers) to reproduce, inserting themselves into the
nucleus of a plant cell to be replicated there. Viroids are usually
transmitted by seed or pollen. Infected plants
can show distorted growth. The first viroid to be identified was the potato spindle
tuber viroid.
Some 33 species have been identified.
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