Definition and Discovery of Viroid

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The word VIROID is composed of two words ‘VIR (means virus)’ and ‘OID (means like)’. Thus, Viroid means virus-like organism/particle. Viroids were first discovered and named by the plant pathologist Theodor O. Diener in 1971 to describe the causal agent of the potato spindle tuber disease. The first viroid identified was Potato Spindle Tuber Viroid (PsTVd).
A viroid is an infectious RNA particle formed from a single-stranded circular RNA. It infects only plants (except Hepatitis D in humans). Viroid may cause disease by triggering RNA slicing. Some are found in the infected host cell nucleus, others found in the chloroplast. More than 40 viroid species and many variants have been characterized.

Definition:
Viroids are infectious, non-protein-coating, highly structured small circular naked RNA molecules able to replicate autonomously and induce diseases in higher plants. The term viroid was coined by T.O. Diener (1971) to describe the causal agent of the potato spindle tuber disease.
In another word, viroid is an infectious particle, composed of a single-stranded circular RNA with no capsid that infects and causes disease in plants. It is basically a virus without a capsid protein coat.

Discovery of Viroid:
The potato spindle tuber disease was first described in 1922 and assumed that the causal agent is a virus.

In 1967 Diener isolated the causal agent and observed that it is sensitive to RNAase enzyme.
Finally, in 1971 Diener discover the causal agent and termed it as Viroid. During this period Potato famine was observed in Ireland.

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