Viroids: Biology, Structure and Possible Functions

The discovery of viroids in 1971/72 as free infectious RNA molecules of low molecular weight and hence the smallest disease agents presently known is an interesting example of serendipity in modern biology, because viroids were found although not actually sought for. In fact, they finally turned out to be completely different from what they were expected to be when the efforts started to characterise the elusive virus-like agents to the corresponding plant diseases. After the early scepticism and even rejection viroid research has now become a rather interesting and well a,ccepted task. Considerable progress has been made in elucidating the structure of viroids and the wealth of information which has recently accumulated renders viroids the most thoroughly studied RNA molecules of today. It is the aim of this report to review the present knowledge on the biology and the structure of viroids and to discuss some of its implications with respect to the functions of viroids which are still largely enigmatic.