Sequence variability in plant viroid RNAs

Viroids are the smallest known pathogens in nature, and to date have been found only in flowering plants. Only 12 have been described so far (Table 1), and they consist of singlestranded, circular RNA molecules which vary in length from 246 to 375 nucleotides and are not encapsidated. The names that have been given to them are generally descriptive of the symptoms they cause. In the field, they continue to cause diseases of varying severity and economic importance. A historical perspective is given in the book by Diener ¹⁷ while several recent reviews^18-22 provide a detailed coverage of the determination of the sequence, secondary structure, and properties of viroids. This chapter presents the most recent data on the sequence and structure of viroids and of approaches and results in the analysis of structure in relation to function. All evidence indicates that viroids do not code for any polypeptides in vivo, so that their biological effects must be exerted via sequence and structural signals, and their replication is completely dependent on host factors.18 22.