ABSTRACT
The subjects of this review may be considered to be “RNA-level” infectious agents, RNAs that replicate in plants, but apparently neither engender any encoded protein nor appear in any DNA form during such replication. The agents are viroids and certain small satellite RNAs, such as those of the nepoviruses 1-3 and of sobemoviruses. 4 Each of these has fewer than 400 nucleotide residues in the most abundant form. Each occurs partly or even predominantly as circles. The small satellite RNA of cucumber mosaic virus is similar to the other satellite RNAs, but may encode a small polypeptide and has not been detected in circular form (Section IV and Chapter 10, Volume III).
