The molecular biology of plant DNA viruses

Two major groups of plant DNA viruses have been described in the literature: the double‐stranded cauli‐moviruses and the single strand geminiviruses. The genomes of viruses from both groups have peculiar properties that reveal unexpected features in either the replication or expression of their genomes. For example, cauliflower mosaic virus appears to replicate through a mechanism involving the reverse transcription of viral RNA, similar to steps described for the replication of animal retrovirus and hepatitis B‐group virus genomes. Viral genomes from both groups have been entirely sequenced and the anatomy of the genomes deduced from the sequence data shows fundamental differences in genome organization. Viron DNA in caulimoviruses is double‐stranded, but only one strand is a sense or coding strand and is transcribed in plants. Although virion DNA in geminiviruses is single‐stranded, both the virion strand and its complement are sense strands and presumably transcribed in plants. While caulimovirus genomes are composed of a single DNA component, geminivirus genomes are composed of one or two components, depending on the virus.