Viral sequences integrated into plant genomes.

Sequences of various DNA plant viruses have been found integrated into the host genome. There are two forms of integrant, those that can form episomal viral infections and those that cannot. Integrants of three pararetroviruses, Banana streak virus (BSV), Tobacco vein clearing virus (TVCV), and Petunia vein clearing virus (PVCV), can generate episomal infections in certain hybrid plant hosts in response to stress. In the case of BSV and TVCV, one of the parents contains the integrant but is has not been seen to be activated in that parent; the other parent does not contain the integrant. The number of integrant loci is low for BSV and PVCV and high in TVCV. The structure of the integrants is complex, and it is thought that episomal virus is released by recombination and/or reverse transcription. Geminiviral and pararetroviral sequences are found in plant genomes although not so far associated with a virus disease. It appears that integration of viral sequences is widespread in the plant kingdom and has been occurring for a long period of time.