Subviral DNAs associated with geminivirus disease complexes.

Ageratum conyzoides (ageratum) is a common and widespread weed species that may act as a reservoir host for geminivirus diseases. Ageratum plants growing in Singapore and exhibiting yellow vein disease are infected with a complex mixture of viral, subviral and recombinant DNA components. This whitefly-transmitted disease is caused by the monopartite begomovirus ageratum yellow vein virus (AYVV) in association with a recently discovered satellite component referred to as DNA beta. Diseased plants also contain a subviral component, referred to as DNA 1, that has probably become associated with the begomovirus and adapted to whitefly transmission during mixed infection with an aphid-transmitted nanovirus. Unlike DNA beta, the nanovirus-like component is not essential for the disease. Recombination between the viral and subviral DNAs occurs frequently and may play an important evolutionary role by generating component diversity. The identification of a similar complex associated with cotton leaf curl disease (CLCuD), a serious constraint to cotton growing in Pakistan, and the isolation of DNA beta homologues from diverse plant species growing in widespread geographical locations suggests that such disease complexes are common and may have a significant impact on agriculture in the eastern hemisphere.