A unique virus complex causes Ageratum yellow vein disease

Ageratum conyzoides L., a weed species widely distributed throughout southe ast Asia, frequently exhibits striking yellow vein symptoms associated with infection by Ageratom yellow vein virus (AYVV), a member of the Geminiviri dae (genus Begomovirus). Most begomoviruses have bipartite genomes (DNAs A and B), but only a DNA A has been identified for AYVV. We demonstrate that yellow vein disease of A. conyzoides results from co-infection by AYVV DNA A (2,741 nt) and a circular DNA that is approximately half its size (1,347 nt) that we designate DNA beta. Apart from the sequence TAATATTAC, common t o all geminiviruses and containing the initiation site of rolling circle re plication, DNA beta shows negligible sequence homology either to AYVV DNA A or to DNA B associated with bipartite begomoviruses. DNA beta depends on D NA A for replication and is encapsidated by DNA A-encoded coat protein and so has characteristics of a DNA satellite. However, systemic infection of A . conyzoides by DNA A alone is sporadic and asymptomatic, and DNA A accumul ation is reduced to 5% or less of its accumulation in the presence of DNA b eta, Therefore, DNA A and DNA beta together form a previously unrecognized disease-inducing complex. Our data also demonstrate that the nanovirus-like DNA 1 component associated with infected A. conyzoides plays no essential role in the disease and represents a satellite-like DNA, Furthermore, the s atellite DNA previously found associated with tomato leaf curl virus is pro bably a defective DNA beta homologue.