Comparative analysis of tissue tropism of bipartite geminiviruses

Abutilon mosaic virus (AbMV), a bipartite geminivirus of the genus Begomovi rus, has been vegetatively propagated for many years in Abutilon sellovianu m in which it is strictly phloem-restricted. Using in situ hybridization an d immunological analyses, the tissue tropism of AbMV in the laboratory host Nicotiana benthamiana was compared with that of two other bipartite begomo viruses, African cassava mosaic virus (ACMV) and tomato golden mosaic virus (TGMV). Analysis of the first systemically infected leaves and longitudina l sections of axillary and flower buds revealed that all three viruses are initially confined to the vascular traces, although both ACMV and TGMV are later detectable in nearly all tissue types. In contrast, AbMV remained str ictly phloem-limited in this host throughout the course of infection. The a bility of ACMV and TGMV to move out of N. benthamiana phloem tissues is cor related with the development of severe symptoms in comparison with the mild symptoms associated with AbMV infection. It was also demonstrated that Sid a micrantha mosaic virus, a virus that is closely related to AbMV, is phloe m-limited in Malva parviflora even though it induces severe leaf curl, stun ting and necrosis in this host. The present data demonstrate that bipartite begomoviruses can exhibit strikingly different patterns of tissue tropism.