Comparative Analysis of Tissue Tropism of Bipartite Geminiviruses

Abutilon mosaic virus (AbMV), a bipartite geminivirus of the genus Begomovirus, has been vegetatively pro- pagated for many years in Abutilon sellovianum in which it is strictly phloem-restricted. Using in situ hybridization and immunological analyses, the tissue tropism of AbMV in the laboratory host Nicotiana benthamiana was compared with that of two other bipartite begomoviruses, African cassava mosaic virus (ACMV) and tomato golden mosaic virus (TGMV). Analysis of the ®rst systemically infected leaves and longitudinal sections of axillary and ¯ower buds revealed that all three viruses are initially con®ned to the vascular traces, although both ACMV and TGMV are later detectable in nearly all tissue types. In contrast, AbMV remained strictly phloem-limited in this host throughout the course of infection. The ability of ACMV and TGMV to move out of N. benthamiana phloem tissues is correlated with the development of severe symptoms in comparison with the mild symptoms associated with AbMV infection. It was also demon- strated that Sida micrantha mosaic virus, a virus that is closely related to AbMV, is phloem-limited in Malva parvi¯ora even though it induces severe leaf curl, stunting and necrosis in this host. The present data demonstrate that bipartite begomoviruses can exhibit strikingly di€erent patterns of tissue tropism.