Genetic requirements for local and systemic movement of tomato golden mosaic virus in infected plants.

Tomato golden mosaic geminivirus (TGMV) has two DNA components, A and B. Replication of DNA A can be detected in inoculated leaves, but DNA B is additionally required for virus movement in planta. Using viral DNA accumulation as an indication of the number of infected cells, we show here that both the BL1 and BR1 genes are necessary for local TGMV movement. We also demonstrate that transient expression of BL1 and BR1 together allows wild-type TGMV DNA A to move systemically. When the transient movement assay was used to analyze various A component mutants, all were found to move locally in inoculated leaves, and only an ar1 (coat protein) mutant was unable to move systemically. In addition, we confirm that a TGMV al2 (AR1 and BR1 trans-activator) mutant has a defect in local movement which can be rescued by provision of exogenous BR1, but not BL1. Finally, we show that the ability of TGMV coat protein mutants to accumulate single-stranded (ss) DNA is dependent on BR1. These results provide experimental evidence obtained in planta which supports three predictions of published models for bipartite geminivirus movement: (i) BL1 and BR1 have distinct and essential roles in cell-to-cell movement as well as systemic movement; (ii) BR1 may interact with viral ssDNA in vivo; and (iii) AL2 is indirectly required for movement through its effect on BR1 expression. In addition, our data suggest that specific models of bipartite geminivirus systemic movement should accommodate a role for the coat protein.