Plant viruses establish a systemic infection by moving through plasmod
esmata, but little is known of the mechanism(s) involved. The roles of
two movement-associated proteins of a single-stranded DNA virus were
investigated in vivo, using functional proteins expressed in E. coli a
nd microinjection into plant cells. We report here that the all protei
n of bean dwarf mosaic geminivirus moves extensively from cell to cell
, increases mesophyll plasmodesmal size exclusion limit, and potentiat
es the movement of double-stranded DNA from cell to cell. Movement of
single and double-stranded DNA out of the nucleus is mediated by the B
R1 protein. These results provide direct experimental evidence for int
ercellular macromolecular transport in plants, and suggest that the BR
1 and BL1 proteins coordinate the movement of viral DNA across both nu
clear and plasmodesmal boundaries.