Abstract
The 32000 mol. wt. non-structural protein (P3) of alfalfa mosaic virus (AlMV) has previously been shown to accumulate in the cell wall fraction of tobacco leaves infected with AlMV. We now report the ultrastructural location of this protein. P3 was visualized immunocytochemically in the middle lamella of the walls of those parenchymal or epidermal cells which had just been reached by the infection front and in which viral multiplication had just begun. P3 was not found when AlMV had accumulated to high levels in infected cells. These findings support the concept that P3 is involved in the spread of viral infection from cell to cell, i.e. is the transport factor of AlMV.