Enhancement of potato virus X synthesis in doubly infected tobacco occurs in doubly infected cells

Enhancement of potato virus X (PVX) synthesis by potato virus Y (PVY) or tobacco mosaic virus (TMV) in tobacco is the result of increased synthesis per cell, not an increase in the number of cells infected. Ultrastructural features typical of PVX-infected cells and those typical of PVY-infected cells were present together in a majority of cells examined by electron microscopy from leaves supporting enhancement. PVX was detected in over 70% of protoplasts prepared from infected leaves and stained with fluorescein-labeled PVX antibody; the proportion was the same for cells from doubly infected and singly infected leaves. In leaves infected with TMV and PVX, 70% of the cells were doubly infected. The fluorescence intensity of individual cells after treatment with fluorescein-labeled PVX antibody, a measure of relative virus concentration, was greater in cells from leaves doubly infected by PVX and PVY than in cells from leaves infected by PVX alone.