Enhancement by potato virus Y of potato virus X synthesis in doubly infected tobacco depends on the timing of invasion by the viruses

The time required for movement of potato virus Y (PVY) and two strains of potato virus X (PVX) from an inoculated lower leaf of small tobacco plants to the fourth leaf above, where maximal enhancement of PVX synthesis occurs, was determined. PVY required 12 hr longer than one PVX strain and 6 hr longer than the other; timing was from the time of inoculation of lower leaves. The rates of movement of the two viruses were the same in singly or doubly inoculated plants. Lower leaves were then inoculated with PVX and PVY at times so that invasion of fourth leaves by the two viruses occurred at predicted intervals. Maximum enhancement of one PVX strain occurred when PVY invaded fourth leaves 12 hr before PVX; enhancement of the other PVX strain occurred when PVY invaded 6 hr after PVX. Enhancement was always less than maximal when invasion by either virus preceded invasion by the other by more than 24 hr. These results were interpreted as indicating that enhancement occurs only in cells invaded by the two viruses within a relatively short period of time and that maximum enhancement results when critical stages in the replication cycle of each virus coincide; in such cells replication of both viruses was probably occurring simultaneously.