A marked synergistic interaction of potato viruses X and Y in inoculated leaves of tobacco.

Abstract

Abstract When the outer half of a half-leaf of Turkish tobacco (Nicotiana tabacum L.) was inoculated with potato virus X (PVX) and a similar area of the opposite half-leaf was inoculated with potato virus Y (PVY) on the same day, severe necrosis and stimulated PVX synthesis occurred along the midvein in the uninoculated area adjacent to the strip inoculated with PVY. When the inoculation with PVX was made at different intervals prior to that with PVY, the necrosis appeared progressively farther from the PVX-inoculated area as the interval between the inoculations was increased. Increasing the interval between inoculations made in the reverse sequence resulted in the necrosis appearing progressively farther from the PVY-inoculated area. Inoculation of entire tobacco leaves with a 1:1 mixture of juice from PVX-infected tobacco plants and juice from PVY-infected tobacco plants resulted in only mild synergism. When the concentration of PVY was held constant and the concentration of PVX was reduced by tenfold steps to 0.001 of that in the original mixture, symptom severity increased progressively until more than 80% of the leaf became necrotic. Despite the increase in symptom severity, juice from those leaves with the most severe symptoms contained no more than 2.5 times as much PVX as did juice from comparable leaves inoculated with the corresponding concentration of PVX without PVY. Moderate synergism in symptomatology resulted when tobacco leaves systemically invaded by PVY were inoculated with PVX, but little or no synergism occurred when PVX was the systemic virus and PVY the one applied directly. The data are interpreted as indicating that the prime requisite for the strong PVX-PVY interaction is that a cell must be invaded first by PVY and then by PVX while PVY is still undergoing active multiplication. It is suggested that infection by PVX normally induces reactions that act to limit PVX synthesis but is unable to do so in a cell in which PVY is actively multiplying.