Sela, B., The Volcani Institute of Agricultural Research, Bet Dagan, Israel
Loebenstein, G., The Volcani Institute of Agricultural Research, Bet Dagan, Israel
Van Praagh, T., The Volcani Institute of Agricultural Research, Bet Dagan, Israel

The size and extractable infectivity of local lesions incited by tobacco mosaic virus (TMV) on Nicotiana glutinosa L. leaves, and TMV multiplication in Cucumis sativus L. cotyledons, increased significantly in the presence of chloramphenicol. In cucumber cotyledons virus concentration increased 2 and 4 times when cotyledons were injected with 100 μg/ml or 200 μg/ml chloramphenicol, respectively, from 1 day before until 1 day after inoculation, even though the number of starch lesions was markedly reduced. Injections 2 days before or 3 days after inoculation did not affect virus concentration. In N. glutinosa leaves injected with chloramphenicol (200 μg/ml) 24 hours after inoculation, lesion size was more than 3 times greater than that of controls. Subsequent applications of the antibiotic were less effective. The increase in size of lesions in N. glutinosa was correlated with their extractable infectivity. We suggest that chloramphenicol applied near the time of inoculation partially inhibits the mechanism responsible for localizing infection in hypersensitive hosts. These results also strengthen our previous suggestion (Loebenstein et al., 1969) that the localizing mechanism which requires DNA-dependent RNA synthesis (sensitivity to actinomycin D) produces a substance(s), probably a protein (sensitivity to chloramphenicol), that is responsible for localizing the virus. © 1969.