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

The size of local lesions incited by tobacco mosaic virus (TMV) on Phaseolus vulgaris L. var. Pinto leaves and TMV multiplication in Cucumis sativus L. cotyledons increased significantly in the presence of actinomycin D. In Pinto bean leaves injected with actinomycin D prior to TMV inoculation, the average diameter of lesions was 2.5 times greater than that of controls. When the antibiotic was applied subsequent to TMV inoculation, the effect on lesion size decreased; it was negligible from the third day onward. A similar trend was observed with lesions on Nicotiana glutinosa L. and N. tabacum L. var. Samsun NN leaves, although the relative increase in lesion size caused by the antibiotic was much smaller than that observed on Pinto beans. In cucumber cotyledons, virus concentration increased 4 or 11 times when cotyledons were injected with 5 μg/ml or 10 μg/ml actinomycin D, respectively, 1 day after inoculation, even though the number of visible starch lesions was reduced. Injections 3-4 days after inoculation did not affect virus concentration. We suggest that actinomycin D applied close in time to the inoculation partially inhibits the formation of the mechanism responsible for the localization of the infection in hypersensitive hosts. These results also strengthen the idea that localization of virus and acquired resistance are caused by similar mechanisms. © 1969.