Abstract
The growth curve of tobacco mosaic virus (TMV) in inoculated leaves of tobacco cv. Ambalema was far low compared to that of susceptible cv. Xanthi. The virus concentration in Ambalema leaves at 12 days after inoculation is one-eighth or less of the level attained in Xanthi leaves. The amount of TMV in Ambalema leaves increased 2.5- to 3-fold when actinomycin D was applied immediately or 1 day after inoculation, but not increased by the application at 10 days or later after inoculation, suggesting that the virus resistance of Ambalema may depend on transcription from host DNA. Ambalema protoplasts isolated from the inoculated leaves at various time showed a steady increase in the fraction of infected cells (as judged by TMV-specific fluorescent antibody staining) to a value of approximately 31.5% at 15 days after inoculation, but in the protoplasts from Xanthi, the fraction of infected cells rapidly increased to value of approximately 80% at 7 days after inoculation and remain close to that value thereafter. In this case, the level of virus accumulation per infected protoplast of Ambalema was far less than that of Xanthi. When these protoplasts were incubated for 72hr, the level of virus accumulation per infected protoplast increased 3.4- to 5.0-fold in Ambalema and 1.7- to 2.1-fold in Xanthi. The increase of the fraction of infected protoplasts and the amounts of virus were observed in protoplasts isolated from the leaves which virus multiplication were increased by treatment with actinomycin D. When protoplasts isolated from healthy leaves were infected with TMV, protoplasts from Ambalema had a percentage of infection similar to and supported a slightly lower accumulation of virus than Xanthi protoplasts. Actinomycin D did not effect the multiplication of TMV in the protoplasts isolated from both tobacco cultivars. It was concluded that the resistance of Ambalema tobacco to infection by TMV involves the reduced accumulation in cell and spread of the virus in host tissues, but this ability to reduce virus accumulation were not formed in protoplasts.
