Studies on the Resistance of Melon Cultivars to Cucumber Mosaic Virus: (I) Virus Multiplication in Leaves or Mesophyll Protoplasts from a Susceptible and a Resistant Cultivars

A susceptible melon cultivar Earl's (Cucumis melo L. var. reticulatus) reacted to cucumber mosaic virus (CMV) by producing diffuse chlorotic spots on inoculated leaves, which supported a high level of virus multiplication. By contrast, the inoculated leaves of a resistant cultivar Kohimeuri (C. melo L. var. makuwa), produced smaller chlorotic spots, which did not enlarge even at late stage of infection, and the leaf extracts had much lower infectivity than those of Earl's. The fluorescent antibody staining of epidermis or sections from inoculated leaves showed that the virus infected cells in Earl's were increased with time after inoculation, while those in Kohimeuri were localized within a small leaf area. The virus multiplication and distribution in Kohimeuri leaves, however, were markedly enhanced in the presence of actinomycin D. When the mesophyll protoplasts isolated from each cultivar were inoculated with CMV-RNA by the electro-transfection method, similar pattern of virus multiplication was obtained. These results suggested that this type of resistance operates mainly at the intercellular level of response after virus infection.