Increased multiplication of cucumber mosaic virus in a resistant cucumber cultivar caused by actinomycin D

Abstract Multiplication of cucumber mosaic virus in the cotyledons of a resistant cucumber ( Cucumis sativus L.) cultivar (Elem) increased in the presence of actinomycin D. Compared with water-injected controls, infectivity increased 3·5 to 5·8 times when cotyledons were injected with 5 μg actinomycin D/ml from 1 day before until 1 day after inoculation. The effect decreased the longer the interval between the inoculation and the application of the antibiotic and was negligible when the antibiotic was injected 3 to 4 days after inoculation. The number of virions, observed with an electron microscope, in extracts of cotyledons injected with actinomycin D 6 h after inoculation, was also significantly higher than in control extracts. No difference in infectivity titre was obtained when actinomycin D was injected into the cotyledons of a susceptible cucumber variety. Sensitivity of the resistance mechanism to actinomycin D suggests that it depends on the transcription of cellular DNA to RNA, thereby producing a susbstance(s) that is responsible for the resistance, and that this mechanism is activated by the virus.