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Central Research Institute, The Japan Tobacco and Salt Public Corporation, 6-2 Umegaoka, Midori-ku, Yokohama 227, Japan
Tobacco mesophyll protoplasts were successfully inoculated with tobacco necrotic dwarf virus (TNDV), a virus closely related to potato leafroll virus. The optimum conditions for infection as assessed by staining with fluorescent antibody to virus particles were 1 µg/ml TNDV, 1 µg/ml poly-L-ornithine, 0.025 M-phosphate buffer, pH 5.4, and 1 to 2 x 105/ml protoplasts: this combination resulted in 70 to 80% infection. Accumulation of virus antigen was first detected 12 h after inoculation, and both the number of fluorescing protoplasts and intensity of fluorescence increased markedly during the following 20 h. Virus antigen was distributed throughout the cytoplasm. Electron microscopy of thin sections revealed groups of virus-like particles in the cytoplasm and a few similar particles in the nucleus. Virus yield was estimated to be 7.8 x 105 particles per infected protoplast at 30 h after inoculation. Accumulation of virus antigen was completely inhibited by cycloheximide, but was not affected by actinomycin D or chloramphenicol, when they were applied 2 h after inoculation. Studies on the replication of aphidborne, phloem-limited viruses should be greatly facilitated by using the mesophyll protoplast system.
Received 31 August 1978;
accepted 26 September 1978.
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