Pathological Cytology of Tobacco Leaf Infected with Tobacco Mosaic Virus III

The idea that chloroplasts are the sites of virus formation is not new. Since Kausche and Ruska (4) published an electron micrograph showing filamentous particles of tobacco mosaic virus associated with chloroplast fragments, the speculation that virus particles are formed within chloroplasts and released from them into the surrounding cytoplasm, has been widely supported by successive investigators (1, 5, 8, 10). Since these investigations were done at a time when fixation methods of tissue cells were not entirely satisfactory, it may be desirable to repeat these investigations by using more effective techniques. In the present study, the intracellular localization of virus particles within host cells was investigated by the electron microscopy of thin sections of young tobacco leaf (Nicoliana tabacum L. var. Xanthi) showing slight symptoms of systemic infection with tobacco mosaic virus of common strain. The specimens were fixed at 2°C. for 2 hours in a solution of 1 per cent osmium tetroxide buffered with veronal acetate to pH 7.4. These were subsequently dehydrated and embedded ill butyl methacrylate. Thin sections were cut with a JUM-4 ultramicrotome (Japan Electron Optics Laboratory Co., Ltd.) and were examined in a JEM-5G electron microscope (Japan Electron Optics Laboratory Co., Ltd.) without removal of the embedding medium. In Fig. 1, the chloroplasts of slightly diseased leaf cells are built up of regularly organized lamellae and grana embedded in a granular matrix. These structural characteristics correspond exactly to those found in the chloroplasts of healthy tobacco leaves (2, 6) and of some other higher plants (3, 9). The fibrous masses of virus rod particles can be easily recognized within the cytoplasm. In these fibrous masses, the individual virus rod particles are generally disposed parallel to one another. Although the virus rod particles come close to the surface of the chloroplasts, they were never found within the latter. A cytoplasmic region showing more details than Fig. 1 is given in Fig. 2.