Repeated electron microscopic examinations of thin sections of young leaves and shoots from mulberry tree infected naturally or artificially with dwarf disease, have failed to show any such uniform particles, spherical or elongated, as have been described for plant viruses in the past. Presence of specific, pleomorphic bodies, however, have been demonstrated consistently in the siebe tubes and occasionally also in the phloem-parenchyma cells. These bodies are spherical to irregularly ellipsoidal in shape, and 80 to 800mμ in diameter. They possess a two-layered limiting membrane of about 8mμ in thickness, instead of cell wall. The smaller bodies, 100∼250mμ in diameter, are nearly round, and generally filled with ribosome-like granules of about 13mμ in diameter. Sometimes net-strands similar to those found in the nuclear regions of other bacteria were located in the less electron dense area. The larger bodies are occupied by a large central vacuole surrounded with ribosome-like granules at the periphery. Frequently, structures similar to nuclear net-strands are observed inside the vacuolated area.The gross morphology and fine structure of these bodies seem to be similar to the descriptions of either the cells of Mycoplasma species (Pleuropneumonia-like organisms) or agents of Psittacosis-Lymphogranuloma-Trachoma group as given by Domermuth et al. (1964), and others, though any agents of such groups have as yet not been reported from plants, so far as we know. Most of the smaller bodies may correspond to the “elementary bodies”. Occasionally, budding-like protrusion or constriction of the larger bodies, suggestive of small body formation, were observed. The diversity in the size of the bodies found simultaneously in the phloem may represent their developmental stages. Moreover, therapeutic effectiveness of tetracyclines to mulberry dwarf (Ishiie et al., 1967), and disappearance of the specific bodies in the phloem of the plants recovered by tetracycline treatment, may provide an evidence in favor of Mycoplasma hypothesis. In view of the constant association of the organism in considerable amounts in the phloem of dwarfed plants, the consistent absence of those in healthy plants, the failure to demonstrate particles of any true virus nature, and the apparent sensitivity to tetracycline, it is suggested that the Mycoplasma-like organism described above may be the causal agent of mulberry dwarf disease, although further experiments are necessary for an undisputed proof of etiology and taxonomy.Presence of similar Mycoplasma-like organisms in the phloem tissues has been confirmed in the preparations from witches' broom potato, witches' broom paulownia, and petunia infected with aster yellows. Negative results were obtained in those from check plants. An attempt to isolate and culture the organisms in question on artificial media is now in progress.These results suggest that reexaminations of the causal agent would be desirable for the yellows and witches' broom group which are transmitted by leafhoppers, or by grafting, and in which it is difficult to detect “typical virus particles”.
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Nov 29, 2010
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