[in Japanese]

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[in Japanese]

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[in Japanese]

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Callus cultures were derived from rice seedlings by growing on Murashige-Skoogs medium containing 2.2ppm of 2.4-Dichlorophenoxyacetic acid. The callus of cultivars, Te-Tep and Norin 20 turned brown 24hr to 48hr after inoculation with Pyricuralia oryzae P₂, Cochliobolus miyabeanus, Aspergillus oryzae or other pathogenic and non-pathogenic fungi. P. oryzae formed appressoria and infection pegs on rice callus 24-48hr after inoculation. C. miyabeanus, invaded tissues without forming apparent appressorium. A saprophytic fungus, A. oryzae, could also invade the callus tissues. Serial sections of infected callus tissues were prepared. Brown granules or paste^* like materials were present on the outermost layer of callus tissues together with spores and mycelia. Degeneration of nuclei or cytoplasm of callus tissues was not observed at the early stage of fungus infection. Increase of peroxidase activity was observed in Te-Tep and Norin 20 after the inoculation with P. oryzae P₂. But neither P. oryzae nor A. oryzae caused an increase in the rate of oxygen uptake of Te-Tep and Norin 20. It appears that callus tissues of rice turned susceptible to P. oryzae and to other fungi during consequtive transfers to fresh medium.

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Cowpea leaves infected with mulberry ringspot virus (MRSV) or satsuma dwarf virus (SDV) at different growth stages were examined under an electron microscope. In the MRSV infected leaf primordia single rows of virus particles were found in the plasmodesmata of the cells near the apical points. The rows of particles were thrusted into the cytoplasm at one side of the plasmodesmata. In leaves 2-5mm, the thrusted rows were bounded with invaginated plasmalemma. Tubules thus developed grew longer with the growth of leaf, up to 5μm in length, and cell walls were protruded deeply into the vacant spaces caused by invagination of plasmalemma.
In SDV infected cowpea plants, rows of virus particles in the plasmodesmata appeared for the first time in leaves 1mm in length and tubules developed in the similar process as observed in the MRSV infected cells.
Cytoplasmic inclusions consisting of tangled endoplasmic reticulum occurred in the cowpea leaf cells infected with MRSV or SDV. Similar tubules and inclusions occurred in the leaf cells of sesame, kidney bean, petunia, and Nicotiana clevelandii plants infected with SDV. The tubules were found also in the spaces between the plasmalemma and cell wall in the cells of N. clevelandii and kidney beans. The walls of these tubules were joined to plasmalemma. The tubules containing single rows of virus particles were observed in dip preparations of infected leaves.

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Rice leaves produce unidentified ethyl acetate-extractable antibacterial substances in response to infection by Xanthomonas oryzae. This study aims at elucidating the role of these substances in the resistance of leaves against pathogen. In leaves inoculated with incompatible strains, antibacterial substances were detected as early as 24hr post-inoculation, increased rapidly to the maximum level within 3-5 days, and remained constant thereafter. Bacterial multiplication in leaves were inhibited as these substances accumulated, suggesting that they play a significant part in resistance expression. In leaves inoculated with compatible strains, on the contrary, these substances were not detected until lesions became visible, increased gradually as lesions enlarged, and reached a considerably high level at the later stage of pathogenesis. These antibacterial substances were also found in a small quantity in leaves that had not been inoculated. These results were discussed in relation to the development of disease syndrome.

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Seed coat of bottle gourd, Lagenaria siceraria STANDL, remained, in most cases, attached to the pegs of seedlings after germination and serves as primary inoculum for the increment of inoculum potential of Fusarium oxysporum f. sp. lagenariae. Infection rate of seedlings with pegs accompanied by seed coat was 14-18%, while that of seedlings having their seed coats attached to cotyledons was 2-3%. Among various parts of seedlings, the density of Fusarium propagules was highest in pegs followed by roots. When roots or pegs were inoculated, the rate of disease outbreak was as high as 90-100% irrespective of inoculation methods. When hypocotyls or cotyledons were wound-inoculated, seedlings were infected, but practically no disease occurred when these parts were inoculated without wounding. Thus the pegs and roots were the major sites for the penetration by the fungus. Histological observation indicated that the fungus penetrates lower side epidermis of the pegs and extends its hyphae throughout the tissues of the pegs. The results suggest that in the process of germination of seeds the fungus latent in the infected seed coat multiplies in the tissues to build up the inoculum potential to a level that is sufficient for establishing infection and that the penetration of the fungus to seedling takes place mostly from the lower side epidermis of pegs.

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Rice waika virus (RWV) was purified by chloroform clarification and three cycles of differential centrifugations followed by sucrose density gradient centrifugation. RWV particles were spherical ones of about 30nm in diameter and showed an UV absorption spectrum characteristic of a nucleoprotein. The sedimentation coeffcient of the virus determined by analytical centrifugation was about 172S. Antiserum with the titer of 1/1, 024 for the agar gel diffusion test was prepared against RWV. By electron microscopy of thin sections, the presence of RWV particles was found to be restricted to phloem cells, in which two types of inclusion bodies and small vesicles were also observed. At the early stage of RWV infection, electron-dense crystalline inclusions showing latticed or tubular structures of protein nature developed, and the virus particles were produced first at their surface, and later within them. As the virus multiplication advanced, the crystalline inclusions changed to become electronlight granular inclusions, another type of inclusion body, which were easily found in RWV-infected cells and always associated with the virus particles. RWV particles gradually increased in number and accumulated as virus aggregates in the granular inclusions. The small vesicles developed around the inclusion bodies, and more often in the cytoplasm along and near the cell lumen. They contained fibrous materials like nucleic acid strands. Subsequently the virus particles were released from the inclusion bodies into the cytoplasm and vacuoles. From these observations the inclusion bodies were thought to be viroplasms. The virus particles in vacuoles often showed crystalline arrays. Phloem necrosis was very common, and accumulation of starch grains due to the phloem necrosis was often observed in chloroplasts in mesophyll cells.

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Two kinds of rough colony type mutant were obtained at a high rate from Pseudomonas mori isolates cultured in modified King's liquid medium for two months. These rough isolates showed a spontaneous agglutination reaction in distilled water, deionized water and physiological saline, indicating that the occurrence of agglutination has a close relation with both filamentous form and motility of them.

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Kasugamycin-resistant strains of rice blast fungus, Pyricularia oryzae, were isolated with a high frequency from the samples collected from those districts where kasugamycin had been used consecutively as a sole anti-blast fungicide, whereas no resistant strain was found in the samples collected from the districts where kasugamycin had been used by mixing with fthalide or alternately with organophosphorus fungicides. Resistant strains occur in conidia formed on lesions with a frequency below 1/10⁵ by spontaneous mutation irrespective of kasugamycin application and their population density becomes higher under the selection pressure of kasugamycin.

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