Studies on the Pathological Anatomy of Rice Plants Infected by Helminthosporium oryzae Breda de Haan

Abstract

The present paper describes the fine structure of hyphal cells of Helminthosporium oryzae which invaded the coleoptile of two rice varieties, Kameji and Shiga Asahi 27, resistant and susceptible to the brown leaf spot respectively.Leaf tissues were fixed in KMnO4 (2% in distilled water) and postfixed in a 1% solution of phosphate buffered OsO4.1. In the susceptible variety the hyphae of the fungus remarkably increased its diameter after penetrating the cuticle of epidermal cell walls, and developed through the middle layer of radial walls of epidermal cells and then, in most cases, intruded into epidermal cells.2. The cell wall of young immature hyphae which invaded the parenchymatous cells of the resistant and susceptible varieties consisted of only a mucilaginous inner layer, similar to that of immature germ tubes. However, two-layered cell walls were observed in the matured hyphae, electron-dense outer layer (about 0.12μ in thickness) and electron-transparent inner layer (about 0.20μ). Vacuoles in old hyphae developed extensively, some reaching 3μ in diameter, and occupied a greater part of the cytoplasm.3. In the susceptible variety accumulation of electron-dense substances, 0.5μ thickness was recognized around the hyphae in the radial wall of epidermal cells. This might be a response product of the host cell to the exudate of the fungus.4. In the parenchymatous tissues of the susceptible variety the hyphal tips swelled again after penetrating the cell wall by a slender hypha, about 1μ in diameter. In these newly formed hyphal cells many mitochondria and lipid bodies were recognized 5. In an affected epidermal cell of coleoptiles of the resistant variety, substances which appear to be due to the degeneration of cytoplasm of the host cells were recognized around the hyphae. Less amount of these substances was detectable around the hyphae in the susceptible variety.6. The hyphae of the causal fungus in coleoptile of the resistant variety were generally slender, about 1μ in diameter. They had large vacuoles in cytoplasm, and finally they showed a vacuous appearance retaining only the outer layer of cell walls.7. In the resistant variety, Kameji, chloroplasts in parenchymatous cells at first swelled after the invasion of the causal fungus. Thereafter, the intergrana-lamellae expanded and the grana-lamellae changed into many small vesicles, 0.1-0.5μ in diameter. Thus the chloroplasts were finally degenerated. This degenerative process is referred to so-called vacuolar degeneration.