Abstract
The host-parasite interaction of rice blast disease was examined periodically by a sliced-sheath inoculation method. The course of infection in the sliced-sheath tissues of three varieties, carrying individually a resistance gene Pi-k, Pi-ta or Pi-i, to Pyricularia oryzae Cavara, was observed after inoculation with race 047 of this fungus. Race 047 penetrated into the inner epidermal cells of leaf sheath, regardless of the resistance or susceptibility of the varieties used. After penetration, characteristic changes in the processes of hyphal development and cell response became obvious, and the reactions were classified into three types, i.e. highly resistant (HR), resistant (R) and susceptible (S) reactions. In HR reaction the fine particles became visible in the cells soon after penetration (fine granulation) and the infection hyphae failed to develop from the penetrated cell into the adjacent cells. Later the finely granulated cells became pale yellow or slight brown throughout the cell and generally did not brown deeply. In R reaction the hyphal development was observed for 8-12hr after penetration as well as in S reaction. Then the fine particles became visible, and followed by the larger coarse particles (coarse granulation). With the appearance of cytoplasmic granulation, hyphal growth was gradually suppressed and the infection hyphae extended very slowly into the surrounding cells. In S reaction the penetrated cells almost remained visibly unchanged for 20hr after penetration in spite of vigorous hyphal growth, and then later became granulated. In R and S reactions, slightly and/or deeply browned particles were finally observed throughout the cells. These results suggested that the resistance expression was more closely related with the occurrence of cytoplasmic granulation rather than that of deep browning in the infected cells. Moreover, two resistant reactions, HR and R, should be strictly distinguished on the study on resistance mechanism in rice blast disease.
