INVESTIGATIONS ON THE MECHANISM OF RESISTANCE TO RICE BLAST DISEASE

Resistance of rice to Pyricularia oryzae Cav.was considered in relation to environmental factors,such as temperature and humidity,which caused resistance changes by affecting host predisposition or by affecting infection and development of pathogenic fungus.Rice resistance to blast disease was associated with components of the rice plant.Siticification of the epidermis of rice was considered a major factor in physical resistance to blast;the degree of resistance was found to increase in proportion to the amount of silicon accumulated in the plants.A close correlation was found between susceptibility and bontent of soluble nitrogen or amino acids,and a rapid;browning reaction of infected tissues in resistant rice varieties was correlated with the amount of polyphenols in the plants.Further srudies showed that the rapid browning reaction of the host was a hypersensitive reaction caused by penetration of the pathogen,in which phenomenon could be observed broken cell membrane,granulated cytoplasm and disintegrated chlorplaslsin the epidermal and parenchymatous cells.These granules or particles foundin hypersensitive reactions were considered phytoalexins with which thehost arrested further growth of the invading fungus or killed it.However,resistance or pathogenicity is,in fact,a result of gene-to-gene interaction between a variety and a race.Further investigat ions on host-parasite interaction and the real roles,chemical and biological properties of those granules or particles found in rice leaf tissues infected with P.oryzae will be helpful for the perfect understanding of the resistance mechanism and functions of resistant genes in rice plant.