Effects of preinoculation with an avirulent isolate of Pyricularia grisea on infection and development of leaf blast lesions caused by virulent isolates on near-isogenic lines of Sasanishiki rice

Abstract

Eight near-isogenic Sasanishiki rice lines with different genes for complete resistance to rice blast were inoculated with an avirulent isolate 72 h before inoculation with a virulent isolate of Pyricularia grisea to clarify the mechanisms of induced resistance in the leaf blades. Subsequent lesions on the leaf blades were classified as brown spots (b type), as observed on Sasanishiki BL no. 8 with resistance gene Pii, or no symptoms (HR type), as observed on Sasanishiki BL no. 4 with the gene Piz-t and on the six other lines. Lesion expansion was significantly reduced in Sasanishiki BL no. 8 compared with that in Sasanishiki BL no. 4 when the leaf blades were preinoculated with a high concentration of a conidial suspension of the avirulent isolate. Moreover, after preinoculation with the conidial suspension of the avirulent isolate in silicon rings on the leaf blades, induced resistance was expressed only in areas close to the inoculation sites. These resistant areas were larger in Sasanishiki BL no. 8 than in BL no. 4. Hyphal growth was markedly inhibited in the epidermal cells of Sasanishiki BL no. 8, whereas inhibition was weak in those of Sasanishiki BL no. 4. In the epidermal cells of leaf blades of Sasanishiki BL no. 8 preinoculated with the avirulent isolate, the frequency of hyphal penetration of the virulent isolate in the presence of host cell browning decreased, as did the frequency of invading hyphae after inoculation with virulent isolates. The results indicate that induced resistance may play a role in the suppression of lesion development in the Sasanishiki near-isogenic lines and that the lines differ in the extent of suppression.