Genetic Studies on the Resistance of Rice Plant to the Blast Fungus

1. Inheritance of resistance in crosses Sensho×H-79 and Imochi-shirazu×H-79

Abstract

Genic analysis for the blast resistance of rice plants was carried out in two varietal crosses given in the title. The reaction of a plant to a fungus isolate appeared to be conditioned by two or three genes quantitatively. From Sensho×H-79 were foud three independent genes, Rb₁, Rb₂ and Rb₃, whose alleles contributed different degrees of susceptibility cumulatively when the virulence of an isolate and the resistance of a host plant to it were each estimated by an index. For all genotypes, the index-values to the used three isolates were assumed to indicate their degree of resistance. Genotypes were divided into two groups as to each isolate, resistant and susceptible, to correspond with the ratio of the former to the latter given in experimental datum. The recombination fractions between Rb₁ and la⁺ were consistently found to be 0.09-0.10 in five data. The gene-for-gene hypothesis was thus extended to a system of quantitative relationships. The data from Imochishirazu×H-79 could also be interpreted in the same manner: Gene Rb₄ and Rb₅ were responsible for the reactions to one of the isolates, but not to the other two isolates. Rb₄ was found to be linked with la, showing 0.23 of recombination fraction.