Genetic analysis and breeding use of blast resistance in a japonica rice mutant R917

R917, a japonica rice mutant with broad-spectrum of resistance to blast, was selected after treatment of the F1 seeds from the cross between Chengte 232 and Xiushui 37 with 10 krad of 60Co γ-ray. R917 was evaluated for blast resistance in multiple years and locations. It was resistant to 136 of 138 strains of Magnaporthe grisea collected from different regions in China with a resistance frequency of 98.55%, much higher than 86.13% and 10.15% of Chengte 232 and Xiushui 37. To analyze its inheritance of resistance to blast, R917 was crossed with the susceptible cultivar, Nonghu 6, and its two parents. It had a single dominant resistance gene to Chinese races ZB13, ZC15 and ZE3, nonallelic to those of Chengte 232 and Xiushui 37. When inoculated with 7 Japanese differential strains, R917 showed the same reaction pattern as Toride 1 and differed from the other differential varieties. But R917 was different from Toride 1 in its reaction to 7 Chinese strains. Allelism test indicated that the resistance genes between R917 and Toride 1 were nonallelic to Chinese races ZC15 and ZE3. R917 was a semi dwarf with strong stem, narrow and erect leaf, tight plant type and monogenic broad-spectrum resistance to blast. Several lines with desirable agronomic traits newly bred using R917 as a donor of blast resistance had the same broad-spectrum of resistance to blast as R917, indicating there was no linkage drag between the resistance gene and other important agronomic genes in R917. R917 has been used as a donor in rice breeding programs in China.