Determination of Resistance Spectra of the Pi-ta and Pi-k Genes to U.S. Races of Magnaporthe oryzae Causing Rice Blast in a Recombinant Inbred Line Population.

Molecular tagged resistance (R) genes are useful for developing improved cultivar resistance using marker-assisted breeding. In the present study, R genes to common races of Magnaporthe oryzae, the causal agent of blast disease of rice (Oryza sativa), were mapped using an F recombinant inbred line (RIL) population derived from a cross of tropical japonica cv. Katy with breeding line RU9101001. Katy was resistant to 10 common U.S. races: IA-45, IB-1, IB-45, IB-49, IB-54, IC-17, ID-1, IE-1, IG-1, and IH-1 of M. oryzae. RU9101001 was resistant to races IA-45, IB-45, IB-54, IG-1, and IH-1. Katy and RU9101001 were susceptible to race IE-1k. Twenty-three polymorphic simple sequence repeat (SSR) markers were used to map R genes. Segregation ratios of 1:1 (resistant/susceptible) to races IB-1, IB-49, IC-17, ID-1, and IE-1 indicated the presence of a single dominant R gene in Katy. Ratios of 3:1 (resistant/susceptible) to races IA-45, IB-45, IG-1, and IH-1 indicated that a single R gene was present in Katy and a different R gene was present in RU9101001. Resistance to the abovementioned races was correlated with the presences of the Pi-ta gene and 11 Katy SSR alleles, suggesting that Pi-ta confers resistance to IA-45, IB-1, IB-45, IB-49, IC-17, IG-1, ID-1, IE-1, and IH-1. Katy, RU9101001, and all RILs were resistant to race IB-54, which was consistent with the presence of Pi-k in Katy and Pi-k in RU9101001. Resistance to IA-45, IB-45, IG-1, and IH-1 correlated with the presence of Pi-k, suggesting that Pi-k confers resistance to IA-45, IB-45, IG-1, and IH-1. These data suggest that Pi-ta and Pi-k are effective R genes with overlapped resistance to the 10 common races of M. oryzae.