Quantitative trait locus analysis of resistance to panicle blast in the rice cultivar Miyazakimochi

Takeaki Ishihara¹, Yuriko Hayano-Saito^1*, Shinichi Oide^1,2, Kaworu Ebana³, Nghia Tuan La⁴, Keiko Hayashi¹, Taketo Ashizawa¹, Fumihiko Suzuki¹ and Shinzo Koizumi^{1 , 5}

¹National Agricultural Research Center, National Agriculture and Food Research Organization, 3-1-1 Kannondai, Tsukuba, Ibaraki 305-8666, Japan.

²Present address: Molecular Microbiology and Biotechnology group, Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizugawa, Kyoto 619-0292, Japan.

³National Institute of Agrobiological Sciences, 2-1-2 Kannondai, Tsukuba, Ibaraki 305-8602, Japan.

⁴National Plant Resources Center, An Khanh, Hoai Duc Hanoi, Vietnam. ⁵Present address: Technical Adviser, Tsukuba International Center, Japan International Cooperation Agency, 3-6 Koyadai, Tsukuba, Ibaraki 305-0074, Japan.

Background: Rice blast is a destructive disease caused by Magnaporthe oryzae, and it has a large impact on rice production worldwide. Compared with leaf blast resistance, our understanding of panicle blast resistance is limited, with only one panicle blast resistance gene, Pb1, isolated so far. The japonica cultivar Miyazakimochi shows resistance to panicle blast, yet the genetic components accounting for this resistance remain to be determined.

Results: In this study, we evaluated the panicle blast resistance of populations derived from a cross between Miyazakimochi and the Bikei 22 cultivar, which is susceptible to both leaf and panicle blast. The phenotypic analyses revealed no correlation between panicle blast resistance and leaf blast resistance. Quantitative trait locus (QTL) analysis of 158 recombinant inbred lines using 112 developed genome-wide and 35 previously reported polymerase chain reaction (PCR) markers revealed the presence of two QTLs conferring panicle blast resistance in Miyazakimochi: a major

QTL, qPbm11, on chromosome 11; and a minor QTL, qPbm9, on chromosome 9. To clarify the contribution of these QTLs to panicle blast resistance, 24 lines homozygous for each QTL were selected from 2,818 progeny of a BC₂F₇ backcrossed population, and characterized for disease phenotypes. The panicle blast resistance of the lines harboring qPbm11 was very similar to the resistant donor parental cultivar Miyazakimochi, whereas the contribution of qPbm9 to the resistance was small. Genotyping of the BC₂F₇ individuals highlighted the overlap between the qPbm11 region and a locus of the panicle blast resistance gene, Pb1. Reverse transcriptase PCR analysis revealed that the Pb1 transcript was absent in the panicles of Miyazakimochi, demonstrating that qPbm11 is a novel genetic component of panicle blast resistance.

Conclusions: This study revealed that Miyazakimochi harbors a novel panicle blast resistance controlled mainly by the major QTL qPbm11. qPbm11 is distinct from Pb1 and could be a genetic source for breeding panicle blast resistance, and will improve understanding of the molecular basis of host resistance to panicle blast.

Full text: Quantitative trait locus analysis of resistance to panicle blast in the rice cultivar Miyazakimochi