Fine-mapping of qSB-9 TQ , a gene conferring major quantitative resistance to rice sheath blight

qSB-9 is a major quantitative trait locus that confers significant resistance to rice sheath blight (SB). Its presence near the end of the long arm of chromosome 9 has been reported in several studies. However, its precise location has not yet been determined. In this study, we report the fine-mapped location of qSB-9 TQ , the resistant allele(s) underlying qSB-9 derived from indica rice variety Teqing (TQ). We have developed a population containing 235 chromosomal segment substitution lines (CSSLs) that integrated TQ donor segments specific to the qSB-9 region in the Lemont genetic background, which is a japonica rice variety highly susceptible to SB. These CSSLs contained identical genetic backgrounds, as monitored with 111 molecular markers, and showed similar morphologies except for tiller angle (TA). We identified a gene controlling TA, TAC1 TQ , in the qSB-9 region by comparing the TA phenotype and the genotype of each CSSL. Although TAC1 TQ only showed a very mild effect on SB resistance, it affected the accurate evaluation of the contribution of qSB-9 TQ . In order to exclude the influence of TAC1 TQ for better fine-mapping of qSB-9 TQ , we selected a total of 68 CSSLs that displayed the same TA phenotype. These 68 CSSLs were evaluated for their SB resistance in the field, and the phenotyping results were consistent with minor variations across 3 years, allowing us to narrow qSB-9 TQ down to a region defined by molecular markers Y84 and Y86. Furthermore, ten of the 68 CSSLs carried chromosomal recombinations in this region. The development of new molecular markers in this region and accurate determination of the SB resistance phenotypes of these ten CSSLs by conducting both field and greenhouse tests allowed us to fine-map qSB-9 TQ to a 146-kb region defined by markers CY-85 and Y86. These results will greatly accelerate the positional cloning and breeding utilization of qSB-9 TQ .