Fine mapping and candidate gene analysis of qSB12, a gene conferring major quantitative resistance to rice sheath blight.

qSB12, a major quantitative sheath blight resistance gene originated from rice variety YSBR1 with good breeding potential, was mapped to a 289-Kb region on chromosome 12. Sheath blight (ShB), caused by Rhizoctonia solani kühn, is one of the most serious global rice diseases. Rice resistance to ShB is a typical of quantitative trait controlled by multiple quantitative trait loci (QTLs). Many QTLs for ShB resistance have been reported while only few of them were fine-mapped. In this study, we identified a QTL on chromosome 12, in which the qSB12 resistant allele shows significant ShB resistance, by using 150 BC backcross inbred lines employing the resistant rice variety YSBR1 as the donor and the susceptible variety Lemont (LE) as the recurrent parent. We further fine-mapped qSB12 to a 289-kb region by generating 34 chromosomal segment substitution lines and identified a total of 18 annotated genes as the most likely candidates for qSB12 after analyzing resequencing and transcriptomic data. KEGG analysis suggested that qSB12 might activate secondary metabolites biosynthesis and ROS scavenging system to improve ShB resistance. qSB12 conferred significantly stable resistance in three commercial rice cultivars (NJ9108, NJ5055 and NJ44) in field trials when introduced through marker assisted selection. Under severe ShB disease conditions, qSB12 significantly reduced yield losses by up to 13.5% in the LE background, indicating its great breeding potential. Our results will accelerate the isolation of qSB12 and its utilization in rice breeding programs against ShB.