Comparison and Confirmation of Quantitative Trait Loci Conferring Partial Resistance to Rice Sheath Blight on Chromosome 9

Sheath blight (SB), caused by Rhizoctonia solani, is one of the worst rice (Orzya sativa) diseases worldwide. Resistance to the SB disease in rice is a complex trait controlled by quantitative trait loci (QTLs). Through map integration, we found several previously identified SB resistance (SBR) QTLs reported in inconsistent regions on the long arm of chromosome 9. Five of them were detected on 'Jasmine 85' (J85), 'Minghui 63' (MH63), and 'Lemont' (LMNT) rice and were designated qSB-9(J85)-1, qSB-9(J85)-2, qSB-9(MH63)-1, qSB-9(MH63)-2, and qSB-9(LMNT), respectively, in the present study. To further verify and physically map the five potential SBR QTLs, we introduced these SBR QTLs into a common susceptible variety (LMNT) and developed a few chromosomal segment substitution lines through marker-assisted selection. After artificial inoculation with the SB fungus, we were able to validate qSB-9(J85)-2 but not the other four SBR QTLs; whereas, on MH63, an SBR QTL designated qSB-9(MH63)-3 was confirmed in the region defined by markers Y83 and Y91.8 that included qSB-9(J85)-2, covering approximately 1,235 kb. Both qSB-9(J85)-2 and qSB-9(MH63)-3 appeared to be dominant resistance genes and contributed to similar levels to SB resistance, reducing SB disease severity by approximately 1.0 on a 0-to-9 SB disease rating system. After comparing with another confirmed SBR QTL (qSB-9(TQ)) from 'Teqing' rice (TQ), we conclude that qSB-9(J85)-2, qSB-9(MH63)-3, and qSB-9(TQ) are probably controlled by the same allelic resistance genes. These results will accelerate the utilization of this major SBR QTL and its map-based cloning.