Fine mapping of qSB-11(LE), the QTL that confers partial resistance to rice sheath blight.

Sheath blight (SB), caused by Rhizoctonia solani kühn, is one of the most serious global rice diseases. No major resistance genes to SB have been identified so far. All discovered loci are quantitative resistance to rice SB. The qSB-11(LE) resistance quantitative trait locus (QTL) has been previously reported on chromosome 11 of Lemont (LE). In this study, we report the precise location of qSB-11 (LE) . We developed a near isogenic line, NIL-qSB11(TQ), by marker-assisted selection that contains susceptible allele(s) from Teqing (TQ) at the qSB-11 locus in the LE genetic background. NIL-qSB11(TQ) shows higher susceptibility to SB than LE in both field and greenhouse tests, suggesting that this region of LE contains a QTL contributing to SB resistance. In order to eliminate the genetic background effects and increase the accuracy of phenotypic evaluation, a total of 112 chromosome segment substitution lines (CSSLs) with the substituted segment specific to the qSB-11 (LE) region were produced as the fine mapping population. The genetic backgrounds and morphological characteristics of these CSSLs are similar to those of the recurrent parent LE. The donor TQ chromosomal segments in these CSSL lines contiguously overlap to bridge the qSB-11 (LE) region. Through artificial inoculation, all CSSLs were evaluated for resistance to SB in the field in 2005. For the recombinant lines, their phenotypes were evaluated in the field for another 3 years and during the final year were also evaluated in a controlled greenhouse environment, showing a consistent phenotype in SB resistance across years and conditions. After comparing the genotypic profile of each CSSL with its phenotype, we are able to localize qSB-11 (LE) to the region defined by two cleaved-amplified polymorphic sequence markers, Z22-27C and Z23-33C covering 78.871 kb, based on the rice reference genome. Eleven putative genes were annotated within this region and three of them were considered the most likely candidates. The results of this study will greatly facilitate the cloning of the genes responsible for qSB-11 (LE) and marker-assisted breeding to incorporate qSB-11 (LE) into other rice cultivars.