Identification of Two Major Rice Sheath Blight Resistance QTLs, qSB1-1HJX74 and qSB11HJX74, in Field Trials Using Chromosome Segment Substitution Lines

Sheath blight (SB) is among the most destructive rice (Oryza sativa) diseases worldwide. SB resistance (SBR) is controlled by quantitative trait loci (QTL). Only a few SB resistance QTLs were confirmed previously in field trials that were independent of morphological traits, a crucial factor in plant breeding. Here, we employed 63 chromosome segment substitution lines (CSSLs) to identify SBR QTLs derived from 'HJX74'. Importantly, these CSSLs all carried the same genetic background as 'HJX74', except in the substituted segment introgressed from susceptible 'Amol3(sona)'. In contrast to most reports that mapped SBR QTLs under complex genetic backgrounds, this approach allowed many CSSLs to consistently retain the agronomic traits of 'HJX74' with moderate resistance, giving the needed high reproducibility in SBR scoring. We have identified five SBR QTLs in field tests. Two of them, qSB11(HJX74) and qSB1-1(HJX74), conferred the greatest reduction in SB ratings by approximately 0.9 to 1.2 on a 0 to 9 scale. qSB11(HJX74) exhibited nearly perfect recessive heredity, whereas qSB(HJX74) 4 showed dominant heredity. Using a secondary F-2 population and overlapping substitution segment lines, we further mapped qSB11(HJX74) and qSB1-1(HJX74) to regions of approximately 430 and 930 kb, respectively. The results will accelerate the rice breeding process for resistance to SB disease.