Mapping of Rsn2, a locus conferring rice tolerance to toxin from the causal agent of rice sheath blight Rhizoctonia solani

Sheath blight (SB), caused by Rhizoctonia solani K & uuml;hn, is one of the most serious rice diseases and poses a major threat to rice grain yield. Owing to the lack of major SB resistance genes and difficulties in phenotyping, progress in identifying and characterizing QTLs conferring SB resistance has greatly improved. R. solani may secrete a toxin (RS toxin) while infecting rice hosts, which is considered an important determinant of virulence. In this study, we found that the SB-resistant variety YSBR1 has a stronger tolerance to the RS toxin than the SB-susceptible variety Lemont, as reflected by the apparently longer rice radicle of YSBR1 than that of Lemont after treatment with the RS toxin. Subsequently, we verified the significant positive correlation between RS toxin sensitivity and disease susceptibility using an F2 population developed from a cross between YSBR1 and Lemont. Using this F2 population, we preliminarily mapped two new RS toxin tolerance QTLs on chromosomes 6 and 7, named Rsn2 and Rsn3, respectively. With the aid of three chromosome segment substitution lines, we further mapped Rsn2 into a 4.6 cM region and confirmed that introgression of Rsn2 also enhanced rice SB resistance. Collectively, these results are useful for further isolation of Rsn2 and elucidation of the mechanism of rice tolerance to both R. solani and its pathogenic factor, the toxin.