Phenotypic gain from introgression of two QTL, qSB9-2 and qSB12-1, for rice sheath blight resistance

Abstract

F2:3 families from crosses between three rice indica introgression lines and their common japonica recurrent parent were used to evaluate two quantitative trait loci (QTL) for sheath blight (SB) resistance. Three selected TeQing-into-Lemont backcross introgression lines (TILs) were more resistant than their susceptible parent (Lemont) in inoculated field plots, and were molecularly verified to contain TeQing alleles at qSB9-2 and/or qSB12-1. F2 individuals homozygous for qSB9-2 and qSB12-1 provided F2:3 families that fit four genotypic classes: containing the resistant TeQing allele for qSB9-2TQ alone, qSB12-1TQ alone, both qSB9-2TQ and qSB12-1TQ, and neither SB QTL introgression. By comparing the SB resistance of these four genotypic classes in micro-chamber evaluations and inoculated field plots, the phenotypic values of the QTL were measured. Under both study conditions, disease resistance ranked qSB9-2 + qSB12-1 > qSB9-2 > qSB12-1 > no QTL, with both qSB9-2 and qSB12-1 acting as dominant resistance genes. In micro-chamber studies, qSB9-2TQ reduced disease an average of 1.0 disease index units and qSB12-1TQ by 0.7 using a scale of 0–9. Field effects of qSB9-2TQ and qB12-1TQ were less pronounced, with average phenotypic gains of 0.5 and 0.2 units, respectively. TIL:642 proved to contain qSB9-2TQ in an introgression so small it was tagged by just RM205 on the tip of chromosome 9. These studies verify that the indica introgression of qSB9-2TQ or qSB12-1TQ can measurably improve resistance to sheath blight disease in a highly susceptible tropical japonica cultivar, and fine-mapped the qSB9-2 locus. Markers presently verified as linked to these QTL can support marker-assisted breeding to improve disease resistance.