Marker-assisted introgression of the quantitative resistance gene pi21 confers broad-spectrum resistance of rice to blast

The quantitative blast resistance gene, pi21 has only been used to improve the resistance of temperate japonica rice cultivars against Japanese blast isolates. To determine the applicability of pi21 in breeding for leaf blast resistance across rice growing regions outside of Japan, the expression of the gene when transferred to different genetic backgrounds against isolates from geographically diverse sources needs to be established. In this study, pi21 was introgressed from Sensho to the indica background of the breeding line IR63307-4B-13-2 and the pyramiding line IRBB4/5/13/21, as well as to the tropical japonica background of Kinandang Patong by marker-assisted backcrossing. A total of 192 improved lines at the BC4F3 and BC4F4 generations were developed and confirmed to have the gene introgression via genotyping using a pi21-specific, indel marker. Thirteen, randomly selected, improved lines, representing all three genetic backgrounds, demonstrated resistance against leaf blast co