Physical Mapping of a Powdery Mildew Resistance Gene in Chromosome 6St from Wheat-Thinopyrum intermedium Introgression Lines

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is a devastating disease threatening global wheat production. Thinopyrum intermedium, a wild relative of wheat, harbors valuable resistance genes for wheat improvement. In this study, we characterized a wheat-Th. intermedium double disomic substitution line X482 (4St-JS (4D) + 6St (6D)) and wheat-Th. intermedium partial amphiploid TA8034 using ND-FISH and Oligo-FISH painting. Importantly, a unique type of translocation between chromosomes 1B and 6B was identified in line X482. Subsequent immunostaining assays revealed the distinct DNA methylation maintained on Thinopyrum and wheat B-genome chromosomes in X482 and TA8034. Genetic analysis combined with ND-FISH of line X482-derived populations demonstrated that chromosome 6St confers adult-stage powdery mildew resistance. Through 60Co-γ irradiation of a 6St monosomic addition line, we developed seven homozygous 6St translocation lines with various breakpoints. Physical mapping using 56 6St-specific molecular markers delineated the resistance locus to bin 6StL-3, corresponding to 358.03–398.21 Mb in the Th. intermedium reference genome v2.1. Notably, the wheat-6St translocation line exhibited powdery mildew resistance without significant negative effects on agronomic traits. These results indicated that the distal region of homologous group 6 chromosomes harbors novel powdery mildew resistance genes, and the developed translocation lines provide valuable germplasm for wheat breeding programs.