Fine mapping of QPm.caas-4BL.2 for adult-plant resistance to powdery mildew in wheat.

We fine-mapped QPm.caas-4BL.2 for adult-plant resistance to powdery mildew in an 880-kb physical interval, predicted a candidate gene, identified a favorable haplotype in wheat, and developed a CAPS marker for marker-assisted breeding. Powdery mildew, a globally devastating disease, severely impairs both grain yield and quality in wheat. Identification of powdery mildew resistance genes is essential for marker-assisted selection of disease-resistant wheat cultivars. In the present study, we conducted fine mapping of the QTL QPm.caas-4BL.2 for adult-plant resistance to powdery mildew from the Zhou8425B/Chinese Spring recombinant inbred line (RIL) population. By integrating genotypic and phenotypic data from progeny populations of 63 heterozygous recombinant individuals, QPm.caas-4BL.2 was mapped to an 880-kb physical interval (611.71-612.59 Mb) on chromosome 4B, containing 12 high-confidence annotated genes in the Chinese Spring reference genome (IWGSC RefSeq v2.1). TraesCS4B03G0836900 (annotated as an MLO) was considered a potential causal gene of QPm.caas-4BL.2 based on gene sequence polymorphisms, transcriptome profiles, and haplotype analysis. The TraesCS4B03G0836900 gene contains seven SNPs (single nucleotide polymorphisms) and formed six haplotypes in 145 re-sequenced wheat accessions, with Hap5-mlo carrying a missense mutation that showed significantly lower disease severity than other haplotypes. A cleaved amplified polymorphic sequence (CAPS) marker targeting the missense mutation was developed and validated in the Zhou8425B-derived varieties. Varieties carrying Zhou8425B allele exhibited 6.9% reduction in disease severity compared to those with Chinese Spring allele. This study provides genes, molecular markers, and resistant germplasm for improvement of powdery mildew resistance in wheat cultivars.