A mlo gene with a variant promoter confers wheat powdery mildew resistance in cultivated einkorn (Triticum monococcum)

Triticum monococcum , an ancient cultivated diploid wheat species that is resistant to several wheat diseases, is an important genetic resource for wheat improvement. In the present study, T. monococcum accession PI 237659 conferred resistance to Blumeria graminis f. sp. tritici ( Bgt ) isolate BgtYZ01, which is widely virulent on bread wheat. Genetic analysis of an F 2 population derived from a cross between resistant PI 237659 and susceptible PI 326317 showed resistance was controlled by a single recessive gene, designated Pm74 . Bulked segregant exome sequencing (BSE‑Seq) followed by linkage analysis involving 11 co-dominant molecular markers mapped Pm74 to a 0.31-cM interval on chromosome arm 5AL, corresponding to a physical region of approximately 132 kb in the reference genome of T. monococcum accession TA10622. This interval contained an MLO gene whose coding sequence did not differ between PI 237659 and PI 326317. However, PI 237659 carries an approximate 92-kb deletion in the upstream untranslated region of mlo that removes part of the mlo promoter. The remaining promoter sequence contains numerous nucleotide variants relative to PI 326317. Quantitative RT-PCR showed that mlo transcript levels were significantly lower in resistant PI 237659 than in susceptible PI 326317 at both the seedling and adult-plant stages. At the adult-plant stage, PI 237659 also showed spontaneous chlorotic flecks and leaf-tip necrosis, phenotypes frequently associated with mlo -mediated resistance. These results suggest that the mlo gene carrying a variant promoter is the candidate for Pm74 . Marker analysis with Xmlo-pin showed that this mlo allele was present in 60% (30/50) of powdery mildew-resistant T. monococcum accessions. This study provides new insight into the genetic basis of recessive powdery mildew resistance in einkorn wheat and identifies a promoter-associated mlo allele with potential value for resistance breeding in wheat.