Fine-mapping of the brittle rachis locus Qbr.sau-2DL in Tibetan semi-wild wheat (Triticum aestivum ssp. tibetanum Shao)

Abstract

The brittle rachis is the most important trait in wheat domestication and de-domestication. Tibetan semi-wild wheat (Triticum aestivum ssp. tibetanum Shao) is a unique common wheat subspecies that originated through de-domestication. Qbr.sau-2DL on chromosome 2DL is one of the three main brittle rachis loci in the Tibetan semi-wild wheat accession “Q1028.” However, the gene responsible for the brittle rachis trait at the Qbr.sau-2DL locus remains unknown. In this study, “QZ110,” a monogenic line for Qbr.sau-2DL isolated from a recombinant inbred line population derived from hybridization between “Q1028” and the common wheat cultivar “ZM9023,” was crossed with the common wheat cultivar “SM482” to construct F₂, F_2:3, and F₃ populations. Genetic analyses indicated that Qbr.sau-2DL was recessive. Using Bulked Segregant Exome Capture Sequencing data and Kompetitive Allele Specific PCR markers, Qbr.sau-2DL was located within an 836.4-kb interval containing 12 high-confidence predicted genes. One of them, TreasCS2D02G502900, was a homolog of OsLG1, which affects rice shattering. The transcript level of TreasCS2D02G502900 in the disarticulation layer was significantly higher in “QZ110” than in “SM482,” consistent with the transcript profile of OsLG1 in rice. Therefore, TreasCS2D02G502900 was identified as the candidate gene for Qbr.sau-2DL. Microscopic observations indicated that Qbr.sau-2DL reduces cell wall thickness and the lignin content in the disarticulation layer. These results provide a basis for clarifying the function and underlying molecular mechanisms of Qbr.sau-2DL in the brittle rachis trait.