Submitted to: Crop Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/1/2003
Publication Date: 5/1/2004
Citation: Haen, K.M., Lu, H., Friesen, T.L., Faris, J.D. 2004. Genomic targeting and high-resolution mapping of the tsn1 gene in wheat. Crop Science. Vol 44:951-962. Interpretive Summary: Tan spot is a leaf disease of wheat and durum caused by the fungus Pyrenophora tritici-repentis. The fungus produces a protein toxin (Ptr ToxA) that causes severe disease on leaves of sensitive wheat genotypes. Sensitivity to the toxin is controlled by a single gene in the host designated as Tsn1. Tsn1 is located on the long arm of wheat and durum chromosome 5B. The objectives of this research were to identify DNA markers tightly linked to the Tsn1 gene and develop a high-resolution map of the Tsn1 region for purposes of cloning the gene. Using various molecular methodologies in combination with wheat and durum cytogentic stocks, we saturated the Tsn1 region with molecular markers. Two large segregating populations were used to construct high-resolution maps of the Tsn1 region, and we identified markers tightly linked to Tsn1. Comparisons with rice DNA sequences indicated that this region was not well conserved through evolution. This work provides the basis for the cloning of the Tsn1 gene, and once cloned, molecular aspects of the wheat-tan spot interaction can be characterized. This knowledge will aid in the development of more highly resistant wheat and durum cultivars and provide insights into host-toxin interactions in general.
Technical Abstract: Tan spot, caused by the fungal pathogen Pyrenophora tritici-repentis causes severe yield losses in wheat and durum. The Tsn1 gene acts dominantly to confer sensitivity to a host-selective proteinacious toxin (Ptr ToxA) produced by the fungus. Our objectives were to: 1) Target markers to the Tsn1 genomic region, and 2) develop a high-resolution map of the Tsn1 locus. The techniques of methylation-sensitive AFLP, traditional AFLP, and cDNA-AFLP were combined with bulked segregant analysis (BSA) using various wheat and durum cytogenetic stocks to target markers to the Tsn1 genomic region. Over 500 primer combinations were screened resulting in the identification of 18 low-copy markers closely linked to Tsn1. High-resolution mapping of the markers delineated the Tsn1 gene to a 0.2 cM interval in a hexaploid wheat population consisting of 1,266 gametes, and to 0.8 cM in a durum wheat population consisting of 1,860 gametes. Comparisons with rice BAC/PAC sequences indicated the absence of synteny within the Tsn1 genomic region. Tsn1 is located within a gene-rich recombination hot spot region, and the physical distance separating the flanking markers may be as little as 200 kb. Therefore, these markers will serve as a basis for the map-based cloning of Tsn1. The isolation of Tsn1 will further our knowledge of wheat-tan spot interactions as well as host-pathogen interactions in general.