Genetic and physical characterization of theLR1 leaf rust resistance locus in wheat (Triticum aestivum L.)

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Abstract
The objective of this study was to characterize the leaf rust resistance locusLr1 in wheat. Restriction fragment length polymorphism (RELP) analysis was performed on the resistant lineLr1/6*Thatcher and the susceptible varieties Thatcher and Frisal, as well as on the segregating F2 populations. Seventeen out of 37 RFLP probes mapping to group 5 chromosomes showed polymorphism betweenLr1/6*Thatcher and Frisal, whereas 11 probes were polymorphic between the near-isogenic lines (NILs)Lr1/6*Thatcher and Thatcher. Three of these probes were linked to the resistance gene in the segregating F2 populations. One probe (pTAG621) showed very tight linkage toLr1 and mapped to a single-copy region on chromosome 5D. The map location of pTAG621 at the end of the long arm of chromosome 5D was confirmed by the absence of the band in the nulli-tetrasomic line N5DT5B of Chinese Spring and a set of deletion lines of Chinese Spring lacking the distal part of 5DL. Twenty-seven breeding lines containing theLr1 resistance gene in different genetic backgrounds showed the same band asLr1/6*Thatcher when hybridized with pTAG621. The RFLP marker was converted to a sequence-tagged-site marker using polymerase chain reaction (PCR) amplification. Sequencing of the specific fragment amplified from both NILs revealed point mutations as well as small insertion/deletion events. These were used to design primers that allowed amplification of a specific product only from the resistant lineLr1/6*Thatcher. This STS, specific for theLr1 resistance gene, will allow efficient selection for the disease resistance gene in wheat breeding programmes. In addition, the identification of a D-genome-specific probe tightly linked toLr1 should ultimately provide the basis for positional cloning of the gene.