Stem rust resistance gene Sr45 originates from Aegilops tauschii, the D-genome progenitor of Triticum aestivum. This gene was found in accession RL5289, which was also the source material for the leaf rust resistance gene Lr21 (1). The susceptibility of Sr45 to some Canadian Puccinia gramininis spp. tritici races precluded its use in breeding. However, Marais et al. (2) in 1994 showed that it was effective against South African races. Presently, Sr45 provides resistance to the Ug99 and its relatives and is also effective against other P. graminins spp. tritici races from Australia, India and South Africa.
Since both T. aestivum and A. tauschii are close relatives, it is relatively easy to obtain fertile recombinants between them. This characteristic has been used many times to transfer valuable genes from A. tauschii into wheat. There are two strategies to achieve this, via synthetic wheats with T. turgidum (donor of the A and B genomes) or direct crossing between T. aestivum and A. tauschii, followed by introgression in both cases (3).
Sr45 is located on chromosome arm 1DS, around 10 mapping units proximal from Sr33, another stem rust resistance gene originated in A. tauschii. Sr45 is already present in Thatcher + Lr21 (the reference genotype for Lr21), Thornbill and AUS 18911. Periyannan et al. (4) developed additional introgression lines carrying fragments even smaller to the ones present in the lines mentioned.
Markers for Sr45
Periyannan et al. (4) developed a high-resolution mapping populations between Chinese Spring and CD1D5406 (Chinese Spring with a ditelosomic substitution line carrying Sr45). This population and two others were used to fine map Sr45 with a combination of synteny analysis, molecular markers scoring and screening of BAC library of A. tauschii genomic DNA. As a result the authors developed a codominant PCR marker, cssu45, closely linked at 0.39 cM from Sr45.
Primers sequences:
cssu45
cssu45-F 5'- CGA GTT TCA ATA CTT CGC CC -3'
cssu45-R 5'- GAT TAC TAT GCA ATA GGG CCC -3'
Annealing temperature: 60°C
Expected products
The PCR amplification of cssu45 yields a 220-bp product in backgrounds with Sr45, and a 238-bp band when it is absent and in lines carrying Sr21, a gene derived from T. monococcum.
The response of lines carrying either Sr45 or Sr21 to many stem rust isolates is identical. However, Sr21 is ineffective against to Ug99 and several of its related races (5). Users are advised to carefully check the genotype of the parental lines in their breeding programs when selecting for Sr45.
References
1. Inheritance in hexaploid wheat of leaf rust resistance and other characters derived from Aegilops squarrosa. Kerber ER, Dick PL. In: Canadian Journal of Genetics and Cytology, 1969, 11:639-647. [Journal link]
2. An assessment of the variation for stem rust resistance in the progeny of a cross involving the Triticum species aestivum, turgidum and tauschii. Marais GF, Potgieter GF, Roux HS, Roux J. In: South African Journal of Plant and Soil, 1994, 11:15–19. DOI: 10.1080/02571862.1994.10634286
3. Cereal breeding takes a walk on the wild side. Feuillet C, Langridge P, Waughemail R. In: Trends in Genetics, 2008, 24:24-32. DOI: 10.1016/j.tig.2007.11.001
4. Identification of a robust molecular marker for the detection of the stem rust resistance gene Sr45 in common wheat.Periyannan S, Bansal U, Bariana H, Deal K, Luo MC, Dvorak J, Lagudah E. In: Theorical and Applied Genetics, 2014, 127:947-55. DOI: 10.1007/s00122-014-2270-6
5. The emergence of Ug99 races of the stem rust fungus is a threat to world wheat production. In: Annual Review of Phytopathology, 2011, 49:465–481. DOI: 10.1146/annurev-phyto-072910-095423