In the Triticum genus, tetraploid T. turgidum is a useful resource for germplasm improvement of hexaploid common wheat (T. aestivum). Several recent studies demonstrated that Pgt race TTKSK resistant genotypes were abundantly present among seven tetraploid subspecies (T. turgidum subsp. carthlicum , dicoccum , dicoccoides , durum, polonicum , turgidum , and turanicum ). In an effort to improve common wheat for TTKSK resistance, we have been transferring stem rust resistance from tetraploid to hexaploid wheat through production of synthetic hexaploid wheat (SHW) or direct hexaploid × tetraploid hybridization followed by backcrossing. For production of SHW lines, we selected 181 unique tetraploid genotypes from the seven tetraploid subspecies for crosses with 14 accessions of Aegilops tauschii (2 n = 2 x = 14, DD) and developed 200 new SHW lines from these crosses. We are currently characterizing these lines for reaction to stem rust. So far, 80 SHW lines and their parents have been evaluated for reaction to races TTKSK, TRTTF, TTTTF and six other U.S. races and genotyped using molecular markers linked to known resistance genes previously identified in T. turgidum subsp. dicoccum and Ae. tauschii. The evaluation data showed that 42, 40, and 52 SHW were resistant to races TTKSK, TRTTF, and TTTTF respectively, with 21 lines being resistant to all three races. Based on marker analysis and race specificity, we postulated that a number of SHW lines have novel genes conferring resistance to TTKSK and other races. For gene introgression through direct hybridization, we have transferred Sr47, which was recently transferred from Ae. speltoides into durum through marker-assisted chromosome engineering, from durum into adapted hard red spring wheat germplasm. The new SHW lines and adapted germplasm carrying unique stem rust resistance genes from the tetraploids represent new sources of stem rust resistance for hexaploid wheat improvement.
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In durum wheat (Triticum turgidum subsp. durum), the gene Sr47 derived from Aegilopsspeltoides conditions resistance to race TTKSK (Ug99) of the stem rust pathogen (Puccinia graminis f. sp. tritici). Sr47 is carried on small interstitial translocation chromosomes (Ti2BL-2SL-2BL·2BS) in which the Ae. speltoides chromosome 2S segments are divided into four bins in genetic stocks RWG35, RWG36, and RWG37. Our objective was to physically map molecular markers to bins and to determine if any of the molecular markers would be useful in marker-assisted selection (MAS). Durum cultivar Joppa was used as the recurrent parent to produce three BC2F2 populations. Each BC2F2 plant was genotyped with markers to detect the segment carrying Sr47, and stem rust testing of BC2F3 progeny with race TTKSK confirmed the genotyping. Forty-nine markers from published sources, four new SSR markers, and five new STARP (semi-thermal asymmetric reverse PCR) markers, were evaluated in BC2F2 populations for assignment of markers to bins. Sr47 was mapped to bin 3 along with 13 markers. No markers were assigned to bin 1; however, 7 and 13 markers were assigned to bins 2 and 4, respectively. Markers Xrwgs38a, Xmag1729, Xwmc41, Xtnac3119, Xrwgsnp1, and Xrwgsnp4 were found to be useful for MAS of Sr47. However, STARP markers Xrwgsnp1 and Xrwgsnp4 can be used in gel-free systems, and are the preferred markers for high-throughput MAS. The physical mapping data from this study will also be useful for pyramiding Sr47 with other Sr genes on chromosome 2B.
The transfer of alien genes to crop plants using chromosome engineering has been attempted infrequently in tetraploid durum wheat (Triticum turgidum L. subsp. durum). Here, we report a highly efficient approach for the transfer of two genes conferring resistance to stem rust race Pgt-TTKSK (Ug99) from goatgrass (Aegilops speltoides) to tetraploid wheat. The durum line DAS15, carrying the stem rust resistance gene Sr47 derived from Ae. speltoides, was crossed, and backcrossed, to durum 5D(5B) aneuploids to induce homeologous pairing. After a final cross to ‘Rusty’ durum, allosyndetic recombinants were recovered. The Ae. speltoides chromosomal segment carrying Sr47 was found to have two stem rust resistance genes. One gene conditioning an infection type (IT) 2 was located in the same chromosomal region of 2BS as Sr39 and was assigned the temporary gene symbol SrAes7t. Based on ITs observed on a diverse set of rust races, SrAes7t may be the same as Sr39. The second gene conditioned an IT 0; and was located on chromosome arm 2BL. This gene retained the symbol Sr47 because it had a different IT and map location from other stem rust resistance genes derived from Ae. speltoides. Allosyndetic recombinant lines carrying each gene on minimal alien chromosomal segments were identified as were molecular markers distinguishing each alien segment. This study demonstrated that chromosome engineering of Ae. speltoides segments is feasible in tetraploid wheat. The Sr47 gene confers high-level and broad spectrum resistance to stem rust and should be very useful in efforts to control TTKSK.