New germplasm development using synthetic and other approaches to transfer stem rust resistance from tetraploids to hexaploids
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.