Wild relatives of common wheat, Triticum aestivum, and related species are an important source of disease and pest resistance and several useful traits have been transferred from these species to wheat. C-banding and in situ hybridization analyses are powerful cytological techniques allowing the detection of alien chromatin in wheat. C-banding permits identification of the wheat and alien chromosomes involved in wheat-alien translocations, whereas genomic in situ hybridization analysis allows determination of their size and breakpoint positions. The present review summarizes the available data on wheat-alien transfers conferring resistance to diseases and pests. Ten of the 57 spontaneous and induced wheat-alien translocations were identified as whole arm translocations with the breakpoints within the centromeric regions. The majority of transfers (45) were identified as terminal translocations with distal alien segments translocated to wheat chromosome arms. Only two intercalary wheat-alien transloctions were identified, one induced by radiation treatment with a small segment of rye chromosome 6RL (H25) inserted into the long arm of wheat chromosome 4A, and the other probably induced by homoeologous recombination with a segment derived from the long arm of a group 7 Agropyron elongatum chromosome with Lr19 inserted into the long arm of 7D. The presented information should be useful for further directed chromosome engineering aimed at producing superior germplasm.
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Wheat stem rust (SR), caused by Puccinia graminis f. sp. tritici, (Pgt) is considered one of the most destructive diseases of the wheat crop. As Sr24 and Sr31 are the most widely used resistance genes in the Southern Cone of America, wheat crops in this region is under threat of SR outbreaks posed by the potential migration of virulent Pgt Ug99-lineage races (Ug99+). Efforts have to be made to develop adapted lines resistant to Ug99+. Genes Sr26, Sr32 and Sr39 are effective to both Ug99+ and local races of the pathogen. This work is aimed to pyramid two and three of the resistance genes in two locally adapted wheat cultivars (G?nesis 2375 and G?nesis 6.87). Donor lines of Sr26, Sr32 and Sr39 (developed by I. Dundas, University of Adelaide, Australia) and molecular markers Sr26#43, csSr32#1 and Sr39#22r (developed by R. Mago et al., University of Adelaide) were used. Lines with two-gene combinations were developed in two steps. First, tree-way crosses were made by crossing heterozygous F1 plants (derived from crossings donor lines) to either one of the two adapted wheat cultivars. Subsequently, tree-way F1 plants were genotyped and only those with two-gene combinations were backcrossed (BC) twice to the adapted cultivars. Among three-way F1 plants, two-genes combinations were confirmed for Sr26+Sr32 (8 out of 31), Sr26+Sr39 (2 of 115) and Sr32+Sr39 (26 out of 103). In the BC1F1 generation, Sr26+Sr32, Sr26+Sr39 and Sr32+Sr39 combinations corresponded with 9, 9 and 45 out of 99, 27 and 241 plants, respectively. In 2017, 1345 BC2F1 plants are being grown to obtain BC2F2. We plan to intercross plants with two-gene combinations to obtain lines with the three genes which will be used as sources of resistance to develop cultivars with presumably longer lasting resistance to wheat SR.
Resistance offers the best means of control of the cereal rusts, but must be strategically deployed so as to avoid exposure of single major genes, which have faltered so many times in the past. The pyramiding of multiple effective resistance genes is a strategy that has proven effective in a number of wheat production areas around the world. However, the process of incorporating multiple resistance genes into a single cultivar using standard breeding techniques is time consuming, laborious, and hampered by the problem of linkage drag. If a suite of effective resistance genes could be efficiently cloned and transferred into wheat as a cassette, it would accelerate the development of durably resistant varieties without the problem of linkage drag. Toward this end, we have developed a resistance gene cloning technology based on resistance gene enrichment sequencing (RenSeq) of EMS-derived mutant R gene alleles. As a proof of concept test, we successfully ‘re’-cloned the already characterized gene Sr33 and are now targeting the cloning of eight other effective resistance genes. For the identification of susceptible mutants for the cloning of Sr32 from Aegilops speltoides, we screened 1,109 M2 families with race TPMKC — as a surrogate for race TTKSK. Five susceptible M2 mutants were confirmed by progeny testing. These mutants were also susceptible to race TTKSK. For the population involving Sr1644 from Ae. sharonensis, 1,649 M2 families were screened, yielding 33 M2 families that appeared to segregate for susceptibility. Thirteen of 33 families were confirmed as bona fide susceptible mutants by progeny tests in the M3 generation. Identification of susceptible EMS mutants of Sr32 and Sr1644 suggests that the underlying resistance in these lines is conferred by single genes. We will report on progress to clone and characterize these genes using R gene exome capture and sequencing technology (RenSeq).