Stem rust resistance gene Sr43, transferred into common wheat (Triticum aestivum) from Thinopyrum ponticum, is an effective gene against stem rust Ug99 races. However, this gene has not been used in wheat breeding because it is located on a large Th. ponticum 7el2 chromosome segment, which also harbors genes for undesirable traits. The objective of this study was to eliminate excessive Th. ponticum chromatin surrounding Sr43 to make it usable in wheat breeding. The two original translocation lines KS10-2 and KS24-1 carrying Sr43 were first analyzed using simple sequence repeat (SSR) markers and florescent genomic in situ hybridization. Six SSR markers located on wheat chromosome arm 7DL were identified to be associated with the Th. ponticum chromatin in KS10-2 and KS24-1. The results confirmed that KS24-1 is a 7DS·7el2L Robertsonian translocation as previously reported. However, KS10-2, which was previously designated as a 7el2S·7el2L-7DL translocation, was identified as a 7DS-7el2S·7el2L translocation. To reduce the Th. ponticum chromatin carrying Sr43, a BC2F1 population (Chinese Spring//Chinese Spring ph1bph1b*2/KS10-2) containing ph1b-induced homoeologous recombinants was developed, tested with stem rust, and genotyped with the six SSR markers identified above. Two new wheat lines (RWG33 and RWG34) carrying Sr43 on shortened alien chromosome segments (about 17.5 and 13.7 % of the translocation chromosomes, respectively) were obtained, and two molecular markers linked to Sr43 in these lines were identified. The new wheat lines with Sr43 and the closely linked markers provide new resources for improving resistance to Ug99 and other races of stem rust in wheat
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Stem rust resistance gene Sr43, derived from tall wheatgrass (Thinopyrum ponticum), is effective against Ug99 lineage Pgt races. Previous studies indicated that Sr43 was located on large Th. ponticum 7el2 chromosome segments in 7D/7el2 translocation stocks KS10-2 and KS24-1. In the present work, we applied a recently-established chromosome engineering procedure to reduce the size of the alien chromosome carrying Sr43. KS10-2 was crossed and backcrossed to the Chinese Spring (CS) ph1b mutant. BC1F1 plants were screened for stem rust response and Ph1- associated molecular markers. Resistant BC1F1 plants homozygous ph1bph1b were further backcrossed to CS. The resulting population of 706 BC2F1 plants was screened for stem rust response and with six co-dominant SSR markers. Wheat lines RWG33 and RWG34 carry Sr43 on shortened alien segments that are about 15% of that in KS10-2. Two molecular markers closely linked to Sr43 were identified; one was an SSR marker and the other a STS marker based on sequences of deletion bin-mapped expressed sequenced tags in wheat. The two new wheat lines with Sr43 and closely-linked markers may provide new resources for combating the threat of race Ug99 and derivatives.
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.
Two procedures were used to induce homoeologous recombination between Agropyron elongatum (Host) Beauv. chromosome 7el2 and wheat chromosomes. One procedure involved the use of 'Chinese Spring' nullisomic 5B – tetrasomic 5D, and resulted in plants lacking chromosome 5B. In the second procedure, a line carrying the mutant gene ph1b was used, and plants were produced that had only a 5B chromosome carrying ph1b. Both procedures resulted in the transfer of a gene or genes for stem rust (Puccinia graminis tritici Eriks. and Henn.) resistance from chromosome 7el2 to wheat chromosomes. During the transfer process, it was discovered that both the whole Agropyron chromosome and the recombinant chromosomes showed preferential transmission through the female gametes, but not through the male gametes. On heterozygous plants seed set was greatly reduced. Apparently, the Agropyron chromosome or a gene carried by it had a gametocidal action that resulted in female gametes, which did not carry the gene, failing to function. However, homozygous lines showed normal fertility.