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Resistance to wheat stem rust in selected accessions of Iranian wheat landraces

Stem rust is a potentially destructive fungal disease of wheat worldwide. In 1998 Pgt pathotype TTKSK virulent to Sr31 was detected in Uganda. The same pathotype was confirmed in Lorestan and Hamedan provinces of Iran in 2007. We used a derivative of race TTKSK to phenotype 62 Iranian wheat landraces (resistant to stripe rust in a previous study) at the seedling stage to this new pathotype (TTSSK). Twenty eight accessions were evaluated for the presence of resistance genes Sr2, Sr22, Sr24, Sr25, Sr26, Sr35, Sr36 and Srweb using SSR markers. None carried Sr2, Sr24 or Sr26, but the presence of Sr22, Sr25, Sr35 and Sr36 was indicated. Some susceptible landraces predicted to carry Sr2 by marker analysis require further investigation. To evaluate defense gene expression in compatible and incompatible stem rust interactions we sampled resistant and susceptible cultivars at 0, 12, 18, 24, 72 hours post-inoculation (hpi). ?-1,3 glucanase expression was studied using qGLU-S and qGLUU-AS primers and a real-time PCR step-one ABI machine, with ?-tubulin and EF1-? genes used as internal controls. In incompatible interactions defense gene expression was increased at 24 hpi, but in compatible interactions the highest level of expression occurred at 12 hpi and was significantly decreased at 18 hpi. The results revealed that expression of defense genes such as ?-1,3 glucanase was earlier in compatible than in incompatible interactions but the expression level was less in incompatible interactions. On the other hand, in susceptible genotypes the expression of defense genes increased immediately after inoculation and declined sharply after infection. In contrast defense gene expression in resistant genotypes began to increase after establishment of the pathogen.

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Tarbiat Modares University of Tehran, Iran
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Tight repulsion linkage between Sr36 and Sr39 was revealed by genetic, cytogenetic and molecular analyses

Stem rust resistance genes Sr39 and Sr36 were transferred from Aegilops speltoides and Triticum timopheevii, respectively, to chromosome 2B of wheat. Genetic stocks RL6082 and RWG1 carrying Sr39 on a large and a shortened Ae. speltoides segments, respectively, and the Sr36-carrying Australian wheat cultivar Cook were used in this study. This investigation was planned to determine the genetic relationship between these genes. Stem rust tests on F3 populations derived from RL6082/Cook and RWG1/Cook crosses showed tight repulsion linkage between Sr39 and Sr36. The genomic in situ hybridization analysis of heterozygous F3 family from the RWG1/Cook population showed that the translocated segments do not overlap. Meiotic analysis on the F1 plant from RWG1/Cook showed two univalents at the metaphase and anaphase stages in a majority of the cells indicating absence of pairing. Since meiotic pairing has been reported to initiate at the telomere, pairing and recombination may be inhibited due to very little wheat chromatin in the distal end of the chromosome arm 2BS in RWG1. The Sr39-carrying large Ae. speltoides segment transmitted preferentially in the RL6082/Cook F3 population, whereas the Sr36-carrying T. timopheevii segment over-transmitted in the RWG1/Cook cross. Genotyping with the co-dominant Sr39- and Sr36-linked markers rwgs28 and stm773-2, respectively, matched the phenotypic classification of F3 families. The RWG1 allele amplified by rwgs28 was diagnostic for the shortened Ae. speltoides segment and alternate alleles were amplified in 29 Australian cultivars. Marker rwgs28 will be useful in marker-assisted pyramiding of Sr39 with other genes.

Bosco Chemaye, kUrmil K. Bansal, Naeela Qureshi, Peng Zhang, William W. Wagoire, Harbans S. Bariana
Theoretical and Applied Genetics
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Genetic relationship between wheat stem rust resistance genes Sr36 and Sr39

Stem rust resistance genes Sr39 (RL6082) and Sr36 (Cook) were transferred from Aegilops speltoides and Triticum timopheevi to chromosome 2B of wheat. Both genes are located on large translocated segments. Genotypes carrying Sr36 and Sr39 produce infection types (ITs) 0; and 2, respectively, against avirulent pathotypes. This investigation was planned to study the genetic relationship between these genes with the aim of combining them in a single genotype. Seedling tests on RL6082/Cook F3 lines showed complete repulsion linkage [25 Sr39Sr39sr36sr36 (IT2-) : 53 Sr39sr39Sr36sr36 (IT2-, IT0;) : 13 sr39sr39Sr36Sr36 (IT 0;)], and preferential transmission of the Ae. speltoides segment over the T. timopheevi segment was evident from the segregation ratio. The Sr39-carrying translocation was shortened by Niu et al. (2011; Genetics 187: 1011-1021) and the genetic stock carrying the shortest segment was named RWG1. Based on the reported location of Sr39 in the smaller alien segment in RWG1, we predicted that it should recombine with Sr36. F3 lines derived RWG1/Cook were phenotyped for stem rust response at the two-leaf stage and again complete repulsion linkage between Sr39 and Sr36 was observed [23 Sr39Sr39sr36sr36 (IT2-) : 78 Sr39sr39Sr36sr36 (IT0;, IT2-) : 68 sr39sr39Sr36Sr36 (IT 0;)]. In contrast to the cross involving the large Sr39 translocation, preferential transmission of the T. timopheevi segment was observed. These results indicated that a genetic determinant of meiotic drive had been deleted in the shortened Ae. speltoides segment. Genotyping with the co-dominant STS marker rwgs28 matched the phenotypic classification of F3 families. Marker rwgs28 was diagnostic for the Ae. speltoides segment, but the rwgs28 allele amplified in Cook was not T. timopheevi-specific.

The University of Sydney, Plant Breeding Institute, Australia
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Genotyping of U.S. Wheat Germplasm for Presence of Stem Rust Resistance Genes Sr24, Sr36 and Sr1RSAmigo

The stem rust resistance genes Sr24, Sr26, Sr36, and Sr1RSAmigo confer resistance to race TTKSK (= Ug99) of Puccinia graminis f. sp. tritici Pers. (Pgt). A collection of 776 cultivars and breeding lines of wheat (Triticum aestivum L.) from all growing regions of the United States were screened with simple sequence repeat and sequence tagged site markers linked to Sr24, Sr26, Sr36, and Sr1RSAmigo to determine frequencies of these genes in U.S. wheat germplasm. Marker efficacy in predicting the presence of these genes was evaluated via comparison with assayed seedling infection type. Among the lines evaluated, the most predominant gene is Sr24, present in hard winter, hard spring, and soft winter wheat lines. Resistance in soft winter wheat is primarily due to Sr36. The 1RS{middle dot}1AL rye translocation carrying Sr1RSAmigo is present at equal frequencies in hard winter and soft winter wheat. Utilization of marker-assisted selection for stem rust resistance genes can hasten the development of wheat cultivars resistant to TTKSK and its variants and allow for the development of resistance gene pyramids for more durable stem rust resistance.

Olson,Eric L.; Brown-Guedira,Gina; Marshall,David S.; Jin,Yue; Mergoum,Mohamed; Lowe,Iago; Dubcovsky,Jorge
Crop Science
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