Phenotypic and genotypic evaluation of wheat genetic resources and development of segregating populations are pre-requisites for identifying rust resistance genes. The objectives of this study were to assess adult plant resistance (APR) of selected wheat genotypes to leaf rust and stem rust and to develop segregating populations for resistance breeding. Eight selected Kenyan cultivars with known resistance to stem rust, together with local checks were evaluated for leaf rust and stem rust resistance at seedling stage and also across several environments. Selected diagnostic markers were used to determine the presence of known genes. All eight cultivars were crossed with local checks using a bi-parental mating design. Seedling tests revealed that parents exhibited differential infection types against wheat rust races. Cultivars Paka and Popo consistently showed resistant infection types at seedling stage, while Gem, Romany, Pasa, Fahari, Kudu, Ngiri and Kariega varied for resistant and susceptible infection types depending on the pathogen race used. The control cultivars Morocco and McNair consistently showed susceptible infection types as expected. In the field, all cultivars except for Morocco showed moderate to high levels of resistance, indicating the presence of effective resistance genes. Using diagnostic markers, presence of Lr34 was confirmed in Gem, Fahari, Kudu, Ngiri and Kariega, while Sr2 was present in Gem, Romany, Paka and Kudu. Seedling resistance gene, Sr35, was only detected in cultivar Popo. Overall, the study developed 909 F6:8 recombinant inbred lines (RILs) as part of the nested mating design and are useful genetic resources for further studies and for mapping wheat rust resistance genes.
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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.
With the TTKS family of races virulent on most genes currently providing protection against stem rust worldwide, identifying, mapping, and deploying resistance genes effective against these races has become critical. We present here a genetic map of Sr35. Both parents of our diploid mapping population (DV92/G3116, 142 SSD lines) are resistant to TTKSK, but the population segregates for resistance to TRTTF (Yemen) and RKQQC (US). Race analysis suggests that G3116 carries Sr21 and DV92 both Sr21 and Sr35. Resistance to TRTTF and RKQQC was mapped to a 6 cM interval on chromosome 3AmL between markers BF483299 and CJ656351. This interval corresponds to a 178-kb region in Brachypodium which contains only 16 annotated genes and exhibits a small inversion (including 2 genes) and a putative insertion (2 genes) relative to rice and sorghum. This map contains closely-linked markers to Sr35 and provides the initial step for this gene's positional cloning.
Wheat stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is a devastating disease that can cause severe yield losses. A previously uncharacterized Pgt race, designated Ug99, has overcome most of the widely used resistance genes and is threatening major wheat production areas. Here, we demonstrate that the Sr35 gene from Triticum monococcum is a coiled-coil, nucleotide-binding, leucine-rich repeat gene that confers near immunity to Ug99 and related races. This gene is absent in the A-genome diploid donor and in polyploid wheat but is effective when transferred from T. monococcum to polyploid wheat. The cloning of Sr35 opens the door to the use of biotechnological approaches to control this devastating disease and to analyses of the molecular interactions that define the wheat-rust pathosystem.
Puccinia graminis f. sp. tritici is the causal agent of stem rust of wheat. A new race designated TTKSK (also known as Ug99) and its variants (TTKST and TTTSK) are virulent to most of the stem rust resistance genes currently deployed in wheat cultivars worldwide. Therefore, identification, mapping, and deployment of effective resistance genes are critical components of global efforts to mitigate this threat. Multipathotype seedling tests demonstrated that resistance gene Sr35 is effective against the three TTKS variants and another broadly virulent race from Yemen, TRTTF. Two genetic maps of Sr35 are presented in diploid (Triticum monococcum) and two in hexaploid wheat (T aestivum). The Sr35 resistance to TRTTF and RKQQC races was mapped in diploid wheat within a 2.2 to 3.1 cM interval on the long arm of chromosome 3A(m) between markers XBF483299 and XCJ656351. This interval corresponds to a 174-kb region in Brachypodium that includes 16 annotated genes. The Sr35 map location was confirmed in two backcross-derived hexaploid populations segregating for Sr35. Recombination between diploid and hexaploid chromosomes was 10-fold lower than between homologous chromosomes, but was sufficient to reduce the introgressed diploid segment. These maps provide markers closely linked to Sr35 that will be useful to accelerate its deployment and pyramiding with other stem rust resistance genes.