QTL mapping of resistance to race Ug99 of Puccinia graminis f. sp. tritici in durum wheat (Triticum durum Desf.)

Stem rust caused by Puccinia graminis f. sp. tritici was historically one of the most destructive diseases of wheat worldwide. The evolution and rapid migration of race TTKSK (Ug99) and derivatives, first detected in Uganda in 1999, are of international concern due to the virulence of these races to widely used stem rust resistance genes. In attempts to identify quantitative trait loci (QTL) linked with resistance to stem rust race Ug99, 95 recombinant inbred lines that were developed from a cross between two durum wheat varieties, Kristal and Sebatel, were evaluated for reaction to stem rust. Seven field trials at two locations were carried out in main and off seasons. In addition to the natural infection, the nursery was also artificially inoculated with urediniospores of stem rust race Ug99 and a mixture of locally collected stem rust urediniospores. A genetic map was constructed based on 207 simple sequence repeat (SSR) and two sequence tagged site loci. Using composite interval mapping, nine QTL for resistance to stem rust were identified on chromosomes 1AL, 2AS, 3BS, 4BL, 5BL, 6AL 7A, 7AL and 7BL. These results suggest that durum wheat resistance to stem rust is oligogenic and that there is potential to identify previously uncharacterized resistance genes with minor effects. The SSR markers that are closely linked to the QTL can be used for marker-assisted selection for stem rust resistance in durum wheat.     

MicroRNA regulated defense responses in Triticum aestivum L. during Puccinia graminis f.sp. tritici infection

Plants have evolved diverse mechanism to recognize pathogen attack and triggers defense responses. These defense responses alter host cellular function regulated by endogenous, small, non-coding miRNAs. To understand the mechanism of miRNAs regulated cellular functions during stem rust infection in wheat, we investigated eight different miRNAs viz. miR159, miR164, miR167, miR171, miR444, miR408, miR1129 and miR1138, involved in three different independent cellular defense response to infection. The investigation reveals that at the initiation of disease, accumulation of miRNAs might be playing a key role in hypersensitive response (HR) from host, which diminishes at the maturation stage. This suggests a possible host-fungal synergistic relation leading to susceptibility. Differential expression of these miRNAs in presence and absence of R gene provides a probable explanation of miRNA regulated R gene mediated independent pathways.     

The relationship between hypersensitive tissue and resistance in wheat seedlings infected with Puccinia graminis tritici

Data obtained during the first 120 hr. of the infection process in several resistant and susceptible varieties of wheat infected with Puccinia graminis tritici did not show any consistent relationship between the growth of the rust colony and the rate of increase of host tissue showing hypersensitive necrosis. Furthermore, no apparent relationship existed between the area of the rust colony at any given time after inoculation, and the amount of hypersensitive necrotic tissue per unit area of the leaf colonized. Thus, it is suggested that hypersensitive necrosis of the host tissue is a consequence and not the cause of resistance of wheat to infection by P. graminis tritici.     

Transient transformation of the rust fungus Puccinia graminis f. sp. tritici

The biotrophic rust fungus Puccinia graminis f. sp. tritici (Pgt) was transformed by particle bombardment. The promoter from the Pgt translation elongation factor 1alpha (EF-1alpha) gene was fused to the bacterial marker genes hygromycin B phosphotransferase (hpt) and beta-glucuronidase (GUS). Transformation constructs were introduced into uredospores of Pgt, an obligate pathogen of wheat, by biolistic bombardment. Uredospores transformed with the construct containing the hpt gene germinated and initiated branching on selective medium, indicating that they had acquired resistance to hygromycin B. However, transformants stopped growing 5 days after bombardment. GUS activity in uredospores and germlings was histochemically detected 4-16 h after bombardment. GUS expression was also obtained using the INF24 promoter from the bean rust fungus Uromyces appendiculatus, demonstrating that heterologous genes can be expressed in P. graminis under the control of regulatory sequences from closely related organisms.     

Performance and survival of different races of Puccinia graminis f.sp. tritici on detached leaves of wheat

The detached leaf culture of Puccinia graminis f.sp. tritici was tested with eight races and two biotypes of the pathogen. The inherent characteristics of the races were similar to those in intact plants. Continuous maintenance of races in pure culture on detached leaves showed the usefulness of the technique for multiplication of inoculum as well as for maintenance of races in other basic studies.     

Genetic determination of wheat resistance against <Emphasis Type=Italic>Puccinia graminis</Emphasis> (f. sp. <Emphasis Type=Italic>Tritici</Emphasis>) derived from <Emphasis Type=Italic>Aegilops cylindrica,Triticum erebuni</Emphasis>, and amphidiploid 4

Lines of winter soft wheat with introgressed new and effective Sr genes were developed as a result of interspecies hybridization at the Plant Breeding and Genetics Institute. The 85/06 line possesses the SrAc1 gene; the 47/06, 54/06, 82/06, 85/06, 87/06, 238/06, and 367/06 lines carry the SrAc1 and SrAc2 genes originated from Aegilops cylindrica; the 352/06 line has the SrTe1 and SrTe2 genes from Triticum erebuni; and the 12/86-04 line contains the SrAd1 and SrAd2 genes from Amphidipoid 4 (Triticum dicoccoides × Triticum tauschii).     

Production of teliospores by some races of Puccinia graminis f. sp. tritici in detached wheat and barley leaves

Five races of Puccinia graminis f. sp. tritici cultured on wheat and barley leaves in two nutrient solutions were studied for teliospore formation by subjecting them to varying treatments of temperature and light. The early appearance as well as a high percentage of teliospore formation occurred in 100 ppm benzimidazole solution on wheat or barley leaves kept at 30°C and 500 footcandles of light. The feasibility of maintaining and multiplying races of Puccinia graminis f. sp. tritici in continuous cultures on detached leaves depends on various factors, the most important being the onset of the teliostage of the fungus. The appearance of the teliospore denotes the culmination of the sporophytic or repeating stage of the wheat rusts. In this paper, some factors that influence the production of teliospores by certain races of Puccinia graminis tritici in detached leaves are discussed.     

Use of seedlings for predicting field resistance of wheat toErysiphe graminis f. sp.tritici

Seedlings can be used to predict field resistance, if any, of wheat varieties and experimental lines to wheat powdery mildew fungus,Erysiphe graminis f. sp.tritici. First leaves of 8-day-old seedlings of a field-resistant wheat showed a greater frequency of primary infections in subsidiary cells of stomata than those in a susceptible wheat. This method of predicting field resistance of experimental lines from seedling studies should prove time-saving because crosses involving field-resistant experimental lines can be made in the same generation.     

Hyperparasitism of Aphanocladium album on Aecidiospores and Teliospores of Puccinia graminis f. sp. tritici

Aphanocladium album invaded rapidly 100% of aecidiospores of Puccinia graminis f. sp. tritici resulting in total collapse of the cells. Only 2% of teliospores were invaded. A. album penetrated through the germ pores. The precocious formation of teliospores which occurs immediately following uredial infection may be regarded as defence mechanism of the rust against its hyperparasite.     

An EST library from Puccinia graminis f. sp. tritici reveals genes potentially involved in fungal differentiation

The rust fungus Puccinia graminis f. sp. tritici is an obligately biotrophic pathogen on wheat plants and thus difficult to investigate. Hence, little is known about this fungus at the molecular level. We constructed a differential suppression subtractive hybridization cDNA-library from rust-infected vs. healthy wheat plants. The majority of expressed sequence tags (ESTs) showed similarities to fungal sequences. Semiquantitative RT-PCR using mRNA from rust-infected leaves, and from axenically grown, differentiating and nondifferentiating young rust colonies as well as sporulating and nonsporulating mature mycelia revealed rather diverse expression patterns for different ESTs, shedding new light on their potential involvement in differentiation and host-pathogen interaction.