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Doctor of Philosophy / Department of Plant Pathology / Harold N. Trick / Bread wheat (Triticum aestivum L.) is an important staple crop for 35% of the world’s population. One economically important pathogen of wheat is Puccinia triticina, the causal agent of leaf rust, can cause up to 50% yield loss during epidemics. Despite the lack of an alternate host to complete the sexual stages, P. triticina still has variation within the population, which can make achieving durable resistance difficult. This study aims to gain a better understanding of the P. triticina-wheat interaction by identifying wheat genes that are induced by individual and multiple races. Six P. triticina races were evaluated on a susceptible variety of wheat at six days post inoculation. RNA was sequenced and 63 wheat genes were identified that showed varying expression in response to the six P. triticina races. Fifty-four wheat genes were characterized during the first seven days of infection using real-time PCR. Race specific gene expression was found in three wheat genes with race differences on Lr2A, Lr2C, and Lr17A. Wheat genes that had similar expression in response to all six races were also identified. Seven of the characterized genes were then silenced using RNAi hairpin constructs. The transgenic plants were molecularly characterized and inoculated with a virulent P. triticina race in the T₂ generation. However, the endogenous genes were not silenced and the transgenic plants maintained susceptibility. A mutation approach was also used to identify wheat genes involved in infection. A mutant population of 3780 wheat plants was created using EMS. Fifteen hundred mutants from the M1 population were screened for plants with a different infection phenotype compared to the non-mutated control and 570 were selected. After two additional generations of selection, eight resistant mutants were obtained. The gene expression of the seven previously identified genes were evaluated and one mutant showed reduced expression of an ER molecular chaperone gene. This research uses a forward and reverse genetics approach to identify and evaluate the function of wheat genes in the wheat-P. triticina interaction. Although RNAi could not determine the gene function, the knockout mutant shows that the identified genes may have a crucial role in infection.

Wheat (Triticum aestivum L. em. Thell, 2n=6x=42, AABBDD) is subjected to significant yield losses by the endemic leaf rust pathogen, Puccinia triticina (Roberge ex Desmaz. F. sp. tritici). Breeding for resistance to this disease is a more appropriate option both environmentally and economically over fungicidal application. More than 57 leaf rust resistance genes in wheat have been identified and many of the resistance genes have been successfully introgressed into resistant cultivars, yet the continuous shifting of predominant races of P. triticina continues to be a challenge to breeders. Pyramiding multiple resistance genes into a single resistant cultivar is one of the preferred strategies to develop superior disease resistant cultivars. Efficient pyramiding requires the utilization of markers closely linked to the resistance genes. The objectives of this study were to characterize a novel source of resistance to leaf rust introgressed into the breeding line WX93D180-R-8-1, to determine its inheritance, map position, and linkage with molecular markers suitable for marker assisted selection. According to the pedigree of WX93D180, TX86D1310*3/TTCC417, the resistance in this breeding line should be derived from TTCC417 (Turkey tritici cereal collection), which was thought to be Triticum monococcum, which is a diploid species made up of only the A genome. However, our marker analyzes results indicated the resistance gene is located in the D genome and has the same location as the cloned leaf rust resistance gene Lr21. We verified the result in our population using primers from Lr21 and found the same segregation pattern with the phenotypic data (disease response). Therefore the pedigree is incorrect, TTCC417 was misidentified, or the resistance was not from TTCC417.

Wheat (Triticum aestivum L. em. Thell, 2n=6x=42, AABBDD) is subjected to significant yield losses by the endemic leaf rust pathogen, Puccinia triticina (Roberge ex Desmaz. F. sp. tritici). Breeding for resistance to this disease is a more appropriate option both environmentally and economically over fungicidal application. More than 57 leaf rust resistance genes in wheat have been identified and many of the resistance genes have been successfully introgressed into resistant cultivars, yet the continuous shifting of predominant races of P. triticina continues to be a challenge to breeders. Pyramiding multiple resistance genes into a single resistant cultivar is one of the preferred strategies to develop superior disease resistant cultivars. Efficient pyramiding requires the utilization of markers closely linked to the resistance genes. The objectives of this study were to characterize a novel source of resistance to leaf rust introgressed into the breeding line WX93D180-R-8-1, to determine its inheritance, map position, and linkage with molecular markers suitable for marker assisted selection. According to the pedigree of WX93D180, TX86D1310*3/TTCC417, the resistance in this breeding line should be derived from TTCC417 (Turkey tritici cereal collection), which was thought to be Triticum monococcum, which is a diploid species made up of only the A genome. However, our marker analyzes results indicated the resistance gene is located in the D genome and has the same location as the cloned leaf rust resistance gene Lr21. We verified the result in our population using primers from Lr21 and found the same segregation pattern with the phenotypic data (disease response). Therefore the pedigree is incorrect, TTCC417 was misidentified, or the resistance was not from TTCC417.

Effective at the adult plant stage, Lr34 is the most important resistance gene to leaf rust. Usage of closely linked molecular markers is the best strategy to facilitate the incorporation of economically important genes in an adapted plant germplasm. Ten novel molecular markers spanning the Lr34 locus were developed, including six microsatellites (cam), one insertion site-based polymorphism (caISBP), two single nucleotide polymorphisms (caSNP) and one indel marker (caIND). Marker caIND11 is the best diagnostic marker for marker assisted selection of Lr34. Two novel haplotypes of Lr34 were discovered in the germplasm. Analysis of these markers on five segregating populations revealed a recombination between caSNP4 and cam8 which provided further support for the identity of the ABC transporter as Lr34. Using Lr34-specific markers, the world collection (WC) was divided into five major haplotypes (H) of which H1 was consistently associated with the resistance phenotype Lr34+. SNP12-C is the functional unit of Lr34. Maximum parsimony network and other observations revealed that H4, an Lr34- haplotype, was probably the most ancient haplotype and H1 the most recent and that it likely arose after the advent of hexaploid wheat. Analysis of geographical distribution showed that H1 was at a high frequency in the Asian germplasm while H4 was more frequent in the European germplasm. Lr34, a gain of function mutation, was hypothesized to have originated in Asia. The (WC) was characterized for seedling and adult plant resistance using gene specific markers and gene postulation. Fourteen seedling genes were determined or postulated in the collection. Lr1, Lr10, Lr3 and Lr20 were the most highly represented genes while Lr9, Lr14b, Lr3ka and/or Lr30 and Lr26 were rare. The WC was evaluated for field resistance. The rust rating in the field ranged from nearly immune (1R) to highly susceptible (84S). Most Lr34 containing accessions had maximum rust severity (MRS) of 35%. The high levels of resistance in some accessions are likely the result of synergy between APR genes or between APR and seedling genes. Accessions that were highly resistant should be considered potential sources of resistance for future wheat breeding program to improve leaf rust resistance.

Effective at the adult plant stage, Lr34 is the most important resistance gene to leaf rust. Usage of closely linked molecular markers is the best strategy to facilitate the incorporation of economically important genes in an adapted plant germplasm. Ten novel molecular markers spanning the Lr34 locus were developed, including six microsatellites (cam), one insertion site-based polymorphism (caISBP), two single nucleotide polymorphisms (caSNP) and one indel marker (caIND). Marker caIND11 is the best diagnostic marker for marker assisted selection of Lr34. Two novel haplotypes of Lr34 were discovered in the germplasm. Analysis of these markers on five segregating populations revealed a recombination between caSNP4 and cam8 which provided further support for the identity of the ABC transporter as Lr34. Using Lr34-specific markers, the world collection (WC) was divided into five major haplotypes (H) of which H1 was consistently associated with the resistance phenotype Lr34+. SNP12-C is the functional unit of Lr34. Maximum parsimony network and other observations revealed that H4, an Lr34- haplotype, was probably the most ancient haplotype and H1 the most recent and that it likely arose after the advent of hexaploid wheat. Analysis of geographical distribution showed that H1 was at a high frequency in the Asian germplasm while H4 was more frequent in the European germplasm. Lr34, a gain of function mutation, was hypothesized to have originated in Asia. The (WC) was characterized for seedling and adult plant resistance using gene specific markers and gene postulation. Fourteen seedling genes were determined or postulated in the collection. Lr1, Lr10, Lr3 and Lr20 were the most highly represented genes while Lr9, Lr14b, Lr3ka and/or Lr30 and Lr26 were rare. The WC was evaluated for field resistance. The rust rating in the field ranged from nearly immune (1R) to highly susceptible (84S). Most Lr34 containing accessions had maximum rust severity (MRS) of 35%. The high levels of resistance in some accessions are likely the result of synergy between APR genes or between APR and seedling genes. Accessions that were highly resistant should be considered potential sources of resistance for future wheat breeding program to improve leaf rust resistance.

Barley leaf rust (<I>Puccinia hordei</I> G. Otth) has been the cause of numerous and often devastating disease epidemics since the beginning of agriculture. Leaf rust is one of the most important diseases that affect barley (<I>Hordeum vulgare</I> L.) throughout the world. The pathogen <I>Puccinia hordei</I> is an obligate parasite. Symptoms of barley leaf rust may range from small chlorotic flecks to large pustules containing spores. Leaf rust epidemics reduce yields and grain quality. Deployment of resistant cultivars is one of the most effective and economical means of controlling barley leaf rust. Identification and incorporation of new and effective sources of resistance are crucial to the success of barley breeding programs. Two types of resistance have been identified. They are race-specific resistance and partial resistance. A hypersensitive reaction by the host to infection of <I>P. hordei</I> isolates lacking corresponding virulence genes is indicative of race-specific resistance that is controlled by major genes. Sixteen race-specific genes (R<I>ph</I>1 to R<I>ph</I>16) have been identified. Partial resistance is generally polygenic and is often more durable that race-specific resistance. The purpose of this research is to determine the inheritance of resistance to leaf rust in the barley experimental line VA 92-42-46, to identify the gene(s) conferring resistance, identify putative resistance related markers, and to map the gene(s) to one or more barley chromosomes using molecular markers. The Virginia barley line 92-42-46 was selected for this research project because it possesses resistance to <I>P. hordei</I> race 30, which has overcome resistance conferred by R<I>ph</I>7. Crosses were made between VA 92-42-46 and Moore, a susceptible cultivar to leaf rust. Inheritance studies were performed by screening F₂ progeny and F_2:3 families against race 8 and race 30 to determine the number of leaf rust resistance genes in VA 92-42-46. Allelism tests were performed to determine gene identity. A single dominant gene at the R<I>ph</I>5 locus or a tightly linked gene confers the resistance to P. hordei in VA 92-42-46. Two populations, 'Moore' X VA 92-42-46 and 'Bowman' X 'Magnif', were used in this study for mapping molecular markers to provide comparison and confirmation of results. 'Magnif' possesses the resistance gene R<I>ph</I>5. Bulked segregant analysis was used to identify polymorphic RFLP and SSR markers that were used for mapping in each population. Linkage analysis revealed that the R<I>ph</I>5 gene maps to barley chromosome 3 (3H) above the centromeric region in the 'Moore' X VA 92-42-46 population. These findings agree with previous research that identified linkage between R<I>ph</I>5 and R<I>ph</I>7 on chromosome 3. The results obtained in this study do not support previous research that had reported the resistance gene R<I>ph</I>5 was located on barley chromosome 7 (5H). Further research should be conducted to verify the results of this study using the 'Bowman' X 'Magnif' population. The markers screened in the region above the centromere region of barley chromosome 3 were monomorphic for the 'Bowman' X 'Magnif' population except for the marker MWG561. Therefore, additional markers above the centromere of barley chromosome 3 should be screened. / Master of Science

Call number: LD2668 .T4 1949 I71 / Master of Science

No description available.

Call number: LD2668 .T4 1964 R71 / Master of Science