Doctor of Philosophy / Department of Plant Pathology / Jianmin Zhou / Plants activate innate immune responses or innate immunity upon pathogen infection. There are two types of plant innate immunity: PAMP-triggered innate immunity (PTI) and effector-triggered innate immunity (ETI). The molecular basis for ETI has been well documented. However, the study on PTI and its interplay with pathogen virulence is in its infancy. My research focuses on the interplay between PTI and bacterial virulence in Pseudomonas-Arabidopsis interactions. NHO1, a gene required for nonhost resistance to Pseudomonas syringae, encodes for the 3-glycerol kinase in Arabidopsis genome. NHO1 functions, at least in part, by depriving glycerol from nonhost bacteria cells. NHO1 is induced by a well-known bacteria PAMP flg22. The induction of NHO1 correlates well with the resistance against Pseudomonas syringae pv. tabaci because a mutant strain of P. s. pv. tabaci deficient in NHO1 induction gains partial virulence on Arabidopsis plants. P. s. pv. tomato strain DC3000 induces transient NHO1 expression that is suppressed in a type III secrection system-dependent manner. Using protoplast assay, nine DC3000 effectors that are able to suppress NHO1 were identified. One of them, HopAI1, induces leaf chlorosis and helps nonpathogenic bacterial growth when expressed in Arabidopsis plants, suggesting that HopAI1 has virulence activity in planta. To study AvrB virulence activity in Arabidopsis plants, one mutant compromised in AvrB-specific RAR2.6 induction has been characterized in detail. rrb3 is more susceptible to a nonhost bacteria P. s. pv. tabaci strain 6505, a virulent bacteria P. s. pv. tomato strain DC3000 and an avirulent bacteria strain DC3000 (avrB). The mutant allele rrb3 carries a point mutation at the end of RAR1 CHORD II domain. RRB3 (RAR1), together with NDR1, is involved in the type II nonhost resistance to P. s. pv. tabaci but not in the type I nonhost resistance to P. s. pv. phaseolicola. RAR1 participates in basal resistance against DC3000 by antagonizing COI1 activity. AvrB targets RAR1 to trigger AvrB-dependent leaf chlorosis and enhanced bacterial growth. The AvrB-dependent enhanced bacterial growth but not leaf chlorosis requires COI1, suggesting that AvrB targets JA signaling pathway to promote parasitism.

Master of Science / Department of Plant Pathology / Karen A. Garrett / While plants are typically exposed to multiple stressors in the field, studies of genome-wide gene expression and phytohormone responses in wild plant species exposed to multiple stressors are rare. Our objectives were to determine the effects of drought and rust stress on gene expression in Andropogon gerardii, the dominant grass in tallgrass prairie, and associated levels of phytohormone production. In a factorial design, plants experiencing drought or non-drought conditions were either inoculated with the rust pathogen Puccinia andropogonis or not inoculated. Gene expression was evaluated with maize microarrays. Drought-stressed plants significantly decreased expression of genes associated with photosynthesis and the hypersensitive response, while expression of genes associated with chaperones and heat-shock proteins increased. No significant differences in gene expression in response to the rust treatment were detected using a mixed model analysis of variance and false discovery rate protection, probably because of the low infection rate. Phytohormone production increased when both stresses were present. The rust treatment significantly increased benzoic acid (BA) production in the presence of drought, while the drought treatment alone significantly increased salicylic acid (SA) production. Leaf tips usually had higher levels of all phytohormones in all treatments and the leaf section evaluated had a larger effect on phytohormone level than did the treatments applied.

Master of Science / Department of Horticulture, Forestry, and Recreation Resources / Edward E. Carey / With acceptance and utilization of chemical pesticides declining, some vegetable producers are turning to alternative methods to manage plant health issues. Compost tea (CT) has provided control of some foliar pathogens and may provide benefits beyond disease suppression. Despite an increasing body of popular and scientific literature focusing on CT as a biological control option for growers, information on the efficacy of CT is still lacking for many pathosystems. In this study, field trials were conducted to evaluate the efficacy of CT on Septoria lycopersici, causal agent of Septoria leaf spot on tomato, in Kansas, in 2006 and 2007. Previous research done at KSU with a similar CT showed adequate control of this pathogen in field and greenhouse studies conducted. Additional work to develop a rapid screening method for efficacy of CT formulations was carried out in the greenhouse at Manhattan, KS. CT sprayed weekly on tomato plants prior to and after disease onset led to no significant difference in control of the pathogen compared to untreated controls. A contact fungicide (chlorothalonil) provided significant control of the pathogen in 2007, but not in 2006. These results contrast with those obtained in previous K-State research. It is difficult to assess why such striking differences were obtained, but the variation in these results point to the need to identify optimal recipes of CT for this pathosystem. Preliminary investigations standardized plant age, inoculum concentration, incubation conditions, and incubation interval for measurable Septoria leaf spot disease development on young tomato plants in the greenhouse. Ingredients of the field-tested CT were used to make a variety of CTs to test using the greenhouse-screening assay. Further work on identifying effective CT recipes is needed to substantiate the validity of this screening protocol and to evaluate the correlation of this method with disease suppression in the field.

Doctor of Philosophy / Department of Plant Pathology / Robert L. Bowden / John F. Leslie / Gibberella zeae is an ascomycete filamentous fungus and the major cause of Fusarium head blight, also called scab, in small grains. This dissertation contains three related studies of G. zeae. In the first, the genetic map was aligned with the first assembly of the genomic sequence released by The Broad Institute (Cambridge, MA). Approximately 99% of the sequence was anchored to the genetic map, indicating the high quality of the sequence assembly and validity of the genetic map. The alignments grouped the linkage groups and supercontigs into four sets, which is consistent with the hypothesis that there are four chromosomes in this fungus. In the second, the sex pheromone precursor genes (ppg1 and ppg2) and the pheromone receptor genes (pre1 and pre2) were identified and characterized. Deletion of ppg1 or pre2 ([Delta]ppg1 or [Delta]pre2 strains) reduced the number of perithecia produced by self-fertilization, but did not completely block perithecial formation. The proportion of crosses resulting from outcrossing increased when the [Delta]ppg1 strains were used as the female in crosses with male strains containing an intact ppg1 gene. [Delta]ppg2 and [Delta]pre1 mutants had no discernable effect on morphological phenotype or self-fertilization. Thus, one of the pheromone/receptor pairs (ppg1/pre2) found in many Ascomycetes has a role in, but is not essential for, selfing or outcrossing in G. zeae, whereas the other pheromone/receptor pair (ppg2/pre1) no longer has a detectable function in sexual reproduction. In the third study, spore germination of G. zeae was tested in the presence of α- factor-like pheromone peptides of G. zeae or N. crassa. The pheromone peptide of N. crassa more efficiently inhibited spore germination than did the peptide from G. zeae. Arginine and lysine residues were the most important determinants in blocking spore germination. In conclusion, this research has validated the genetic map and the genomic assembly of G. zeae, characterized sex pheromone functions and characterized pheromone peptide ability to inhibit spore germination. The pheromone peptides of G. zeae and N. crassa may be useful as control agents for G. zeae and pheromone peptide efficacy might be further enhanced by judicious substitutions for some of the amino acids.

Doctor of Philosophy / Department of Plant Pathology / William W. Bockus / James P. Stack / Wheat blast, caused by the Triticum pathotype of Magnaporthe oryzae (MoT), is a serious disease of wheat causing yield failures and significant economic losses during epidemic years in Brazil, Paraguay, and Bolivia. Although outbreaks occur only sporadically, wheat blast is considered a major disease affecting wheat production in South America and may be a threat to the wheat crop in the United States. Wheat is a major crop in the U.S. and wheat exports from the U.S. are important to food security of several countries around the World. Thus, it is important to understand the potential for MoT entry and establishment into the U.S. and to test U.S. wheat cultivars for susceptibility to MoT. The hypotheses of this research project were a) importing wheat grain from Brazil does not pose a risk for MoT establishment in the U.S., and b) resistance to MoT head infection does not exist in U.S. hard red winter wheat elite cultivars. Quantitative pathway analysis models were used to estimate the risk of MoT entry and establishment, in the coterminous U.S. and in a more targeted area within southeast North Carolina, via the importation of wheat grain from Brazil. The pathway model predicted that significant risk for MoT entry and establishment exists in some areas of the U.S. However, in approximately 60% of the coterminous U.S. winter wheat production areas the risk of MoT establishment was estimated to be zero. With respect to winter wheat growing areas in the U.S., conditions for MoT establishment and wheat blast outbreak occur only in small, restricted geographic areas. A higher resolution pathway analysis based on a ground transportation corridor in North Carolina indicated that conditions for MoT establishment exist seven out of ten years. Among U.S. cultivars tested, a continuum in severity to head blast was observed; cultivars Everest and Karl 92 were highly susceptible with more than 90% disease severity, while cultivars PostRock, Jackpot, Overley, Jagalene, Jagger, and Santa Fe showed less than 3% infection.

Doctor of Philosophy / Department of Plant Pathology / John F. Leslie / Members of the genus Fusarium are widely distributed in many geographic regions of the world. This genus includes plant pathogens of many important cereal crops, e.g., wheat, maize, rice and sorghum, and of other native and economically important plants. From culture collections at Kansas State University and Universiti Sains Malaysia, strains from Southeast Asia, primarily from Malaysia and Thailand, associated with mango malformation disease, bakanae disease of rice, and stalk rot of sorghum were analyzed in sexual crosses and molecular diagnostics, e.g., Amplified Fragment Length Polymorphisms (AFLPs). Fusarium proliferatum was recovered from all three crops, with each crop also yielding some species unique to the crop, e.g. F. fujikuroi from rice, F. thapsinum from sorghum, and F. mangiferae from mango. These results are consistent with hypotheses that F. proliferatum has a wide host range while other species have much more limited host preferences. The absence from our samples of species associated with these diseases in other parts of the world suggests policies should be developed to reduce the chances of introduction of novel pathogens into Southeast Asia. Fusarium fujikuroi and F. proliferatum are closely related. They usually can be separated by sexual cross-fertility and DNA sequence analysis. However, some strains can cross irregularly and with poor fertility to produce viable interspecific hybrids. From a laboratory cross between F. fujikuroi FGSC8932 and F. proliferatum FGSC7615, 533 progeny were collected. These progeny were characterized for their AFLP genotype, mating type, gibberellic acid production, and pathogenicity on rice, onions, and apples. A recombination-based map from this interspecific cross was constructed. QTLs associated with gibberellic acid production, rice pathogenicity, and onion pathogenicity were identified. Gene segregation amongst the progeny of the F. fujikuroi × F. proliferatum cross was distorted towards F. proliferatum. Both novel and transgressive pathogenicity phenotypes were detected. Overall, this research demonstrates the potential threats that can result from an interspecific cross. These threats include pathogens with novel toxin profiles, new pathogenicity phenotypes, and more virulent strains. The variation observed among the progeny may enable isolation and characterization of genetic factors that have a role in pathogenicity, toxin production, and host specificity.

Doctor of Philosophy / Genetics Interdepartmental Program / Bikram S. Gill / Leaf rust, caused by fungus Puccinia triticina, is an important foliar disease of wheat worldwide. Breeding for race-nonspecific resistant cultivars is the best strategy to combat this disease. Aegilops tauschii, D genome donor of hexaploid wheat, has provided resistance to several pests and pathogens of wheat. To identify potentially new adult plant resistance (APR) genes, 371 geographically diverse Ae. tauschii accessions were evaluated in field with leaf rust (LR) composite culture of predominant races. Accessions from Afghanistan only displayed APR whereas both seedling resistance and APR were common in the Caspian Sea region. Seventeen accessions with high APR were selected for production of synthetic hexaploid wheat (SHW), using ‘TetraPrelude’ and/or ‘TetraThatcher’ as tetraploid parents. Six SHWs were produced and evaluated for APR to LR and resistance to tan spot at seedling stage. Genetic analysis and mapping of APR introgressed from accession TA2474 was investigated in recombinant inbred lines (RIL) population derived from cross between SHW, TA4161-L3 and spring wheat cultivar, ‘WL711’. Genotyping-by-sequencing approach was used to genotype the RILs. Maximum disease severity (MDS) for LR was significantly correlated among all experiments and APR to LR was highly heritable trait in this population. Nine genomic regions significantly associated with APR to LR were QLr.ksu-1AL, QLr.ksu-1BS, QLr.ksu-1BL.1, QLr.ksu-1BL.2, QLr.ksu-2DS, QLr.ksu-2DL, QLr.ksu-5AL, QLr.ksu-5DL and QLr.ksu-6BL. Association of QLr.ksu-1BL.1 with marker Xwmc44 indicated this locus could be slow-rusting APR gene, Lr46/Yr29. QTLs detected on 2DS, 2DL and 5DL were contributed by TA4161-L3 and are novel, along with QLr.ksu-5AL. Tan spot, caused by necrotrophic fungus, Pyrenophora tritici-repentis, has recently emerged as a damaging disease of wheat worldwide. To identify QTLs associated with resistance to Race 1 of P. tritici-repentis, F[subscript]2:3 population derived from cross between SHW, TA4161-L1 and winter wheat cultivar, ‘TAM105’ was used. Two major effect QTLs, QTs.ksu-1AS.1 and QTs.ksu-7AS were significantly associated with tan spot resistance and contributed by TA4161-L1. QTs.ksu-7AS is a novel QTL and explained 17% of the phenotypic variation. Novel QTLs for APR to LR and tan spot identified in SHWs add new variation for broadening the gene pool of wheat and providing resources for breeding of durable resistant cultivars.

Master of Science / Department of Entomology / David C. Margolies / James R. Nechols / A two-year greenhouse experiment was conducted to compare the relative effectiveness of biological control versus chemical control for western flower thrips, Frankliniella occidentalis, as a means of reducing the spread of Tomato spotted wilt virus (TSWV) on tomatoes. To compare efficacy of different thrips management tactics for reducing TSWV incidence, tomatoes were subjected to one of three treatments: 1) biological control based on weekly releases of the predatory mite, Amblyseius cucumeris, at a commercially-recommended rate, 2) a single chemical treatment with Conserve®, a spinosad formulation, or 3) no treatment. TSWV was introduced into the greenhouse either by starting with 20% of the crop already infected and releasing non-viruliferous thrips, or by making a single release of viruliferous thrips. Analyses were done among thrips management tactics for each virus introduction method to examine the cumulative number of weeks plants were infected, the weekly proportion of infected plants, and total marketable yield. The effects of different virus introduction methods were also compared. A comparison of virus introduction methods showed that, among all plants, the average number of weeks they were infected by TSWV was significantly lower when virus was introduced through infected plants than by infected thrips. In addition, when virus was introduced by infected thrips, a significantly greater proportion of plants were infected in any given week than when virus was introduced on infected plants. Finally, crop yields were significantly lower when virus was introduced via infected thrips than on infected plants. Among thrips management methods, plants were infected for significantly less time, and the proportion infected was lower in any given week, when biological or chemical control was applied compared to no thrips management. Tomato yields were not affected by thrips management tactic. There was no significant difference between biological and chemical control in the length of time that plants showed symptoms. However, the proportion of infected plants was marginally greater with biological control in weeks 4 and 5 than with chemical control; differences were not significant thereafter. My findings suggest that inundative releases of biological control may provide as adequate a level of protection from TSWV as chemical control in commercial greenhouse tomato crops.

Master of Science / Department of Plant Pathology / Karen A. Garrett / Ari Jumpponen / Traditional fallow periods in the Bolivian highlands are being shortened in an effort to increase short-term crop yields, with potential long-term impacts on soil communities. Using 454-pyrosequencing, we characterized fungal and bacterial community responses to (1) the length of fallow period and (2) the presence of the plants Parasthrephia sp. or Baccharis sp. (both locally known as ‘thola’). Thola is widely considered by farmers as beneficial to soil health, although it is also frequently harvested as a source of fuel by farmers. Soils in one study area, Ancoraimes, had higher levels of organic matter, nitrogen and other macronutrients compared to the other study area, Umala. In our analyses, Ancoraimes soils supported more diverse fungal communities, whereas Umala had more diverse bacterial communities. Unexpectedly, the longer fallow periods were associated with lower fungal diversity in Umala and lower bacterial diversity in Ancoraimes. Fungi assigned to genera Verticillium, Didymella, and Alternaria, and bacteria assigned to genera Paenibacillus, Segetibacter, and Bacillariophyta decreased in abundance with longer fallow period. The presence of thola did not significantly affect overall soil fungal or bacterial diversity, but did increase the frequency of some genera such as Fusarium and Bradyrhizobium. Our results suggest that fallow period has a range of effects on microbial communities, and that the removal of thola from the fields impacts the dynamics of the soil microbial communities.

Texte remanié de : Dissertation : ? : Philosophische Fakultät IV (Sprach- und Lietraturwessenschaften) der Universität Regensburg : 2003. / Bibliogr. p. [311]-332. Notes bibliogr. Index.