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  • ItemEmbargo
    Involvement of CYP72A219 in herbicide-resistant Palmer amaranth and the role of P450 reductase in the mechanism of metabolic resistance
    (Colorado State University. Libraries, 2023) Rigon, Carlos A. G., author; Gaines, Todd A., advisor; Dayan, Franck E., advisor; Beffa, Roland, committee member; Peebles, Christie, committee member
    Herbicide resistance in weeds poses a major challenge to modern agriculture worldwide, impacting effective weed control strategies. Metabolic resistance stands out as the major and more complex resistance mechanism due to its ability to metabolize a wide range of herbicides within weed species. Metabolic resistance involves herbicide metabolism through three key phases: activation, conjugation, and sequestration. These phases involve the action of important enzymes such as cytochrome P450 monooxygenases, glutathione S-transferases, and ABC transporters. Metabolic resistance mechanisms have gained prominence in the past decade, posing significant challenges to sustainable agriculture and weed management practices. Amaranthus palmeri (Palmer amaranth) one of the most troublesome weeds globally has evolved metabolic resistance to HPPD inhibitor tembotrione. Understanding and addressing the mechanism are crucial for developing effective strategies to combat herbicide resistance and ensure global crop production. In the present study, four upregulated P450 genes were identified in HPPD-resistant Palmer amaranth from Nebraska (NER), a troublesome weed species. Among these genes, CYP72A219_4284 demonstrated the ability to deactivate the herbicide tembotrione in a heterologous system. This gene was also upregulated in metabolic HPPD-resistant Palmer amaranth plants from different fields across the United States, indicating its involvement in conferring herbicide resistance. Our study also investigated the regulation of these resistance genes, including the promoter sequences and transcription factors involved. Additionally, quantitative trait loci associated with herbicide resistance were identified. This work represents the first identification and validation of genes responsible for herbicide metabolism in Palmer amaranth. Validation of the metabolic resistant gene and the exploration of regulatory mechanisms contribute to a better understanding of metabolic herbicide resistance in weeds, facilitating the development of effective weed management strategies. Cytochrome P450 reductase (CPR), an essential enzyme localized in the endoplasmic reticulum, provides electrons for P450 enzymes during monooxygenase reactions. The transfer of electrons from NADPH to the P450 active site occurs through a complex CPR:P450 interaction. Despite the numerous P450 genes in plant genomes, CPR genes are limited, typically consisting of two or three copies. In Arabidopsis, the two CPR genes, ATR1 and ATR2, have distinct roles in primary and inducible metabolism, respectively. Our study investigated the function of ATR1 and ATR2 in transgenic Arabidopsis plants overexpressing the CYP81A12, which is known to metabolize a wide range of herbicides. The hypothesis was that silencing these ATR1 or ATR2 genes would lead to a reduction of P450 activity involved in herbicide metabolism. ATR1 predominantly transfers electrons to CYP81A12, as knocking down ATR1 led to a significant reduction in herbicide resistance. Knockouts of the ATR2 gene also resulted in decreased herbicide resistance, although the effect was less pronounced. Variation in the number and function of CPR genes among different weed species suggests diverse genetic pressures and potential targets for herbicide resistance management. Inhibition of CPR activity could be a promising approach to restore herbicide effectiveness against metabolic herbicide-resistant weeds. This is the first study to our knowledge that explores the involvement of CPR genes in herbicide resistance in weeds, providing valuable insights into their crucial role. The findings significantly advance our understanding of the mechanisms underlying CPR-mediated herbicide resistance and offer potential targets for the development of effective weed management strategies.
  • ItemOpen Access
    Developing integrated pest management tactics for alfalfa mosaic virus and its aphid vector in chile peppers
    (Colorado State University. Libraries, 2023) Janecek, Taylor, author; Szczepaniec, Adrianna, advisor; Nachappa, Punya, committee member; Uchanski, Mark, committee member
    Alfalfa mosaic virus (AMV, Bromoviridae: Alfamovirus) is a virus transmitted to plants by aphids in a non-persistent manner. The virus was first identified in chile peppers Capsicum annuum L. (Solanales: Solanaceae) in Southern Colorado in 2019. The goal of this research was to explore management strategies to suppress the virus given its devastating impact on the yield and quality of chile peppers. The objectives were to: 1) determine whether chile peppers have innate resistance to AMV, 2) test the effectiveness of host plant resistance and planting date modifications to suppress the virus in the field, 3) determine whether AMV is seed transmissible, and 4) survey abundance and diversity of aphids (Hemiptera: Aphididae) that likely transmit AMV in the system. In the greenhouse experiment, I found significant differences among varieties of chile peppers in the severity of AMV symptoms and identified a variety suitable for a field experiment. In the field, I found that the susceptible variety, Joe Parker, which tended to have high AMV symptoms in the greenhouse, was also highly susceptible to AMV in the field. Conversely, Mira Sol, which appeared to have resistance to the virus in the greenhouse screening assay had low incidence of AMV symptoms and low AMV titers in the field as well. Planting date also played an important role in symptom severity, where late planted peppers (mid-June) had significantly lower severity of AMV symptoms than peppers planted at conventional and early planting dates (the middle and end of May). Despite this, the yield and quality of peppers planted early was significantly greater than that of peppers planted later in the season. In addition, there was evidence of seed transmission of AMV in chile peppers, with 10% for Mira Sol and 2% for Joe Parker, from seeds collected from infected peppers had AMV. Lastly, I found high diversity of aphid species within my experimental plots (14-16 species) and lower diversity in nearby alfalfa fields (4-5 species). Moreover, severity and titers of AMV were positively correlated with earlier planting date, which was likely related to higher aphid densities early in the season. This research contributed to formulating integrated tactics that chile pepper producers can implement in their production to suppress the impact of AMV on the crop. Finally, this is the first report of AMV transmission through seed in peppers and is the first study describing this pathosystems in Colorado.
  • ItemEmbargo
    Sweet surprise: the search for genes conferring beet curly top virus resistance
    (Colorado State University. Libraries, 2023) Withycombe, Jordan, author; Nachappa, Punya, advisor; Nalam, Vamsi, committee member; Nishimura, Marc, committee member; Dorn, Kevin, committee member
    Sugar beets (Beta vulgaris L.) are grown across the western United States and suffer economic loss annually to curly top disease. Curly top disease is caused by the beet curly top virus (BCTV) and is spread by the only known insect vector the beet leafhopper, Circulifer tenellus Baker (BLH). Current management strategies for BCTV include chemical control using neonicotinoid seed treatments and foliar insecticidal sprays, as well as the use of BCTV-resistant sugar beet varieties. However, the underlying genetic mechanism surrounding resistance in sugar beet is unknown. The overarching goal of this study was to identify the mechanism of resistance in sugar beet to BCTV and identify potential genes conferring resistance. The objectives for this study were: 1) classify the nature of BCTV resistance in a resistant (EL10) and susceptible (FC709-2) genotype of sugar beet using host suitability and host preference insect assays, as well as assess viral load within each genotype and 2) characterize the transcriptional response to BCTV infection using RNA-sequencing. To classify the nature of BCTV resistance in each genotype of sugar beet, host suitability and preference assays were conducted using virus infected and uninfected BLH. In host suitability assays, the percentage of surviving BLH adults and the number of nymphs produced when reared on a single plant of either genotype was determined over a 3-week period. There was no difference in adult survival, or the number of nymphs produced on either genotype for the virus infected or uninfected leafhoppers. Host preference assays were used to assess settling behavior of BLH over time when given a choice between the two genotypes. It was concluded that virus infected leafhoppers had a clear choice to settle on the susceptible genotype at all timepoints after 4 hours, while uninfected leafhoppers did not make as strong of a settling choice. Average viral load for each genotype across three timepoints was estimated using qPCR. The results showed that the average viral load increased in each genotype over time, yet there was no difference in the average viral load between the genotypes at any individual timepoint. The global transcriptional response to BCTV infection over time for a resistant and susceptible genotype of sugar beet was conducted using RNA-sequencing technology. Mock-inoculated and BCTV-inoculated plants from each genotype were sampled on day 1, 7 or 14 post inoculation resulting in the preparation of 36 mRNA sequencing libraries. Comparison between mock-inoculated and BCTV-inoculated plants of each genotype and timepoint were conducted separately to generate six list of differentially expressed transcripts (DETs). Each transcript was annotated with a description and further classified for its role in the plant biological, cellular or molecular processes. The results showed that both genotypes of sugar beet had a dynamic response to BCTV infection over time, although there was minimal overlap between the responses to one another. EL10, the resistant genotype, had DETs associated with phytohormone production including jasmonic acid and abscisic acid, along with proteins linked to stress reduction and the downregulation of plant primary metabolic processes. In contrast FC709-2, the susceptible genotype, was found to produce opposing phytohormones like salicylic acid and auxins, as well as the production of volatile organic compounds and an increase of primary plant metabolic processes. These opposing responses shed light on the differences in the transcriptional response of a resistant and susceptible genotype of sugar beet. Understanding and classifying the mechanisms of resistance or susceptibility to BCTV infection in sugar beet is beneficial to researchers and plant breeders as it provides a basis for further exploration of the host plant-virus-vector interactions.
  • ItemEmbargo
    Integrated weed management: insights from a weed resistance survey and non-chemical weed seed control in the Central Great Plains
    (Colorado State University. Libraries, 2023) Simões Araujo, André Lucas, author; Gaines, Todd, advisor; Dayan, Franck, committee member; Fonte, Steven, committee member
    With the impending release of genetically engineered sugar beet varieties with resistance to glyphosate, dicamba, and glufosinate, significant changes are expected in weed management practices, particularly with regards to in-crop weed control. Glyphosate is used during fallow and in-crop periods, while dicamba is commonly employed in fallow applications, specifically targeting glyphosate-resistant weed species. This study provides insights into the resistance status and frequency of resistance in problematic weed species to the three active ingredients in sugar beet systems across Colorado, Nebraska, and Wyoming. While numerous studies have highlighted the widespread prevalence of glyphosate-resistant kochia and Palmer amaranth across the United States, there is limited research focusing on these species within the context of a sugar beet system. Additionally, our findings reveal the first occurrence of glyphosate-resistant and dicamba-resistant Palmer amaranth populations in Colorado, and dicamba-resistant kochia populations within a sugar beet system. Furthermore, we report that all dicamba-resistant kochia populations tested in Colorado lack a known target-site resistance mechanism, suggesting the involvement of a novel resistance mechanism. Surveys assessing glufosinate resistance in the sugar beet system have not been conducted until now, and we provide valuable baseline information on the resistance frequency for this herbicide prior to an anticipated increase in glufosinate use. To address the widespread issue of herbicide resistance in various crop systems, it is crucial to adopt alternative strategies that mitigate resistance evolution and maintain the long-term effectiveness of available herbicides. One promising approach is chaff lining, a harvest weed seed control method that has gained popularity in Australia due to its effectiveness in reducing populations of herbicide-resistant ryegrass, especially when combined with other weed control methods. However, the efficacy of chaff lining may be influenced by several factors, including crop and environmental factors, as reported in Australian literature. Scientific studies assessing the applicability and effectiveness of chaff lining in the United States are limited. Recognizing this research gap and aiming to explore the potential of chaff lining, our study investigated its applicability in field settings within the Central Great Plains region of the United States. Through our research, we provide insights into chaff lining efficacy of and highlight the potential inconsistencies that may arise in suppressing weed seeds using this method. Notably, we demonstrate that various factors, including location and environmental conditions, may be involved and impact the effectiveness of chaff lining as a weed management strategy. These findings underscore the importance of integrating chaff lining with other weed management methods to achieve effective and sustainable weed control. Chaff lining, like any other weed management strategy, should not be solely relied upon. Instead, it should be implemented as part of an integrated approach to ensure its long-term effectiveness.
  • ItemEmbargo
    Uncoupling plant growth and defense through phytohormone crosstalk modification
    (Colorado State University. Libraries, 2023) Johnston, Grace Allen, author; Argueso, Cris, advisor; Leach, Jan, committee member; Prasad, Ashok, committee member
    Phytohormones are essential regulators of development and response to biotic and abiotic stresses. Activation of the plant immune system by pathogen attack often results in changes in plant growth, frequently leading to smaller plants with reduced seed set. Previously, we discovered that cytokinin (CK), a hormone known for its role in the regulation of cell division and plant growth, also has an important role in the activation of defense against pathogens through a synergistic interaction with the defense hormone salicylic acid (SA). Here, we address whether these two phytohormones also regulate the negative effect of immune activation on plant growth. Differential gene expression analysis and physiological assays were used to characterize the crosstalk between CK and SA in growth and defense in Arabidopsis thaliana plants with altered states of immunity. We show that the interplay between the phytohormones CK and SA regulates both defense responses to pathogens and plant development. Endogenous levels of these two hormones were modulated in the snc1 ckx3 ckx5 (s35) triple mutant. The three mutations result in increased CK and SA content simultaneously and yields a novel reproductive growth phenotype. When challenged with pathogens from diverse lifestyles, the s35 mutant conserves an autoimmune phenotype. Transcriptome analysis of s35 reproductive tissue reveals differential regulation of genes associated with nitrogen response and regulation of redox status. Our data suggests that the increased content of both CK and SA hormones contributes to a rebalancing of redox homeostasis and perception of nutrient availability within the shoot apical meristem (SAM), resulting in the uncoupling of reproductive growth and pathogen defense. Further experimentation and investigation into the mechanistic interactions mediating the balance between plant growth and defense could lead to implementation of phytohormone crosstalk engineering to target specific advancements in crop species.
  • ItemOpen Access
    Phytoalexin deficient4 (PAD4): a plant defense regulatory gene with distinct alternative splicing patterns in tomato (Solanum lycopersicum) and soybean (Glycine max)
    (Colorado State University. Libraries, 2023) Schmidt, Rebecca, author; Nalam, Vamsi, advisor; Argueso, Cristiana, committee member; Reddy, Anireddy, committee member
    Alternative splicing is an important post-transcriptional regulatory mechanism that contributes to a plant's ability to perceive and respond to a variety of biotic and abiotic stressors. Alternative splicing has a documented role in plant immunity, as many R genes, which are important for plant defense against specialized pathogens, undergo alternative splicing in response to pathogen perception. Despite this, the role of alternative splicing in other components of plant defense responses is not well documented. As transcriptome data diversify to include more species and conditions, the extent of alternative splicing in plants has become apparent. PHYTOALEXIN DEFICIENT4 (PAD4), plays an integral role in plant defense signaling to biotic stressors, and in regulating responses to abiotic stresses. PAD4 undergoes alternative splicing in Soybean (Glycine max). Additionally, the expression pattern of Glycine max PAD4, GmPAD4, and its splice variant GmPAD4-AS1 are further characterized in early growth stages. We hypothesize PAD4 produces full-length and alternatively spliced transcripts in multiple species, and that PAD4 gene structure may influence the occurrence of alternatively spliced transcripts. Here we characterize alternative splicing of PAD4 in tomato (Solanum lycopersicum), identifying two splice variants. We also investigate the conservation of PAD4 intron-exon structure conservation across diverse species. PAD4 expression patterns are characterized using available expression data.
  • ItemOpen Access
    Factors affecting potato early dying in the San Luis Valley, Colorado
    (Colorado State University. Libraries, 1994) Davidson, Robert Day, author; Harrison, Monty D., committee member; Knutson, Kenneth W., committee member; McIntyre, Gary, committee member; Schwartz, Howard, committee member
    A three year comparison of Russet Burbank potatoes field grown in microplots containing combinations of Verticillium (V), Erwinia carotovora subsp. carotovora (Ecc) and E. c. subsp. atroseptica (Eca) inoculated into the seed and, in the case of Ecc, also applied in the irrigation water was conducted. Three levels of irrigation were used for all treatments. Verticillium appeared to have minimal impact. Plant stand, height and tuber numbers were not significantly different among treatments. Verticillium wilt or potato early dying (PED) progress was, in general, significantly greater than normal maturity in controls. Yield was significantly depressed in only one of three years. In the two warmest years, yields increased as irrigation increased. In the coolest year, the reverse was true with yields increasing as irrigation decreased. Greenhouse studies completed in 1989 showed that air temperature can play a major role in PED symptom development. Specialized chambers were held at three different air temperatures 15, 25 and 30°C with treatments similar to those used in the field studies. PED symptom progression was fastest under the highest temperature (30°C), but did not reach the same level of severity as found under lower temperatures. PED was greatest under the 25°C temperature, while almost non-existent at 15°C. Soil fumigation with BusanR resulted in reductions in Verticillium microsclerotial counts which were maintained for at least two further growing seasons. However, unfumigated soil also showed similar reductions in microsclerotial counts during the same time period. Ecc and Eca appeared to be the primary pathogens causing disease in the PED complex in the San Luis Valley, while V. dahliae appeared to have a secondary role. Synergistic V + Erwinia interactions were found. Progress of PED in V + Erwinia seed treatments was similar to or sometimes significantly greater than PED progress in plants exposed to either Erwinia or Verticillium alone. Yields were significantly lower than the control or either pathogen alone in three of the V + Erwinia seed treatments. Under pathogen combination treatments there was an obvious trend toward reduced yields and, as irrigation increased, toward increased PED severity, disease progress and yield loss. Erwinia carotovora treatments had significant reductions in stand in two of three years with Erwinia seed treatments, but not when Ecc was introduced through irrigation water. Overall, as irrigation and Erwinia inoculum density increased, stand loss increased. Tuber numbers and yields were, in general, depressed significantly under Erwinia seed treatments. In 1990 the high Ecc irrigation treatment under optimum moisture also significantly decreased yields. Erwinia infection of daughter tubers was greatest in the two cooler seasons. Overall, the higher the inoculum level used or the greater the water stress, the higher the infection rate.
  • ItemOpen Access
    Soil microbial communities associated with forest root diseases and Rocky Mountain forests
    (Colorado State University. Libraries, 2022) Lalande, Bradley, author; Stewart, Jane, advisor; Stromberger, Mary, committee member; Tinkham, Wade, committee member; Trivedi, Pankaj, committee member
    To view the abstract, please see the full text of the document.
  • ItemOpen Access
    Developing integrated pest management (IPM) strategies for hemp russet mite (Aculops cannabicola Farkas) on hemp (Cannabis sativa L.)
    (Colorado State University. Libraries, 2022) Hayes, Christopher, author; Nachappa, Punya, advisor; Cranshaw, Whitney, advisor; Prenni, Jessica, committee member
    Cannabis sativa L. is a plant that is rapidly becoming a crop of global agricultural importance. However, because of the historical peculiar regulatory status of this crop little has been developed on the pests and pest management needs of the crop. Among the more serious pests that have become established with this plant is hemp russet mite (HRM) Aculops cannabicola Farkas (Acari: Eriophyidae). In order to assess the efficacy of various IPM approaches to mitigating HRM infestations, a series of field and lab experiments were conducted including: 1) evaluation of the effects of sprays of sulfur on control of hemp russet mite, yield, and cannabinoid produduction; 2) evaluation of the efficacy of field release of the phytoseiid mites Amblyseius andersoni, A. swirskii, Neoseiulus fallacis, and N. californicus on HRM-infested hemp plants; and 3) evaluation of hot water immersion as a potential disinfestation method for HRM-infested cuttings used in propagation. The results of the sulfur sprays in field trials showed excellent ability to suppress HRM by up to 98%. Yields of treated plants improved by up to 33% and there was a further increase in the percentage of phytocannabinoids by up to 45% relative to untreated plants. Greatest effects were seen in all trials with plants receiving two applications, one during the vegetative period in July and the second at the initiation of flower production in August. Mass releases of N. fallacis and A. swirskii, but not N. californicus and A. amblysieus, did produce a significant reduction in HRM populations, but no treatments significantly affected yield or percentage of phytocannabinoids, relative to untreated plants. No reproduction was observed of any of the released mites on HRM-infested plants. Immersion treatments to disinfest cuttings included use of a water bath at temperatures of 106°F or 109°F for 10 or 15 minutes, and dips in room temperature surfactant solution of Dr. Bonner's Pure-Castile lavender soap at 1.0% and 0.1% concentrations. All treatments were able to cause significant reduction of HRM on infested hemp cuttings, although none caused complete elimination. No phytotoxicity, as evidenced by effects on subsequent rooting, were observed with any treatment. This study provides novel effective approaches to mitigating HRM at multiple stages in hemp production operations. Outcomes of this research may provide hemp producers and other stakeholders with key pest management strategies needed to produce hemp plants that are free of HRM.
  • ItemOpen Access
    The role of chemical canopy sprays and irrigation methods on the incidence of the perennial canker, Cytospora plurivora in western Colorado peach orchards
    (Colorado State University. Libraries, 2022) Wright, Sean, author; Stewart, Jane E., advisor; Charkowski, Amy, committee member; Uchanski, Mark, committee member
    Cytospora plurivora is a secondary pathogen that has reached near epidemic levels in peach orchards on the western slope of Colorado. C. plurivora is responsible for Cytospora canker disease and is a limiting factor in peach production in the Grand Valley. Peach growers have limited management methods available to combat this disease, which prompted an investigation into irrigation practices as well as prophylactic chemical sprays following freeze events. In late 2020, the western slope received a freeze event that caused severe damage to peach shoots, buds, and twigs. Freeze damage provides infection courts within tree tissues that C. plurivora can infect. This freeze event prompted growers to apply prophylactic chemical sprays of Captan, lime sulfur, and lime sulfur with the addition of NuFilm. An efficacy threshold of three-months post chemical spray was determined for both Captan and lime sulfur treatments. Lime sulfur with the addition of NuFilm showed a loss of efficacy at two-months post spray. Additionally, an investigation into the movement of C. plurivora conidia under differing irrigation techniques was conducted. Both drip and micro-sprinkler treatments had positive detections for C. plurivora over the course of the study. In these studies, conidia traveled much greater distances than previously shown, traveling up to 135m from the closest canker. Understanding how chemical canopy sprays and different watering practices affect the incidence of Cytospora canker disease will assist in preserving the peach industry on the western slope of Colorado.
  • ItemOpen Access
    TALE-bound QTL: a computational investigation of bacterial effector association with resistance quantitative trait loci in Oryza sativa
    (Colorado State University. Libraries, 2022) Sharkey, Jacob Emmett, author; Leach, Jan E., advisor; Huerta, Alejandra I., committee member; Nishimura, Marc, committee member; Roberts, Robyn, committee member
    Durable resistance to Xanthomonas oryzae pathovars oryzae (Xoo) and oryzicola (Xoc), which cause bacterial blight and bacterial leaf streak, respectively, is highly sought after in rice (Oryza sativa) due to the pathogens ability to impact maximum attainable yields. Regions of the rice genome associated with quantitative resistance to multiple strains of Xoo and Xoc, known as quantitative trait loci (QTL), were previously identified using a multi-parent advanced generation intercross (MAGIC) rice population and a combination of genome wide association studies and interval mapping. These QTL have been associated with decreased lesion lengths by Xoc and Xoo on rice. What remains unknown is the molecular basis for the induction of genes under these QTL during pathogen infection. Considering our biological question "what is the molecular basis for regulation of resistance QTL associated with Xoo and Xoc?", we predicted that part of the answer could be found by investigating the bacteria's direct interaction with the O. sativa genome. Upon infection, Xoo and Xoc injects the host with DNA-binding TALE (transcription activator-like effector) proteins. These effectors, when bound to their target plant gene promoter, induce gene transcription. We hypothesize that differential interactions of TALE with promoters of rice genes under the QTL lead to the resistant/susceptible phenotypes exhibited across varieties. To test this, we designed a pipeline that predicts TALE-regulated candidate genes involved in quantitative resistance. This pipeline identifies genes that meet three criteria: (1) the presence of a binding site for an X. oryzae TALE in the gene's promoter, a strong correlation between binding site presence, and disease phenotypes and overlap of the gene with a resistance QTL. We used this pipeline with genomic and phenotypic data for the eight MAGIC founders to identify candidate genes involved in resistance against seven Xoo and Xoc strains. Candidate genes identified include ones encoding a patatin-like phospholipase and multiple NB-ARC containing proteins such as the Mla1 protein. Here, we exploit the abundant genomic data for the rice-X. oryzae systems and the ability to predict direct associations between bacterial proteins and plant genomes, to propose a method that could streamline the identification of genes involved in quantitative resistance to TALE- harboring Xanthomonas.
  • ItemOpen Access
    Pavement, pests, & parasitoids, oh my! Elm herbivores and their natural enemies in the urban forest
    (Colorado State University. Libraries, 2022) Buenrostro, Jacqueline, author; Hufbauer, Ruth, advisor; Cranshaw, Whitney, committee member; Redmond, Miranda, committee member; Stewart, Jane, committee member
    Urban areas are the fastest growing habitat type in the world, and an increasing proportion of the United States and global population lives in urban areas. Urban forests provide essential ecosystem services to rapidly expanding urban populations, but their health is threatened by damaging herbivory from non-native, invasive insects. To address this problem, my masters research investigated two critical questions that limit our ability to sustainably manage invasive insects in urban forest ecosystems: (1) How do urban environments impact the density of invasive herbivores in the urban forest? and (2) Are predator and parasitoid natural enemies present, and, if so, what is the level of parasitism? I investigated these questions in the context of the elm-herbivore study system, analyzing a globally distributed host tree and its complex of invasive insect pests in Colorado, USA. In my first chapter, I address the first question and explore how a variety of environmental factors that vary across urban habitats influence the density of several invasive insects. Specifically, I evaluate how vegetational complexity, distance to buildings, impervious surface, canopy temperature, host availability, and density of co-occurring herbivores impact three invasive pests of elm trees: the elm leaf beetle Xanthogaleruca luteola Müller (Coleoptera: Chrysomelidae), the European elm flea weevil Orchestes steppensis Korotyaev (Coleoptera: Curculionidae), and the elm leafminer Fenusa ulmi Sundevall (Hymenoptera: Tenthredinidae). I found that insect responses to these factors were species-specific, and all environmental factors were associated with density of at least one pest species except for distance to buildings. Elm leafminer density decreased with higher temperatures and was influenced by an interaction between vegetational complexity and impervious surface. Elm flea weevil density increased with greater host availability, and elm leaf beetle density increased with higher temperatures. Both elm leaf beetle and elm flea weevil density decreased with greater leafminer density, suggesting that insect density is mediated by species interactions. Results of this study can be used to inform future tree planting efforts through the selection of "safe sites", or locations where trees will be less likely to experience damaging outbreaks of insect pests. Additionally, these results can be used to strategize preventative management on trees that are located in outbreak "hotspots", or locations where environmental conditions make trees predisposed to insect outbreaks. Finally, results of this study contribute to our knowledge of the dynamic ways in which multiple invasive insects interact in urban environments. This information will be especially valuable as non-native insect introductions continue to increase into the future. In my second chapter, I narrow my focus to two historically important and particularly damaging pests of elm, the elm leaf beetle and elm flea weevil, to address the second question listed above and explore the complex of natural enemies attacking these two pests. In many areas of these insects' invaded range, outbreaks severely damage elm hosts. Natural enemies are thought to be important in regulating elm leaf beetle and elm flea weevil populations in other regions, but whether natural enemies are present in Colorado is largely unknown. As such, the aim of chapter 2 was to identify which predators and parasitoids of these pests are present in Colorado, a state with frequent pest outbreaks and where the natural enemy community is almost entirely undescribed. In June – August 2021, I identified predators through field observations and laboratory feeding trials, finding seven species of predators from six arthropod orders that fed on elm leaf beetle or elm flea weevil. Additionally, I reared 58 elm leaf beetle egg clusters, 539 elm leaf beetle larvae, and 435 elm flea weevil mines to detect parasitoids. Two parasitoids of elm leaf beetle, the egg parasitoid Oomyzus gallerucae and the larval-adult parasitoid Erynniopsis antennata, are present in Colorado, representing novel records of these species in the state. However, combined parasitism of elm leaf beetle eggs and larvae was low at <3% across the season, with parasitoids nearly absent early in the season and peak parasitism occurring in late summer. I found five families of parasitoid wasps that emerged from leaves containing weevil mines: Chalcididae, Encyrtidae, Eulophidae, Euplemidae, and Pteromalidae. Parasitoids emerged from <20% of leaves containing weevil mines with almost no parasitism early in the season. Given the low parasitism rates and few predators observed in our study, it seems unlikely that predator and parasitoid natural enemies exert effective control over elm leaf beetle and elm flea weevil in Colorado. This finding challenges the assumption that natural enemies are a driving force of elm leaf beetle and elm flea weevil control in Colorado. Additional research is needed to confirm species identifications for parasitoids of the elm flea weevil, disentangle elm leaf beetle and elm flea weevil population dynamics, and establish effective and sustainable control methods amidst frequent pest outbreaks. Together, these two research projects enhance our knowledge of what triggers outbreaks of the elm leaf beetle Xanthogaleruca luteola, the elm flea weevil Orchestes steppensis, and the elm leafminer Fenusa ulmi in urban areas while also laying the groundwork for a renewed interest in biological control of elm leaf beetle and elm flea weevil. It is my hope that this work can be applied to other invasive insect pests in urban forest ecosystems and make urban forests more resilient in an era when they are increasingly vulnerable to insect attack.
  • ItemOpen Access
    Russian wheat aphid, Diuraphis noxia (Kurdjomov), ecology and reproduction on five noncultivated grass hosts in high elevation environments
    (Colorado State University. Libraries, 2010) Pucherelli, Sherri F., author; Peairs, Frank B., advisor; Cranshaw, Whitney, committee member; Hansen, Neil, committee member
    Russian wheat aphid ecology, occurrence, movement, and reproduction on five noncultivated grass hosts was examined in high elevation environments. The objectives of the first study were to: 1) identify Russian wheat aphid occurrence and abundance on five common grass hosts at elevations between 1,829- 2,743 m during June-October; 2) monitor Russian wheat aphid movement and flight patterns between 1,524- 2,743 m, with the use of suction traps; 3) describe the predator and competitor assemblages encountered by the Russian wheat aphid in high elevation environments; and 4) confirm Russian wheat aphid holocycly in North America by collecting sexual forms and eggs. Weekly aphid and predator collections were made from crested wheatgrass, intermediate wheatgrass, slender wheatgrass, western wheatgrass, and foxtail barley at 23 sites, between 1,829-2,743 min the Cache La Poudre River canyon, Larimer County, Colorado. Four suction traps were installed at 1,655, 1,992, 2,206, and 2,372 m to collect alate aphids. Russian wheat aphids were collected from all five grass hosts sampled. Russian wheat aphids were most prevalent in July and most commonly collected from crested wheatgrass. Other cereal aphid species were collected, including Diuraphis frequens, Sipha elegans, Rhopalosiphum padi, Sitobion avenae, and Schizaphis graminum. Sipha elegans was the most abundant aphid collected. The most Russian wheat aphids were collected in June in the trap at 1,655 m, coinciding with peak captures in nearby wheat production. Trap captures declined with increasing elevation. Russian wheat aphid populations in the Cache La Poudre canyon are likely maintained by both immigrant aphids and from local metapopulations within the canyon. Environmental conditions in high elevation environments could prompt the production of novel biotypes. The second study determined Russian wheat aphid biotype R W A2 reproductive and development rates on the same five noncultivated grass hosts to gain information about host quality, potential refuges and sources of selection pressure. Russian wheat aphid reproductive and development rates were measured at 18-24°C, on the five grass hosts sampled in the first study, and at 24-29°C, and 13-18°C on intermediate and crested wheatgrass. The intrinsic rates of increase for all five hosts were lower than those reported for susceptible and resistant wheats. Aphids feeding on crested and intermediate wheatgrass at the 13-18°C temperature had lower fecundity, fewer nymph production days, longer generational times, and lower intrinsic rate of increase than aphids feeding at the 18-24 °C temperature regime. Poor hosts pose greater selection pressures. The five noncultivated hosts tested were poor hosts in comparison to wheat, and their greater selection pressure might promote Russian wheat aphid genetic variability.
  • ItemOpen Access
    Mountain pine beetle-caused lodgepole pine mortality from the 1980's and subsequent fire occurrence in Colorado
    (Colorado State University. Libraries, 2010) West, Daniel R., author; William Jacob, advisor; Negron, Jose, advisor; Veblen, Thomas, committee member; Rocca, Monique, committee member
    A need for understanding the interaction between post-epidemic bark beetle forest stands and subsequent fire occurrence has escalated due to recent (1999-2010) unprecedented epidemic mountain pine beetle in lodgepole pine (Pinus contorta) forests of Colorado. Outbreaks of mountain pine beetle (Dendroctonus ponderosae Hopkins) populations in the early 1980' s resulted in substantial tree mortality across Colorado and provide a means to study past outbreak areas and subsequent fire occurrence. Beetle outbreaks on the Arapaho National Forest (NF) (1980-1987) and the White River National Forest (1981-1987), delineated by the USDA Forest Service Aerial Detection Survey (ADS), indicated approximately 76,900 ha were affected, with approximately 450,000 trees killed per forest. Mountain pine beetle-caused tree mortality is generally thought to increase subsequent fire occurrence and intensity but little scientific research supports this hypothesis. Thus, my objectives were to 1) determine whether there were differences in fire occurrences between lodgepole pine forests in Colorado, impacted or not impacted by previous outbreaks of D. ponderosae and 2) determine if fire occurrences in areas with mountain pine beetle-caused lodgepole pine mortality were related to topographic attributes, ignition type, and meteorological conditions. We used historic USDA Forest Service Aerial Detection Survey maps (1980 - 1990) in conjunction with USDA Forest Service digital fire location records to look for mountain pine beetle and ignition relationships. Sixty eight maps were scanned to spatially identify D. ponderosae-caused mortality in lodgepole pine forests over the Arapaho NF and White River NF. Using a GIS, the spatial relationship between mountain pine beetle caused mortality areas and subsequent fire occurrence was identified. During the summer of 2008, 57 ignition points were field assessed on the Arapaho NF and White River NF to verify the presence of mountain pine beetle-caused mortality prior to the fire as well as confirm the location of the recorded fire. Two of the 57 ignition points had evidence of trees killed by mountain pine beetle prior to the ignition. Tests for independence of fire occurrence and mountain pine beetle-caused mortality were conducted for the Arapaho NF and White River NF. Combined human and lightning-caused fire densities did not differ (a=0.05) between areas with and without mountain pine beetle-caused mortality on the Arapaho NF, however, there were more fires in areas with mortality from the mountain pine beetle than non-impacted areas on the White River NF. Densities of lightning-caused fires alone did not differ between nonbuffered and 50 m buffered mountain pine beetle-caused mortality areas and areas outside the mortality on the Arapaho NF or White River NF. Logistic regression was used to model the probability of an ignition occurring within the area of the 1980's mountain pine beetle-caused lodgepole pine mortality on each forest. Elevation of fire occurrence was the most significant variable explaining the cooccurrence of fire with mountain pine beetle-caused mortality. Spatial autocorrelation was significant within human and lightning-caused fires but not lightning-caused fires alone. Thus, lightning-caused fires were used to eliminate the non-random nature of human-caused fires. Probability density functions were created using elevation ranges of mountain pine beetle-caused lodgepole pine mortality, lightning-caused fires prior to 1980, and elevations of fire occurrences intersecting aerially detected mountain pine beetle-caused mortality post-1980 through 2005 ( during and post-outbreaks) to identify areas of highest probability independent of one another. Maximum probabilities for the occurrence of a lightning fire in a 1980' s mountain pine beetle-caused mortality area within the Arapaho NF was between 2710 m and 2815m while on the White River NF the highest probability was between 2600 m and 2900 m. Probability density functions for a lightning fire occurring in an area with mountain pine beetle-caused mortality were applied across each National Forest using a GIS. Our analysis suggests that 1980's mountain pine beetle-caused lodgepole pine mortality has not contributed to an increase in fire frequency over the subsequent twenty five years. The variability in fire occurrence and the homogeneity of mountain pine beetle outbreak-caused mortality, past and present, between the Arapaho NF and White River NF demonstrates the limited nature of comparability of this study to other locations.
  • ItemOpen Access
    Xenobiotics translocate in aquatic plants: a case study using three aquatic herbicides
    (Colorado State University. Libraries, 2022) Ortiz, Mirella F., author; Dayan, Franck, advisor; Nissen, Scott, advisor; Bedinger, Patricia, committee member; Heilman, Mark, committee member
    When invasive aquatic weeds dominate aquatic ecosystems there are numerous negative impacts. Milfoil (Myriophyllum spp.) and hydrilla [Hydrilla verticillata (L.f.) Royle] are the most costly aquatic plants to manage in the U.S. per year. These invasive plants form extensive surface canopies that negatively affect water quality and native plant communities, and can also impact recreational uses such as swimming, fishing, and boating. Synthetic auxins, such as 2,4-dichlorophenoxyacetic acid (2,4-D), have been widely used for selective control of milfoil since 1959. Since then, several populations of hybrid watermilfoil (M. sibiricum x M. spicatum; HWM) have showed lower sensitivity to this herbicide. In 2015, a HWM population with lower sensitivity to 2,4-D was found in Idaho, USA. Using the same 2,4-D-resistant population and a known susceptible Eurasian watermilfoil (M. spicatum; EWM) population from Colorado, the mechanism of 2,4-D resistance was examined by conducting 14C-2,4-D absorption, translocation, desorption, and metabolism experiments. 2,4-D resistance in HWM is not due to non-target-site resistance as no differences in herbicide absorption, translocation, desorption and/or metabolism were identified; therefore, target-site resistance is the most likely resistance mechanism. More research is needed to identify the molecular basis for the 2,4-D-resistant trait in HWM. Herbicide combinations are widely recommended to alleviate the evolution of herbicide resistance. The aquatic herbicide endothall is often used in combination with 2,4-D for HWM management as an effective control option and a resistance management strategy, but it is still unknown how combining herbicides might impact the behavior of each herbicide. Experiments combining radiolabeled with non-radiolabeled herbicides were conducted to evaluate herbicide absorption, accumulation, and translocation from shoots to roots in HWM. Endothall accumulation was not impacted when these herbicides were applied in combination, but its translocation from shoots to roots was reduced by 50% when applied in combination with 2,4-D. When 2,4-D, was applied in combination with endothall shoot absorption increased by 80%; however, 2,4-D movement from shoots to roots was reduced from 24.8% ± 2.6 to only 3.93% ± 0.4 when in the presence of endothall. The overreliance on a single mode of action resulted in evolved fluridone resistance in hydrilla in the late 1990s. 2,4-D is not effective for hydrilla control at label rates, but the most recently registered auxinic herbicide, florpyrauxifen-benzyl, is highly active against hydrilla. Where fluridone-resistant hydrilla is present, endothall is being used in combination with florpyrauxifen-benzyl for its control. In order to test experiments combining radiolabeled and non-radiolabeled endothall and florpyrauxifen-benzyl were conducted to evaluate herbicide absorption, accumulation, and translocation in two hydrilla biotypes, monoecious (MHV) and dioecious (DHV). Herbicide accumulation in both biotypes was not impacted when these herbicides were applied in combination. Endothall translocation from shoots to roots in DHV did not appear to be impacted (alone = 18.7% ± 1.4; combination = 23.2% ± 2.2); however, endothall shoot-to-root translocation in MHV was reduced from 16.2% ± 1.3 to 2.2% ± 0.1 when applied in combination with florpyrauxifen-benzyl. Florpyrauxifen-benzyl shoot-to-root translocation was reduced by 16 and 6 times in DHV and MHV when applied in combination with endothall, respectively. These data highlight differences in herbicide behavior when herbicides are applied in combination. Future research is needed to determine if these differences negatively impact the operational effectiveness when herbicides are applied in combination. Lastly, endothall and 2,4-D have been used to control aquatic weeds for more than 60 years, and still there is very little information available about the in planta behavior of these herbicides in aquatic weed species. 2,4-D is purportedly systemic in aquatic plants based almost entirely on its behavior in terrestrial plants. It was demonstrated in this dissertation that radioactive 2,4-D and endothall can translocate from shoots to root systems; however, it was not determined if the radioactivity in the roots was parent herbicide or a metabolite(s). Therefore, the last chapter of this dissertation used multiple analytical methods to answer the question if 2,4-D and endothall are truly systemic in aquatic plants. The intact 2,4-D detected in HWM shoots was 1.31 µg g-1 dry weight (DW) and 0.11 µg g-1 DW was detected in the roots. For endothall, 1.08 and 0.12 µg g-1 DW was detected in DHV shoots and roots, respectively. We therefore conclude that 2,4-D and endothall have similar in planta behavior, with about 8-10% of absorbed intact active ingredient translocating to the roots of these aquatic plants.
  • ItemOpen Access
    Lophodermella needle cast pathosystem: the phylogenetic relationships, host-mycobiota interactions, and molecular diagnosis of Lophodermella pathogens on Pinus
    (Colorado State University. Libraries, 2022) Ata, Jessa Pude, author; Stewart, Jane E., advisor; Abdo, Zaid, committee member; Kim, Mee-Sook, committee member; Mondo, Stephen J., committee member; Norton, Andrew P., committee member
    The impact of needle diseases in conifer stands has increased worldwide due to regional variations of warmer and wetter climates that spur the activity of needle pathogens. Heavy needle cast infection results in loss of growth among pine stands which can lead to losses in biomass production and decline in ecosystem goods and services. Despite this threat, a well-informed disease management strategy is lacking due to limited research on many needle pathogens that remain to have unclear taxonomy, uncharacterized fungal biology, and unknown trophic lifestyles and interactions. Thus, this research applied molecular tools to understand conifer needle pathosystems, particularly Lophodermella needle casts that have caused epidemics on Pinus contorta stands in Colorado, USA. Specifically, this research aims to analyze the phylogeny of Lophodermella species using molecular data and identify shared derived characters for taxa delimitation; investigate the interaction of the mycobiota and the P. contorta host in healthy versus diseased states; and develop molecular tools for the rapid diagnosis of Lophodermella needle cast. To achieve these objectives, this research is divided into five chapters. The first chapter gives an overview of the emerging needle diseases worldwide and the needle cast epidemics on P. contorta in Colorado caused by Lophodermella concolor and L. montivaga. It discusses current knowledge on the Lophodermella pathogens and management strategies for needle diseases. The second chapter highlights the relationship of Lophodermella species from North America (L. arcuata, L. concolor and L. montivaga) and Europe (L. sulcigena and L. conjuncta), and their potential synapomorphic characters. It also revealed a newly identified, genetically unique rhytismataceous species on Pinus flexilis that is morphologically similar to L. arcuata. The third chapter discusses the adverse impact of the diseases to needle mycobiota and the defense strategies of the P. contorta host. It further shows, for the first time, the endophytic lifestyle of Lophodermella pathogens on P. contorta. The fourth chapter details the efficiency of the PCR- based markers developed from multi-copy and single-copy gene regions to identify and detect L. concolor and L. montivaga on P. contorta, and L. arcuata and Bifusella linearis on P. flexilis. And lastly, the fifth chapter summarizes the important results of this research and discusses their potential implications on the management of emerging needle diseases. My dissertation closes with recommendations on future research that will address further questions of needle diseases caused by Lophodermella species and other pathogens.
  • ItemOpen Access
    Quizalofop-resistant wheat: biochemical characterization of the AXigen™ trait and corresponding metabolism
    (Colorado State University. Libraries, 2021) Bough, Raven A., author; Dayan, Franck E., advisor; Gaines, Todd A., committee member; Haley, Scott, committee member; Pearce, Stephen, committee member
    A new weed management tool in wheat, the CoAXium™ Wheat Production System, incorporates quizalofop-resistant wheat, a specialized formulation of quizalofop (Aggressor™), and a stewardship management program for effective management of annual grasses with otherwise limited control options. The AXigen™ trait confers resistance primarily through a single-point mutation in ACC1. The mutation causes an alanine to valine substitution at position 2004 in wheat acetyl-CoA carboxylase (ACCase) relative to the Alopecurus myosuriodes reference. Through greenhouse and biochemical studies paired with protein homology modelling and simulations, the research presented herein provides strong evidence that a conformational change imparted by the amino acid substitution results in quizalofop-resistant ACCase. Conversely, the mutation conveys negative cross-resistance to haloxyfop, a similar herbicide to quizalofop with a smaller molecular volume. The remaining research objectives focus on quizalofop metabolism in CoAXium™ wheat. Liquid chromatography-mass spectrometry measurements of quizalofop content over time from liquid demonstrate cooler temperature conditions (4.5°C) delay quizalofop metabolism by 4 times compared to warmer temperature conditions (19°C). Reduced temperatures also delay quizalofop metabolism to the same extent in the following annual grass weed species: Aegilops cylindrica, Bromus tectorum, and Secale cereale. Further, additional studies suggest herbicide metabolism mechanisms enhance overall CoAXium™ wheat quizalofop resistance. Despite similar ACCase resistance, resistant winter and spring wheat varieties convey varying degrees of whole-plant resistance. In winter wheat but not spring wheat, increased resistance corresponds to a shorter quizalofop half-life, implying faster metabolism boosts overall resistance. Treatment of resistant spring wheat varieties with cloquintocet, a metabolism-boosting safener, increases overall resistance. Follow-up differential expression analysis of cloquintocet-treated plants may support differential metabolism findings and lead to identification of putative candidate genes associated with upregulated herbicide metabolism, such as cytochrome P450 monooxygenases, glutathione-S-transferases, and glycosyltransferases.
  • ItemOpen Access
    Protoporphyrinogen oxidase: origins, functions, and importance as an herbicide target site
    (Colorado State University. Libraries, 2021) Barker, Abigail, author; Dayan, Franck, advisor; Snow, Christopher, committee member; Pilon, Marinus, committee member; Gaines, Todd, committee member
    Protoporphyrinogen IX oxidase (PPO)-inhibiting herbicides are effective tools to control a broad spectrum of weeds, including those that have evolved resistance to glyphosate. Their utility is being threatened by the appearance of biotypes that are resistant to PPO inhibitors. While the chloroplastic PPO1 isoform is thought to be the primary target of PPO herbicides, evolved resistance mechanisms elucidated to date are associated with changes to the mitochondrial PPO2 isoform, suggesting that the importance of PPO2 has been underestimated. Our investigation of the evolutionary and structural biology of plant PPOs provides some insight into the potential reasons why PPO2 is the preferred target for evolution of resistance. The most common target-site mutation imparting resistance involved the deletion of a key glycine codon. The genetic environment that facilitates this deletion is apparently only present in the gene encoding PPO2 in a few species. Additionally, both species with this mutation (Amaranthus tuberculatus and Amaranthus palmeri) have dual targeting of PPO2 to both the chloroplast and the mitochondria, which might be a prerequisite to impart herbicide resistance. The most recent target-site mutations have substituted a key arginine residue involved in stabilizing the substrate in the catalytic domain of PPO2. This arginine is highly conserved across all plant PPOs, suggesting that its substitution could be equally likely on PPO1 and PPO2, yet it has only occurred on PPO2, underscoring the importance of this isoform for the evolution of herbicide resistance. As glyphosate resistance becomes widespread, weed control turns to older mechanisms of action with less resistance. Protoporphyrinogen oxidase (PPO) inhibitors are a versatile class of herbicides that have been used since the 1960's, with active ingredients that work in pre-emergent and post-emergent applications. Differential efficacy of PPO inhibitors applied pre-emergent, early post-emergent and late post-emergent has been observed in multiple species and settings. Understanding the cause of higher efficacy in younger plants could preserve these important weed control tools. To understand the differing efficacies elements that affect the mechanism of action of PPO inhibitors were analyzed over the course of plant growth including target site transcript levels and protein levels, herbicide uptake, antioxidant capacity, and indicators of flux through the pathway. Data show levels of PPO do not explain differential efficacy. Increases of glutamate, the pathway precursor, do increase damage due to PPO inhibitor treatment, but increased levels are not observed in younger plants. Differential efficacy is likely due to a combination of increase in antioxidant capacity and a decrease in herbicide uptake. Other possible factors such as metabolism will need to be measured in future work. Protoporphyrinogen oxidase (PPO) is a critical enzyme across life as the last common step in the synthesis of many metalloporphyrins. The reaction mechanism of PPO was assessed in silico and the unstructured loop near the binding pocket was investigated. The substrate, intermediates, and product were docked in the catalytic domain of PPO using a modified Autodock method, introducing flexibility in the macrocycles. Sixteen PPO protein sequences across phyla were aligned and analyzed with Phyre2 and ProteinPredict to study the unstructured loop from residue 204–210 in the H. sapiens structure. Docking of the substrate, intermediates, and product all resulted in negative binding energies, though the substrate had a lower energy than the others by 40%. The α-H of C10 was found to be 1.4 angstroms closer to FAD than the β-H, explaining previous reports of the reaction occurring on the meso face of the substrate. A lack of homology in sequence or length in the unstructured loop indicates a lack of function for the protein reaction. This docking study supports a reaction mechanism proposed previously whereby all hydride abstractions occur on the C10 of the tetrapyrrole followed by tautomeric rearrangement to prepare the intermediate for the next reaction. Weed control is essential in modern agriculture, though it becomes more difficult with increasing resistance levels to current herbicides and a slow process to register a new mechanisms of action because of safety concerns and current methods. Agrematch provides a new method to identify possible herbicide candidates using an artificial intelligence algorithm that takes into effect biological parameters with the goal of reducing R&D time on new herbicides. Herein we describe the discovery of 4-chloro-2-pentenamides as novel inhibitors of protoporphyrinogen oxidase, a known herbicide target site, by the Agrematch AI. The herbicidal activity is confirmed in greenhouse assays, with the highest performing AGR001 showing good activity pre-emergent at 150 g/ha and post emergent as low as 50 g/ha on the troublesome weed palmer amaranth (Amaranthus palmeri). A lack of activity is shown on PPO resistant palmer amaranth carrying the ΔG210 deletion mutation. The mechanism of action is confirmed by the herbicide dependent accumulation of protoporphyrin IX, subsequent light dependent loss of membrane integrity, and direct inhibition of protoporphyrinogen oxidase in an in vitro assay. Modeling of the docking of these inhibitors in the active site of protoporphyrinogen oxidase confirms the target.
  • ItemOpen Access
    Cytokinin-mediated processes promote heat-induced disease susceptibility of plants to bacterial pathogens
    (Colorado State University. Libraries, 2021) Shigenaga, Alexandra Marie, author; Argueso, Cristiana, advisor; Bush, Dan, committee member; Leach, Jan, committee member; Heuberger, Adam, committee member; Nishimura, Marc, committee member
    As global human populations continue to grow and temperatures are expected to rise, the pressure to increase food productivity and develop more stress-resistant crop varieties intensifies. Increased temperatures, a consequence anticipated as a result of global climate change, is expected to have an overall negative impact on crop productivity and agricultural systems. When exposed to non-optimal, high temperature conditions plant defense responses to pathogen attack are attenuated, leading to a process referred to here as heat-induced disease susceptibility. The plant growth hormone cytokinin is known to regulate responses to both biotic and abiotic pressures, making it an ideal target to study heat-induced disease susceptibility. The overarching goal of this dissertation was to understand the role of cytokinin in heat-induced disease susceptibility, to identify novel strategies to combat this process and design new ways to teach future generations about the impact of climate change on agricultural systems and science policy. First, I identified that a plant lacking a functional cytokinin signaling pathway, ahk2,3 mutated on the cytokinin signaling receptors AHK2 and AHK3, was less susceptible at elevated temperatures to the bacterial pathogen, Pseudomonas syringae pv. tomato DC3000 (Pst DC3000). My results show that ahk2,3 plants are less susceptible under high temperature conditions with Pst DC3000 populations proliferating at a lower rate compared to wild-type plants overtime, suggesting that heat-induced susceptibility is partially dependent on cytokininiii signaling. Our results show that differences in susceptibility under elevated temperatures of ahk2,3 and wild-type plants is not attributed to an increase in defense responses, but rather by a possible change in the availability of nutrients for Pst DC3000. Together the data reveals that under high temperature conditions cytokinin promotes late-physiological processes, centered around primary metabolism, that are contributing to increased pathogen proliferation. These results led to the identification of cytokinin-regulated genes that could be utilized for breeding efforts to obtain loss-of-heat induced disease susceptibility that could be translated to crop species. Second, I identified that another member of the Brassicaceae family, Brassica napus, also exhibited heat-induced disease susceptibility to the bacterial pathogen, P. syringae pv. maculicola (Psm ES4326). Gene expression analysis confirms that similar to Arabidopsis, B. napus plants increase cytokinin signaling in response to high temperature stress. To further address if cytokinin was important for heat-induced disease susceptibility of B. napus, I utilized a chemical approach. B. napus plants were sprayed with the cytokinin-signaling antagonist, PI-55, prior to inoculation and results show that a single application of PI-55 led to a loss of susceptibility under heat to Psm ES4326. Additionally, this application of PI-55 did not lead to any adverse vegetative growth parameters, suggesting a potential novel chemical approach to combat heat-induced disease susceptibility in Brassicaceae crops. Lastly, I constructed a new approach to teach future generations about the impact of climate change on plant diseases in agricultural systems. "Plant Diseases and Climate Change" is an active learning activity designed to give college students experience in synthesizing information and developing a solution, in the context of plant pathology. This exercise uses the issue of heat-induced susceptibility of rice in the Philippines to improve student understanding of the interactions between abiotic and biotic factors affecting global food security. By using an international agricultural pathosystem, I aim to inform students how environmental pressures can impact economically important plant systems, the role scientists and experts play in policy making to preserve food security, and the importance of agriculture on a global scale.
  • ItemOpen Access
    Field population genetics of global Puccinia striiformis f. sp. tritici populations in wheat reveal a dynamic molecular battlefield
    (Colorado State University. Libraries, 2021) Lyon, Rebecca, author; Trivedi, Pankaj, advisor; Broders, Kirk, committee member; Pearce, Stephen, committee member; Hufbauer, Ruth, committee member
    To view the abstract, please see the full text of the document.