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Browsing Theses and Dissertations by Author "Argueso, Cristiana T., committee member"

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    Aspects of weed resistance to auxinic herbicides
    (Colorado State University. Libraries, 2020) Rodrigues Alves de Figueiredo, Marcelo, author; Gaines, Todd A., advisor; Argueso, Cristiana T., committee member; Dayan, Franck E., committee member; Reddy, Anireddy S. N., committee member
    Synthetic auxins have been widely used for selective control of broadleaf weeds since the mid-1940s. After more than 70 years using synthetic auxin herbicides, there are 41 different resistant species reported. Weed resistance to auxin herbicides is poorly understood and in most reported cases, no studies have been done to investigate the mechanistic changes that occur in resistant populations. The mechanisms of herbicide resistance in weeds are classified as 1) target-site when mutations reduce the interaction of the herbicide molecule to its binding site and/or changes of gene expression of the targeted enzyme to compensate for herbicide inhibition; and as 2) non-target-site mechanisms, which include any genetic mutations that will prevent or reduce the herbicide reaching its site of action. In this present research, two 2,4-D resistant weed species were studied, and the mechanisms of resistance were elucidated, where one species evolved metabolic non-target-site resistance to 2,4-D and the second species evolved a novel mechanism of target side modification. In 2009, an Amaranthus tuberculatus (common waterhemp) population with ten-fold resistance to 2,4-D was found in Nebraska, USA. Using the same 2,4-D-resistant and a known susceptible A. tuberculatus population from Indiana, the mechanism of 2,4-D resistance was examined by conducting [14C] 2,4-D absorption, translocation and metabolism experiments. No differences were found in 2,4-D absorption, but resistant plants translocated more of the radioactive material than susceptible A. tuberculatus. Resistant plants metabolized [14C] 2,4-D more rapidly than susceptible plants. The main metabolites were purified and their structures were solved by NMR and HRMS. Susceptible plants conjugate 2,4-D to 2,4-D Aspartic Acid (2,4-D-Asp). Resistant plants showed a distinct metabolic profile where 2,4-D is hydroxylated into 5-OH-2,4-D, conjugated in a sugar metabolite (5-OH-2,4-D-Glucoside) and malonylated into 5-OH-2,4-D-(6-O-Malonyl)-Glucoside. Pre-treatment with the cytochrome P450 inhibitor malathion inhibited 2,4-D hydroxylation. Toxicological studies in waterhemp and Arabidopsis confirmed that the hydroxylated metabolite lost its auxinic action and toxicity. In contrast, the 2,4-D-Asp metabolite induced auxin inhibition to the plants tested. These results demonstrate that resistant A. tuberculatus evolved novel detoxification reactions that rapidly metabolize 2,4-D, potentially mediated by cytochrome P450. That novel mechanism is more efficient and produces metabolites with lower toxicity compared to the innate aspartic acid conjugation. Metabolism-based herbicide resistance poses a serious challenge for weed management due to the potential for cross-resistance to other herbicides. Sisymbrium orientale (Indian hedge mustard) is an important weed species in Australia reducing yields in crops and pastures. In 2005, a 2,4-D and MCPA resistant population was reported in the Port Broughton region in South Australia. Aux/IAAs are dynamic repressor proteins that regulate Auxin Response Factors (ARFs) to activate auxin related genes and are also co-receptors for auxins and synthetic auxin herbicides. The degradation of Aux/IAAs is done by the enzyme complex E3, called SCFTIR1/AFB, which, in the presence of auxin, performs ubiquitination on Aux/IAA making it a target of proteasome 26S, an enzyme responsible for proteolysis in eukaryotes. An RNAseq study showed that a 27 bp deletion in Aux/IAA2 (IAA2) degron tail was correlated to the resistant phenotype. The mutant allele was functionally validated to confer 2,4-D resistance by transforming Arabidopsis thaliana with the wild type SoIAA2 and SoIAA2Δ27 alleles. Performing binding analysis by surface plasmon resonance, the association of TIR1 in the presence of auxin (IAA, 2,4-D and dicamba) showed slower association and faster dissociation to the resistant IAA2 peptide compared to the susceptible IAA2 peptide. Our results suggest that the loss of 9 amino acids located in the degron tail may reduce the capacity of IAA2 to "embrace" TIR1 in the presence of auxin, reducing ubiquitination rate, resulting in higher stability to repress auxin response factors and ultimately conferring resistance to 2,4-D.
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    ItemOpen Access
    Understanding broad-spectrum disease resistance in rice: prompting a genome-wide uprising
    (Colorado State University. Libraries, 2017) Tonnessen, Bradley William, author; Leach, Jan E., advisor; Ben-Hur, Asa, committee member; Jahn, Courtney E., committee member; Argueso, Cristiana T., committee member; Bush, Daniel R., committee member
    Rice is the main staple food crop of the world, and thus, the detriments caused by rice diseases are a threat to international food security. The emergence of new virulent strains of pathogens can significantly reduce yields, and there are continual efforts to develop more resistant rice cultivars. Utilization of single R-genes is effective, but has proven inadequate due to rapid pathogen evolution. Thus, there is a need for breeding multigenic, broad-spectrum disease resistance in new varieties. This study aims to understand the aspects of basal resistance and its contribution to tolerance to multiple, diverse pathogens. Phenylalanine ammonia-lyase is a key enzyme in phenylpropanoid metabolism, which contributes to the basal defense response (DR). In this project, the DR gene, OsPAL4, which colocalizes with a disease resistance Quantitative Trait Loci (QTL), was shown to contribute to resistance to three important rice diseases, rice blast, bacterial blight, and sheath blight, in experiments using an ospal4 mutant. The functional element of resistance QTL haplotypes of DR genes such as OsPAL4 are largely unknown, and this work searched for sequence patterns in the promoters of DR genes to discern a regulatory mechanism specific to DR. Multiple cis-regulatory Modules (CRMs), or groups of DR-related sequence motifs were identified in promoters of DR genes. These CRMs harbor structural organizations of cis-elements known to be involved in the DR, and also motifs involved in a putative epigenetic regulatory mechanism. Polymorhpisms in CRMs are found in resistant relative to susceptible QTL haplotypes in DR gene promoters. These CRMs are sequence patterns found across DR gene promoters. Thus, we hypothesize that DR-associated CRM can be used as breeding markers to select loci on a genome scale that encode traits supporting broad spectrum basal resistance to important rice diseases.
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    Understanding weed biology and herbicide resistance to improve weed management
    (Colorado State University. Libraries, 2020) Soni-Castillo, Neeta, author; Gaines, Todd A., advisor; Dayan, Franck E., committee member; Argueso, Cristiana T., committee member; Haley, Scott D., committee member
    Weed management is essential in agriculture, natural areas, and rangelands. Weed control has mainly relied on herbicides. These chemical compounds are a low-cost option, easy to apply, and very efficient to eliminate weeds. However, as part of survival strategies weed species have evolved mechanisms to overcome herbicides and continue their life cycle. Thus, it is imperative that we increase our knowledge in weed biology and resistance mechanisms to develop better management strategies. Here I present three chapters that cover these areas of study. First, as an intent to promote more tools for management strategies in winter wheat, a field survey was conducted to identify the potential to implement harvest weed seed control for problematic winter annual grasses in this cropping system. The second chapter covers the results of a herbicide resistance survey to screen for imazamox and quizalofop resistance of troublesome winter annual grasses in winter wheat and rangeland areas. The third chapter aimed to determine the distribution of native and introduced Phragmites australis haplotypes which is a riparian species problematic in rangeland and natural areas. Harvest weed seed control methods showed potential to manage downy brome, feral rye, and jointed goatgrass. Seed retention of these winter annual grasses was over 75% indicating that the majority of seeds could be collected during wheat harvest. After screening over 280 samples of winter annual grasses, only two feral rye populations showed resistance to imazamox. Further studies on resistance mechanisms showed that one population (A) can rapidly metabolize the herbicide compared to a susceptible and the second population (B) contained a target site mutation in the imazamox target enzyme. Introduced Phragmites australis haplotypes were identified in Colorado using molecular markers. In addition, a low-cost and quick genotyping tool was developed to encourage land managers to conduct more frequent monitoring. Main results from this dissertation are expected to contribute with the big endeavor of promoting integrated weed management solutions and better weed biology understanding.