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Aspects of weed resistance to auxinic herbicides

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dc.contributor.authorRodrigues Alves de Figueiredo, Marcelo, author
dc.contributor.authorGaines, Todd A., advisor
dc.contributor.authorArgueso, Cristiana T., committee member
dc.contributor.authorDayan, Franck E., committee member
dc.contributor.authorReddy, Anireddy S. N., committee member
dc.date.accessioned2020-08-31T10:12:03Z
dc.date.available2023-11-07T10:12:03Z
dc.date.issued2020
dc.description.abstractSynthetic 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.
dc.format.mediumborn digital
dc.format.mediumdoctoral dissertations
dc.identifierRodriguesAlvesdeFigueiredo_colostate_0053A_16194.pdf
dc.identifier.urihttps://hdl.handle.net/10217/211808
dc.languageEnglish
dc.language.isoeng
dc.publisherColorado State University. Libraries
dc.relation.ispartof2020-
dc.rightsCopyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright.
dc.titleAspects of weed resistance to auxinic herbicides
dc.typeText
dcterms.embargo.expires2023-11-07
dcterms.embargo.terms2023-11-07
dcterms.rights.dplaThis Item is protected by copyright and/or related rights (https://rightsstatements.org/vocab/InC/1.0/). You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).
thesis.degree.disciplineAgricultural Biology
thesis.degree.grantorColorado State University
thesis.degree.levelDoctoral
thesis.degree.nameDoctor of Philosophy (Ph.D.)

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