Graduate Thesis Or Dissertation
 

Insights into Novel Non-Fumigant Nematicides: Physiological and Cellular Responses of Meloidogyne Incognita and Other Plant-Parasitic Nematodes

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  • Plant-parasitic nematodes (PPN) pose a severe threat to crop production with the economic losses due to nematode parasitism in excessive of US$ 80 billion each year. Meloidogyne incognita, a globally distributed pest with a diverse host range, contributes significantly to this economic loss. The most reliable way to manage PPN is often through chemical controls, however, many fumigant and non-fumigant chemical controls are being phased out due to their harmful effects to both applicators and the environment. As new nematicide enter the market, there is a need for basic information on the toxicity of these new nematicides to PPN including M. incognita. The aim of this dissertation research is to determine foundational knowledge of how novel nematicides, fluopyram, fluensulfone, and fluazaindolizine affect M. incognita movement and motility, reproduction, and transcription in comparison with the traditional nematicide oxamyl. In Chapter 2, 24hr dose-response curves were established for M. incognita second-stage juveniles (J2) exposed to fluopyram, fluensulfone, fluazaindolizine, and oxamyl. Using the dose-response curves, sublethal doses were obtained to examine their effect on M. incognita J2 motility and fecundity on a susceptible host. Fluopyram was determined to be the most toxic nematicide to J2, but did not reduce nematode reproduction at sublethal doses. Fluensulfone and oxamyl both had similar levels of toxicity at 24-hrs and fluazaindolizine was the least toxic. Oxamyl, fluensulfone, and fluazaindolizine all were able to suppress nematode reproduction even at sublethal doses. Motility was unaffected by any compound, however, the level of activity of motile nematodes varied amongst compounds. The novel nematicide fluazaindolizine has been shown to be PPN specific in its toxicity, necessitating the exploration of what PPN genera are susceptible to this compound and how might populations within a species vary in their susceptibility to this compound. Therefore, in Chapter 3 dose-response curves were generated for populations of 3 different Meloidogyne species along with Globodera ellingtonae, Xiphinema americanum, and Pratylenchus neglectus, and P. penetrans. In addition to dose-response curves, the effects of a pre-exposure to fluazaindolizine on reproduction of 3 different Meloidogyne species was also examined. Sensitivity to fluazaindolizine varied >10-fold within species of Meloidogyne, but this did not translate to a change in the ability of fluazaindolizine to suppress reproduction. Pratylenchus species were found not to be susceptible to fluazaindolizine, but Globodera ellingtonae, Xiphinema americanum were; expanding the range of PPN known to be sensitive to this compound. Chapters 4 and 5 examined the changes in M. incognita transcription in response to 24-hr exposure to fluopyram, fluensulfone, fluazaindolizine, and oxamyl. The aim of these two chapters was to provide context for the physiological responses seen in M. incognita to nematicides, determine potential modes-of-action for fluensulfone and fluazaindolizine, and gain greater understanding how nematicides are toxic to nematodes. Transcriptional changes were examined of various components of cellular function including cellular detoxification, fatty-acid retinoid-binding proteins, transcriptional regulators of oxidative stress, beta-fatty acid oxidation, acetylcholine neuron potential, the citric acid cycle, and oxidative phosphorylation. Although the most toxic nematicide, fluopyram had limited impacts on expression of cellular detoxification or other stress-responses, but expression patterns in the citric acid cycle and oxidative phosphorylation pathways support the mode-of-action, succinate dehydrogenase inhibitor, described in fungi. Fluensulfone and fluazaindolizine both resulted in robust transcriptional responses with 1,208 and 2,611 DE genes, respectively. These compounds had strong impacts on cellular detoxification, causing differential regulation of transcription factors and mixed regulation of genes in the detox pathway. Expression data indicated two potential pathways of interest for mode-of-actions: β-fatty acid oxidation pathway and 2-Oxoglutarate dehydrogenase of the TCA cycle for fluensulfone and fluazaindolizine, respectively. Oxamyl negatively affected expression of cellular detoxification, but expression changes in acetylcholine neuron components also provided strong evidence to support its functionality as an acetylcholinesterase inhibitor. Overall, the research in this dissertation aimed at understanding nematicide toxicity to PPN from the organismal to the cellular level to support more well-informed use of these nematicides in managing PPN.
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