Mode of action studies on the nematicide fluensulfone

Abstract

Plant parasitic nematodes (PPNs) place a heavy burden on agriculture throughout the world. This burden is accentuated by a lack of effective and safe methods for the control of these crop pests, with many nematicides having been banned due to unacceptable non-target toxicity. Few alternatives have emerged to fill this gap. Fluensulfone is a newly registered nematicide that has a favourable toxicity profile relative to previously used nematicides. Fluensulfone has proven efficacy in the field against a range of nematode species. The mode of action of fluensulfone is however currently unknown. The aim of this thesis was to investigate the effects of fluensulfone on nematodes and to gain insight into its mechanism of action. Caenorhabditis elegans is a model organism with well characterised genetics, neurophysiology and behaviour. C. elegans has proven useful in previous mechanism of action studies on anthelmintics. Fluensulfone was found to have nematicidal activity against C. elegans but at concentrations ?100-fold greater than those reported to be effective against the PPN Meloidogyne javanica. Fluensulfone affected a number of C. elegans behaviours, including locomotion, pharyngeal pumping, egg laying and development. A reverse genetic approach did not identify any probable targets but did indicate that fluensulfone has a novel mechanism of action relative to established anticholinesterase and macrocyclic lactone nematicides. Electrophysiological analysis found that fluensulfone has complex effects on the C. elegans pharyngeal system with lower concentrations exciting pharyngeal activity and higher concentrations inhibiting. The pharyngeal effects suggested that fluensulfone might influence feeding behaviour in PPNs. In PPNs, the behaviour of the stylet mouth spear is critical in feeding and host invasion. The action of fluensulfone on the stylet of the PPN Globodera pallida was therefore investigated, to validate observations made with C. elegans. The pharmacology of the stylet was also investigated, using knowledge of the C. elegans pharynx as a guide. The pharmacological regulation of the stylet was found to be similar to the C. elegans pharynx, with 5-HT an important regulator. Fluensulfone stimulated stylet activity and blocked 5-HT-stimulated activity. Studies with a 5-HT receptor antagonist on G. pallida and C. elegans indicated that fluensulfone interacts with 5-HT signalling to stimulate stylet and pharyngeal activity. The concentrations of fluensulfone that had acute effects on both G. pallida and C. elegans behaviour were still ?50-fold greater than those reported to have nematicidal activity against PPN species. Immotility assays revealed that 1-30 ?M fluensulfone elicited a progressive increase in paralysis over 14 days, leading to death. No such effect occurred in C. elegans adults or dauers. Staining with the metabolic marker MTT revealed that fluensulfone is nematicidal and that metabolic impairment may be the cause of this paralysis and death. Nile Red staining indicated that lipid consumption is reduced in the presence of fluensulfone. Fluensulfone also profoundly inhibited G. pallida hatching from cysts. The effects of fluensulfone on G. pallida physiology and metabolism require further investigation via rigorous measures such as oxygen consumption. If metabolic impairment is identified as the causative agent of fluensulfone nematicidal activity this must be further studied through metabolomics and other techniques.

More information

Identifiers

ORCID for Linda Holden-Dye: orcid.org/0000-0002-9704-1217

Catalogue record

Export record