Detoxification of host plant phenolic aglycones by the spruce budworm

Abstract

This study examines the post-ingestive fate of two host-plant derived small-molecule phenolics (the acetophenones piceol and pungenol) that have previously been shown to be toxic to the outbreaking forest pest, spruce budworm (Choristoneura fumiferana). We test first whether these compounds are transformed during passage through the midgut, and second whether the budworm upregulates activity of the detoxification enzyme glutathione-s-transferase (GST) in response to feeding on these compounds. Insects were reared on either foliage or artificial diet to the fourth instar, when they were transferred individually to one of two treatment diets, either control or phenolic-laced, for approximately 10 days, after which midguts were dissected out and used for Bradford soluble protein and GST enzyme activity analysis. Frass was collected and subjected to HPLC-DAD-MS. HPLC showed that the acetophenones do not autoxidize under midgut pH conditions, but that glucose- and glutathione-conjugates are present in the frass of insects fed the phenolic-laced diet. GST enzyme activity increases in insects fed the phenolic-laced diet, in both neutral pH and alkaline assays. These data show that the spruce budwom exhibits counter-adaptations to plant phenolics similar to those seen in angiosperm feeders, upregulating an important detoxifying enzyme (GST) and partially conjugating these acetophenones prior to elimination, but that these counter-measures are not totally effective at mitigating toxic effects of the ingested compounds in the context of our artifical-diet based laboratory experiment.

Hydroxyacetophenone defenses in white spruce against spruce budworm

“…The concentrations of hydroxyacetophenones utilized in bioassays may underestimate their effect on C. fumiferana. In both studies, the maximum concentrations (i.e., 5.4 mg/g for pungenol and 6.7 mg/g for piceol per dry mass) are lower than their averages in most sampling designs tested (Delvas et al, 2011;Donkor et al, 2019; Table 1). In addition, C. fumiferana feeds on previous year's foliage before budburst, which has high levels of hydroxyacetophenones in R trees (Parent et al, 2017).…”

“…The fate of piceol and pungenol after ingestion by the C. fumiferana was also studied in a bioassay showing an upregulation of the detoxifying enzyme glutathione S-transferase and partial conjugation of the hydroxyacetophenones prior to elimination during their ingestion by C. fumiferana (Donkor, Mirzahosseini, Bede, Bauce, & Despland, 2019). However, these counter-measures are not completely effective at mitigating toxic effects of the ingested compounds in bioassays.…”

“…Insects can develop counter‐adaptations to plant defensive traits that are heritable and enable the herbivores to withstand plant defense. For instance, C. fumiferana can respond to plant hydroxyacetophenones by upregulating a GST to partially eliminate the compounds (Donkor et al, ). This counter‐adaptation could challenge the survival of the plant.…”

Gene selection for studying frugivore-plant interactions: a review and an example using Queensland fruit fly in tomato

“…The genes identified through this process were associated with different phases of the insect chemical-detoxification process, which occurs for both the detoxification of plant secondary chemicals and pesticides ( Heidel-Fischer & Vogel, 2015 ; Heckel, 2018 ). During Phase I, genes/enzymes such as P450 monooxygenases (P450s) and carboxylesterases (COEs) are involved in oxidation, hydrolysis or reduction of toxic compounds; subsequently, Phase II involves the conjugation of the modified toxins with hydrophilic groups such as glutathiones, sulphate and sugars by glutathione S-transferases (GSTs) and UDP-glucosyltransferase (UGT) to enhance the polarity of the molecules and so help excretion; while finally, in Phase III, ATP-binding cassette transporters export the conjugated toxins out of the cell ( Donkor et al, 2019 ). Each phase of the detoxification process is associated with major gene families ( Fig.…”