Phytochemical Background Mediates Effects of Pyrrolizidine Alkaloids on Western Flower Thrips

Abstract

Plants produce an extremely diverse array of metabolites that mediate many aspects of plant-environment interactions. In the context of plant-herbivore interactions, it is as yet poorly understood how natural backgrounds shape the bioactivity of individual metabolites. We tested the effects of a methanol extract of Jacobaea plants and five fractions derived from this extract, on survival of western flower thrips (WFT). When added to an artificial diet, the five fractions all resulted in a higher WFT survival rate than the methanol extract. In addition, their expected combined effect on survival, assuming no interaction between them, was lower than that of the methanol extract. The bioactivity was restored when the fractions were combined again in their original proportion. These results strongly suggest synergistic interactions among the fractions on WFT survival rates. We then tested the effects of two pyrrolizidine alkaloids (PAs), free base retrorsine and retrorsine N-oxide, alone and in combination with the five shoot fractions on WFT survival. The magnitude of the effects of the two PAs depended on the fraction to which they were added. In general, free base retrorsine was more potent than retrorsine N-oxide, but this was contingent on the fraction to which these compounds were added. Our results support the commonly held, though seldom tested, notion that the efficacy of plant metabolites with respect to plant defence is dependent on their phytochemical background. It also shows that the assessment of bioactivity cannot be decoupled from the natural chemical background in which these metabolites occur.

Keywords: Antagonism; Frankliniella occidentalis; Liquid-liquid partition; Plant defence; Predominant storage; Synergism.

Fig. 1

Chemical structures of retrorsine and retrorsine N-oxide

Fig. 2

Western flower thrips (WFT) survival rates of the five fractions (S_F) and the recombined fractions (a), the expected survival assuming no interaction (white bars), the observed survival of the re-combined fractions (black bars) (b), and the interaction effect among fractions (S_F*F) (c) at the five tested concentrations of Jacobaea mass equivalents

Fig. 3

Log-transformed survival rates of 2^nd instar Western flower thrips (WFT) (Frankliniella occidentalis) against the concentration of retrorsine (solid dots) and retrorsine N-oxide (open dots)

Fig. 4

Magnitude of the interaction effect (mean ± SE) against concentration of shoot fractions expressed as Jacobaea mass equivalents (g/mL) for PA at 1.4 mM (white bars) and at 7.0 mM (grey bars). The combination of n-hexane fraction (a), CHCl₃ fraction (b), EtOAc fraction (c), n-BuOH fraction (d) and H₂O fraction (e) with retrorsine (on the left) and with retrorsine N-oxide (on the right). Deviation of the interaction effect S_PA*F from the dashed line indicates a synergistic (<1) or an antagonistic effect (>1). Two-way ANOVAs were used to analyze whether the overall interaction effect of PA concentration and fractions (S_PA*F) deviated from one (Table S1). * The survival rate of WFT in one replicate of the combination of n-BuOH and retrorsine at 7.0 mM was zero, therefore it was considered a missing value

Fig. 5

∆ mortality rate of Western flower thrips (WFT) against Jacobaea mass equivalents (g/mL) (mean ± 95% confidence interval) of different fractions. ∆WFT mortality rate is the difference between WFT mortality rate on a diet with retrorsine and a particular fraction and WFT mortality rate on a diet with retrorsine N-oxide and the same fraction (a) n-hexane fraction, (b) CHCl₃ fraction, (c) EtOAc fraction, (d)n-BuOH fraction, (e) H₂O fraction. Open diamonds: PA concentration of 1.4 mM; Solid squares: PA concentration of 7 mM