Abstract

Asatone and isoasatone A from Asarum ichangense Cheng were determined to be defensive compounds to some insects in a previous investigation. However, the anti-insect activity mechanisms to caterpillar are still unclear. The compounds asatone and isoasatone A from A. ichangense were induced by Spodoptera litura. The anti-insect activity of asatone and isoasatone A to S. litura was further tested by weight growth rate of the insect through a diet experiment. Isoasatone A showed a more significant inhibitory effect on S. litura than asatone on the second day. The concentration of asatone was higher than isoasatone A in the second instar larvae of S. litura after 12 h on the feeding test diet. Both compounds caused mid-gut structural deformation and tissue decay as determined by mid-gut histopathology of S. litura. Furthermore, some detoxification enzyme activity were measured by relative expression levels of genes using a qPCR detecting system. Asatone inhibited the gene expression of the cytochrome P450 monooxygenases (P450s) CYP6AB14. Isoasatone A inhibited the relative expression levels of CYP321B1, CYP321A7, CYP6B47, CYP6AB14, and CYP9A39. Asatone increased the relative gene expression of the glutathione transferases (GSTs) SIGSTe1 and SIGSTo1, in contrast, isoasatone A decreased the relative gene expression of SIGSTe1 by about 33 fold. Neither compound showed an effect on acetylcholinesterase SIAce1 and SIAce2. The mechanism of anti-insect activity by both compounds could be explained by the inhibition of enzymes P450s and GSTs. The results provide new insights into the function of unique secondary metabolites asatone and isoasatone A in genus Asarum, and a new understanding of why A. ichangense is largely free of insect pests.

Highlights

  • Most plants have evolved a range of defense systems to protect themselves when they are infected by insects

  • The two peaks were identified as asatone and isoasatone a using standards, respectively (Figure 1)

  • Concentrations of asatone and isoasatone a were much higher in roots than in stems or leaves from fresh plants, and concentration of asatone was 21.3 times higher than isoasatone a in the roots (Figure 1C,D)

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Summary

Introduction

Most plants have evolved a range of defense systems to protect themselves when they are infected by insects. Constitutive and inducible secondary metabolites may increase after being fed upon by insects to prevent further damage [1]. Metabolites that were induced in damaged plants could prove useful for preventing and controlling insects. Some extracts from plants can inhibit growth of Spodoptera litura Fab. larvae or kill them [2,3,4]. Asarum ichangense is a species of the genus Asarum. Yi et al reported that all parts of A. sieboldii caused 100% mortality in both Lycoriella ingenua and Coboldia fuscipes larvae [5].

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