Plant defense and herbivore counter-defense: benzoxazinoids and insect herbivores

Abstract

Benzoxazinoids are a class of indole-derived plant chemical defenses comprising compounds with a 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one skeleton and their derivatives. These phytochemicals are widespread in grasses, including important cereal crops such as maize, wheat and rye, as well as a few dicot species, and display a wide range of antifeedant, insecticidal, antimicrobial, and allelopathic activities. Although their overall effects against insect herbivores are frequently reported, much less is known about how their modes of action specifically influence insect physiology. The present review summarizes the biological activities of benzoxazinoids on chewing, piercing-sucking, and root insect herbivores. We show how within-plant distribution modulates the exposure of different herbivore feeding guilds to these defenses, and how benzoxazinoids may act as toxins, feeding deterrents and digestibility-reducing compounds under different conditions. In addition, recent results on the metabolism of benzoxazinoids by insects and their consequences for plant-herbivore interactions are addressed, as well as directions for future research.

Keywords: Chemical ecology; Detoxification; Metabolism; Nutritional indices; Poaceae.

Fig. 1

Glucoside hydrolysis of naturally occurring benzoxazinones and degradation to benzoxazolinones via oxo-cyclo/ring-chain tautomerism

Fig. 2

BXD biosynthesis and compartmentalization in plant cell

Fig. 3

Growth curves (± SEM) for S. frugiperda larvae fed on artificial diets containing MBOA

Fig. 4

Representation of detrimental effects on aphids caused by different levels of DIMBOA in diet. Above a certain DIMBOA concentration threshold (dashed line), the antifeedant effect of DIMBOA overcomes its toxicity, decreasing DIMBOA intake and diminishing mortality in aphids. In higher DIMBOA concentrations, the mortality curves matched those of unfed groups

Fig. 5

Structures of MBOA metabolites detected in plants and insect frass

Fig. 6

Specific activity of MBOA glucosylation (±SEM) in gut homogenates of S. frugiperda and O. nubilalis (univoltine and bivoltine strains) (N = 3)