Plant Defense Against Herbivores: Chemical Aspects

Abstract

Plants have evolved a plethora of different chemical defenses covering nearly all classes of (secondary) metabolites that represent a major barrier to herbivory: Some are constitutive; others are induced after attack. Many compounds act directly on the herbivore, whereas others act indirectly via the attraction of organisms from other trophic levels that, in turn, protect the plant. An enormous diversity of plant (bio)chemicals are toxic, repellent, or antinutritive for herbivores of all types. Examples include cyanogenic glycosides, glucosinolates, alkaloids, and terpenoids; others are macromolecules and comprise latex or proteinase inhibitors. Their modes of action include membrane disruption, inhibition of nutrient and ion transport, inhibition of signal transduction processes, inhibition of metabolism, or disruption of the hormonal control of physiological processes. Recognizing the herbivore challenge and precise timing of plant activities as well as the adaptive modulation of the plants' metabolism is important so that metabolites and energy may be efficiently allocated to defensive activities.

Alteration of plant primary metabolism in response to insect herbivory

“…Metabolites and energy can be allocated more efficiently if there is not only recognition of herbivory, but also reprogramming over time (Mithöfer and Boland, 2012). Within an hour after exposure to herbivore feeding, expression of genes encoding diverse plant metabolic pathways is altered (Fürstenberg-Hägg et al, 2013).…”

The role of L-DOPA in plants

“…22,23 Nonprotein amino acids accumulate massively in many plants and seem to play an important role in resistance to herbivores. 24 Velvet bean seeds, as aforementioned, are known to accumulate large amounts of L-DOPA and are not attacked by small mammals or insects, suggesting a feeding repellent property.…”

Leaf trichomes affect caterpillar feeding in an instar-specific manner

“…In the Nicotiana tabacum-Manduca sexta system, glandular trichomes have also been shown to release volatiles following caterpillar damage that are attractive to predators [9]. In contrast, non-glandular trichomes are thought to act primarily as mechanical barriers that restrict herbivore movement and thereby prevent feeding damage [3,10]. Although various defensive functions of both glandular and non-glandular trichomes have been characterized, their relative efficacy in deterring feeding by different herbivores or herbivores at different stages of development are not well understood.…”