Secondary Metabolites: Deterring Herbivores

AbstractAll plants produce and store secondary metabolites (SM), which are not important for primary or energy metabolism of a plant. However, SMs are not waste products, but important for the ecological fitness and survival of the plants producing them. Apparently, plants have evolved the production and storage of SM as a means to defend themselves against herbivores, bacteria, fungi, viruses as well as other competing plants. Most SM can interfere with basic molecular targets of animals or microbes and thus provide plants with an adequate protection against a multitude of enemies. Plants usually produce complex mixtures of SMs, which can work in an additive or even synergistic way. Some defence chemicals address a single target, such as a neurotransmitter receptor or ion channel, others have a broad activity spectrum and exhibit pleiotropic activities on several targets.Key concepts:Plants cannot run away when attacked by herbivores; they use defence chemicals to ward off enemies.Plants do not have an immune system to defend themselves against microbes hence use antimicrobial secondary metabolites instead.Plants produce and store complex mixtures of secondary metabolites with additive or even synergistic activities.Secondary metabolites are important for chemical defence but are also used to attract pollinating insects and fruit‐dispersing animals.The process of biosynthesis, transport and storage of SM is energetically costly.Some SM specifically interact with a particular molecular target in herbivores, whereas other SM are nonselective.Synthesis, transport and storage of SM are optimized in space and time to fulfil the ecological functions.Although some SMs are stored in a constitutive way, others are inducible and are only made in case of danger.Herbivores have evolved effective biochemical adaptations towards the defence chemistry of plants, involving cytochrome p450 enzymes and ABC transporters.