Secondary Metabolites: Deterring Herbivores

AbstractAll plants produce and store secondary metabolites (SMs), 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 and viruses, as well as other competing plants. Most SMs 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 an ion channel; others have a broad‐activity spectrum and exhibit pleiotropic activities on several targets. SMs also serve as signal compounds attracting pollinating and fruit‐dispersing animals.Key ConceptsPlants cannot run away when attacked by herbivores; they use defence chemicals to ward off enemies.Plants do not have an elaborate immune system with specific antibodies to defend themselves against microbes, and instead use antimicrobial secondary metabolites (SMs) in their innate immune system.Plants produce and store complex mixtures of SMs with additive or even synergistic activities.SMs are important for chemical defence but are also used to attract pollinating and fruit‐dispersing animals.The process of biosynthesis, transport and storage of SMs is energetically costly.Some SMs specifically interact with a particular molecular target in herbivores or microbes, whereas other SMs are nonselective, thus being able to address a multitude of enemies.Synthesis, transport and storage of SMs are optimised 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.Jasmonic acid and salicylic acid are important signal molecules in the regulation of SM biosynthesis.Herbivores have evolved effective biochemical adaptations towards the defence chemistry of plants, involving cytochrome p450 enzymes and ABC transporters.