Revisiting Plant-Herbivore Co-Evolution in the Molecular Biology Era

Important theories that define the role of secondary metabolites in the co‐evolution of plants and insect herbivores were proposed in a series of publications in the 1950s and 1960s. Now, roughly 50 years later, it is possible to revisit these early theories and determine how well they hold up in the light of sequenced genomes, molecular phylogenies, completely elucidated biosynthetic pathways and evidence for specific biochemical detoxification mechanisms in insect herbivores. Two classes of plant secondary metabolites stand out as being particularly well‐studied: the glucosinolates, which are characteristic of cruciferous plants; and the benzoxazinoids, which are found in many grass species. Using these metabolic pathways as examples, it is possible to demonstrate at the molecular level how biosynthesis of plant secondary metabolites evolved from primary metabolism, how the development of defensive chemicals can drive plant‐herbivore co‐evolution and how insects are able to respond to plant defences at the molecular level.