Role of secondary metabolites in plant defense against insect herbivores

In all natural habitats, plants are surrounded by an enormous number of potential enemies (biotic) and various kinds of abiotic environmental stress, greatly responsible for heavy reduction in crop productivity. To counter the herbivore attack, plants produce defensive compounds that appear to have no direct function in growth and development. These substances are known as secondary metabolites. Secondary metabolites, including terpenes, phenolics and nitrogen (N) containing compounds, defend plants against a variety of insect herbivores. These compounds are either produced constitutively or in response to plant damage, and affect feeding, growth, and survival of herbivores. 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. The role of secondary metabolites in defense may involve deterrent and anti-feedant activity, toxicity or acting as precursors to physical defense systems. By virtue of their biological activities against insect herbivores, many of these substances are employed commercially as insecticides, while others find uses as fragrances, flavorings, medicinal drugs, and industrial materials.