4: Plants’ Defense Against Insects Check Access

Plants have evolved a wealth of highly diverse chemical compounds in response to insect attacks. This chapter addresses the major groups of chemical plant defenses. Many unrelated plants produce viscous secretions, known as resins or gums, in response to wounding or insect attacks. Volatile monoterpenes with strong odors in leaves deter insects. In response to injury, plants can synthesize defensive volatiles de novo, like green-leaf volatiles related to hexanol and increased amounts of mono- and sesquiterpenes. Many examples of multipartite relationships link a plant, its herbivores, and the herbivores’ natural enemies. Cyanogenic glycosides are widespread defensive plant compounds that generate hydrogen cyanide when leaves or other plant parts are damaged. Structures of common cyanoglycosides and mechanisms of their reactions in plants are shown. Glucosinolates, also known as mustard oils, are sulfur- and nitrogen-containing plant compounds found mainly in plants of the cabbage family (Brassicaceae). Numerous insects have learned to tolerate and to adapt to glucosinolates. Diverse defensive compounds, that to humans have a distinct bitter taste, deter insect herbivores. Some of them, like neem, have been used as insect repellents since ancient times. Chemical structures of bitter-tasting plant compounds are introduced, like azadirachtin from neem, tannins, and the alkaloid quinine. Plant toxins, including the major family of alkaloids, harm or kill invading insects. The occurrence and typical structures of alkaloids are introduced. Characteristic ring structures in alkaloids are commonly used to classify these plant defenses. Plant latex occurs in about 10% of all angiosperms. The structures of characteristic latex components are introduced, like rubber, bitter compounds like lactucin and the alkaloid morphine, and the cardenolide calotropin from milkweeds.