Local and Systemic Production of Volatile Herbivore-induced Terpenoids: Their Role in Plant-carnivore Mutualism

Abstract

Plants may defend themselves by promoting the effectiveness of enemies of herbivores, which is termed <indirect defense>. In the past 10 years it has become known that plants may respond to being damaged by herbivorous arthropods by producing volatiles that attract carnivorous arthropods. These volatiles are different from those emitted in response to mechanical damage and terpenoids are a major class among the herbivore-induced carnivore attractants. Herbivore secretions such as caterpillar regurgitate elicit the plant response. The carnivore-attractants are produced at the site of herbivore damage as well as systemically throughout the plant. An elicitor that is exported from herbivore-infested leaves and induced carnivore-attractant production in uninfested leaves has recently been extracted. Herbivore cues are not very detectable to carnivores at a distance, herbivore-induced plant volatiles increase herbivore detectability enormously. Moreover, herbivore-induced plant volatiles may also provide carnivores with information about the plant species, the plant cultivar and the herbivore species that infests the plant and thus are more reliable to carnivores than general damage-related plant volatiles. Therefore, herbivore-induced volatiles are an essential information source for carnivores that search for herbivores. Among the terpenoids the two homoterpenes 4,8-dimethyl-1,3(E),7-nonatriene and 4,8,12-trimethyl- 1,3(E),7(E),11-tridecatetraene are the most often reported herbivore-induced plant volatiles. They can be synthesized by plants of many species from the terpene alcohols nerolidol and geranyllinalool without any mediation of herbivory. Whether the production of nerolidol and geranyllinalool is effectuated by herbivore enzymes or by herbivore-induced plant enzymes remains to be elucidated. That plants repond to herbivory by producing terpenoid carnivore attractans should challenge phytochemists, plant biochemists and plant molecular biologists to investigate the signal-transduction pathway and the regulation mechanisms of this intriguing defense of plants.