A Maize Sesquiterpene Cyclase Gene Induced by Insect Herbivory and Volicitin: Characterization of Wild-type and Mutant Alleles

Plants can defend themselves from herbivorous insects by emitting volatile chemical signals that attract natural enemies of the herbivore. For example, maize seedlings attacked by beet armyworm larvae (Spodoptera exigua) produce a mixture of terpenoid and indole volatiles that serve to attract parasitic wasps. A key step in terpenoid biosynthesis is the conversion of acyclic prenyl diphosphates to terpenoid compounds by specific terpenoid synthases (cyclases). We have cloned a maize sesquiterpene cyclase gene, stc1, by transposon tagging and have identified two deletion mutations of the gene. The stc1 gene is located on chromosome 9S and does not seem to have a closely related ortholog in the maize genome. It is induced 15- to 30-fold in maize leaves by beet armyworm larvae feeding or by application of purified volicitin, the insect-derived elicitor, at a mechanically wounded site. stc1 induction is systemic, because undamaged leaves of the same plant show a similar increase in stc1 transcription. Analysis of volatiles from volicitin-treated seedlings revealed that a major naphthalene-based sesquiterpene was present in wild-type seedlings but absent in the Ac-insertion and x-ray-deletion mutants. Therefore, we have identified a maize gene that responds to caterpillar herbivory by producing a chemical defense signal that most likely serves to attract natural enemies of the herbivore.