Volicitin, an elicitor of maize volatiles in oral secretion of Spodoptera exigua: Isolation and bioactivity

Plants respond to insect-inflicted injury by systemically releasing relativ ely large amounts of several volatile compounds, mostly terpenoids and indo le. As a result, the plants become highly attractive to natural enemies of the herbivorous insects. In maize, this systemic response can be induced by the uptake via the stem of an elicitor present in the oral secretions of c aterpillars. Such an elicitor was isolated from the regurgitant of Spodopte ra exigua larvae, identified as N-(17-hydroxylinolenoyl)-L-glutamine, and n amed volicitin. Here we present details on the procedure that was used to i solate volicitin and the biosasays that demonstrate its potency as an elici tor of maize volatiles that attract parasitoids. With a series of liquid ch romatography purification steps, volicitin was separated from all other ina ctive substances in the regurgitant of larvae of the noctuid moth S. exigua . Maize seedlings that were incubated in very low concentrations of pure na tural volicitin released relatively large amounts of terpenoids and became highly attractive to the parasitoid Microplitis croceipes. The identificati on of this and other insect-derived elicitors should allow us to determine their precise source and function, and better understand the evolutionary h istory of the phenomenon of herbivore-induced volatile emissions in plants.