Quantitative relationships between induced jasmonic acid levels and volatile emission in Zea mays during Spodoptera exigua herbivory

Jasmonic acid (JA) has long been hypothesized to be an important regulator of insect-induced volatile emission; however, current models are based primarily on circumstantial evidence derived from pharmacological studies. Using beet armyworm caterpillars (BAW: Spodoptera exigua) and intact corn seedlings, we examine this hypothesis by measuring both the time-course of insect-induced JA levels and the relationships between endogenous JA levels, ethylene, indole and sesquiterpenes. In separate Morning and Evening time-course trials, BAW feeding stimulated increases in JA levels within the first 4-6 h and resulted in maximal increases in JA, indole, sesquiterpenes and ethylene 8-16 h later. During BAW herbivory, increases in JA either paralleled or preceded the increases in indole, sesquiterpenes and ethylene in the Morning and Evening trials, respectively. By varying the intensity of the BAW herbivory, we demonstrate that strong positive relationships exist between the resulting variation in insect-induced JA levels and volatile emissions such as indole and the sesquiterpenes. To address potential signaling interactions between herbivore-induced JA and ethylene, plants were pretreated with 1-methylcyclopropene (1-MCP), an inhibitor of ethylene perception. 1-MCP pretreatment resulted in reduced production of ethylene and volatile emission following BAW herbivory but did not alter the insect-induced accumulation of JA. Our results strongly support a role for JA in the regulation of insect-induced volatile emission but also suggest that ethylene perception regulates the magnitude of volatile emission during herbivory.