Armyworm table manners: quantification of caterpillar elicitors transferred to the plant leaf surface during herbivory.

Mentor: Eric Schmelz

Abstract:   The fatty acid-amino acid conjugate N-(17-hydroxylinenoyl)-L-glutamine, termed volicitin, is a potent elicitor of plant volatile emission and present in the regurgitate of the beet armyworm larvae (Spodoptera exigua).  These caterpillar-induced plant volatiles serve as important ecological signals used by parasitoids and predators in the location of suitable hosts and prey.  The mechanistic interaction between volicitin, stimulation of the plant defense hormones (i.e. jasmonic acid) and initiation of volatile emission has been well described.  What has not been made clear is the amount of volicitin, if any, that comes in contact with the leaf surface during actual beet armyworm herbivory.  Volicitin levels in caterpillar regurgitate are routinely measured using HPLC and UV detection; however, to quantify trace levels of volicitin that may exist on the leaf surface following caterpillar attack more sensitive detection method is needed.  We explore the use of chemical ionization-gas chromatography-mass spectroscopy (CI-GC-MS) using cool-on-column sample introduction and selected ion monitoring.  Due to the high molecular weight of volicitin (MW 421) and poor chromatographic properties, a method was explored to cleave the glutamine from the 17-hydroxy-linolenic acid.  In theory, the unique 17-hydroxy-fatty acid should be available as a marker of herbivory and demonstrate the presence of insect elicitors on the leaf surface after attack.

Setting up samples for cool-on-column gas chromatographic analyses.

Preparation of fatty acid methyl esters for vapor phase extraction based purification.

Anallysis of flame ionization detection response of hydroxy-fatty acid methyl esters separated by gas chromatography.

Construction of summarized figure detailing the use of chemical ionization GC/MS in metabolic profiling.

Collection of volatile metabolites and fatty acid methyl esters using vapor phase extraction.