Helicoverpa zea genomic response to defense compounds encountered in Nicotiana tabacum

Although plant secondary metabolites have typically been studied for their individual roles in defending plants against herbivory, the actual defenses encountered by insect herbivores are thought to emerge from the totality of these secondary compounds acting in concert. In one known example of such defensive synergism, the combination of the toxic antifeedant nicotine acting in conjunction with the anti-digestant trypsin protease inhibitor has a greater net impact on the growth of lepidopteran larvae than either compound acting alone. In this study, we employed microarray techniques and feeding assays to examine how sixth instar Corn Earworm (Helicoverpa zea Boddie) caterpillars negotiate the synergistic effects of nicotine and trypsin protease inhibitors. Caterpillars were allowed to feed on Tobacco (Nicotiana tabacum L.), as well as two artificial diet treatments laced with nicotine and trypsin protease inhibitor. Total RNA was extracted from caterpillars and hybridized onto oligo microarrays. We also considered the possibility of foraging behavior as a coping mechanism, by performing preference assays on caterpillars provided with a choice between artificial control diet and each of the two experimental treatments. We find that H. zea counters nicotine and trypsin protease inhibitors by modulating expression of genes coding for protease inhibitors and cytochrome P450 enzymes. We note that caterpillars feeding on nicotine-laced diet engage in cannibalism more frequently than caterpillars feeding on control diet.