Chemical warfare in the insect gut: Genomic approaches for identifying plant defense compounds and insect counter defenses

It estimated that over one million insect species feed on plants. During the coevolution of plant-insect relationships, plants developed defensive strategies to minimize insect herbivory, which in turn led to the development of counter adaptations by herbivores. Anti-nutritional and toxic host defenses that act post-ingestively often involve plant proteins that are adapted to the harsh environment of the insect gut. The plant hormone jasmonate (JA) plays a central role in regulating the induced expression of many of these defensive proteins, most notably proteinase inhibitors. We are employing a shotgun proteomic approach to identify JA-regulated proteins that are stable during passage through the digestive tract of various insect herbivores. Application of this approach to Manduca sexta larvae reared on tomato led to the identification of two JA-regulated enzymes, arginase and threonine deaminase, that retard the growth of lepidopteran insects by degrading essential amino acids (arginine and threonine, respectively) in the midgut. To gain insight into potential mechanisms that enable lepidopteran insects to adapt to JA-regulated plant defenses, we prepared cDNA libraries from midgut tissue of larvae reared either on wild-type plants or on mutant plants that are defective in JA signaling. High throughput sequencing of these libraries, using next-generation sequencing technology (e.g., Illumina), is being used: 1) to elucidate the midgut transcriptomes of various insect species, and 2) to identify insect genes that are differentially expressed as a result of host plant chemistry. Results obtained from these experiments are providing new insight into the mechanisms by which lepidopteran insects actively modify their digestive physiology to adapt to host plant defenses.