Using nutritional indices to study LOX3-dependent insect resistance.

Induced resistance to biotic attackers is thought to be mediated by responses elicited by jasmonic acid (JA), a subset of which are lipoxygenase 3 (LOX3) dependent. To understand the importance of LOX3-mediated insect resistance, we analysed the performance of Manduca sexta larvae on wild-type (WT) and on isogenic Nicotiana attenuata plants silenced in NaLOX3 expression and JA signalling, and we used Waldbauer nutritional indices to measure the pre- and post-ingestive effects. LOX3-mediated defenses reduced larval growth, consumption and frass production. These defenses reduced how efficiently late-instar larvae converted digested food to body mass (ECD). In contrast, LOX3-mediated defenses decreased approximate digestibility (AD) in early instar larvae without affecting the ECD and total food consumption. Larvae of all instars feeding on defended WT plants behaviourally compensate for their reduced body mass by consuming more food per unit of body mass gain. We suggest that larvae feeding on plants silenced in NaLOX3 expression (as-lox) initially increase their AD, which in turn enables them to consume more food in the later stages and consequently, to increase their ECD and efficiency of conversion of ingested food (ECI). We conclude that N. attenuata's oxylipin-mediated defenses are important for resisting attack from M. sexta larvae, and that Waldbauer nutritional assays reveal behavioural and physiological counter responses of insects to these plant defenses.