Defense pathway elicitors differentially mediate eastern hemlock resistance to the hemlock woolly adelgid
Conifers have evolved varied constitutive and inducible defenses to defend themselves against antagonists. Exogenous application of elicitors, such as jasmonates and salicylates, have proved effective at inducing defensive responses that mirror those induced by herbivores and pathogens. Using these elicitors, we can explore plant responses to attack, as well as the consequences of these changes for subsequent herbivore interactions.
Eastern hemlock (Tsuga canadensis) is experiencing significant mortality due to the invasive hemlock woolly adelgid (HWA; Adelges tsugae). Research to date indicates that hemlocks largely lack resistance to the adelgid and that HWA may manipulate its host to increase susceptibility. The presence of a second invasive herbivore, elongate hemlock scale (Fiorinia externa), appears to confer adelgid resistance, but the mechanism is largely unknown. To determine if herbivore-induced resistance might be mediated by elicitors, we examined the effects of methyl jasmonate (MeJA) and benzothiadiazole (BTH), a salicylic acid pathway elicitor, on chemical defenses and resistance to adelgid attack. We quantified total phenolics in tissues and assessed effects of induction on resistance by performing a preference assay with adelgids in the mobile first-instar ‘crawler’ phase. Adelgid crawlers were presented paired tissues originating from elicited or non-elicited trees, and their settlement preference was scored.
Exogenous application of plant chemical elicitors yielded varying effects on chemical defenses and adelgid behavior. The application of MeJA increased total phenolic content (TPC) of needles, and adelgid settlement was lower on Me-JA foliage than on non-induced control tissue. In contrast, the application of BTH affected neither TPC nor adelgid settlement. Taken together, our results suggest that application of MeJA has the ability to enhance resistance of susceptible hemlock trees by inducing endogenous plant defenses. We suspect that resistance of native hosts against exotic invasive herbivores may often be mediated by induced responses to other herbivores.