Background and aims – Arbuscular mycorrhizal fungi (AMF) are ubiquitous soil organisms that interact with plant roots and exchange soil nutrients for plant-derived carbohydrates, frequently resulting in growth and fitness benefits for plants. These benefits may be due partly to AMF effects on plants' resistance to insect herbivores, particularly through enhancement of induced defenses. Here we studied the impacts of AMF colonization on constitutive and induced resistance in two species of nightshades, Solanum ptycanthum and S. dulcamara.
Methods – We used a factorial design manipulating AMF presence and jasmonic acid application to determine if constitutive and induced resistance differ in the presence and absence of mycorrhizae. We measured three protein-based chemical defenses and performed a bioassay by feeding leaves of experimental plants to a specialist herbivore, Manduca sexta.
Key results – The presence of AMF influenced chemical defenses in S. dulcamara, including an interaction with jasmonic acid application for polyphenol oxidase activity. Solanum ptycanthum defenses were unaffected by AMF. Caterpillar growth was also unaffected by AMF but reduced by jasmonic acid treatments, indicating that, while AMF may influence certain chemical defenses in some plant species, this does not always translate to resistance against herbivores.
Conclusions – Our results emphasize the context dependency of fungi–plant–herbivore interactions and suggest that mycorrhizal effects on plant defense may vary with other plant traits or life history strategies.

ABOVEGROUND; ARBUSCULAR MYCORRHIZAL FUNGI; BELOWGROUND; HERBIVORE; INDIRECT EFFECTS; INDUCED DEFENSE; SOLANUM DULCAMARA; SOLANUM PTYCANTHUM