Support from the underground: Induced plant resistance depends on arbuscular mycorrhizal fungi

1. Mycorrhizal symbiosis is thought to affect interactions between plants and herbivores by its influence on plant growth, nutrition and the plants defence system. Moreover, arbuscular mycorrhizal fungi (AMF) may enhance the inducibility of resistance responses. Until now, induction of plant resistance has not been considered to be a mechanism affecting the outcome of mycorrhization for plant-herbivore interactions. 2. Here, we test the hypothesis that the resistance of plants against herbivores depends on the induction of plant resistance by previous herbivory and mycorrhization. With a full factorial experiment in a greenhouse, we examined responses in growth of seven herbaceous plant species to AMF and the induction of resistance. To evaluate whether induced resistance is higher in plants with AMF we analyzed the combined effects of AMF and induction on herbivory, using bioassay caterpillars (Spodoptera littoralis). 3. Across all species, mycorrhization increased growth of plants and performance of the bioassay herbivore feeding on them. If, however, we induced resistance by allowing a caterpillar to feed for a short period on the plants, mycorrhization did not increase plant growth and performance of a subsequent herbivore that fed on the plant. This suggests that the increased plant resistance after induction was dependent on the symbioses with AMF. 4. Our results indicate that induction interacts with the allocation of the additional resources provided by mycorrhization towards plant growth and plant resistance. Therefore, mycorrhiza may play an important but hitherto overlooked role in the induction of plant resistance against herbivores.