Emergency Alert: Plant communication about herbivory through arbuscular mycorrhizal fungi

By responding to volatile signals of neighbors, plants avoid costly defenses when unnecessary, and induce defenses before insects arrive. However, field tests have shown that volatile communication is typically limited to immediate neighbors. Recent greenhouse research has indicated arbuscular mycorrhizal fungi can act as a conduit for distress signals. Mycorrhizal networks may allow plants to communicate over long distances because they often connect thousands of individuals, but no experiments have tested the role of mycorrhizal fungi in signaling herbivory between plants in the field. In this study, we tested for communication about herbivory between tomato plants through mycorrhizal fungi in an agricultural setting. Tomatoes were planted in a field and allowed to connect to the common mycorrhizal network which was augmented by broadcast application of G. intraradices.  Half of the plants acted as triggers, while the other half acted as signal receivers. The receivers were divided into 3 treatments: Mycorrhizal exclusion, root exclusion, and control. Trigger plants were induced by Manduca sexta for 24 hours. Plants in the root exclusion and control treatments exhibited higher levels of protease inhibitors and endogenous methyl jasmonate compared to plants in the mycorrhizal exclusion bag, suggesting plants connected only via mycorrhizae shared information about herbivory. Manduca larvae fed on tomato leaves from the control and root exclusion treatments grew less than those fed on leaves from the mycorrhizal exclusion treatment, indicating that mycorrhizal communication led to reduced herbivore performance. We conclude that mycorrhizae may play an important role in plant defenses in the field.