Alliance Between Conifer Trees and Endophytic Fungi Against Insect Defoliators

Fungal endophytes can alter plant resistance against herbivores by indirectly influencing plant secondary metabolism or through direct effects of their own metabolism. However, the role of fungal endophytes in conifer defences to insect herbivores remains largely unknown. We characterised the endophytic fungal communities and terpene concentrations of 30 white spruce families across two sites. We determined the effects of fungal endophytes on a defoliating insect, eastern spruce budworm, by testing the budworm responses to media amended with fungal endophytes or exposing them to their volatile organic compounds. We further examined whether the changes in the endophytic fungal communities and abundance alter the terpene concentrations of white spruce by inoculating seedlings with endophytic fungi. Terpene and fungal community compositions in mature trees varied among families and sites. The bioassays showed fungal endophytes can kill budworms or reduce their fitness due to the toxicity of fungal mycelium or volatile compounds. The inoculation experiments demonstrated that the changes in fungal communities and abundance can alter the terpene concentrations in seedlings. We developed a "Plant Partnership Hypothesis" to reflect the role of fungal endophytes in plant resistance to insect herbivores, demonstrating a co-evolutionary relationship among fungal endophytes, tree defences, and insect herbivores.