Ectomycorrhizal fungal species differentially affect the induced defensive chemistry of lodgepole pine.

Plants interact simultaneously with multiple organisms, including ectomycorrhizal (EM) fungal symbionts which benefit plants by facilitating resource acquisition. Yet, their role in induced plant defenses that rely on the allocation of plant resources has received little attention. We investigated whether EM fungi can affect the induction of defense-related monoterpenes in greenhouse-grown lodgepole pine (Pinus contorta var. latifolia) seedlings, and whether such effects differed between EM fungal species occurring alone or in combination. Fungal interactions on growth media were also assessed to complement the greenhouse study. Our study revealed that the production of certain monoterpenes is influenced by the fungal species colonizing pine roots. Furthermore, pine seedlings did not necessarily benefit from having associations with multiple EM fungi, as we found contrasting effects of single vs. multiple species of fungi on induced monoterpene responses. Finally, monoterpene responses were altered when early-colonizing species inhibited the colonization or development of later-arriving species. We conclude that the presence of EM fungi can impact host susceptibility to insect and pathogen attack, suggesting that seedlings establishing in areas lacking fungi that promote the induction of tree defense chemicals may suffer from increased susceptibility to future pest damage.