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.

Rapid monoterpene induction promotes the susceptibility of a novel host pine to mountain pine beetle colonization but not to beetle-vectored fungi.

Chemical induction can drive tree susceptibility to and host range expansions of attacking insects and fungi. Recently, mountain pine beetle (Dendroctonus ponderosae Hopkins; MPB) has expanded its host range from its historic host lodgepole pine (Pinus contorta var. latifolia Douglas ex Loudon) to jack pine (Pinus banksiana Lamb) in western Canada. Beetle success in jack pine forests likely depends upon the suitability of tree chemistry to MPB and its symbiotic phytopathogenic fungi. In particular, how rapid induced defenses of jack pine affect MPB colonization and the beetle's symbionts is unknown. In the field, we characterized and compared differences in rapid induced phloem monoterpenes between lodgepole and jack pines in response to various densities of Grosmannia clavigera (Robinson-Jeffery and Davidson)-a MPB symbiotic fungus used to simulate beetle attack-inoculations. Overall, lodgepole pine had higher limonene and myrcene, but lower α-pinene, concentrations than jack pine. However, myrcene concentrations in jack pine increased with inoculation density, while that in lodgepole pine did not respond to density treatments. We compared the growth and reproduction of MPB's symbiotic fungi, G. clavigera, Ophiostoma montium (Rumford) von Arx and Leptographium longiclavatum Lee, Kim and Breuil, grown on media amended with myrcene, α-pinene and limonene at concentrations reflecting two induction levels from each pine species. Myrcene and α-pinene amendments inhibited the growth but stimulated the reproduction of G. clavigera, whereas limonene stimulated its growth while inhibiting its reproduction. However, the growth and reproduction of the other fungi were generally stimulated by monoterpene amendments. Overall, our results suggest that jack pine rapid induction could promote MPB aggregation due to high levels of α-pinene (pheromone precursor), a positive feedback of myrcene (pheromone synergist) and low levels of limonene (resistance). Jack pine is likely as susceptible to MPB-vectored fungi as lodgepole pine, indicating that jack pine induction will likely not adversely affect symbiont activities enough to inhibit the invasion of MPB into jack pine forests.

Fungal Volatiles Can Act as Carbon Sources and Semiochemicals to Mediate Interspecific Interactions Among Bark Beetle-Associated Fungal Symbionts.

Mountain pine beetle (Dendroctonus ponderosae) has killed millions of hectares of pine forests in western North America. Beetle success is dependent upon a community of symbiotic fungi comprised of Grosmannia clavigera, Ophiostoma montium, and Leptographium longiclavatum. Factors regulating the dynamics of this community during pine infection are largely unknown. However, fungal volatile organic compounds (FVOCs) help shape fungal interactions in model and agricultural systems and thus may be important drivers of interactions among bark beetle-associated fungi. We investigated whether FVOCs can mediate interspecific interactions among mountain pine beetle's fungal symbionts by affecting fungal growth and reproduction. Headspace volatiles were collected and identified to determine species-specific volatile profiles. Interspecific effects of volatiles on fungal growth and conidia production were assessed by pairing physically-separated fungal cultures grown either on a carbon-poor or -rich substrate, inside a shared-headspace environment. Fungal VOC profiles differed by species and influenced the growth and/or conidia production of the other species. Further, our results showed that FVOCs can be used as carbon sources for fungi developing on carbon-poor substrates. This is the first report demonstrating that FVOCs can drive interactions among bark beetle fungal symbionts, and thus are important factors in beetle attack success.