Linking fungal symbiosis with tree chemical defenses: Implications for forest health
L. BULLINGTON , E. MARTIN, N. ERBILGIN, P. KENNEDY, A. KEGLEY, B. LARKIN, Y. LEKBERG, D.SIX, R. SNIEZKO
Department of Ecosystem and Conservation Sciences, 32 Campus Drive, University of Montana, Missoula, MT, USA 59812

Invasive pathogens threaten the sustainability of forest ecosystems worldwide. Trees possess intrinsic pathogen-defense mechanisms, including major-gene resistance (MGR) and quantitative-disease resistance (QDR). Symbiotic fungi can enhance tree resistance, influencing entire ecosystems.  However, it remains unclear whether this enhanced resistance through symbiosis triggers changes in the chemical defense profiles of trees, and the extent and duration of such alterations remain unexplored. We inoculated Pinus monticola seedling families exhibiting resistance (MGR or QDR) or high susceptibility to Cronartium ribicola , the causative agent of white pine blister rust (WPBR), with endophytic and ectomycorrhizal fungi individually and in combination. We monitored growth, terpenes, and disease progression before and after C. ribicola infection and in controls without C. ribicola infection. We observed prolonged changes in the inducible defenses of trees and evidence of immune priming across all treatments. In control treatments, shifts in thost defenses coincided with increased seedlings growth rate. Among WPBR-infected seedlings, symbiont treatments reduced disease symptoms notably in families with QDR and to a lesser extent in susceptible, but not those with MGR. Our study underscores the influence of endophytic and ectomycorrhizal fungi in enhancing tree growth and defense while reducing disease symptoms. We also show that these fungi can induce long-lasting changes in conifer foliar terpenes, expanding our understanding of their ecological significance in forest ecosystems and suggesting potential applications in tree protection from invasive species.