The impact of polyphenolic compounds on the in vitro growth of oak-associated foliar endophytic and saprotrophic fungi

Foliar fungal endophytes are horizontally transmitted symbionts that inhabit healthy, photosynthetic tissues of all lineages of land plants where they influence plant health and productivity. Endophyte communities often are more similar among closely related hosts, potentially as a result of a preference for particular morphological, ecophysiological, or chemical host traits. However, the various ecological and evolutionary factors that drive community assembly often are difficult to disentangle. Here, we examined the impact of six polyphenolic compounds on the growth of 15 phylogenetically diverse Quercus (oak)-associated fungal species and assessed whether tolerance to phenolics is associated with their degree of specialization to oaks in nature. Despite frequently reported antifungal properties of phenolics, we found that oak-associated fungi grew the same or better than positive controls in 78% of trials with all compounds, although fungal sensitivity differed as a function of compound type and concentration. Overall, species with high specificity to Quercus had the greatest tolerance to phenolics, whereas generalists were more sensitive. Differences between generalists and specialists suggest that variation in phenolic abundance and composition among oaks may act as a selective filter that influences endophyte host associations in nature.