Antifungal leaf-surface metabolites correlate with fungal abundance in sagebrush populations

A central component in understanding plant-enemy interactions is to determine whether plant enemies, such as herbivores and pathogens, mediate the evolution of plant secondary metabolites. Using 26 populations of a broadly distributed plant species, sagebrush (Artemisia tridentata), we examined whether sagebrush populations in habitats with a greater prevalence of fungi contained antifungal secondary metabolites on leaf surfaces that were more active and diverse than sagebrush populations in habitats less favorable to fungi. Because moisture and temperature play a key role in the epidemiology of most plant-pathogen interactions, we also examined the relationship between the antifungal activity of secondary metabolites and the climate of a site. We evaluated the antifungal activity of sagebrush secondary metabolites against two fungi, a wild Penicillium sp. and a laboratory yeast, Saccharomyces cerevisiae, using a filter-paper disk assay and bioautography. Comparing the 26 sagebrush populations, we found that fungal abundance was a good predictor of both the activity (r2 = 0.36 for Saccharomyces, r2 = 0.37 for Penicillium) and number (r2 = 0.34 for Saccharomyces) of antifungal secondary metabolites. This suggests that selection imposed by fungal pathogens has led to more effective antifungal secondary metabolites. We found that the antifungal activity of sagebrush secondary metabolites was negatively related to average vapor pressure deficit of the habitat (r2 = 0.60 for Saccharomyces, r2 = 0.61 for Penicillium). Differences in antifungal activity among populations were not due to the amount of secondary metabolites, but rather to qualitative differences in the composition of antifungal compounds. Although all populations in habitats with high fungal prevalence had secondary metabolites with high antifungal activity, different suites of compounds were responsible for this activity, suggesting independent outcomes of selection on plants by fungal pathogens. The location of antifungal secondary metabolites on the leaf surface is consistent with their putative defense role, and we found no evidence supporting other functions, such as protection from ultraviolet light or oxidation. That the antifungal activity of sagebrush secondary metabolites was similar for two different fungi provides support for broad antifungal defenses. The incidence and severity of fungal disease in the field (caused by Puccinia tanaceti) were similar in moist and dry habitats, possibly reflecting an equilibrium between plant defense and fungal attack, as sites with greater fungal abundance compensated with more effective secondary metabolites. The geographic correlation between fungal abundance and antifungal secondary metabolites of sagebrush, coupled with our other data showing heritable variation in these metabolites, suggests that pathogenic fungi have selected for antifungal secondary metabolites in sagebrush.