Getting by with a little help from fungal friends: seed endophytes contribute to seed defense against fungal pathogens in keystone tropical trees

1. Soil-borne fungal pathogens are known to be major causes of seed mortality in tropical forests; however, not all fungal infections of seeds are necessarily lethal. It has been hypothesized that the seed microbiome may contribute to seed defense and persistence in soil seed banks and is especially important for non-dormant seeds with limited physical and chemical defenses. 2. This study examined microbially mediated seed defense in two fig (Ficus) species from a tropical forest in the Western Ghats, India. One species, Ficus callosa has a free-standing habit, and the other, Ficus beddomei is a hemi-epiphyte (seeds germinate in the canopy of host trees and send roots to soil). We inoculated seeds using pre-dispersal and soil-borne seed fungal isolates obtained from the same fig species, classified their effects as endophytic or pathogenic, then tested the effect of pre-treatment of seeds with endophytes before exposure to pathogens. 3. We predicted that (i) fungi infecting seeds prior to dispersal will have less negative effects on survival compared to soil-borne fungi (ii) the hemi-epiphytic figs are more susceptible to soil borne fungi, and (iii) prior infection by non-pathogenic, pre-dispersal and soil-borne endophytic fungi excludes pathogens and increases seed survival. 4. We found that inoculation with pre-dispersal fungal taxa did not affect seed germination for either fig species, while the soil borne fungi were frequently pathogenic, especially in the hemi-epiphytic fig. Incubation with endophytic fungi prior to pathogen exposure increased seed survival depending on endophyte and pathogen identities, and was more likely to occur in the free-standing fig. 5. Synthesis: Our findings show that prior seed infection by endophytes could be a plausible mechanism of seed defense against fungal pathogens. It highlights the potential for dynamic seed-fungal interactions to increase seed survival, with implications for tree populations and diversity in tropical forests.