Getting by With a Little Help From Fungal Friends: Seed‐Associated Endophytes Contribute to Seed Defense Against Fungal Pathogens in Keystone Tropical Trees

ABSTRACT

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 seed‐associated fungi may contribute to seed defense and persistence in soil seed banks and are especially important for non‐dormant seeds with limited physical and chemical defenses. 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 (seeds germinate in the soil), and the other, Ficus beddomei is a hemi‐epiphyte (seeds germinate in the canopy of host trees and send roots to soil). We predicted that (i) fungi infecting seeds prior to soil exposure will have less negative effects on seed 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, endophytic fungi excludes pathogens and increases seed survival. We inoculated seeds using seed‐associated fungal isolates obtained from the two fig species prior to and after exposing seeds to soil, classified their effects as endophytic or pathogenic, then tested the effect of pre‐treatment of seeds with endophytes before exposure to putative pathogens. We found that inoculation with fungal taxa isolated from seeds prior to soil exposure did not affect seed germination for either fig species, while 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, with stronger effects in the free‐standing fig. Synthesis : Our findings show that prior seed infection by seed‐associated 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.