Context-dependent and variable effects of endohyphal bacteria on interactions between fungi and seeds

Plant-associated fungi often harbor endohyphal bacteria (EHB) that modulate fungal phenotypes. We quantified the effects of EHB on interactions between fungi and seeds of neotropical pioneer trees, which fungi colonize naturally in forest soil. Seeds were exposed to six fungal isolates that harbored EHB, and to clones of those fungi from which EHB were removed by antibiotic treatment. Seed colonization by fungi was evaluated for five tree species, and germination and viability were evaluated for three tree species. EHB influenced seed colonization by fungi in 5 of 30 fungus-tree species combinations, but the magnitude of their effects was small and the direction of effects depended upon fungal isolate-tree species pairs. EHB had rare and context-dependent effects on seed germination and viability, but their effects were strong when observed. Rare but powerful effects of EHB on fungal interactions with seeds highlight important and context-dependent aspects of plant and fungal ecology.