Variation in the ability of fungi in the extrafloral nectar of Mallotus paniculatus to attract ants as plant defenders

Many plant–ant interactions are considered mutualisms. In Mallotus paniculatus (Euphorbiaceae) (also known as Turn-in-the-wind), the extrafloral nectaries (EFNs) on the base of the leaf laminas can produce sugar-rich secretions to attract ants as effective agents against herbivores or plant competitors. Growing evidence reveals that microorganisms are important “hidden players” in insect–plant interactions. Understanding which microorganisms act as such third-party species and how they operate is a major challenge in the study of mutualistic interactions. In this study, we showed that two dominant fungal species, the yeast Jaminaea angkorensis and the hyphal fungus Gibellulopsis nigrescens both from EFNs and the interiors of bodies of the ant Pheidole megacephala (Formicidae) on their own were sufficient for ant attraction. Our results also revealed that different fungal species on the host plant influenced ant behavior differently. These results can be applied in agriculture to increase ants or herbivore predators to protect plant hosts using fungal baits. Moreover, they indicate that fungal odors represent the critical signal to establish the plant–microbe–insect interactions. The traditional plant– defender concept must be updated to include the role of microorganisms.