Toxin-Producing Endosymbionts Shield Pathogenic Fungus against Micropredators

The fungus Rhizopus microsporus harbors a bacterial endosymbiont (Mycetohabitans rhizoxinica) for the production of the antimitotic toxin rhizoxin. Although rhizoxin is the causative agent of rice seedling blight, the toxinogenic bacterial-fungal alliance is, not restricted to the plant disease. It has been detected in numerous environmental isolates from geographically distinct sites covering all five continents, thus raising questions regarding the ecological role of rhizoxin beyond rice seedling blight. Here, we show that rhizoxin serves the fungal host in fending off protozoan and metazoan predators. Fluorescence microscopy and coculture experiments with the fungivorous amoeba Protostelium aurantium revealed that ingestion of R. microsporus spores is toxic to P. aurantium. This amoebicidal effect is caused by the dominant bacterial rhizoxin congener rhizoxin S2, which is also lethal toward the model nematode Caenorhabditis elegans. By combining stereomicroscopy, automated image analysis, and quantification of nematode movement, we show that the fungivorous nematode Aphelenchus avenae actively feeds on R. microsporus that is lacking endosymbionts, whereas worms coincubated with symbiotic R. microsporus are significantly less lively. This study uncovers an unexpected ecological role of rhizoxin as shield against micropredators. This finding suggests that predators may function as an evolutionary driving force to maintain toxin-producing endosymbionts in nonpathogenic fungi. IMPORTANCE The soil community is a complex system characterized by predator-prey interactions. Fungi have developed effective strategies to defend themselves against predators. Understanding these strategies is of critical importance for ecology, medicine, and biotechnology. In this study, we shed light on the defense mechanisms of the phytopathogenic Rhizopus-Mycetohabitans symbiosis that has spread worldwide. We report an unexpected role of rhizoxin, a secondary metabolite produced by the bacterium M. rhizoxinica residing within the hyphae of R. microsporus. We show that this bacterial secondary metabolite is utilized by the fungal host to successfully fend off fungivorous protozoan and metazoan predators and thus identified a fundamentally new function of this infamous cytotoxic compound. This endosymbiont-dependent predator defense illustrates an unusual strategy employed by fungi that has broader implications, since it may serve as a model for understanding how animal predation acts as an evolutionary driving force to maintain endosymbionts in nonpathogenic fungi.