Endofungal bacteria as hidden facilitators of biotic interactions

Fungi play pivotal roles in ecology and human health, driving nutrient cycling, supporting antibiotic production, and posing threats through toxin production. Less well-recognized, however, is their ability to harbour endosymbiotic bacteria. Advances in genomics and microscopy have revealed the prevalence of endofungal bacteria across diverse fungal phyla, though their functions are primarily inferred from genomic and transcriptomic studies. Recent functional research has begun to shed light on their influence on fungal pathogenicity, physiology, and ecology. These findings raise fundamental questions about the establishment and benefits of bacterial-fungal endosymbiosis, as well as the role of endosymbionts in mediating fungal interactions with other organisms. This review provides an in-depth analysis of the molecular mechanisms involved in the establishment and persistence of these symbioses. It also summarizes the current understanding of how endofungal bacteria impact fungal interactions with other organisms. For instance, endofungal bacteria contribute to the beneficial effects of fungi on plant health and fitness, protect fungal hosts from fungivorous predators, and enhance fungal virulence against plants, animals, and humans. These discoveries highlight the need for holistic investigations into bacterial-fungal endosymbiosis to fully understand their role in natural ecosystems. A deeper understanding of these multipartite partnerships offers exciting opportunities to improve ecosystem management, food safety, disease control, and crop productivity.