Endohyphal Bacteria in Fungi: Diversity, Molecular Interactions, Ecological Significance and Biotechnological Applications
Gedela Venkata Hima Sameera *
Department of Plant Pathology, G.B Pant University of Agriculture and Technology, Uttarakhand, India.
Sonam Bhatt
Department of Plant Pathology, G.B Pant University of Agriculture and Technology, Uttarakhand, India.
Ashish Singh Bisht
Department of Plant Pathology, G.B Pant University of Agriculture and Technology, Uttarakhand, India.
Ritvik Sharma
Department of Plant Pathology, G.B Pant University of Agriculture and Technology, Uttarakhand, India.
Oendri Ghosh
Department of Plant Pathology, G.B Pant University of Agriculture and Technology, Uttarakhand, India.
Anshika Rawat
Department of Plant Pathology, G.B Pant University of Agriculture and Technology, Uttarakhand, India.
*Author to whom correspondence should be addressed.
Abstract
Fungi are increasingly recognised as multipartite biological systems in which the phenotype of the fungal host may depend not only on its own genome and environment but also on associated microorganisms. Among these associations, endohyphal bacteria (EHB), also termed endofungal bacteria or fungal endobacteria, occupy a particularly intimate niche within living fungal hyphae and reproductive structures. Early observations of bacteria-like organisms in mycorrhizal fungi were followed by molecular, microscopic and genomic studies demonstrating that intracellular bacteria occur across diverse fungal lineages and can influence fungal growth, reproduction, metabolism, stress tolerance, virulence and interactions with plants. The functional consequences of these associations range from mutualistic and growth-promoting interactions to context-dependent or pathogenic outcomes. Notable examples include the production of the phytotoxin rhizoxin by Mycetohabitans rhizoxinica within Rhizopus microsporus, the contribution of endohyphal bacteria to indole-3-acetic acid production in fungal endophytes, nitrogen fixation associated with Ustilago maydis, and alterations in carbon and lipid metabolism in early diverging fungi. Recent studies further indicate that endofungal bacterial communities may be more diverse and dynamic than previously assumed, with both vertically inherited and horizontally acquired associations. Advances in microscopy, bacterial curing and reinfection, long-read sequencing, transcriptomics, metabolomics and experimental manipulation are providing increasingly direct evidence for causal interactions. This review synthesizes the available information on the history, diversity, classification, establishment, molecular mechanisms and ecological functions of EHB, with particular emphasis on plant-associated fungi and agricultural relevance. Current limitations and future opportunities for exploiting fungal–bacterial holobionts in sustainable agriculture, biological control and natural-product discovery are also discussed.
Keywords: Endohyphal bacteria, endofungal bacteria, fungal endobacteria, fungal holobiont, bacterial–fungal symbiosis, plant–fungus interaction, secondary metabolism