Abstract: Microbial symbioses are ubiquitous in nature wherein fungalbacterial complexes can interact in diverse ways and collectively perform various ecological functions. Some bacteria reside within the hyphae of fungi and regulate the growth, development, distribution, and secondary metabolism of the fungi; these bacteria are referred to as endohyphal bacteria (EHB). Recent studies on EHB have revealed a novel chapter in microbial ecology that represents the most intimate interaction among fungi and bacteria symbioses. EHB have been found to regulate key components of host reproductive machinery, induce the production of phytohormones, and play a complementary protective role for fungi under stress conditions. The most well-studied fungi-EHB symbiotic system is the phytopathogenic fungus Rhizopus sp. and its EHB Burkholderia sp., the phytotoxin essential for causing rice seedling blight is produced by the endosymbiont rather than the host fungus. EHB were found to affect the ecology and diversity of endophytic fungi that colonize higher plants. In some cases, EHB help in activation of genes involved in the recognition processes, transcription regulation, and primary metabolic protein synthesis. Although methods have been developed to isolate EHB in axenic culture, many fungi-EHB symbiotic systems still remain unexplored. This review article focuses on EHB and endohyphal symbioses with host fungi, the complex and delicate relationships among these partners, and the effects of EHB on the growth and development of host fungi and plants and proposes future research directions in this field.

Key words: endobacteria, endohyphal bacteria (EHB), fungi, symbiosis