Endohyphal bacteria; the prokaryotic modulators of host fungal biology

Fungi interact with bacteria in specific environmental niches through microbial cell–cell interactions and carry out various ecological functions collectively. However, there are a number of known associations wherein some bacteria reside within the hyphae of fungi, leveraging their growth, development, distribution and secondary metabolism. These bacteria are referred to as endohyphal bacteria (EHB). The EHB have been found to regulate key components of host reproductive machinery, induce the production of phytohormones, and play a complementary protective role for the host fungus under stress conditions. In a unique endohyphal association of Burkholderia with Rhizopus sp., it was found that the phytotoxin which is essential for pathogenecity of the fungus, was produced by the endosymbiont rather than the host fungus causing the rice seedling blight. The EHB were also found to influence the ecology and diversity of endophytic fungi colonizing higher plants. In some cases, the EHB help in activation of genes involved in the recognition processes, transcription regulation, and synthesis of primary metabolism proteins. Although, methods have been developed to isolate EHB in axenic culture, this symbiotic association provides enormous opportunities for new discoveries and new insights into fungal biology. In this review article, we present a discussion on EHB, their significance, and diverse functions in nature as unraveled by the latest research to understand this unique microbial association.