P-366 - Pangenome of endohyphal bacteria reveals putative mechanisms for symbiosis with fungal plant pathogen

The phytobiome includes interkingdom microbial interactions, such as endohyphal bacteria (EHB) that can control their fungal host, resulting in altered plant growth and stress tolerance. The rice seedling pathogen Rhizopus microsporus (Rm) harbors toxin-producing endohyphal Mycetohabitans species, acting as an emerging model system to study fungi-EHB interactions. Approximately 10-20% of Rm accessions harbor Mycetohabitans and these host isolates require EHB in order to sporulate. The reduced genome size of Mycetohabitans spp. further supports a specialized symbiosis and offers potential insight into the partners’ coevolution, but too few complete Mycetohabitans genomes exist for reasonable comparative studies. To identify the core and accessory genes for the Mycetohabitans genus, we isolated bacteria from EHB-positive fungal hyphae, extracted and sequenced gDNA, and assembled 15+ new genomes, creating a dataset of 28 complete genomes. We found  unevenly present secondary megaplasmids that have high mobilome content, low sequence similarity, and less than 10% shared core gene clusters, suggesting genetic exchange. We also identified prophages, putative toxin-antitoxin systems, and diverse secondary metabolite clusters in the Mycetohabitans genomes. EHB are increasingly apparent across plant-associated fungi, making lessons learned from this and other symbioses useful for understanding impacts of microbe-microbe coevolution on plant health.