Bacterial endosymbionts enhance fungal virulence by disrupting the disease-suppressive rhizobiome.

The mechanisms by which bacterial endosymbionts (bacteria living within fungal cells) enhance the fitness and virulence of fungal pathogens remain poorly understood. Here, we report that the tomato Fusarium wilt pathogen Fusarium oxysporum f. sp. lycopersici (FOL) hosts Achromobacter spp. endosymbionts that enhance fungal virulence. This virulence potentiation is partially dependent on interactions with the native rhizosphere microbiota. We show that bacterial endosymbiont-harboring FOL reshapes the rhizosphere bacterial community during pathogen infection and decreases the abundance of disease-suppressive bacteria, including Streptomyces spp. taxa. This inhibitory effect is mediated by bacterial endosymbiont-stimulated production of beauvericin (an antibacterial cyclic hexadepsipeptide) by FOL. Together, our findings reveal a tripartite interaction in which a fungal pathogen leverages its bacterial endosymbiont to weaken rhizosphere microbiome-based disease suppression by inhibiting plant-protective bacterial taxa. This work highlights how cross-kingdom symbioses can modulate pathogen ecology and virulence in soil environments.