Hypoxia induces phenotypic and metabolic shifts in endophytic Flavobacterium sp. 98

Oxygen plays a crucial role in shaping microbial physiology, functions, and behavior. Endophytic bacteria, residing within plant tissues, inhabit microenvironments where oxygen availability can be limited. However, the magnitude of hypoxic conditions in the endosphere and how these affect functional microbial traits are largely unknown. Here, we showed with a microsensor that oxygen levels in roots of sugar beet seedlings drop drastically to variable, low oxygen levels when going from epidermal to endodermal root tissue into the vasculature. Subsequently, we investigated phenotypic and metabolic responses of endophytic Flavobacterium sp. 98 at oxygen levels of 100 ppm. Under these low oxygen conditions, Flavobacterium sp. 98 showed reduced growth, enhanced motility, and an altered extracellular metabolite profile. Flavobacterium sp. 98 colonies spread out in response to oxygen limitation and more effectively restricted hyphal growth of the sugar beet root pathogen Rhizoctonia solani than Flavobacterium sp. 98 grown at ambient oxygen conditions. Exometabolome analysis revealed enhanced accumulation of lysophosphatidylethanolamine (lysoPE) and N-acetyl-phenylalanine under low-oxygen conditions, along with a reduced level of the antifungal compound 5,6-dimethylbenzimidazole. These responses reflect physiological and metabolic plasticity of Flavobacterium sp. 98, highlighting significant changes in the expression of specific traits under hypoxic conditions. Our findings provide insights into niche-adaptive strategies of endophytic bacteria and pinpoint functional traits in microbe-plant interactions operating inside plant tissue.