Wild Luteibacter populations exhibit climate-associated divergence and distinct genomic strategies for osmotic stress tolerance

Soil bacteria play a crucial role in supporting ecosystem health, yet little is known about how wild populations adapt to abiotic stressors, like limited water availability. We used 186 Luteibacter isolates from maize roots grown in never-irrigated prairie soils collected from a steep precipitation gradient in Kansas, US to test whether climate history is associated with osmotic stress tolerance and genomic differentiation. In vitro, strains from semi-arid sites were more tolerant to osmotic-stress treatment than those from humid sites. Comparative genomics of a subset of 96 isolates identified four strongly biogeographically structured lineages and extensive accessory-genome diversity. Semi-arid climate-associated strains were enriched for genes involved in osmotic homeostasis, oxidative stress protection, motility, and type IV secretion. Humid climate-associated strains were enriched in environmental sensing, metabolic versatility, biofilm formation, iron acquisition, type VI secretion, and resource competition. GWAS identified 22 candidate genes associated with osmotic-stress tolerance, including those related to type IV pili, biofilm production, DNA repair, and phage. No candidates were shared between semi-arid and humid strains, suggesting distinct genetic mechanisms of osmotic tolerance. Predicted prophage composition was also geographically structured, although total prophage gene content did not predict osmotic-stress tolerance. Together, these results show that adaptation to long-term differences in water availability likely involves coordinated shifts in stress physiology, genome content, and ecological interactions in wild bacterial populations, providing insight into predicting microbial responses to changing climates.