Metabolomic and phytohormonal responses of Pseudomonas fluorescens and Bacillus subtilis to PEG-induced osmotic stress

Abstract Plant growth-promoting rhizobacteria are widely associated with plant drought tolerance, but their intrinsic metabolic responses to reduced-water-availability conditions remain incompletely characterized. This study compared the phytohormonal and culture-associated metabolite profiles of Pseudomonas fluorescens PKR1 and Bacillus subtilis PKR5 in axenic cultures exposed to 30, and 40 w/v % PEG 6000 treatments. Under control conditions, the two strains differed in their production of indole-3-acetic acid, indole-3-butyric acid, and gibberellic acid. Increasing PEG concentration led to strain-specific metabolites and hormone-dependent changes in the phytohormone profile. Multivariate analysis showed significant effects of both strain background (PERMANOVA: F = 3.3409, R² = 0.1727, p = 0.0426) and PEG stress level (F = 8.8334, R² = 0.3557, p = 0.0001) on the qualitative metabolite profiles. PKR1 retained a comparatively similar number of detected features across PEG concentrations, whereas PKR5 showed a broader feature set under 40 w/v % PEG. Several features tentatively assigned as proline derivatives, fatty acids, lipid amides, and phenolic compounds showed strain or treatment-associated distributions. These findings reveal distinct culture-associated metabolite signatures under defined PEG-induced osmotic stress and identify candidate features for targeted analytical validation and subsequent functional investigation.