Transcriptomics and Metabolomics Analyses Reveal How Rhizobacteria Acinetobacter calcoaceticus Enhance the Growth and Stress Tolerance in Lespedeza davurica

Background: Lespedeza davurica is an important perennial leguminous shrub endemic to China’s Loess Plateau, and it plays a crucial role in ecosystem restoration and soil erosion control. However, phosphorus deficiency and environmental stresses limit its growth potential and ecological function. Methods: In the present study, the interaction between Acinetobacter calcoaceticus DP25, a phosphate-solubilizing rhizobacterium isolated from L. davurica rhizosphere, and L. davurica was investigated. We performed biochemical analyses of leaves from L. davurica planted in saline–alkali soil to monitor antioxidant defense systems and stress-related metabolites, and conducted a combination of transcriptomics and metabolomics approaches to elucidate the bacteria-mediated enhancement of growth and stress tolerance in L. davurica. Results: DP25 inoculation substantially enhanced L. davurica growth performance, increasing plant height by 47.68%, biomass production by 102.54–132.42%, and root architecture parameters by 62.68–78.79% (p < 0.0001). Catalase activity, a key antioxidant enzyme, showed a marked increase of 41.53% (p < 0.001), while malondialdehyde and free proline contents decreased by 18.13% and 19.33%, respectively (p < 0.05). Transcriptomic analysis revealed 263 differentially expressed genes, with enrichment in carotenoid biosynthesis, ABC transporters, and pentose and glucuronate interconversion pathways. Metabolomic profiling identified 246 differentially accumulated metabolites, highlighting enhanced secondary metabolite production and stress response mechanisms. Integration of multi-omics data revealed 19 co-regulated pathways involved in growth promotion and stress tolerance. Conclusions: A. calcoaceticus DP25 enhances L. davurica growth through coordinated regulation of metabolic pathways involved in photosynthesis, antioxidant defense, and secondary metabolite biosynthesis. These findings provide molecular insights into beneficial plant–microbe interactions and support the development of sustainable strategies for ecosystem restoration in degraded environments.