Pseudomonas fluorescens G3 Enhances the Salt Stress Tolerance of Maize and Modulates Soil Microbial Community Composition in the Rhizosphere

Soil salinization impacts over one billion hectares, threatening global food security. Here, a salt-tolerant bacterial strain, Pseudomonas fluorescens G3, was isolated from the rhizosphere of maize (Jinongyu-719) growing in saline–alkali soils in Gansu Province, China. This strain demonstrated the ability to secrete indole-3-acetic acid (IAA), 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, and extracellular polysaccharides. It also exhibited notable phosphate-solubilizing activity and robust siderophore production capabilities. Under salt stress conditions (200 mM NaCl), the P. fluorescens G3 strain significantly improved maize’s growth parameters, namely its plant height, root length, and dry weight. Further, it enhanced antioxidant enzyme activity while reducing the accumulation of malondialdehyde (MDA), mitigating stress-induced oxidative damage. In P. fluorescens G3-inoculated plants, leaf and root Na+ contents decreased by 34.90% and 33.91%, while their K+ contents increased by 40.20% and 33.47%, respectively. Inoculation with P. fluorescens G3 enhanced taxonomic richness (ACE, Chao1) and evenness (Shannon, Simpson) in the rhizosphere bacterial community, leading to a significantly greater relative abundance of several bacterial genera: Pseudomonas, Methylophaga, Enhygromyxa, Desulfuromonas, and Devosia. These shifts in the microbial community composition suggest a potential restructuring of functional profiles, possibly enhancing processes beneficial to plant salt tolerance, such as ion homeostasis and stress mitigation: the biosynthesis of cofactors and secondary metabolites; bacterial secretion and two-component systems; porphyrin metabolism; flagellar assembly; biofilm formation; and bacterial chemotaxis. Redundancy analysis revealed positive correlations between microbial composition at both the phylum and genus levels and the activity of stress resistance enzymes after treatment with Pseudomonas fluorescens. This study provides important theoretical foundations and microbial resources for utilizing microbial community regulation in saline–alkali soil bioremediation.