Endophytic Streptomyces sp. H49 enhances salt tolerance in soybean by regulating physiological responses,...

Endophytic actinobacteria have gained growing attention for their ability to enhance plant salt tolerance. As the dominant genus of actinobacteria, Streptomyces represents a group of important plant growth-promoting (PGP) microorganisms with great potential for improving plant stress resistance. However, studies on endophytic Streptomyces enhancing salt tolerance in soybean remain limited. Here, we elucidated the effects and underlying mechanisms of endophytic Streptomyces sp. H49 on salt tolerance in soybean (Glycine max L.) using physiological, biochemical, genomic, transcriptomic, and 16S rRNA gene sequencing approaches. Streptomyces sp. H49 produces indole-3-acetic acid (IAA), secretes siderophores, exhibits nitrogen-fixing activity, and solubilizes inorganic phosphate, with all related functional genes identified in its genome. Under salt stress, H49 inoculation significantly promoted soybean growth, enhanced photosynthetic capacity, and maintained cellular ion homeostasis by reducing Na⁺ accumulation and increasing K⁺ content in plant tissues. H49 also enhanced osmotic adjustment capacity and antioxidant defense systems, thereby mitigating reactive oxygen species (ROS) accumulation. Transcriptomic analysis revealed that H49 modulated multiple metabolic and signaling pathways, including photosynthesis, ion homeostasis, osmotic regulation, antioxidant defense, and hormone signaling. Additionally, 16S rRNA gene sequencing revealed that H49 inoculation altered the composition and increased the diversity of the rhizosphere bacterial community, with notable changes in the relative abundance of several phyla (e.g., Actinobacteriota, Bacteroidota). This study provides theoretical insights and valuable microbial resources for developing PGP microbial inoculants to improve soybean productivity in saline-alkali soils.