Salt-alkali-tolerant growth-promoting Streptomyces sp. Jrh8-9 enhances alfalfa growth and resilience under saline-alkali stress through integrated modulation of photosynthesis, antioxidant defense, and hormone signaling.

Streptomyces is a group of plant growth-promoting microorganisms with considerable potential for enhancing plant tolerance to environmental stress. However, the mechanisms by which Streptomyces strains induce systemic tolerance to saline-alkaline stress remain unclear. Here, we evaluated the properties of Streptomyces sp. Jrh8-9, isolated from the halophyte rhizosphere soil, and its effects on alfalfa growth and response to saline-alkali stress. Jrh8-9 exhibited multiple plant-beneficial traits, including phosphate solubilization, nitrogen fixation, indole-3-acetic acid production, and high saline-alkali tolerance. Jrh8-9 inoculation considerably promoted growth in stressed alfalfa by increasing shoot fresh weight, root fresh weight, leaf area, plant height, root length, and root vigor by 46.7 %, 250.8 %, 36.0 %, 31.8 %, 47.4 %, and 103.0 %, respectively. It also improved the chlorophyll content, maximum photochemical efficiency of photosystem II, and the net photosynthetic rate. Physiological and biochemical analyses revealed that Jrh8-9 facilitated ion homeostasis by reducing Na and increasing Mg levels, improving osmotic regulation by increasing soluble sugar and relative water contents, and enhancing antioxidant defenses by increasing superoxide dismutase, catalase, and ascorbate peroxidase activities. Transcriptomic profiling identified key differentially expressed genes associated with auxin and jasmonic acid signaling in response to Jrh8-9 inoculation, with auxin- and jasmonic acid-related genes linked to antioxidant pathways. Further analysis showed that increased auxin and jasmonic acid levels induced by Jrh8-9 mitigated reactive oxygen species accumulation and supported photosynthetic function. These findings highlight the multifaceted mechanisms underlying Streptomyces-induced saline-alkali tolerance and provide a potential strategy for improving forage crop resilience in saline-alkali soils.