The Application of Saline–Alkali-Tolerant Growth-Promoting Endophytic Bacteria for Enhancing the Saline–Alkali Tolerance of Alfalfa

Saline–alkali stress is a severe abiotic factor that limits plant growth and development. Endophytic bacteria can improve plant tolerance to such stress through various mechanism, including osmoregulatory substance accumulation and antioxidant enzyme activity. In this study, four saline–alkali-tolerant endophytic strains, designated SYM-2, SYM-4, SYM-9, and SYM-15, were isolated from the roots of alfalfa grown in saline–alkali soil. Though 16S rDNA sequencing, morphological observations, and physiological–biochemical characterization, the strains were identified as closely related to Bacillus cereus, B. thuringiensis, B. halotolerans, and Pantoea agglomerans, respectively. These strains demonstrated the ability to produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase, siderophores, and indole-3-acetic acid (IAA), as well as solubilizing phosphorus. Under saline–alkali conditions, inoculation with these strains significantly increased alfalfa growth parameters. Plant height increased by 4.07–33.90% and root length by 7.49–27.94%, and fresh and dry weight (both above and below ground) increased compared with the control. Strain SYM-15 showed the highest promoting effects, increasing plant height by 33.90%, root length by 27.94% and shoot dry weight by 59.26%. Additionally, root activity increased by 11.23–40%, proline content by 19.09–129–87%, and soluble protein by 7.71–42.49%, and the activities of catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) were significantly elevated across treatments. At the same time, inoculation reduced the levels of hydrogen peroxide (H2O2), superoxide anion (O2−), and malondialdehyde (MDA). Compared with the control and other treatments, including SYM-9, the peroxidase activity and superoxide dismutase activity of alfalfa significantly increased after the SYM-15 treatment, while hydrogen peroxide content, phosphorus content, and neutral detergent fiber and acid detergent fiber contents decreased (p < 0.05). Therefore, SYM-15 plays an important role in promoting growth and represents a promising, high-quality strain resource for the large-scale development of microbials aimed at improving alfalfa tolerance under saline–alkali conditions.