Legume Genomics and Genetics 2025, Vol.16, No.3, 108-127 http://cropscipublisher.com/index.php/lgg 116 plants reduce salt damage. Their mechanisms of action include: secreting mucopolysaccharides to fix salt outside the cell, reducing the concentration of free Na+ in the soil solution, thereby reducing the amount of salt ions absorbed by the plant; producing growth hormone (IAA) to promote plant root development to avoid surface salt; synthesizing ACC deaminase to reduce plant ethylene levels and alleviate premature aging caused by salt stress; and inducing the plant's antioxidant system and osmotic regulation. Sridhar et al. (2025) isolated a salt-tolerant Bacillus flexus, which was inoculated into sesame to significantly improve the survival and growth of plants under 100 mM~200 mM NaCl salt stress. The chlorophyll, soluble sugar and proline contents of sesame leaves treated with the inoculated bacteria increased significantly, while membrane damage indicators such as malondialdehyde decreased, and the activities of antioxidant enzymes such as superoxide dismutase and catalase were significantly enhanced, indicating that oxidative damage in the plant body was reduced and cell membrane stability was improved. Finally, the treated sesame plants maintained a good growth state and yield under high salt conditions. Similarly, many studies have confirmed that inoculation with suitable PGPR strains can improve the salt tolerance of legume crops. For example, after inoculation of peas and alfalfa with salt-tolerant Pseudomonas and Bacillus brevis, the electrolyte leakage and peroxide levels of the plants decreased, indicating that salt stress damage was alleviated. The screening and application of salt-tolerant rhizobium strains have also made progress. It has been reported that a Chinese rhizobium strain that can effectively nodulate and fix nitrogen in a relatively high-salt environment has been screened out. Inoculating it into legumes can improve nodulation and growth on saline-alkali soils to a certain extent. These results are of positive significance for improving saline soils and developing salt-tolerant legume crops using symbiotic systems. 4.3 Activation of plant immunity and biocontrol potential In addition to direct nutrient supply, the interaction between beneficial microorganisms and legumes can also indirectly improve the resistance of legumes to diseases and pests by inducing the plant's own immune defense mechanism, which is the so-called "induced systemic resistance" (ISR) effect. Many rhizosphere growth-promoting bacteria and endophytic nitrogen-fixing bacteria have been found to have the effect of activating plant immunity. They can induce plants to produce broad-spectrum disease resistance without causing diseases and are considered to be potential biological control forces. In legumes, some rhizobia can induce plant resistance by themselves. For example, during the invasion of symbiotic rhizobia, although plants reduce some immune responses to facilitate symbiosis, they also initiate low-level activation of certain defense pathways. This "mild" immunity is believed to increase the vigilance of plants against other pathogens. Studies have shown that after inoculation with rhizobia, the salicylic acid and jasmonic acid signaling pathways in legumes will change, which will accelerate the response of plants to subsequent pathogen infections. For example, Diaz-Valle et al. (2019) reported that inoculating beans with Rhizobium etli, which is effective in nitrogen fixation, not only increased their growth, but also induced the expression of defense-related genes in plants, showing stronger resistance to insect pests. This shows that symbiotic nitrogen-fixing bacteria can also act as "immune inducers." More extensive research is on PGPR. After many fluorescent Pseudomonas, Bacillus subtilis, etc. colonize the rhizosphere of legumes, they will secrete some volatile organic compounds or secondary metabolites to stimulate plants to initiate ISR. The results show that the activity of resistance-related enzymes (such as phenylalanine ammonia lyase and peroxidase) in plant leaves increases, and defense substances such as lignin accumulate, which can limit the expansion of pathogens more quickly when encountering them. Experiments on crops such as peas and alfalfa have shown that the incidence of powdery mildew and damping-off disease decreases after the application of growth-promoting agents, which is exactly the ISR at work. Microbial-induced resistance is usually broad-spectrum and can have a certain inhibitory effect on multiple diseases and pests at the same time, unlike disease-resistant varieties that are specific. Therefore, using beneficial microorganisms as biopesticides or biocontrol agents is one of the green measures to reduce dependence on chemical pesticides.
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