Salinity is a serious limiting factor for the growth of rhizobia. Some rhizobia are tolerant to salt stress and promote plant growth, but the mechanisms underlying these effects are poorly characterized. The growth responses and osmoprotectants in four Bradyrhizobium strains were examined under salt stress in this study. Two-dimensional electrophoresis (2-DE) and mass spectrometry were conducted to investigate protein profiles in rhizobia exposed to salt stress. Subsequently, salt tolerance in stylo (Stylosanthes guianensis) inoculated with rhizobia was further detected in hydroponics. Results showed that the Bradyrhizobium strain RJS9-2 exhibited higher salt tolerance than the other three Bradyrhizobium strains. RJS9-2 was able to grow at 0.35 M NaCl treatment, while the other three Bradyrhizobium strains did not grow at 0.1 M NaCl treatment. Salt stress induced IAA production, and accumulation of proline, betaine, ectoine, and trehalose was observed in RJS9-2 but not in PN13-1. Proteomics analysis identified 14 proteins regulated by salt stress in RJS9-2 that were mainly related to the ABC transporter, stress response, and protein metabolism. Furthermore, under saline conditions, the nodule number, plant dry weight, and N concentration in stylo plants inoculated with RJS9-2 were higher than those in plants inoculated with PN13-1. These results suggest that the tolerance of RJS9-2 to salt stress may be achieved by the coordination of indole-3-acetic acid (IAA) production, osmoprotectant accumulation, and protein expression, thus promoting stylo growth.

Rhizobia inoculation is widely used in legumes to improve the yield and quality of forage. However, the symbiotic interactions of different legumes are specific. The search for efficient strains plays an important role in legume forage and in practical production. In this study, Desmodium intortum was first inoculated with rhizobia from different regions, and then the production traits, nutritional quality, and fermentation of D. intortum in the rumen were evaluated according to the rhizobia strains. The results showed that, compared with the control, inoculation with different rhizobia strains significantly increased the number of nodules, shoot fresh weight, crude protein content, plant protein yield, and ether extract content of D. intortum, and inoculation with the R3 strain (Bradyrhizobium elkanii) increased these values by 61.0%, 29.6%, 16.1%, 62.9%, and 30.4%, respectively. In the basal diet of Pennisetum purpureum Rich. × P. americana King grass cv. Reyan No.4, D. intortum inoculated with different rhizobia was added (75:25), and the combination of the inoculated R3 and R5 strains increased the rumen’s ammonia-nitrogen (NH₃-N) levels and in vitro dry matter digestibility (IVDMD): R3 increased the contents of NH₃-N and IVDMD by 50.1% and 12.3%, respectively, while R5 increased them by 50.9% and 15.4%, respectively. Based on a comprehensive evaluation through principal component analysis, R3 was ranked first overall. These results support the development of competitive inoculants from indigenous strains as a strategy to improve the nutritional value of D. intortum and ruminal fermentation.