Co-inoculation with Bacillus and exopolysaccharide Cp2-exopolysaccharides promotes salt stress tolerance and productivity in alfalfa

Abstract

Background

Alfalfa is one of the most important forage crops in the world, and its performance is significantly affected by salt stress. Although plant growth promoting rhizobacteria (PGPR) can alleviate salt stress, their colonization in the rhizosphere is often compromised by high salinity. Exopolysaccharides (EPS) not only enhance the stress resilience of PGPR but also directly improve plant salt tolerance. Consequently, a thorough investigation of the synergistic effects between EPS and PGPR is of significant theoretical and practical importance for developing advanced microbial fertilizers.

Methods

To investigate the underlying mechanisms, we applied salt stress using NaCl and introduced EPS isolated from Erwinia persicina strain Cp2 (Cp2-EPS). The effects of co-inoculation of Cp2-EPS alone and Bacillus DN2 on the growth and salt tolerance of alfalfa were investigated.

Results

Pot experiments demonstrated that co-inoculation exerted stronger effects than single treatments, with Cp2-EPS showing a more pronounced impact than DN2 alone. The alfalfa seedlings after combined inoculation showed increased photosynthesis and greater accumulation of osmotic substances, such as proline, soluble sugars, and soluble proteins. Increased activity of superoxide dismutase and catalase effectively reduced cell membrane damage, enhanced the ability to scavenge reactive oxygen species, and alleviated oxidative stress symptoms in plant cells. The colonization density of DN2 in the alfalfa rhizosphere from the co-inoculation was significantly higher than that in the single DN2 inoculation.

Conclusions

Alfalfa salt tolerance and productivity were synergistically enhanced by Cp2-EPS through its promotion of Bacillus DN2 colonization and persistence in the rhizosphere.