The poly-γ-glutamic acid-producing bacterium Bacillus amyloliquefaciens W25 enhanced the salt tolerance of lettuce by regulating physio-biochemical processes and influencing the rhizosphere soil microbial community

Soil salinization is considered a serious agricultural problem that affects the growth and yield of plants in the world. Poly-γ-glutamic acid (γ-PGA) can adsorb cations and increase plant salt tolerance. However, the role of γ-PGA-producing bacteria in regulating plant salt stress response and the involved mechanisms remain unknown. This study characterized the effects of plant growth-promoting and γ-PGA-producing strain Bacillus amyloliquefaciens W25 on the growth and salt stress response in lettuce, and the microbial composition and structure in rhizosphere soil. This strain tolerated up to 8% NaCl and showed K solubilization and γ-PGA production in salt stress. Strain W25 significantly decreased the concentration of water-soluble Na+ in solution and the Na+ high storing capability of γ-PGA produced by strain W25 was 27.48 mg g-1. Inoculation with strain W25 could improve lettuce biomass production by decreasing the plant Na/K and Na/Ca ratios, improving the plant antioxidant levels, and regulating the plant IAA level. Strain W25 inoculation enhanced soil quality by reducing soil exchangeable sodium percentage, and increasing available K and P and soil enzyme activities. Furthermore, inoculation with strain W25 not only increased the diversities but also changed the compositions of bacterial and fungal communities in the lettuce rhizosphere soils. The abundances of beneficial soil bacteria, such as Pseudomonas, Massilia, and Bacillus, were enhanced, which showed significantly positive correlations with the biomass of lettuce. While the abundances of plant pathogenic fungal genera like Fusarium and Cephaliophora were decreased in strain W25 inoculated rhizosphere soils. This study showed the great potential of γ-PGA-producing PGPB for the enhancement of plant growth in salt-affected soil.