Halotolerant rhizobacteria mitigate the effects of salinity stress on maize growth by secreting exopolysaccharides

Salinity has emerged as a crucial factor negatively affecting crop productivity. Exopolysaccharides (EPS) play a vital role in plant growth promoting rhizobacteria (PGPR)-mediated enhancement of plant stress resistance. In this study, we screened three halotolerant rhizobacteria that produce large amounts of EPS. The strains were characterized in vitro for plant growth-promoting traits, and their capacity to grow maize seedlings inoculated with these strains at 0, 100, and 200 mM NaCl was investigated. The 16S rRNA gene sequence and phylogenetic analysis showed that these strains belong to the genera Pseudarthrobacter, Acinetobacter, and Pseudomonas. These three rhizobacteria showed tolerance to NaCl ranging 6–8 % in LB medium. Interestingly, under salt stress in vitro, salt triggered more EPS accumulation in strain MHR6 than in the positive control Y1. All three strains had the ability to fix nitrogen, produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase, and dissolve organic and inorganic phosphate, while strains MHR1 and MHR6 had the ability to secrete indole-3-acetic acid (IAA). EPS-producing bacteria inoculation improved maize growth in terms of seedling growth performance, antioxidant levels, chlorophyll and osmotic regulator accumulation under salt stress compared to uninoculated controls. Moreover, seedlings inoculated with EPS-producing bacteria exhibited lower levels of malondialdehyde (MDA) and electrolyte leakage (EL) under salt stress. Among the three PGPR strains, Pseudomonas simiae MHR6 was identified as a promising microbe for mitigating the effects of salinity stress on plant growth. These results provide a theoretical basis and new insights into the comprehensive understanding and application of EPS-producing bacteria as microbial inoculants.