Rhizosphere inoculation of PGPR strain Bacillus cereus BC56 enhances salt tolerance of cucumber

Salt stress is a major cause of crop failure worldwide. In this study, a plant growth-promoting rhizobacteria (PGPR) strain Bacillus cereus BC56 was isolated from the rhizosphere soil of cucumber. BC56 has the ability to produce NH₃ and siderophore, and to solubilize phosphorus to promote cucumber growth. Under 100 mM NaCl treatment, BC56 significantly increased shoot length (1.28-fold), root length (1.16-fold), shoot fresh weight (1.19-fold), root fresh weight (1.54-fold), and chlorophyll fluorescence parameter of ABS/CS_m (1.19-fold), TR₀/CS_m (1.22-fold), ET₀/CS_m (1.52-fold), SPAD (1.12-fold) of cucumber seedlings compared to the cucumber without BC56 inoculation. Physiological and biochemical analysis showed that BC56 could increase the content of total soluble sugars (TSS, 1.36-fold) in cucumber seedlings. BC56 also increased peroxidase (POD, 1.17-fold) and glutathione reductase (GR, 2.59-fold) activity of cucumber seedlings, which can scavenge reactive oxygen species (ROS) to reduce salt toxicity. However, cucumber seedlings inoculated with BC56 showed a 0.59-fold decrease in abscisic acid (ABA) compared to those uninoculated with BC56. RNA-seq results showed that BC56 induced changes in the expression of a significant number of genes related to photosynthesis, phytohormones, transcriptional regulation, metabolite synthesis and metabolism, and cellular components in cucumber under salt stress, suggesting its role in reducing the deleterious effects of salinity. We concluded that BC56 can alleviate salt stress in cucumber seedlings by affecting photosynthesis, phytohormone levels, osmotic and antioxidant regulation.