Halotolerant Bacillus spizizenii FMH45 promoting growth, physiological, and antioxidant parameters of tomato plants exposed to salt stress.

KEY MESSAGE: Bacillus spizizenii is for the first time described as a plant growth salt-tolerant bacterium able to alleviate salt stress in crop plants by improving physiological parameters and antioxidant defense mechanisms. Agricultural soil salinization is a serious issue worldwide affecting agricultural yield. Plant growth promoting bacteria can enhance salt tolerance and plant yield. Bacillus spizizenii FMH45 has been shown to inhibit fungal attacks in tomato fruits and to augment tomato seed germination in presence of abiotic stresses. During this study, we reported for the first time B. spizizenii as a salt-tolerant bacterium able to alleviate salt stress in tomato plants. B. spizizenii FMH45 was examined in vitro for its potential to produce several plant growth promoting characters (siderophores, IAA, and phosphate solubilization) and hydrolytic enzymes (cellulase, glucanase and protease) in the presence of saline conditions. FMH45 was also investigated in vivo in pot experiments to evaluate its ability to promote tomato plant growth under salt stress condition. FMH45 inoculation, enhanced tomato seedling length, vigor index, and plant fresh and dry weights when compared to the non-inoculated controls exposed and not exposed to a regular irrigation with salt solutions containing: 0; 3.5; 7; and 10 g L-1 of NaCl. FMH45-treated plants also presented improved chlorophyll content, membrane integrity (MI), and phenol peroxidase (POX) concentrations, as well as reduced malondialdehyde (MDA) and hydrogen peroxide (H2O2) levels under saline conditions with a significant salinity × strain interaction. Furthermore, FMH45 inoculation significantly decreased endogenous Na+ accumulation, increased K+ and Ca2+ uptake, and thereby improved K+/Na+ and Ca2+/Na+ ratios. This study proves that bio-inoculation of FMH45 efficiently increases salt tolerance in tomato plants. This sustainable approach can be applied to other stressed plant species in affected soils.