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One critical environmental stress that limits plant production and development is salinity stress. Recently it has been shown that application of plant growth-promoting rhizobacteria (PGPR) can alleviate the deleterious effects of environmental stresses. Present study aimed to evaluate the effects of some bacterial strains on proline, sugar, total phenolic compounds (TPC), Phenylalanine ammonia lyase (PAL), photosynthetic pigments and antioxidant activities (guaiacol peroxidase, polyphenol oxidase and superoxide dismutase) of cucumber plants under salinity stress. A completely randomized design was applied with a factorial arrangement of two factors: salinity at three levels (0, 50 and 100 mM) and Pseudomonas fluorescens and Bacillus subtilis strains, with three replications. The results showed that cucumber plants that were inoculated with Pseudomonas and Bacillus strains possessed noticeable variations in proline, sugar, TPC, PAL and enzymes activity compared to un-inoculated control. These results suggest that use of these bacterial strains overcame harmful effect of salinity by accumulation of proline, TPC, sugar, PAL activity and enzymes activity that can be considered as a suitable method to manage salinity stress.

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Biofilm formation and rhizosphere colonization of the plants are the main infrastructures for the biological control of the plant diseases. Bacteria accumulation in the protective layer, which results from their self-production of Exopolysaccharides (EPS), is called the biofilm. The formation of these complex structures originates from the multicellular behaviors of bacteria. Various elements can play a role in these mechanisms. In this study, we examined biofilm formation, root colonization, and salt tolerance to four concentrations of NaCl in the strains of Bacillus velezensis (Q12, US1, and UR1). The results showed that the biofilm strength plays an important role in the efficiency of tomato root colonization. Furthermore, UR1 that had defects in producing the surfactin, iturin, and fengycin using Ultrahigh-Performance Liquid Chromatography-High Resolution Electrospray Ionization Mass Spectrometry (UHPLC-HRESIMS), was incapable of tolerance to salinity, biofilm formation, competition, and rhizosphere colonization. Confocal Laser Scanning Microscopy (CLSM) studies showed that strains US1 and Q12 differed in the biofilm strength, the position of the bacteria that are located laterally, polar, or both, and root colonization. Q12 was introduced as the best strain in all these experiments. Also, based on the findings of this and previous studies, the possibility to create the subpopulations influenced by genetic diversity in Bacillus velezensis strains during biofilm formation is suggested.