Halotolerant Plant Growth-Promoting Rhizobacteria Enhance Growth, Antioxidant Defense and Salinity Tolerance in Vitis vinifera Cultivars

Abstract

Salinity stress is one of the major constraints on plant growth and productivity, especially in arid and semi-arid regions. A promising strategy to improve plant tolerance is the use of Salt-Tolerant Plant Growth-Promoting Rhizobacteria (ST-PGPR), which are naturally adapted to saline environments. In this study, we tested two halophilic bacterial strains, Bacillus subtilis POE26 and Oceanobacillus sp. 76, isolated from halophytic plants, for their ability to enhance salinity tolerance in three grapevine cultivars: Fakhri, Bidaneh Sefid (syn. Thompson Seedless) and Gaznehei. Plant responses were evaluated by measuring growth traits, osmoprotectant accumulation, antioxidant enzyme activity and K⁺/Na⁺ balance. Salinity stress markedly reduced all growth parameters, although inoculation partially mitigated this reduction. Inoculation with Oceanobacillus sp. 76 significantly increased root length and root biomass, while B. subtilis POE26 improved stem fresh and dry weights. Both strains enhanced antioxidant defense, as shown by higher Ascorbate Peroxidase (APX), Superoxide Dismutase (SOD) and Catalase (CAT) activities and promoted proline accumulation under salinity stress (35.12% and 16.58% increase over control for B. subtilis and Oceanobacillus, respectively). They also increased glycine betaine levels under saline conditions and reduced malondialdehyde. The K⁺/Na⁺ ratio was higher in treatments without bacterial inoculation. These results indicate that halophilic bacteria can improve growth and stress physiology in grapevine seedlings exposed to salinity. Further studies are needed to validate these findings under field conditions and to better understand the underlying mechanisms.