Enhanced salt stress tolerance in tomato plants following inoculation with newly isolated plant growth-promoting rhizobacteria

Salinity is a major abiotic stress that affects the growth and productivity of many crops, especially tomato plants. Plant inoculation with plant growth-promoting rhizobacteria (PGPR) has gained agronomic importance in improving salinity stress tolerance. In the present study, twenty-four bacterial strains were isolated from rhizospheric samples in arid and semi-arid regions of Tunisia, and subsequently, their potential in enhancing plant salinity stress tolerance was studied. The predominant identified taxonomic classes were Bacilli and Proteobacteria. Most of the isolates were able to fix nitrogen, solubilize phosphate, and produce IAA (indoleacetic acid). They also exhibited antagonistic activity against Fusarium pathogens. The strains were screened to assess their salinity stress alleviation potential on tomato seeds and Arabidopsis thaliana under 150 mM salt stress. Strain Gb67 (Bacillus halotolerans) demonstrated significant enhancement of germination parameters in tomato seeds, while strains All3 (Bacillus subtilis) and Gb7 (Bacillus mojavensis) showed a positive influence on Arabidopsis growth parameters. In addition, these isolates enhanced tomato plant tolerance to salinity. Gb67 induced the greatest plant growth promotion in terms of fresh weight (41.85%), plant height (80.59%), root length (66.66%), total chlorophyll content (24.51%), and carotenoid content (64.96%) over the control untreated, followed by All3 or Gb7 inoculation. The selected isolates exhibited salt tolerance traits producing polyamines, volatile compounds, and ACC deaminase. Furthermore, quantitative estimation of exopolysaccharides indicated that Gb67 produced significantly more exopolysaccharides than All3 and Gb7. The findings suggest that the selected Bacillus strains could be employed as PGPR to enhance tomato crop growth in saline agricultural systems.