Evaluation of the salt-stress tolerance potential of indigenous Stenotrophomonas rhizophila and Bacillus marisflavi isolates

Bacillus marisflavi and Stenotrophomonas rhizophila are considered important microorganisms in agriculture, largely due to their biocontrol potential and their ability to mitigate biotic and abiotic stresses in crop plants. Despite this, a limited number of studies have been conducted on rhizospheric interactions and stress tolerance between these microbes and crop plants. This study, therefore, aimed to evaluate the potential of the isolates S. rhizophila EU.2B33 and B. marisflavi EU.18 to improve salt stress tolerance in watermelon plants. Bacterial isolates were exposed to different concentrations of NaCl (5 mM, 10 mM, 20 mM, 40 mM, 60 mM, 100 mM, 500 mM, 1 M, and 2 M) in a salt-free nutrient broth medium. Isolates were incubated at 37℃ for 48 h with continuous shaking of 150 rpm, and optical density taken thrice per every 24 h period using a microplate reader. After 4 and 6 hours of incubation, growth peaks were observed in both isolates, although S. rhizophila EU.2B33 exhibited a more efficient growth than B. marisflavi EU.18. Interestingly, a significant increase in growth was observed at salinity concentrations above 100 mM, although this level is regarded as the stress threshold. Protein analysis revealed an increase in band intensity with increasing dose and electrophoretically heterogeneous structure of the strains. According to SDS-PAGE results, similar bands were observed in both strains at approximately 35 and 25 kDa. At 60 mM salinity, B. marisflavi (214.8 ± 24.7 mg) and S. rhizophila (204.8 ± 25.0 mg) showed higher hypocotyl fresh weight values than the control (180.4 ±22.9 mg). At 200 mM, the control (54.9 ± 4.9 mg) treatment decreased significantly, while S. rhizophila (91.7 ± 8.5 mg) remained higher (P