Enhancement of salinity stress tolerance and plant growth in finger millet using fluorescent pseudomonads

Soil salinity, the most critical environmental stress hampering crop productivity in drier agro-ecosystems, is mitigated through various management strategies. A preference for a non-chemical, eco-friendly, and economical approach has led to the investigation of fluorescent pseudomonads, a common group of plant growth-promoting rhizobacteria (PGPR), known to alleviate stress. In the present study, salinity-sensitive finger millet (Indaf-9) seeds treated with 40 fluorescent pseudomonad isolates were obtained from different saline regions of India. From these, 20 were selected based on growth promotion studies. These isolates were again subjected to growth promotion studies but under varied saline concentrations (0-350 mM NaCl). The results obtained were validated with studies on growth kinetics under a gradient of NaCl concentrations. Three strains, SPF-5, SPF-33, SPF-37, were selected based on their performance and characterized. They were screened for their plant growthpromoting traits (indole acetic acid, biofilm, siderophore, and ACC deaminase), stress-related physiological traits (enzymatic antioxidants, free proline, H2O2, lipid peroxidation, total protein, phenolics, flavonoids, total chlorophyll, leaf relative water content, and dry weight) and growth parameters under green-house conditions. Under increased salinity stress, strain SPF-33 showed increased activity of enzymatic antioxidants and elevation in proline content along with decreased lipid peroxidation and H2O2, with increased plant height and number of spikelets. Similarly, treatment with SPF-37 exhibited a significant increase in germination, vigor index, increased plant height, and the number of spikelets, total chlorophyll, phenolics, flavonoids, proteins, and leaf relative water content under 350 mM NaCl as compared to the control. Overall, our results indicate the fluorescent Pseudomonad strains SPF-33 and SPF-37 are strong candidates for alleviating salt stress in the field.