Saline soil microbiome: A rich source of halotolerant PGPR

Abstract

Twenty halotolerant free-living, nitrogen-fixing bacterial strains were isolated from the Lonar Soda Lake and screened for growth at different NaCl concentrations. Out of 20, only Four isolates DL2, DL3, and DL6 showed optimum growth at 4% NaCl, and DL11 showed optimum growth at 6% NaCl concentration and tolerated 10% NaCl concentration. Biochemical and 16S rRNA sequencing identified DL2 as P. mendocina, DL3 as A. beijerinckii, DL6 as P. stutzeri, and DL11 as B. subtilis. Further, these four isolates were characterized for plant growth-promoting traits and all are indole acetic acid and ammonia producers and they also solubilize the phosphate under 4% NaCl concentration. The salt tolerance of these isolates was also increased from 10 to 14% by using a physical mutagen like UV and mutant strains were also screened for PGP traits. To determine the efficacy of these isolates as plant growth-promoting bacteria, Bioinoculum I (mixed culture of wild strains) and Bioinoculum II (mixed culture of mutant strains) were prepared and tested on Catharanthus roseus at 4% NaCl concentration. Plants treated with Bioinoculum II showed 12.61 and 82.35% increase in shoot and root length, respectively, and 71.42% increase in number of flowers as compared to control plants under saline conditions. These results indicate that halotolerant bacteria isolated from saline environments have the potential to enhance plant growth under saline stress through direct or indirect mechanisms and would be most appropriate as bioinoculants under such conditions.