Auxin and proline producing rhizobacteria mitigate salt-induced growth inhibition of barley plants by enhancing water and nutrient status

Abstract Although inoculation with plant growth-promoting rhizobacteria (PGPR) can improve crop salt tolerance, it is not always clear how these beneficial microbes affect plant ionic and water status. To investigate this phenomenon, three salt-tolerant bacteria were used to inoculate barley plants cultivated under salt stress. The bacterial strains Bacillus mojavensis S1, B. pumilus S2 and Pseudomonas fluorescens S3 showed an increase in IAA and proline production when cultivated on 100 and 200 mM NaCl supplemented medium. A salt-sensitive barley cultivar Rihane was grown at 0 and 100 mM of NaCl, with or without bacterial inoculation. Salt treatment decreased shoot dry weight of uninoculated plants by about 25%. In uninoculated plants, salinity decreased leaf water potential 0.8 MPa, increased roots and shoots Na+ concentrations by approximately 59% and 89% respectively and increased proline concentration. Bacterial inoculation with strains S1 and S3 prevented salt-induced reduction of shoot dry weights respectively by 54% and 46%, and root dry weights by almost 40%. These two bacterial strains enhanced stomatal conductance and CO2 assimilation of plants subjected or not to salt. Under saline conditions, all bacterial strains decreased root and shoot Na+ concentrations, improved leaf water potential and the strain S1 maintained it close to control values. Salinity stress was mainly alleviated in barley plants after inoculation with S1 and S3 strains, having respectively high proline and IAA productions under 200 mM of NaCl, which contribute to the alleviation of salt stress and stimulate root development, enhancing water and nutrient uptake by plants. Future prospects of the PGP potential of halotolerant bacterial species will extend their application in agriculture.