Response to salt stress is modulated by growth-promoting rhizobacteria inoculation in two contrasting barley cultivars

There is an increasing interest for plant growth-promoting rhizobacteria (PGPR), particularly those associated with plants originating from extreme environments like saline habitats. We are assessing, here, whether the inoculation with three PGPR bacteria strains isolated from the rhizosphere of Hordeum maritimum naturally growing in saline soil could mitigate the impact of high salinity (200 mM of NaCl) on two contrasting local barley cultivars. The affinity of interaction between plant and bacteria in response to this environmental constraint was also evaluated. At 200 mM of NaCl, the strains S1 of Bacillus mojavensis and S2 of B. pumilus maintained the highest level of indole acetic acid production and the strain S3 of Pseudomonas fluorescens the highest number of viable cells. In the salt-sensitive cultivar Rihane, salinity reduced significantly plant biomass, chlorophyll and shoots water contents and enhanced malondialdehyde leaf content. Salt impact was also related to higher Na+ uptake. However, these parameters were slightly altered under salinity in the tolerant cultivar Kerkna which is likely due to its ability to transport Na+ to shoots for osmotic adjustment. The effect of bacteria inoculation on barley growth and tolerance to salt stress was dependent on the bacteria and cultivar genotypes and their interactions with the salinity of the soil. At 0 mM of NaCl the strain S2 increased significantly the plant fresh biomass of both cultivars. At 200 mM of NaCl, a positive effect on Rihane plant biomass was observed after S1 strain inoculation, while the Kerkna plant biomass did not change significantly after bacteria inoculation. Overall, the sensitive cultivar Rihane responds better to bacteria inoculation in comparison to the tolerant cultivar under control and salt conditions, which demonstrate a certain affinity of interaction between plant cultivar and bacterium strain modulated by the salinity of the soil. The multitude of soil-plant-microbe interactions, and in particular this affinity-effect observed between plants and rhizobacteria modulated by soil conditions, constitute a challenge for developing bio-promoting inoculum at the commercial level. This constraint can possibly be managed by developing an inoculum containing a consortium of PGPR bacterial strains having broad spectrum interactions with different plant cultivars that function optimally under several environmental constraints.