Native bacteria promote plant growth under drought stress condition without impacting the rhizomicrobiome

Inoculation of plants with beneficial plant growth-promoting bacteria (PGPB) emerges a valuable strategy for ecosystem recovery. However, drought conditions might compromise plant-microbe interactions especially in semiarid regions. This study highlights the effect of native PGPB after 1 year inoculation on autochthonous shrubs growth and rhizosphere microbial community composition and activity under drought stress conditions. We inoculated three plant species of semiarid Mediterranean zones, Thymus vulgaris, Santolina chamaecyparissus and Lavandula dentata with a Bacillus thuringiensis strain IAM 12077 and evaluated the impact on plant biomass, plant nutrient contents, arbuscular mycorrhiza fungi (AMF) colonization, soil rhizosphere microbial activity and both the bacterial and fungal communities. Inoculation with strain IAM 12077 improved the ability of all three plants species to uptake nutrients from the soil, promoted L. dentata shoot growth (>65.8%), and doubled the AMF root colonization of S. chamaecyparissus. Inoculation did not change the rhizosphere microbial community. Moreover, changes in rhizosphere microbial activity were mainly plant species-specific and strongly associated with plant nutrients. In conclusion, the strain IAM 12077 induced positive effects on plant growth and nutrient acquisition with no impact on the rhizosphere microbiome, indicating a rhizosphere microbial community resilient to native bacteria inoculation.