Soil parameters drive the diversity of Citrus sinensis rhizosphere microbiota which exhibits a potential in plant drought stress alleviation

Plant associated microorganisms, particularly those exhibiting a plant growth promoting (PGP) effect, play an important role in plant nutrition and health and the adaptation to unfavorable climatic conditions, such as drought which threatens the productivity of agricultural crops. The selection of specific microbial populations in the soil habitats associated to plants depends upon the soil physico-chemical parameters besides the 'rhizosphere effect' played by each plant species through rhizodepositions. In this study, we investigated the community structure and PGP potential of the microbiota associated to Citrus sinensis plants located in different geographical regions of Tunisia. The bacteria community structure was correlated to soil physiochemical parameters and we identified potassium, carbon and organic matter content as drivers of the C. sinensis microbiota composition. The evaluation of the potential of selected bacteria as biofertilizer and bio-stimulator under drought stress was achieved through the phylogenetic and functional characterization of a large collection of bacterial strains isolated from the rhizosphere of C. sinensis. The strains were screened in vitro for putative plant growth promoting traits, and the six most promising isolates were tested in vivo on Solanum lycopersicum and Capsicum annuum model plants. The bacterized plants were cultivated under drought stress and compared with not bacterized and fully irrigated control plants. All the tested bacteria induced a significant increase in the number of leaves and in root biomass of both plant species compared to not inoculated plants. Our results highlighted that the strains Ensifer adhaerens S1B1.5 and Pseudomonas resinovorans S4R2.6 were, in particular, effective in promoting plant growth under water shortage, indicating them as promising strains for the development of sustainable biofertilizers suited for agriculture in arid and semi-arid regions characterized by water scarcity.