Plant growth promoting microbiome associated to halophytes in Tunisian hypersaline soils

Plant Growth Promoting (PGP) bacteria are known for their role in enhancing plant fitness both in conventional and extreme environments. PGP bacteria can directly and indirectly boost plant growth by providing nutrients and metabolites and by reducing the incidence of pathogenic attacks. Plant growth and production enhancement in salt and drought affected lands can be pursued by the selection of PGP bacteria adapted to the extreme geochemical conditions of saline soils. In this perspective we investigated by cultivation dependent and independent methods the microbiome composition associated to Salicornia rhizosphere and bulk soils collected from Sebkhet and Chott hypersaline ecosystems in Tunisia. Denaturing Gradient Gel Electrophoresis showed the occurrence of a rich and highly diverse microbiome associated to Salicornia rhizospheric hypersaline soils. A large halophilic bacterial collection was established from bulk soils and Salicornia rhizospheres. The PGP traits of the isolates were assessed in vitro in presence of 5% of sodium chloride, to simulate those environmental condition that likely hamper the efficacy of PGP inocula isolated from conventional ecosystems. Tolerance to abiotic stresses, such as extreme temperature, salt concentration and low water availability, was widespread within the bacterial collection. A high number of isolates showed the ability to influence the plant nutrients and hormonal balance and expressed biocontrol activity in vitro. This study allowed the identification of several Halomonas strains resistant to abiotic stresses and performing different PGP activities, including ammonia and indol-3-acetic acid production, phosphate solubilization and potential nitrogen fixation. The results confirmed that halophytes are valuable source of PGP bacteria.