Halophyte associated plant growth promoting bacteria : key tool for agriculture and phytoremediation

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. Soil salinization and drought are among the most severe environmental stressors affecting crop production around the world. High salt content in the soil is a typical feature of arid and semiarid regions, hosting different halophyte species including those belonging to the genus Salicornia. Halophytes comprise several plant species of emerging interest as source of valuable products and services, including phytoremediation technology. One of the most promising and sustainable approach to improve plant growth and production in salt and drought affected lands is represented by the selection of PGP bacteria able to cope with the extreme geochemical conditions of saline soils. In this perspective we investigated by cultivation dependent and molecular fingerprinting the microbiome composition associated to Salicornia rhizosphere and bulk soils collected from Sebkhet and Chott hypersaline ecosystems in Central Tunisia. Denaturing Gradient Gel Electrophoresis (DGGE) and Automated Ribosomal Intergenic Spacer Analysis (ARISA) were applied to depict the overall bacterial community structure of the hypersaline soils, displaying the occurrence of a rich and highly diverse microbiome associated to the Salicornia plants roots. A large collection of bacteria was established from bulk soils and Salicornia rhizospheres. Plate counts showed that culturable halophilic/halotolerant bacteria were abundant in the bulk soil as well as in the rhizosphere of Salicornia specimens. The PGP traits of the halophilic/halotolerant bacteria collection were assessed in vitro in presence of 5% of sodium chloride, to simulate those environmental condition that likely hamper the efficacy of PGP inocula selected from conventional ecosystems. Tolerance to abiotic stresses typical of desertic and semi-desertic areas, such as extreme temperature, salt concentration and low water availability, was widespread within the bacteria collection and represented a common trait in strains belonging to different phylogenetic groups. A high number of isolates showed the ability to influence the nutrients and hormonal balance in vitro at high salinity value. In addition, some strains expressed protease activity, potentially correlated to biocontrol capacity against phytopathogens. In particular, the screening allowed the identification of several strains resistant to the tested abiotic stresses and performing different PGP activities, including ammonia and indol-3-acetic acid production and phosphate solubilization. A Halomonas PGP strain was successfully gfp-labelled and used to demonstrate that is capable of successfully colonize Salicornia roots under laboratory conditions. Furthermore, in vivo test were performed on a subset of bacteria, identifying strains able to sustain the growth of Salicornia plantlets. The results confirmed that halophytes are valuable source of PGP bacteria. The halophilic/halotolerant strains isolated in the present study could be exploited to setup specifically designed biofertilizer to sustain plant growth in soils impacted by salinity and drought stresses in a sustainable perspective. According to the climate change scenario and the future extension of salt affected lands this powerful approach should be carefully considered for a successful land restoration through phytoremediation and to reclaim land traditionally considered unsuitable for agriculture.