## 4. Discussion
These PGPR tolerated salinity up to at least 3% NaCl [29] and were able to survive in the rhizosphere of plants due to their persistence and competitiveness in saline and arid soil conditions. However, most isolates had optimal growth between 30 and 37 \({}^{\circ}\)C and at pH 7 except of some strains which were able to grow at 40 \({}^{\circ}\)C and at alkaline pH (pH 9) or even very alkaline pH (pH 11) for strain BR5. Temperature is one of the most important factors governing the physiology and growth of microorganisms. The strain's ability to grow over a wide pH range is evidence to its ability to survive. Strain tolerance is a strain-specific trait. Indeed, the stressful sampling environment promotes the development of efficient bacteria. It is agreed that stress is an inducer of bacterial species activity [30]. Fluctuating ecosystems promote the development of bacteria with good stress tolerance, this supposes the existence of selection pressure exerted by the natural environment. Three representative strains from each sample site with the best PGP activities and the best stress tolerance capacity were chosen and identified after 16S-rDNA sequencing. The BR5, OR15, and RB13 strains belong to _Bacillus atrophaeus_. The Bacilli represent a significant fraction of the microbial community in soil. This genus is ubiquitous and can survive in several environments based on its sporulating capacity. Moreover, _Bacillus_ strains exhibit several characteristics which improve their survival in the rhizosphere and their efficiency as PGPR. It has been shown that when environmental conditions are more selective, microbiome variability can be simplified, and taxonomic diversity is limited [31]. It has also been reported that in saline ecosystems native haplotypes such as _Salicornia_ sp. retain a rhizosphere bacterial community, homogeneous in the composition of taxa [22]. To test the PGP potential of the selected strains of _Bacillus_ (BR5, OR15, and RB13) on plants, plants of _Arabidopsis thaliana_ were inoculated with each strain and their different combinations compared to the control plants not inoculated under saline stress conditions (0, 50, 100, and 150 mM NaCl). The results showed a significant decrease in the different growth parameters (total foliar area, plant weight, root length, and chlorophyll content) under salinity stress. The responses of plants to salinity induce various alterations in the biochemical and physiological structural aspects. Different studies have reported that salinity affects plants in several ways, the rate of leaf area expansion decreases along with a reduction in the rate of leaf production resulting in plant death [32]. Salinity can increase the biosynthesis of certain hormones such as ethylene to high enough levels which can lead to physiological changes in plant tissue. Inoculation with the bacteria considerably improved the growth parameters of _A. thaliana_. Similar results of promoting the growth of _A. thaliana_ plants (total leaf area, plant weight, chlorophyll content, and root length) have already been reported by several works using various species of PGPR such as _Burkholderia phytofrimans_[33], _Phylobacterium brassicearum_ STM196 [34], and _Bacillus subtilis_ GB03 [35]. At high concentrations of NaCl (100 and 150 mM), the various morphological parameters were significantly reduced. Bacterial inoculation (including OR15 and RB13) and their combination improved the aerial part of the plants (fresh weight and leaf surface) and the chlorophyll content. While root length did not seem to be affected by the increase in the salt. It could be interpreted that auxin produced by these strains could be responsible for the promotion of growth and salt tolerance. Auxin is one of the important plant hormones responsible for the formation of lateral roots at different stages of development. The IAA is the plant growth regulator; it can enter plant cells and stimulate root growth [36].