## 4 Discussion
This is in agreement with the literature reporting a density of endophytic bacteria ranging from \(1\times 10^{2}\) to \(1\times 10^{9}\) per gram of plant tissue depending on the plant species, the plant organ considered, the phenological stage of the plant, the plant cultivar and the interaction with other organisms, as well as other environmental factors [55]. These five strains belonged to _Pseudomonas migulae_, _P. brassicacarum_ and _Pseudomonas_ sp. Interestingly, other strains belonging to these bacterial species have been previously reported as endophytic [56; 22; 57]. All selected bacterial strains were able to improve plant development for at least one of the growth parameters considered. Therefore, in general, all strains behave as plant growth promoters on sorghum. In more detail, the SVB3R3 strain (able to synthesize siderophores and solubilize phosphate, but unable to produce IAA) significantly stimulated all plant parameters, while the SVB6R1 strain (positive for all three physiological traits tested) increased only the biomass of the plant and the weight of the roots. This suggests that in vitro tests of bacterial physiological activities are not always able to define the activity of the bacterial strain in vivo. Moreover, it must be considered that other unknown mechanisms may be involved in promoting plant growth. Sorghum is one of the best adapted cereals to water limited soils and is classified as one of the most drought tolerant crops cultivated [58]. However, it was shown that huge variations in sorghum salt tolerance exist among the different cultivated genotypes [59]. Therefore, possible support from bacteria able to enhance stress tolerance in plants could be desirable. The effects of drought and salinity stress on plants partially overlap and impair several plant physiological processes, such as seed germination, seedling growth and vigor, and flowering and fruit production [60]. Numerous reports have indicated the involvement of soil bacteria in increasing plant tolerance to the inhibitory effects of salt. However, only a small fraction of those reports deal with bacterial endophytes. For example, [PERSON] et al. [61] performed a meta-analysis on plant stress mitigation by endophytes based on the results presented in 209 papers. This statistical analysis showed that endophytes inducing plant stress alleviation belonged to bacteria (53%), filamentous fungi (41%) and yeast (6%). The results showed the positive impacts of endophytes on plants exposed to drought, nitrogen deficiency, and salinity stress conditions. In agreement with previous observations, the growth of the uninoculated plants treated with a salt solution was dramatically reduced in our study. However, strain SVB3R4 increased all measured plant growth parameters. Consistent with this result, [PERSON] et al. [62], showed that 11 endophytic bacterial isolates from Jizan (Saudi Arabia) exhibited plant growth promotion abilities on _Arabidopsis thaliana_ exposed to 100 mM NaCl. Similarly, bacterial endophytes _P. fluorescens_ YsS6 and _P. migulae_ 8R6 were shown to enhance plant growth in non-stressed condition and subsequently protect tomato plants against salinity (165 and 185 mM) by facilitating their growth under such a stressful condition [63]. Interestingly, in plants inoculated with rhizosphere bacteria (model strain _P. putida_ UW4), excess salt was accumulated in the vacuole, a confined area far away from cytosolic metabolism [63]. On the contrary, plant growth-promoting bacterial endophytes were found to limit the concentration of sodium in plant shoots through a still unknown mechanism. The evaluation of the expression of salt stress related symptoms in sorghum plants indicated that SVB3R3 and SVB3R4 strains reduced the severity of symptoms across a narrow time range (7-14 days) and then returned to the level expressed by control plants. On the other hand, strain SVB6R1 delayed and reduced the expression of symptoms throughout the experiment, also increasing the survival of the plants.