## 4 Discussion  
In this work, three non-perturbed soils subjected to different management regimes were considered as a possible source for bacterial endophytic strains with potential in plant growth promotion. Moreover, three economically relevant plant species (tomato, cucumber and sorghum) were used as traps for endophytes. Characterization of plant beneficial traits showed that each considered plant species hosted a plethora of endophytes with multiple plant growth promotion abilities, some of which could significantly enhance seed germination, as well as plant growth and development. This kind of physiological characterization allowed us to select some strains that can be considered potentially efficient biofertils and to highlight the relationships between bacteria with specific physiological activities and a particular plant species. In fact, our results showed that 56% of the endophytes isolated from cucumber displayed indole 3-acetic acid (IAA) production, 55% of the strains from tomato showed phosphate solubilization activity, and 42% of the strains from sorghum showed siderophore production. Moreover, the occurrence of bacterial endophytes able to synthesize IAA, solubilize phosphate and produce siderophores was highest in pasture soil (55.5%, 76.2% and 57.1%, respectively). In particular, our results demonstrated that pasture soil, the least perturbed soil among those considered, is probably the one with the highest biodiversity and richness in bacterial species. For this reason, the highest number of endophytic strains with plant beneficial physiological traits were isolated from this soil. This picture is obviously restricted to culturable endophytic bacteria, which are likely a very small fraction of the inner microbiota of the plant, that in turn represent an even smaller portion of the total rhizosphere microbiota. However, it is well established in the literature that plant genotype drives not only the development of plant phenotypes, but also the establishment of a specific plant microbiota [31]. In addition, the type of soil used to cultivate a plant species can also shape its endophytic community. Consistently, different endophytic populations can be recorded in the same plant species growing in different soils [22]. About 27% of the isolates showed the ability to synthesize IAA. Production of IAA in bacteria occurs though six different metabolic pathways, with five of them based on tryptophan as a precursor [32; 33]. Bacterial IAA affects the plant endogenous auxin pool; as a consequence, the amount of endogenous auxin is often the determinant of the bacterial IAA effect on plant growth. High levels of total auxin enhance ethylene synthesis in plants, leading to leaf and fruit abscission and inhibition of stem growth. In this way, IAA synthesized by bacteria is directly related to plant growth and development [34]. The low bioavailability of P and Fe in most soils is one of the main factors limiting plant growth [34]. Therefore, bacteria able to solubilize and mineralize P (phosphate-solubilizing bacteria, PSB) and synthesize siderophores have great potential as biofertilizers. In this work, 30% of the bacterial endophytes isolated were able to solubilize phosphate and 32% were able to produce siderophores. Solubilization of inorganic P (Pi) by PSB occurs via the production of low molecular weight organic acids, leading to acidification of soil and then to increased solubilization of inorganic phosphate [35]. The evidence showing that PSB can directly affect plant Pi acquisition inside the root is still missing [36]. However, a linear relationship between the phosphate solubilization activity of PSB and increased phosphorus levels in tissues of plants inoculated with PSB was reported by [PERSON] et al. [37]. Similarly, iron bioavailability in aerobic soil is very low, ranging from about 1 \(\times\) 10\({}^{-7}\) at pH 3.5 to 1 \(\times\) 10\({}^{-23}\) M at pH 8.5 [38].