## 4 Discussion
In the meantime, IAA stimulates the synthesis of ethylene in the plant, through the transcription of the enzyme ACC synthase that catalyzes the formation of ACC, which can be released by the plants, and then taken up by bacteria, acting as an ACC sink [62]. A further step forward, is when bacteria possessing the _acdS_ gene can cleaved ACC by ACC deaminase. Bacterial ACC deaminase, which is generally present at a relatively low level, is induced by the increasing amounts of ACC deriving from the stress induction by ACC synthase in the plant [62]. The net result is the lowering of plant ethylene synthesis, which is particularly helpful in reducing stress ethylene, which is deleterious for the plants. Comprehension of the competitiveness of PGPB, based on their characteristics, is rather complex, though several studies are consistent with the possession of ACC deaminase activity facilitating bacterial competitiveness or persistence in the environment. The phosphate solubilizing capability underlined for the four strains has important environmental and economic consequences. Nowadays, the phosphate supplementation in agriculture is commonly obtained by means of synthetic chemical fertilizers. The application of these products not only represents a huge cost for the farmers but also contributes to environmental pollution. The phosphate supplementation is obtained only through continuous fertilization campaigns and the chemical compounds applied, which lead to a stratification of inaccessible forms of phosphates in soils. Inoculation of PSB, instead of synthetic chemical compounds, could support plant nonurishment whilst mobilizing immobilized phosphorous in lightly productive and unproductive agricultural soils [17] and preserving soils from losses of this essential chemical element [63]. Inoculation of PSB--belonging mainly to _Rhizobium_, _Pseudomonas_, _Azotobacter_, and _Bacillus_ genera--has been already used as an alternative fertilization strategy, obtaining increasing phosphorous availability, without changing the soil composition [17]. Most of the literature concerning interactions of PGPB and plants, describe in vitro or greenhouse growth experiments; only a few articles contain field experiment and consider the PGPB effects on plant growth promotion and soil health. In our case, we obtained a positive influence of PGPB treatment on all investigated plant-growth and development parameters and, notably, in soil fertility status. Thus, the PGPB stimulant properties--underlined with the broth cultures assays--were confirmed by the field experiment results. Indeed, soil health was positively influenced in terms of fertility. The application of PGPB consortium increased total organic carbon, organic matter, and available P and allowed to keep good concentrations of the other nutrients until harvest. Soil resources have been exploited to the fullest, allowing the plants to access resources not otherwise available. This phenomenon must be ascribed to the ability of PGPB strains to mobilize nutrients and to fix atmospheric nitrogen [64]. Similar results are already reported by other authors [65; 66] and are mandatory for an effective plant growth promotion. Bacterial inoculum presence also promoted positive changes in soil microbial community physiological profile. PGPB are already known to increase exudation from roots of host plants, which may, in turn, support the growth of microorganisms and/or influence their metabolic activity [67]. Amplitude and variability of substrate spectra metabolized by soil microbial populations are considered an indirect index of community biodiversity [68], including in the case of ectomycorrhizal [69] and endophytic fungi [70]. Based on NGS results, seed bacterization was able to alter the soil bacterial community composition. Diversity and Richness values were higher in the presence of the bacterial treatment. Also, the comparison of the abundances among the common genera found in the control and SIB samples underlined an alteration of the community structure.