## 4 Discussion  
Soil salinization limits plant growth and agricultural productivity by up to 50%, and it is a very common phenomenon in developing countries and in extremely exploited agricultural lands [31]. Therefore, many researchers are looking for new biotechnologies that are able to improve crop cultivation in these areas. In this regard, the isolation and selection of halotolerant bacteria showing PGP features could be a quick, effective, and sustainable solution at this problem. In this study, we characterized novel halotolerant/halophilic PGPR and evaluated the effect of increasing the NaCl concentration on their growth-promoting features (PGP). Comparing the 16S rDNA sequences of QH23 with those of QH24, and those of M21 with M23, we obtained for both strain pairs identity values close to 97% (97.37 and 96.81%, respectively), below the accepted molecular limits for species definition (>98.5%) [32]. Therefore, we can hypothesize that the identified strains may be four different species. The molecular differences were in accordance with the results of the biochemical characterization, which highlighted different profiles for each of the strains. The phylogenetic analysis showed that QH23 was closest to _H. alkaliophila_ species (99.93% identity), a strain isolated for the first time from a salt pool located in Monte-fredane in Campania Region (South of Italy). QH24 was closest to _H. titanicae_ species (98.93% identity), whose type strain, BH1, was isolated from the RMS Titanic wreck site. However, the high identity value, above the threshold of 98.5% for members of this genus, is not enough to hypothesize that both strains belong to their closest respective species. In fact, from our comparative analysis of multiple 16S sequences, it appears that distinct species of _Halomonas_ genus can exhibit up to (and more then) 99.93% identity. M21 and M23 are taxonomically close to B. _haynesii_, _B. sonorensis_, and _Bacillus_ sp. OSS 35 (an unclassified strain of _B. subtilis_); nevertheless, the 16S rDNA sequence identity among these species, for both strains, was close to 97%, and based on threshold of 98.5%, it is too low to consider possible classification at the species level. Therefore, further molecular and phenotypic characterization studies are needed to exactly define the species of our four isolates [33]. Several bacterial strains of _Halomonas_ and _Bacillus_ genera are known for their abilities to tolerate moderate/high concentrations of NaCl and to exhibit many PGP features (IAA production, ACC deaminase activity, etc.) [34; 35]. _Halomonas neptunia_, a very similar species to the QH24 strain, and isolated for the first time from a deep-sea hydrothermal-vent [36], exhibits the ability to produce IAA and moderately siderophores, slightly solubilizes phosphate, and shows ACC deaminase activity [37]. Likewise, we showed that QH24 produces significant amounts of siderophores and IAA. According to [PERSON] et al. [27], who considered moderate ACC deaminase activity, when it ranged from 20 to 300 nmol \(\alpha\)-ketobutyrate per mg protein\({}^{-1}\cdot\)h\({}^{-1}\), QH24 exhibits similar ACC deaminase activity (79.6 \(\pm\) 18.7 nmol\(\cdot\)per mg protein\({}^{-1}\cdot\)h\({}^{-1}\)). In this regard, the ACC deaminase activity lowered plant ethylene levels, promoting its growth, and therefore, PGPR could metabolize ACC exuded from seed or plant roots, decreasing its concentration [38]. The IAA is known to regulate the growth and developmental processes of plants, such as cell division, elongation, tissue differentiation, and apical dominance [39]. Moreover, the roots are most sensitive to fluctuations in the IAA level. The QH23 strain, based on our data, is phylogenetically close to _Halomonas venusta_. They have common PGP properties. In fact, [PERSON] et al. [40] observed that this species is able to produce high amounts of IAA (16.2 \(\pm\) 0.1 ug\(\cdot\)mL\({}^{-1}\)). [PERSON] et al. [41] evaluated the effect of _H.