# Plant-Growth-Promoting Bacteria Mitigating Soil Salinity Stress in Plants
## 5 Role of PGPB in Alleviating Salinity Stress to Plants
### PGPB Support Nutrient Acquisition in Plants
#### 5.2.3 Iron Mobilization  
Iron is an essential micronutrient for all living cells, in particular plant cells, because it is involved in plenty of processes, proteins, and enzymes and is a cofactor in photosynthesis, N2-fixation, and respiration, including the reparation processes of RNA and DNA. It is one of the most common elements on Earth (second or fourth, according to different sources) [5; 114; 115; 116]. Despite the above, iron is not widely accessible in soils forming complexes of hydroxides and oxyhydroxides; thus, its bioavailability decreases below 10% in loamy soils [117]. In soil matrix, iron exists in two convertible states as Fe2+ and Fe3+, depending on pH, oxygen, and organic matter content. There are reports that soil salinity is an additional negative factor for plant iron nutrition [5; 118]; thus, growth, development, and yield suffer. Plants respond to such limitations through accreting organic molecules by the root, acidifying the rhizosphere and resulting in a certain increase of iron availability. An additional toll is the synthesis of some compounds that bind Fe3+ by the plants to make it more accessible to the roots [119], but even that is not sufficient to satisfy the plants' need for iron [38]. Different PGPB, such as _Pseudomonas_, _Bacillus_, and _Serratia_, possess the ability to synthesize low molecular weight (<1000 Da) compounds named siderophores, having a high affinity to Fe3+ and forming complexes with them. They act as solubilizing agents for many more ions (with less affinity) from minerals or organic compounds such as Al, Cd, Pb, Cu, and Zn. Although the siderophores are very diverse, the iron-chelating group of the siderophores includes three types: hydroxycarboxylic acid, catechol, or hydroxamic acid [120]. These complexes are taken up by the same PGPB, but in the rhizosphere, the bacterial population is numerous due to the specific conditions created by the root secretions, so the concentration of siderophores becomes significant. This leads to the mobilization of available iron, which is largely absorbed by the plant roots. [121]. Table 1 presents the different studies on how PGPB-producing siderophores mitigate some of the salinity stress effects. In one study with sunflower plants and maize, improvement in growth was reported [122]. A similar investigation was made using wheat plants and PGPR populations isolated from halophytes, presenting multiple PGP traits, including siderophores. That investigation confirmed the utility of PGPR in saline soils as the plants had enhanced salinity tolerance, increased growth and K+/Na+ ratio, and reduced endogenous ethylene production. Moreover, diverse genes were reported to be upregulated in the roots of salt-stressed plants [77]. In other studies on iron-deficient soil, the growth of _Arabidopsis thaliana_ was induced by the inoculation of siderophores producing a _Pseudomonas fluorescens_ strain [123]. Researchers suggested that siderophore production could even be preventive in response to environmental stress; thus, _Azospirillum brasiliense_ produced siderophores in the iron-rich rhizosphere of cucumber [124].