# 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
#### 5.2.2 Phosphate Solubilization  
Phosphorus (P) is an essential plant nutrient that limits plant growth and, in many agricultural lands, is of low bioavailability. This nutrient is required for diverse metabolic processes such as respiration, biosynthesis, photosynthesis, energy transfer, and signal transduction, so plants need it in proper amounts. There are two forms of P compounds: organic and inorganic phosphates. Soil bacteria are capable of converting insoluble forms into bioavailable forms; thus, plant growth and development is promoted [105; 106]. Generally, the concentration of soluble P in soil is quite low, usually at levels of 0.001% or less. Plants can take up diverse forms of P, but of the greatest importance is absorption in the forms of HPO\({}_{4}\)\({}^{-2}\) or H\({}_{2}\)PO\({}_{4}\)\({}^{-1}\), while the state of the compounds in the soil depends on pH and soil type [107]. apatite is a common mineral form of P that is characterized by low solubility [108], while the most common organic forms are phytate and phosphomono(di-, tri-)esterase, being mostly bioavailable to plants [5; 109].  
Phosphate-solubilizing PGPB involve the genus _Arthrobacter_, _Bacillus_, _Beijerinclia_, _Burkholderia_, _Enterobacter_, _Pseudomonas_, _Ervinia_, _Mesorhizobium_, _Flavobacterium_, _Rhodococcus_, and _Klebsiella_, among others [9; 37; 54; 59; 110]. Some of them may fix soluble P in their cells or convert P from organic to inorganic forms using different phosphatases [91; 111]. In saline soils, the bioavailability of nutrient elements is low because the salt modifies the balance of ions. However, halotolerant PGPB are able to mitigate, on a certain level, some of the deleterious effects of salinity, improving plant growth, biomass accumulation, and yield [2; 111]. _Achromobacter piedaudii_ ARV8 had a quite positive influence on the growth of tomato plants under salinity, improving P uptake and water balance [112].  
It is clear that halotolerant PGPB improve plant nutrient status, but the effect in most of the studies cannot be attributed to only one trait, e.g., P-mobilization or production of IAA and siderophores, or ACC-deaminase activity. The application of beneficial bacterial populations results in multiple positive results on the whole plant. Salt-tolerant bacterial strains expressing PGP abilities were found to produce EPS, auxin, and proline, reducing sugars and total soluble sugars, and to solubilize phosphate [17]. These abilities of the native isolate of the wheat rhizosphere are associated with plant growth and biomass enhancement in saline soil. Plant-growth-promoting strains support the restriction of Na\({}^{+}\) uptake by the roots caused by a decreased quantity of the apoplasmic flow of sodium ions. The authors also informed us that the use of a consortium of four strains, two _Bacillus_ sp. increasing EPS production and two _Enterobacter_ sp., was the most efficient way to reduce cation uptake by plants in soil salinity environments. Populations of two isolates determined as _Azotobacter chroococcum_ alleviated the saline stress in maize through the integration of several mechanisms that improve plant growth and mineral nutrition. These mechanisms included Na+ exclusion and K+ uptake in maize, thereby increasing their K+/Na+ ratio, enhancing polyphenol and chlorophyll content, and decreasing the proline concentration, increased by the salinity, in leaves. At the same time, both _Az. chroococc_ isolates possessed several PGP traits such as nitrogen fixation, phosphate solubilization, and auxin synthesis [113].