# The Role of Plant Growth-Promoting Rhizobacteria (PGPR) in Mitigating Plan's Environmental Stresses
## 4 PGPR to Face Salinization and Drought in Marginal Soils
### Drought and Salinity Pressure
The phenomenon of salinization concerns all the salts which, if in excess, can cause severe damage, but they are mainly sodium chloride (NaCl), sodium sulphate (Na\({}_{2}\)SO\({}_{4}\)), magnesium sulphate (MgSO\({}_{4}\)), and magnesium chloride (MgCl\({}_{2}\)); however, NaCl is certainly the most critical one. Excessive salinity modifies the biochemical and physiological processes of the plant, and photosynthetic efficiency can also be significantly altered [110]. Numerous soil microorganisms able to tolerate the presence of salt even at high concentrations have been identified and characterized. We describe bacteria belonging mainly to the genera _Bacillus_, _Pseudomonas_, _Agrobacteriumum_, _Enterobacter_, _Klebsiella_, _Streptomyces_, and _Ochromobacterium_, tolerant up to 150 g/L of NaCl [111], and this makes them excellent candidates to alleviate the salt stress of plants. Generally, salinity does not significantly alter soil microbial communities, perhaps due to a sort of buffer effect [112; 113]. In some cases, soil microbial diversity was mainly influenced by soil salinity [42]. In the presence of high salinity and scarcity of water, plants are subjected to considerable stress, which leads to numerous adverse effects, including reduced photosynthetic activity, limited assimilation of nutrients, oxidation and consequent deterioration of cell membranes, and dehydration of the plant, which can lead to death. Abiotic stresses, such as drought and salinity, have a strong and severe impact on agriculture, further aggravating the functionality of ecosystem services. The selection of stress-tolerant species and understanding the biochemical mechanisms underlying this tolerance is certainly a way forward. However, undoubtedly, the contribution that PGPR can make in mitigating the damage caused to plants by salinity and drought is the most important [114]. To date, there are many microorganisms isolated from soils, which show interesting properties for promoting plant growth. The search and selection of the best multifunctional PGPR, safe for the environment and humans, is the right way to meet the needs and production needs of agriculture in the world [115]. PGPR can increase the tolerance of plants to salinity, acting primarily through the accumulation of osmolytes, increasing the absorption of nutrients, the nitrogen fixation, the solubilization of P and other essential elements, but similarly with the activity of ACC deaminase, the production of auxins, siderophores, and exopolysaccharides [116]. Furthermore, the regulation implemented by aquaporins whose synthesis is induced by water stress is particularly critical for mitigating the adverse effect of salinity. Aquaporins influence the ability to absorb water, thus maintaining adequate photosynthesis [117]. Several halotolerant bacteria have been isolated from halophilic plants of _Salicornia_[118] and _Sueda_[119], species grown in the desert, saline, and alkaline-saline soils. These halotolerant bacteria can survive in high salt environments by different fielding mechanisms [120]. For example, ion pumps can regulate intracellular ion concentrations, accumulate sugars (i.e., sucrose, trehalose, glycerol), produce exopolysaccharides (EPS), thus creating hydrating biofilms [121].