# 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  
Soil microorganisms can modify their membrane structure and thus counteract the scarcity of humidity. For example, they can produce polysaccharides. Thanks to their hygroscopicity, they can trap and hold water molecules in their structure, helping maintain the acceptable moisture content in the microenvironment surrounding them [100]. A recent study showed that _Bacillus subtilis_ and _Azotobacter brasilense_ strains, isolated from an arid region of Pakistan, could produce high amounts of EPS. The inoculation into wheat seedlings, both individually and in a consortium, led to a significant increase in plants' osmotic potential, water potential, and chlorophyll content [101]. Unfortunately, due to climate changes, the phenomenon of drought is constantly increasing and represents one of the most fearful abiotic stresses. It modifies the physiology and morphology of plants by reducing the diffusion within the tissues of water-soluble nutrients, such as sulphates, nitrates, calcium, magnesium, etc. [102]. PGPR can support plants in counteracting the detrimental effects of drought stress. Experimental evidence shows how some endophytic microorganisms can mitigate water stress even in plants other than those from which they have been isolated. This evidence indicates that the interaction of PGPR is not specific, and this could facilitate mitigation interventions. The continuous worsening of climate change is modifying the global geography, with a fast transformation of many agricultural areas into marginal soils inappropriate for the life of populations, making the role of PGPR progressively more and more critical. Taking advantage of plant-microbe interactions that can positively impact the response of plants to drought and, in general, to all abiotic stresses is a compelling strategy. The roots that show a high structural and morphological plasticity are the first to undertake the chemical-physical modifications of the soil in the presence of abiotic stress. Drought leads to significant changes in the phospholipid composition of the root membrane. Under water stress conditions, many PGPR produce the hormone ABA capable of regulating plant cell dehydration by controlling stomatal closure, decreasing leaf transpiration, and stress signal transduction [103]. Still, the action of PGPR, which increases the elasticity of the root membranes, can respond to these adverse effects on the root and allow the new roots to penetrate deeper into the soil to reach basins of water [104; 105]. PGPR activity in drought conditions is also represented by the storage of solutes within plant cells to act on cellular osmotic regulation. The substances generally accumulated are amino acids, such as proline, glycine, phenylalanine, organic acids, sugars, and inorganic ions, which can cooperate to protect plants from possible oxidative damage [106]. The other abiotic factor that is particularly damaging and often associated with drought is the salinization of soils, which is an ever-increasing global threat. The high salt concentration often harms the soil, particularly the organic material decomposition, nitrification, denitrification, microbial activity, and the biodiversity [107]. Removing excess salt from soils by physical and chemical methods is a time-consuming and costly operation, unsustainable and ineffective with high salt concentrations [108]. Salt stress can cause a drastic reduction in the productivity of various crops and mainly fruit crops, such as the species of the genus _Prunus_, which are considered very sensitive to salt. To prevent, at least in part, these conditions that may limit the normal development of these fruit trees, one of the agronomic strategies successfully adopted is that which uses rootstocks tolerant to biotic and abiotic factors [109].