## 3 Plant Growth-Promoting Rhizobacteria, A Potential Approach for Bioremediation  
One of the safer tools to alleviate environmental deterioration is the application of ecofriendly living agents such as bacteria, fungi, algae and higher plants to eliminate the toxic chemicals, oil spills and toxic metals present in the altered sites, which is known as \"bioremediation\" [57]. To discriminate between the use of these biological agents for polluted sites' decontamination and their use in biorecycling processes designed to reduce the emission of toxins at source, bioremediation is defined as \"a biological response to environmental abuse\" [58; 59]. Apart from their implications as plant growth enhancers, PGPR are also known to have a primordial role in practically all bioremediation aspects of contaminated soils (PTE, fungicides, organic pollutants, etc.) [60; 61; 62]. For example, some PGPR can interact with the soil's toxic metals by binding them to their cell surfaces or incorporating them in some metabolic functions after captivation. Some microorganisms can reduce metal ions such as Hg\({}^{2+}\) and Ag\({}^{+}\) to Hg\({}^{0}\) and Ag\({}^{0}\) and provide a perfect model for total metal removal from the soil [63]. It has been experimentally demonstrated that some rhizobacteria produce extracellular enzymes such as peroxididases, reductases, Cytochrome P\({}_{450}\), lacases and glutathione-S-transferase, having important roles in polycyclic aromatic hydrocarbon degradation, molecules present in crude oil and known for their toxicity and carcinogenicity [64]. [PERSON] [65] produced a detailed review about PGPR application in soil decontamination, focusing on the genus _Pseudomonas_, and some of the most important strategies used by PGPR in bioremediation such as the production of biosurfactants, biofilms, toxic metal solubilization and siderophore production. Plant Growth-Promoting Rhizobacteria utilize a wide range of mechanisms to improve plant growth and soil quality under stress conditions. Under ionic stress, mostly related to high salinity, drought and desiccation, certain bacteria can provide compatible solutes (proline, glycine betaine, sugars and derivatives, etc.) for root cells to avoid ion accumulation in the cytoplasm and, thus, water deficiency. Others can synthesize exopolysaccharideaccharides (EPS) in the rhizosphere. Bacterial EPS enhance water and ion (K\({}^{+}\), Ca\({}^{+}\), Na\({}^{+}\)) uptake. They also play a major role in soil structure stabilization and aggregation under high ion concentrations. Certain PGPR express ion antiporters in their membranes (Na\({}^{+}\) (K\({}^{+}\))/H\({}^{+}\)) to maintain their water balance in the cytoplasm under ionic stress. They also produce stress mitigation enzymes such as 1-aminocyclopropane-1-carboxylate (ACC) deaminase (see Section 5.2) and antioxidant enzymes such as peroxidase (POD), catalase (CAT) and superoxide dismutase (SOD) to eliminate the high amounts of reactive oxygen species (ROS) produced under abiotic stresses. In addition, PGPR phytohormones such as auxins, gibberellins, abscisic acid and cytokines (see Section 5.4) are inevitable for the stabilization of plants' physiology under water stresses [66]. In PTE bioremediation, several mechanisms are used by beneficial soil bacteria to adsorb, transform and uptake toxic elements in the