## 3 Plant Growth-Promoting Rhizobacteria, A Potential Approach for Bioremediation
[74] realized a pot experiment in a growth chamber, proving the ability of _Azospirillum lipoferum_ strain 5 to enhance the development of wheat root systems growing in crude oil-contaminated soil. In addition, a greenhouse experiment was conducted by [PERSON] et al. [75] on the wheat growth stimulation potential of the two bacteria _Azospirillum lipoferum_ and _Rhizobium leguminosarum_ under different concentrations of Zn and Cd. The treatments: _Azospirillum_; _Azospirillum_ + _Rhizobium_; _Rhizobium_ + 200 ppm Zn; _Azospirillum_ + _Rhizobium_ + 300-ppm Cd; _Rhizobium_ + 300 ppm Zn and _Azospirillum_ + 300 ppm Zn resulted in an increase in wheat growth parameters compared to controls without bacterial inoculation. Similarly, a _Pseudomonas_ sp. strain, isolated from hydrocarbon-contaminated soil, showed an important effect in stimulating rice's root and shoot elongation and enhanced its final yield in hydrocarbon and toxic metal-contaminated soils [76]. Plant Growth-Promoting Rhizobacteria Implication in Induced Systemic Tolerance and Induced Systemic Resistance  
Currently, modern agriculture is facing extremely dangerous biotic and abiotic problems. Some of these constraints are the result of the human populations' growth explosion in some regions of the planet. Others are the direct consequence of environmental degradation due to drought, salinity, oxidative stress, toxic metals, nutrient deficiency and pathogens, loss of biodiversity and global climate changes [77; 78; 79; 80; 81; 82]. In addition, the biggest part of the agricultural loss is due to abiotic factors. For example, the average yield of wheat is about 1500 kg/ha, while crop damage was estimated at 2000 and 14,500 kg/ha due to biotic and abiotic stresses, respectively [83]. Several works have highlighted the role of some PGPR as inducers of plant tolerance to abiotic stress by provoking physiological and biochemical changes in their tissues, which result in enhancing their tolerance to environmental stresses such as drought, salinity and toxic metals. Such complex interactions between plants and bacteria are known under the term \"Induced Systemic Tolerance\" (IST) [80; 84; 85; 86; 87]. Among other bacteria, _Arthrobacter_, _Azospirillum_, _Azobacter_, _Klebsiella and Pseudomonas_ are known for their ability to promote wheat and rice growth under high salinity conditions. [84; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101]. Other works have described PGPR's effect on drought stress mitigation in wheat (_Burkholderia_, _Bacillus_, _Paenibacillus_, _Azospirillum_ and _Azobacter_) and rice (_Pseudomonas_, _Bacillus_, _Arthrobacter_, _Azospirillum_) [102; 103; 104; 105; 106; 107; 108; 109; 110]. Recently, PGPR conferring plant tolerance to toxic metals (Cd, Zn, etc.) have increasingly attracted researchers' attention. Thus, a _Pseudomonas_ sp. SNA5 was efficiently used by [PERSON] et al. [111] to promote wheat growth under high concentrations of cadmium, which is associated with high phosphate fertilization. [PERSON] et al. [112] found that a _P. aeruginosa_ was an ideal candidate for wheat growth enhancement against Zn-induced oxidative stress by improving the necessary nutrients' availability, as well as by lowering Zn metal uptake. In addition, ref [113] used PGPR to stimulate wheat growth under high Cr concentrations. Moreover, [PERSON] et al. [114]found that _Enterobacter ludwigii_ (HG 2) and _Klebsiella pneumoniae_ (HG 3) could significantly promote wheat seedling growth under mercury stress. Furthermore, the two bacteria _Ochroductrum_ sp. and _Bacillus_ spp. revealed a high potential for rice growth promotion under toxic metal-contaminated soil [115]. It is largely recognized that many PGPR are implicated in plant defense stimulation against phytopathogens (viruses, bacteria, fungi and insects), which is designated \"Induced Systemic Resistance\" (ISR) [80; 84; 85; 86; 87].