## 3 Plant Growth-Promoting Rhizobacteria, A Potential Approach for Bioremediation
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**Bacterial Genera** & **Soil-Related Problem** & **Targeted Crops** & **Reference** \\ \hline _Bacillus_ & Drought & _Triticum aestivum_ & \\ _Enterobacter_ & Drought & _Zaa mays_ & [PERSON] et al., 2019 [49] \\ \hline _Bacillus_ & Drought & _Glycyrrhiza uralensis_ & [PERSON] et al., 2019 [50] \\ \hline _Agrobacteriumium Leclercia_ & & & \\ _Pseudomonas_ & Drought & _Triticum aestivum_ & Zanar-ul-Hye, 2019 [51] \\ _Bacillus_ & & & \\ \hline _Pseudomonas_ & Heat & _Triticum_ & [PERSON] et al., 2019 [53] \\ \hline \multirow{2}{*}{_Bacillus_} & Fungicidal toxicity and & _Oryza sativa_ & [PERSON] et al., 2019 [54] \\  & soil-borne diseases & _Arachis hypogaea_ & [PERSON] et al., 2019 [55] \\ \hline _Nostoc_ & Fungicidal toxicity and & _Oryza sativa_ & [PERSON] et al., 2020 [56] \\ _Anabena_ & soil-borne diseases & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._soil. In this context, the toxic species is either passively adsorbed (biosorption) to the cell surface or internalized (bioaccumulation) to its interior through \"metabolic independent mechanisms\". Inside the cytoplasm, toxic metal behavior follows more complex and, often, \"metabolic dependent mechanisms\" that may include: (1) metal compartmentation within specific organelles, (2) enzymatic detoxification (methylation, oxidation, dealkylation, reduction, etc. ), and (3) efflux pumps that transport the modified and harmless toxic metal forms outside the cytoplasm (Figure 1) [67]. At a cellular level, microbial populations permanently communicate between themselves, but at the same time with their surrounding root systems. In addition, some positive PGPR effects are controlled by small diffusible signaling molecules, mainly N-acyl homoserine lactose molecules (AHLs), that regulate gene expression in response to bacterial population density and interactions with plants, particularly under stress conditions [68; 69]. For example, the N-3-oxo-hexanoyl homoserine lactone (3 OC6-HSL) was found to be highly effective in restoring wheat root length, shoot length and fresh weight under salt stress conditions. In fact, 3 OC6-HSL upregulated salt-responsive gene expressions such as (1) abscisic Acid (ABA)-dependent osmotic stress genes: _COR15a, RD22, ADH_ and _PSCSI_, (2) ABA-independent gene: _ERD1_, and (3) ion-homeostasis regulation genes: _SOS1_, _SOS2_ and _SOS3_ in Arabidopsis under salt stress conditions [70]. Moreover, [PERSON] et al. [71] carried out impressive work on the role of quorum sensing systems during microbial biofilm formation while degrading the pollutant 1,2,4-trichlorobenzene (1,2,4-TCB). For them,  
Figure 1: Principal mechanisms of bacterial bioremediation and plant growth regulation under abiotic stresses. * (IST): see Section 3.  
there was no doubt about the positive correlation between the 1,2,4-TCB mineralization, microbial biofilm abundance and AHL (3-oxo-C\({}_{12:1}\)-HSL; 3-oxo-C\({}_{10:1}\)-HSL, 3-oxo-C\({}_{14:1}\)-HSL; OH-C\({}_{14:1}\)-HSL, etc.) production. Among other PGPR, a _Pseudomonas aeruginosa_ PS1 significantly promoted root and shoot nitrogen, root and shoot phosphorus and the seed yield of greengram (_Vigna radiata_) plants at all tested concentrations of tebuconazole, a fungicide belonging to the triazole group and largely used in agriculture. At high concentrations, tebuconazole may be accumulated in soils and plants, becoming toxic, damaging plant tissues and affecting crop yield [60]. [PERSON] et al. [72] studied the effect of a _Pseudomonas protegens_ S5 LiBe on barley growth restoration under toxic metal contamination. The results showed an increase in germination rate, shoot and root fresh weight, shoot and root dry weight and shoot length. Moreover, the two siderophore producers, _Alcaligenes faecalis_ RZS2 and _Pseudomonas aeruginosa_ RZS3, showed a high ability to promote wheat growth when seeds were sown in PTE-contaminated soil; their bioremediation potential was higher than other metal chelators such as EDTA or citric acid [73]. [PERSON] et al.