## 2 Functional Attributes of PGPR and Their Mechanisms
### Biofertilization  
Atmospheric nitrogen can be converted to plant-absorbable forms (NH\({}_{4}\)+) by PGPR with biological nitrogen-fixing (diazotrophy) ability [18]. Nitrogen-fixing bacteria (NFB) are categorized into two main groups including (i) symbiotic bacteria associated with leguminous plants (e.g., _Rhizobium_) and nonleguminous plants (e.g., _Frankia genus_ and _Azospirillum_ species associated with some dicotyledonous species and cereal grasses, respectively) and (ii) nonsymbiotic free-living bacteria (e.g., cyanobacteria and some genera, including _Azotobacter_, _Arthbroacter_, _Beijerinckia_, _Pseudomonas_, and _Diazotrophicus_) [47]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**PGPR Strain** & & **Mode of Action** & \begin{tabular}{c} **Plant (PGPR Isolated** \\ **from and/or Affected** \\ **by PGPR)** \\ \end{tabular} & **Reference** \\ \hline \multirow{4}{*}{_Pseudomonas fluorescens_ CHA0T} & - & Biosynthesis of the antibiotics & \multirow{4}{*}{\begin{tabular}{c} Wheat \\ [39] \\ \end{tabular} } & \multirow{4}{*}{\begin{tabular}{c} Mo\(\overline{\text{u}}\)[PERSON] et al. \\ [39] \\ \end{tabular} } \\  & & (2,4-diacetylphroglucinol, \\  & phenazine-1-carboxylic acid, \\  & pyoluteorin, and pyrrolitrin) & & \\ \hline \multirow{4}{*}{_Streptomyces rochei IDWR19,_S. caripinensis IDWR53,_S. thermollicatus IDWR81_} & - & Biocontrol (antifungal) & \multirow{4}{*}{\begin{tabular}{c} Wheat \\ [40] \\ \end{tabular} } \\  & - & Enzyme production (e.g., chitinase, \\  & cellulase, and phytase) & & \\ \hline \multirow{4}{*}{_Bacillus megaterium_} & - & Ethylene inhibition & \multirow{4}{*}{\begin{tabular}{c} Tomato \\ [41] \\ \end{tabular} } & \multirow{4}{*}{\begin{tabular}{c} [PERSON] et al. \\ [42] \\ \end{tabular} } \\  & - & Phosphate solubilization & & \\  & - & Phytohydromes (ABA) production & & \\ \hline \multirow{4}{*}{_Bacillus sonorensis_} & - & Bioprotection (Chitinase production) & \multirow{4}{*}{\begin{tabular}{c} Sweet and chili \\ peppers \\ \end{tabular} } & \multirow{4}{*}{\begin{tabular}{c} [PERSON] et al. \\ [42] \\ \end{tabular} } \\  & - & Phosphate solubilization & Sweet and chili & \\  & - & Production of HCN, IAA, and & peppers & [42] \\  & ACC deaminase & & \\ \hline \multirow{4}{*}{_Orthobactrum_ sp. (Accession No. JQ514559)} & - & Biodegradation of chlorpyifos & \multirow{4}{*}{\begin{tabular}{c} Rice \\ [43] \\ \end{tabular} } & \multirow{4}{*}{\begin{tabular}{c} [PERSON] and \\ [PERSON] \\ [43] \\ \end{tabular} } \\  & - & Phosphate solubilization & & \\  & - & Production of HCN and IAA & & \\ \hline \multirow{4}{*}{_Sinorhizobium mellioti 1021_} & - & Vitamins (biotin) production & Alfalfa & \
\begin{tabular}{c} [PERSON] et al. \\ [44] \\ \end{tabular} \\ \hline \multirow{4}{*}{_Sinorhizobium mellioti 1021_} & - & Vitamins (riboflavin and lumichrome) & \multirow{4}{*}{Alfalfa} & \multirow{4}{*}{
\begin{tabular}{c} [PERSON] et al. \\ [45] \\ \end{tabular} } \\  & production & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._Phosphate solubilizing bacteria (PSB) are among the known PGPR, with a notable capability in solubilizing insoluble complexes of P in soil and making them available to plants using various mechanisms [48; 49]. Typically, PSB affect a soil's biological and physicochemical characteristics, particularly through the release of various organic acids that lead to the chelation of mineral ions and decrease the environmental pH, providing soluble forms of P into the soil [5]. Moreover, the secretion of some enzymes (e.g., phytases and phosphatases) by PSB into the soil can lead to the breaking down of complex organic P forms by catalyzing the mineralization process [49]. The application of potassium solubilizing bacteria (KSB) was proposed as one of the sustainable efficient practices in plant production by transforming insoluble K from feldspar and aluminosilicate minerals into available K and improving the K uptake by plants [24; 50].