## 4 Plant-Bacteria Interaction Regulates Plant Protein and Metabolites Production
### Bacteria-Mediated Plant Metabolite Biosynthesis for Salt Stress Mitigation (Indirect Mechanism)  
#### 4.2.1 Osmolytes Synthesis in Plant  
Osmolytes are osmoprotectant solutes that improve the cell's ability to retain water without interfering with regular metabolism. The primary function of these organic metabolites is to control osmotic adjustment. These osmotic solutes or osmoprotectants aid plants in withstanding extreme osmotic stress throughout the plant's life cycle [49].  
These substances help to maintain the osmotic differences that exist between the cell's surroundings and the cytoplasm. Osmolytes are neutral substances that protect proteins and other cell membranes from different stress events that affect cellular metabolism. Proline, sucrose, polyols, trehalose, glycine betaine (GB), and alanine betaine are among the metabolites. _Bacillus fortis_ SSB21 inoculation altered proline levels and increased the expression of stress-related genes such as _CAPIP2_, _CaKR1_, _CaOSM1_, and _CAChi2_ in plants [50]. Salt stress-induced proline buildup was observed to be considerably higher in salt challenged (300 mM) _Sulla carnosa_ plants that were inoculated with either _Acinetobacter_ sp. B3, _Pseudomonas putida_ Br18, or _Curbobacterium_ sp. Br20 bioinoculants compared to uninoculated plants [51]. Inoculation of salt-stressed soybean plants with _Bacillus firmus_ SW5 increased proline levels by up to 23.1% [52]. Notably, the proline synthesis gene (_P5 CS1_) expression was dramatically up-regulated in _Enterobacter_ sp. EJ01-infected plants under stressful conditions [53]. Details about bacterial mediated modification of plant stress related genes and their impact on plants are given in Table 1.  
Trehalose, a non-reducing disaccharide, is a key osmoprotectant for osmotic adjustment in stressed plants. It is not only a source of energy during desiccation, but also an effective stabilizer for dehydrated enzymes, proteins, and lipid membranes, as well as other biological structures. The use of trehalose was found to increase the antioxidant enzyme activity in _Arabidopsis_ sp. seedlings grown in 250 mM salt stress [54]. One of our elite strains, _Brevibacterium linents_ RS16, alleviated salt stress (150 mM) by mediating the accumulation of osmolytes such as proline and glycine betaine in rice plant cells [55].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Crop** & **Salinity** & **Microorganism** & **Metabolites/Process** & & & \\
**Level** & **Used** & **Involved** & & & & \\ \hline \multirow{4}{*}{_Solanum_ _lycoopersicum_} & \multirow{4}{*}{0–60 mM} & _Pseudomonas_ & & & & \\  & & _azoclurans_ CHB & & & & \\  & & 1107 & & & & \\ \cline{1-1}  & & _Bacillus subtilis_ & & & & \\ \cline{1-1}  & & IB22 & & & & \\ \cline{1-1}  & & _Nocardiules_ sp. & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Various metabolites produced by plant growth promoting bacteria in increasing the salt stress mitigation and growth of various plants/crops.