## 4 Plant-Bacteria Interaction Regulates Plant Protein and Metabolites Production
### Bacteria-Mediated Plant Metabolite Biosynthesis for Salt Stress Mitigation (Indirect Mechanism)
#### 4.2.6 Salt Overly Sensitive (SOS) Pathway in Plant  
The SOS signalling system, which includes SOS3, SOS2, and SOS1, has been postulated to mediate cellular signalling under salt stress in order to maintain ion homeostasis. SOS1 is one of many plasma membrane Na\({}^{+}\)/H\({}^{+}\) antiporters involved in salt tolerance, and it is essential for regulating long-distance Na\({}^{+}\) transport from root to shoot. SOS3 encodes a pyridoxal kinase that is involved in the production of pyridoxal-5-phosphate, which is a cofactor for many cellular processes. SOS3 regulates Na\({}^{+}\) and K\({}^{+}\) homeostasis by altering ion transporter activity (Figure 3). It has also been proposed that SOS3 performs a specific role in _Arabidopsis_ root hair development. A study conducted by [PERSON] et al. [63] showed an increase in SOS1 and SOS3 expression levels in roots of wheat grown under salt stress. Similarly, inoculation of _Azorobacter chroococum_ 76A up-regulated the SOS signaling pathway and mitigated salt stress in tomato plants [75].  
4.2.7. Hopanoids: Recent Frontier in Bacteria-Mediated Stress Mitigation  
Hopanoids are a class of sterol-like lipids found in bacteria, archaea, and plants that help to maintain membrane stiffness under a variety of stress circumstances [76; 77]. Bacterial hopanoids have been investigated intensively over the last five decades, and they've lately been discovered to be a key role in bacteria-microbe interactions [76; 77; 78]. Environmental stressors, such as changes in the extracellular milieu's osmolarity, can cause increased hopanoid production, which provides stiffness to the bacterial membrane [79]. In agro-ecosystems, the majority of research has found a link between hopanoid production and nitrogen fixation [77]. _Bradyrhizobium_ sp. is an example of a rhizbial species. When higher hopanoids are generated, nodule-forming bacterial strains symbiotically associated with legumes have been observed to fix increased atmospheric nitrogen [80]. Bacterial strains that synthesize hopanoid include _Anabaena_ sp. _Azotobacter_ sp., _Beijerinkia_ sp., _Burkholderia_ sp., _Franka_ sp., and _Nostoc_ sp. are among the bacteria that have been identified and are capable of fixing nitrogen from the atmosphere [80; 81; 82].  
Recent omics-based research has linked particular hopanoids, such as hpnP and shc, to methanol use in bacterial strains [83]. A particular bacterial genus _Methylobacterium_ spp. are well-known plant-associated bacteria that can colonize the plant endosphere and use methanol as their primary carbon source [84]. _Methylobacterium_ sp. has been shown to improve plant growth and stress tolerance in a variety of plant species under a variety of environmental circumstances [85; 86; 87; 88; 89]. The genome sequencing data of _Methylobacterium oryzae_ CBMB20, a well-known plant growth-promoting endophytic bacteria responsible for reducing salt stress in plants [22; 89], identified two distinct gene sequences, _HpnK_ (WP 003601714.1) and _HpnJ_ (WP 012317976.1). The functions of these newly discovered hopanoid genes in plant-bacteria interactions or stress mitigation mechanisms have yet to be determined. As a result, more research is needed to fully comprehend the role of hopanoids in plant growth promotion and salt stress mitigation, which will open up a new frontier in plant-bacterial interaction.