# Plant-Growth-Promoting Bacteria Mitigating Soil Salinity Stress in Plants
## 5 Role of PGPB in Alleviating Salinity Stress to Plants
### Phytohormone Signals
#### 5.1.1 Auxins  
The auxin is a phytohormone that affects several plant processes, including root hair and stem cell formation and elongation, and has a momentous function on plant growth under salinity stress [59; 61]. One of the most common and studied auxins is indole-3-acetic acid (IAA), also produced by PGPB, functioning as a signaling molecule. The level of endogenous IAA in roots can be defined as optimal or suboptimal, so PGPB excreting that phytohormone could change the level and, thus, promote or suppress plant growth [62; 63]. IAA is synthesized by different pathways that use tryptophan released from root exudates as a precursor. Most of the PGPB synthesize that phytohormone through the indole-3-pyruvate pathway, while the biosynthesis in plants is inducible [55; 64]. Thus, the ability to generate IAA seems to be a common trait for halotolerant PGPB [59].  
In a study, the halotolerant strain _Kocuria rhizophila_ Y1, isolated from maize rhizosphere and tolerating up to 10% NaCl, presented two growth-promoting traits: phosphate solubilization and IAA production [65]. The inoculation of the strain showed it could protect maize from salt stress by regulating plant hormone (IAA and ABA) levels and improving nutrient acquisition. This resulted in significantly improved growth, photosynthetic capacity, biomass production, antioxidant levels, relative water content, seed germination rate, and chlorophyll accumulation in maize under saline conditions compared to the noninoculated plants. The inoculated maize also showed higher transcript levels of genes encoding antioxidants (ZmGR1 and ZmAPX1) and genes involved in salt tolerance (ZmNHX1, ZmNHX2, ZmNHX3, ZmWRKY58, and ZmDREB2A) than in noninoculated plants.  
In the present review, most of the authors reported IAA production by the beneficial bacteria, tolerant to salt (Table 1). This ability is expressed even in salinity stress conditions, which is a beneficial effect and a valuable result of the environmental adaptation [4; 16; 17; 66; 67].  
In an investigation, _Sulla carnosa_, an important legume for animal feed grown in salt-affected areas, was positively influenced by PGPB _Bacillus subtilis_, with or without coinoculation of the actinomycorrhizal fungus _Rhizophagus intraradices_[16]. The authors suggested that the efficiency of _B. subtilis_ was due to the high IAA production by this strain under salinity stress. An isolate, identified as rhizobacteria _Arachis hypogaea_, was able to stimulate the growth of peanuts by producing IAA and siderophores and reducing the nitrate to nitrite. The authors stated that the strain was also able to adapt to the tested saline environment through phospholipid and fatty acid (to the degree of unsaturation) modifications; that is, the most important mechanism for maintaining the physical properties of the plasma membrane [68].  
Inoculation of a _Bacillus firms_ SW5 population that exhibited the production of IAA and siderophores, and phosphate mobilization increased the growth and biomass yield of salinity-stressed soybean plants, expressed as chlorophyll content, nutrient uptake, osmolytes levels, total phenolic and flavonoid contents, and antioxidant enzyme activities [69]. The antioxidant enzyme-encoding genes and stress-related genes exhibited the highest expression levels in soybean plants inoculated with _B. firmus_ SW5 and treated with 80 mM NaCl. Generally, the authors suggested that _B. firmus_ SW5 bacteria play an important role in the amelioration of adverse effects of salinity on soybean, improving root system architecture, stimulating the antioxidant systems, and expressing stress-responsive genes.