# Plant-Growth-Promoting Bacteria Mitigating Soil Salinity Stress in Plants
## 5 Role of PGPB in Alleviating Salinity Stress to Plants
### Phytohormone Signals
#### 5.1.3. Modulation of Plant Ethylene Levels  
The ethylene is generated endogenously by the plant for the regulation of plant growth (roots, stems, leaves, flowers). It is associated with environmental stress, so when the plant is stressed, it synthesizes ethylene, thus increasing the endogenous concentration of that phytohormone [79]. This response to an environmental stress, including salinity, drought, and toxic elements, decreases the plant's own root elongation, alters the root structure, decreases biomass, and reduces whole plant development [80]. This is done by the activation of plants' precursor of ethylene, called S-adenosyl methionine, which is further converted to 1-amino-cyclopropane-1-carboxylic acid (ACC) by the ACC-synthae pathway, followed by conversion to ethylene by the enzyme ACC-oxidase [81]. In addition, PGPB are able to use the ACC, thus preventing the ethylene synthase. Some PGPB may produce the enzyme ACC-deaminase that splits the ACC molecule into \(\alpha\)-ketobutirate and NH\({}_{4}\)\({}^{+}\), which is the most widely found mechanism among PGPB. Thus, the concentration of endogenous plant ethylene decreases, affecting the expression of the genes encoding the enzymes ACC-synthae and ACC-oxidase [82].  
This trait of PGPB is well known under different stressful conditions, such as heavy metals, drought, and salinity. The expression of ACC-deaminase under salinity results in the induction of plant growth, alleviating the adverse effects of ethylene by the modulation of ACC accumulation and ACC-synthae activities [83]. The _Bacillus_ sp. consortium, presenting halotolerant species and applied in maize rhizosphere, also induced a plant response for protection enzymes, proline, chlorophyll, and soluble sugars, in a salt stress environment (100 mM NaCl).  
Rice is a very important crop all over the world, especially in Asia. In a salinity stress study, _Bacillus amyloidateaciens_ NBRISN13 confers salt tolerance in rice by modulating the response oftranscription in a set of 14 genes [84]. The study was conducted in hydroponic and soil conditions, expressed in the inoculation of a bacterial population in such a salty environment, thus increasing plant growth and tolerance. The authors suggested that this is a stimulation of osmoprotectants, utilizing microbial populations as a mechanism of inducing salt tolerance in rice, and it is possibly due to increased root colonization, ACC-deaminase activity of _B. amyloliquefaciens_, maintained chlorophyll content, and enhanced proline accumulation [85].  
In another project, the effect of PGPB _P. fluorescens_ biotype A, containing ACC-deaminase maize under salt stress, showed more than a 3-fold increase in root length at the EC of 9 dS.m\({}^{-1}\), and an increase in plant height of 2.3-fold after the inoculation of a _P. putida_ biotype A population at the EC of 12 dS.m\({}^{-1}\)[86]. In addition, _P. fluorescens_ and _Enterobacter_ spp. expressing ACC-deaminase increased maize yield under salinity, but also improved NPK nutrition and the K\({}^{+}\)/Na\({}^{+}\) ratio [87]. In a recent study, several _Bacillus_ isolates showed high ACC-deaminase activity, siderophore production, and phosphate solubilization, besides extracellular enzymes such as lipase, amylase, and cellulose, among others [88]. The isolates were able to promote the growth of pepper seedlings, alleviating the harmful effects of salinity and inducing the plants to accumulate proline. Some of the strains also induced plant antioxidant enzymes.