# Plant-Growth-Promoting Bacteria Mitigating Soil Salinity Stress in Plants
## 5 Role of PGPB in Alleviating Salinity Stress to Plants
### PGPB Support Nutrient Acquisition in Plants
#### 5.2.1 Nitrogen Fixation  
Nitrogen is one of the most important elements for life on Earth, and it is an integral part of proteins, nucleic acids, and chlorophyll. It is a limiting factor in terms of production from agricultural plants. Although 78% of the Earth's atmosphere is nitrogen, it cannot be used by organisms other than microorganisms that bind and molecules that convert it to organic and mineral forms. In salinity-rich environments, access to N is restricted by its availability, but the same process of nitrogen fixation is negatively affected through low access to soil ammonium salts, nitrates, and sulfates. [42; 91]. As N availability is an important obstacle for yield production in salinity, lots of farmers excessively use agrochemicals in their agricultural practices, thus resulting in increased contamination (eutrophication and acidification) and financial costs [92]. The excellent ability of N-fixing bacteria to convert atmospheric nitrogen into easily accessible mineral compounds is a substantial help in the growth of plants and their nutrition under salinity stress.  
The ability to fix soil nitrogen is a vital characteristic of two kinds of microorganisms, symbiotic and free-living, which contribute to the global nitrogen load of about 180 \(\times\) 10\({}^{6}\) t.y\({}^{-1}\), divided into 80%/20% [93]. Organic inputs in the rhizosphere from the roots modify microbial diversity, resulting in an enormous number of free-living N-fixing bacteria (diazotrophs) that colonize plant roots of nonleguminous plants. They are mainly from genera _Azotobacter_, _Azospirillum_, _Pseudomonas_,Agrobacterium, Ervinia, Bacillus, Serratia, Klebsiella, and Burkholderia, among others [94; 95]. Because of elevated energy requirements and low metabolic activity, the intensity of N-fixation in diazotrophs is limited, and their contribution to the nitrogen nutrition of plants can reach 20 kg.ha\({}^{-1}\).y\({}^{-1}\)[96]. Undoubtedly, salinity affects the activity of N\({}_{2}\)-fixing bacteria as some of them are sensitive, but others are tolerant to salt. PGP diazotrophs may ameliorate the osmotic-stress-producing osmolites that permit plants to maintain their metabolisms at acceptable levels [37]. Authors have suggested that halotolerant PGP diazotrophs can be found and easily isolated from the rhizosphere of halophytes [97]; thus they can be useful as biofertilizers in salinity-inhibited crops [54].  
Investigators have reported that salinity inhibits nodule formation in mutualistic legume-rhizobia relationships [98]; thus, the nitrogen nutrition of legumes is negatively affected. Inoculation of PGPB, together with _Rhizobium_, helps in the increase in nodule numbers under salinity in various cases [99; 100; 101; 102]. In addition, the plant secretes flavonoids from the root that provoke the _Rhizobium_ bacteria to secret a Nod-factor, initiating nodule formation [103]. These Nod-factors may also act as stress-response signals of salinity stress in legumes, especially when they are generated by PGPB. In that sense, _Bradyrhizobium japonicum_ significantly improved nodule formation and the biomass of soybean (more than 20%) under salinity stress, and such positive effects become more evident with time [104].