Throughout limited rainfall or summer season, salinity is on the peak at the surface because a drought-kind situation prevails and water is lost from the system. The coastal region shallow groundwater is saline and because of quick vaporization during the summer times, the salts are deposited on the surface through capillary action. Thus, the salts form a layer on the surface of the soil, increasing the salinity of the horizon and resulting EC peaked in the summer season (0–20 cm) (Tripathi et al. 2006). These continuous processes of ‘salinity increase followed by rapid decrease’ go on cyclically. Thus, stress-tolerant ability of bacterial isolates is of great significance in high salt accumulated soils. It is also reported that calcium phosphates can be formed by phosphate precipitation, which includes rock phosphate (fluorapatite and francolite), which are insoluble in soil. Their solubility is indirectly proportional to soil pH, if solubility increases soil pH decreases. PSMs improve P availability by producing organic acids that drop the soil pH (Satyaprakash et al. 2017).
On the basis of stress-tolerant activity of all isolates, we optimized the environment growth parameters and maintained it throughout the studies for all isolates, i.e., 5% (w/v) supplement of NaCl for SB 2 and SB 3 while 10% (w/v) for SB 1, SB 4 and SB 5, pH 8 and temperature 30 °C. Recently finding showed that, in the alkaline soils of the tropics, concentrations of salt and pH may be as high as 2% and 10.5, respectively, and temperatures may range between 35 and 45 °C (Srinivasan et al. 2012). The interesting spatiality on the growth behavior of SB3 was observed with non-adaptability to withstand salinity (more than 5% NaCl concentration). SB 1, SB 4 and SB 5 actively pumped out salt, keeping the inside of the cell at a normal salt concentration with the increasing salinity and survive over a longer span of duration, while the others gradually stop growing as the limits of their salinity tolerance are reached. Pseudomonas alcaligenes PsA15, Bacillus polymyxaBcP26, and Mycobacterium phlei MbP18, three PGPR isolates were thermo and halotolerant and therefore confer on them prospective economical benefit to survive in arid and salt-affected soils such as calcisol (Egamberdiyeva 2007). Maybe halotolerant microbes isolated in this study adapt the interior protein chemistry of the cell to high salt concentration. High NaCl concentrations normally disrupt membrane transport systems and denature proteins, some microorganisms, possess unusual plasma membranes and many unusual enzymes to survive over a longer duration under such extreme stress conditions. It has been found that the osmo-tolerance reactions of MSP- 393 viz. de novo synthesis of osmolytes and excess production of salt stress proteins efficiently abolished the harmful influences of high osmolarity. Pseudomonas fluorescens MSP-393 can be used as a model bioinoculant for crop production in saline soils (Paul and Nair 2008). More research work is needed to evaluate their efficiency and mechanisms under salt stress conditions. The relative efficiency of plant inoculation was higher under extreme conditions of soil temperature in different trials. The seasonal bound salinity, the temperature in farm plots may be the most possible reason behind that depicts their true applicability of these stressful characteristics of isolates. However, their dominance in soil with season can’t be stated affirmatively. Our results showed that bacteria-isolated from saline soil having a great potential to survive in the stress environment over a longer span of duration.
Among the sequenced isolates, 1 belonged to Firmicutes, 3 were proteobacteria and 1 was actinobacteria. The phylogenetic tree of nucleotide acid sequences of SB 1 showed the grouping of SB 1with Halomonas piezotolerans MN435603, Halomonas aquamarina AJ306888, Halomonas meridiana AJ306891 in a single cluster at the genus level. SB 2 revealed grouping with Serratia rubidaea AB004751 while SB 3 displayed the grouping with Serratia ureilytica AJ854062 and Serratia marcescens JMPQ01000005 in a single cluster at the species level. Nucleotide analysis of SB 4 represents cluster together with Glutamicibacter mishrai KT444697, a member of Arthrobacter, and formed a single cluster pattern at the genus level. Nucleotide acid sequence-based phylogenetic tree analysis showed a single cluster pattern for SB 5, Halobacillus alkaliphlius AM295006 and Halobacillus halophilus HE717023 at the genus level. On the basis of phylogenetic analysis, Serratia sp. was found to have similarity at species level compared to the Halomonas sp., Arthrobacter sp. and Hallobacillus sp. (Fig. 2).
All the isolates were demonstrated maximum P solubilization within 4 days of incubation in the range of 50.674 to 116.669 P2O5 (ppm). A drastic drop in pH was also noticed throughout the experiment. These results are in agreement with previously reported results (Son et al. 2006), which identified bacteria capable of solubilizing P under up to 50 g l−1. The reducing phosphate solubilization with improvement in the incubation time has been stated for few microorganisms, which could be attributed to the depletion of nutrients in the culture medium (Vyas et al. 2007). The P concentration in the culture broth as a signal of phosphate solubilizing ability should be carefully noticed, and a kinetic study of this parameter would suggest a more trustworthy picture of cellular behavior toward P. Soil phosphates mostly the apatites and metabolites of phosphatic fertilizers are fixed as calcium phosphate under alkaline condition. P solubilization by the response of microorganisms is the outcome of the combined effect of pH decline and organic acid production. The drop in pH clearly suggests acid production, which could be a possible reason for P solubilization. It is suggested that microorganisms, which decrease the medium pH during growth, hold a great promise to convert insoluble P into soluble (Selvi et al. 2017; Park et al. 2011). The mineralization of organic P in soil is the result of organic acid production and acid phosphatases. Production of low molecular weight organic acids results in acidification of the microbial cell and its surrounding. Consequently, may be proton substitution for Ca2+ is important to released Pi from a mineral phosphate. The excretion of H+ to the external surface in interchange for cation uptake or with the support of H+ translocation ATPase could form different ways for solubilization of mineral phosphates (Vessey 2003).
This is already reported that member of genus Arthrobacter and Serratia are good solubilizers of TCP. They solubilized 519.7 and 421.8 g l−1 of insoluble TCP, respectively. Hence, the presence of these members in the rhizosphere might be helpful to plant P nutrition and development (Chen et al. 2006). On the basis of phenotypic and molecular characterization, members of the genus Halomonas and Halobacillus are being reported first time as effective phosphate solubilizers from saline soil. The bacterial isolates were isolated from alkali soil, which was alkali and had high salinity levels. The soil was also subject to desiccation under periodic spells of drought.
These phosphate-solubilization microbial isolates from saline soils were capable of solubilizing a considerable amount of Pi and are likely to be more successful as microbial-based inoculants than the microorganisms isolates from other soils because of their capability to survive the stress factor that affects crops productivity. These phosphate solubilizing soil bacteria could use as effective bio-fertilizer applicants for improving the phosphate nutrition of crop plants helps to minimize the phosphate fertilizer application. The pH of the saline soil in CSSRI varies with seasonal bound alkalinity; such alkaline soils are rich in calcium phosphate. In another way, we can say that in alkaline soils, the activity of calcium is high and thus favors the formation of insoluble TCP. These alkaline soils containing free calcium carbonate phosphate ions coming with solid-phase CaCO3 are precipitated on the surface of these particles. Therefore, widespread studies to analyze the effect of single and dual inoculations of saline-alkaline soil-based PSB species on yields of different crops are required immediately.
Regarding this characteristic of phosphate solubilizing capacity of the isolates in the bulk, soils have been studied, all the cultures had shown remarkably high ability to solubilize insoluble-phosphate, a character quite interesting and important from ‘application in agriculture’ point of view. These, after their field evaluation in saline soil, if found suitable, can be commercially used to substitute the application of costly ‘Phosphate fertilizer’ like Single Super Phosphate by much lesser costly and rather cheaply available ‘Rock-Phosphate’ and dolomite. Further studies on these kinds of stress-tolerant cultures are to be very interesting and important.
Stress-tolerant PSB isolates were capable to produce a remarkable amount of IAA with and without L-tryptophan. SB 3 showed the highest value of IAA production (0.183 g l−1) followed by SB 4 (0.178 g l−1) and SB 5 (0.151 g l−1) after 4 days of inoculation in the presence of 100 mg l−1 tryptophan, while 0.062, 0.057 and 0.054 g l−1 IAA production were observed without L- tryptophan, respectively (Fig. 4). In a previous study, it is conveyed that the range of IAA productions in several PSB isolates was 57–288 μg/ml culture media (Shahab et al. 2009). Some of the PSM act as plant growth promoters due to their ability to produce IAA but there is a different IAA production potential among PSM (Souchie et al. 2007).
PSM isolates such as Bacillus polymyxa, Bacillus pulvifaciens, Pseudomonas striata, Aspergillus awamori, Aspergillus niger and Penicillium digitatum tested for the synthesis of auxin and gibberellins. Maximum auxin produced by P.striata and the least auxin activity was recorded by A.awamori (Vassilev et al. 2006). Production of IAA by isolated PSB ranged between 0.054 and 0.183 g l−1 indicating that the isolated PSB has a plant growth-promoting effect. The IAA production capability of PSB is remarkable in the application of the isolated PSB to phytostabilization (Park et al. 2011). The reactions most frequently invoked to describe the direct effects of plant growth-promoting bacteria on plants is the production of phytohormones, including auxins such as indole acetic acid or IAA (Patten and Glick 2002; Haque et al. 2020). B. amyloliquefaciens, a gram-positive bacterium is capable to produce and secrete a significant amount of IAA. Improved IAA production after supplementing of L-tryptophan and drastic decline of IAA production in engineered trp mutants indicate that the main pathway of IAA biosynthesis in this bacterium is dependent on L-tryptophan (Idris et al. 2007). To demonstrate that IAA synthesis in bacteria is dependent on tryptophan concentration, a mutant of the gram-negative plant-beneficial bacterium Pseudomonas putida was used, deficient in ipdc gene product (indole-3-pyruvate decarboxylase) (Patten and Glick 2002). IAA production by plant growth-promoting microbes is an important feature in the development of plant growth. There exist sufficient research studies that several soil microorganisms are keenly involved in the auxins synthesis. IAA was identified in around 80% of bacterial isolated of the rhizosphere.
In most cases, the production of IAA has been analyzed in saline-free conditions (Barua et al. 2012). This is the first evidence that PSM augments plant growth due to the biosynthesis of growth-promoting substances. As per our best knowledge, this is the first time report that saline soil-associated isolates could potentially promote plant growth under saline conditions because they are capable to produce plant growth promoters under such stress conditions. These isolates are very useful to saline soil agriculture point of view which enhances crop productivity.
Interaction studies on PGPR and other microbial community and their effect on the biochemical function of crop plants in the presence of soil salinity regimes are still in an incipient stage. Potent PGPR and other microbe’s inoculations might work as the prospective mechanism for reducing the stress of salinity in salt-sensitive crops. Therefore, broad-spectrum investigations are required in this field, and the usage of PGPR and other symbiotic microorganisms can be beneficial in developing strategies to enable sustainable agriculture in saline soils.