Management of Phosphorus in Salinity-Stressed Agriculture for Sustainable Crop Production by Salt-Tolerant Phosphate-Solubilizing Bacteria—A Review

: Among the environmental factors, soil salinity is one of the most detrimental factors affecting plant growth and productivity. Nutritional-imbalance is also known as one of the negative effects of salinity on plant growth and productivity. Among the essential plant nutrients, phosphorus (P) is a nutrient in which the uptake, transport, and distribution in plant is adversely affected by salinity-stress. Salinity-stress-mediated low a P availability limits the crop production. Adding additional P fertilizer is generally recommended to manage P deﬁcit in saline-soils; however, the low-efﬁciency of available P fertilizer use in salt-affected soils, restricts P availability, and P fertilizers are also a cause of signiﬁcant environmental concerns. The application of salinity-tolerant phosphate– solubilizing-bacteria (ST-PSB) can be as a greatly effective and economical way to improve the P availability, and recover the P-deﬁcit in saline-land. This review focuses on soil salinization and its effect on P availability, the mechanisms of P solubilization by ST-PSB, ST-PSB diversity, their role in alleviating salinity stress in plants, the current and future scenarios of their use, and the potential application of this knowledge to manage the sustainable environmental system. According to this review, adding ST-PSB to saline soils could be an alternative for alleviating the negative effects of salinity on plants and may ameliorate salinity tolerance. uptake in root, increasing water intake, soil water holding capacity and increase root adhering-soil (RAS); Volatile organic compounds (VOCs) can induce the high-afﬁnity K + transporter (HKT1) in shoots and decrease of HKT1 in roots, restricting Na + entry into roots and promoting shoot-to-root Na + recirculation; Enhance nutrient uptake of N, P, K and Fe through nitrogen ﬁxation, P and K solubilization, and siderophore production). ( II ) Hormonal modulation in plant (e.g., decrease the excessive ethylene level by ACC deaminase; regulation of ABA through cytokinin; and overall growth facilitated by IAA and GA) ( III ) Mitigation of osmotic and oxidative stress in plant (e.g., induction of synthesizing plant antioxidative enzymes ( POD , SOD , CAT , APX , and GR ) to scavenge ROS; and increasing osmolyte accumulation (Proline, glycine betaine, soluble sugar and choline, etc.) to maintain osmotic balance into plant cells) ( IV ) Reduced Plant Susceptibility to Pathogens (e.g., production of HCN, siderophores, salicylic acid, jasmonic acid, and lytic enzymes to control phytopathogen infections).


Introduction
Human population overshoot is profoundly connected with resource depletion, biodiversity loss, ecosystem collapses, climatic change, and an increase in poverty and hunger [1,2]. This overpopulation potentially jeopardizes both ecologists and agronomists, where the dilemma remains the question of maintaining the ecosystem as well as feeding large populations. To ensure global food demand, improving crop yield and sustaining soil fertility (mainly in terms of improving nitrogen (N), phosphorus (P), and potassium (K) availability to plants), is required in the worldwide agriculture sector. Soil fertility Salinization occurs in soil by the presence of salt, composed with various soluble ions (e.g., cations: Na+, K + , Ca 2+ , and Mg 2+ and anions: Cl − , NO 3 − and HCO 3 − ) [46]. Soil salinity is categorized into three groups based on electrical conductivity (ECe), exchangeable sodium percentage (ESP), and pH. Accordingly, a soil with pH < 8.5, ECe > 4 dS/m and ESP < 15 is a saline soil, a soil with pH > 8.5, ECe > 4 dS/m, and ESP > 15, is a saline-sodic soil, and a soil with pH > 8.5, ECe < 4 dS/m, and ESP > 15 is a sodic soil [47]. Soilsalinization is a growing environmental problem over time that can have a natural origin (primary salinization) like the weathering of parent materials, fossil salts, rising sea levels, rainfall, etc., and be caused by from anthropogenic activities (secondary salinization), such as poor irrigation, inadequate drainage, over-extraction of groundwater, inappropriate cropping patterns, rotations, overuse of agrochemicals and the use of waste effluents, etc. [48][49][50].
Salinization induces a nutritional disorder in salt-tolerant plants through changing nutrient availability, competitive absorption, transport, or partitioning within the plant [51]. The interaction of salinity and primary mineral nutrition is a very complex process that adversely affects N (e.g., a decrease in N availability and a disturbance in nodule formation), P (e.g., a decrease in availability and uptake of P by the plant), and K (e.g., an inhibition of transportation and decrease of K uptake by the plant) in soil [25].

Salinity Effect on P Availability in Soil
The phosphorus cycle is considered mainly as a "sedimentary cycle", because the interchange involved is only between water and soil; limited to the atmosphere [32]. P is a significantly dynamic and highly reactive element in soil that persists in three forms depending on its solubility and chemical nature, such as (i) insoluble inorganic phosphate (Pi); (ii) soluble orthophosphate (bioavailable phosphate); and (iii) insoluble organic phosphate (Po). Generally, insoluble Pi exists in the form of more stable primary P minerals (apatites, strengite, and variscite), secondary P minerals (e.g., Ca-P, Fe-P, and Al-P), and sorbed P (e.g., in clay and in Fe and Al oxides) in soil [52]. The insoluble Po presents approximately 30% to 65% of total P, where it remains in different forms such as inositol phosphate (soil phytate: 50% of total organic P), phosphomonoesters, phosphodiesters including phospholipids, nucleic acids and phosphotriesters [53,54]. Phosphorus transformation/dynamicity occurs from one P pool to another through natural processes (physical and chemically) via weathering, desorption-adsorption, dissolution-precipitation, immobilization; and by microbial activity such as solubilization (insoluble Pi) and mineralization (insoluble Po) ( Figure 1). and chemically) via weathering, desorption-adsorption, dissolution-precipitation, immobilization; and by microbial activity such as solubilization (insoluble Pi) and mineralization (insoluble Po) ( Figure 1). The orthophosphate in soil is mostly valuable due to its soluble nature, and it exists in three forms viz; mono-hydrogen phosphate (HPO4 2-), di-hydrogen phosphate (H2PO4 -), and tri-hydrogen phosphate (H3PO4) depending upon the pH condition [54]. The plant cell might consume soluble P in the form of HPO4 2-or H2PO4 - [31]. The orthophosphate in soil is mostly valuable due to its soluble nature, and it exists in three forms viz; mono-hydrogen phosphate (HPO 4 2− ), di-hydrogen phosphate (H 2 PO 4 − ), and tri-hydrogen phosphate (H 3 PO 4 ) depending upon the pH condition [54]. The plant cell might consume soluble P in the form of HPO 4 2− or H 2 PO 4 − [31]. Phosphorus availability and its uptake by plants in saline environments is affected by several factors in soil-plant environments. A High ionic strength, due to the electrolyte concentration in saline soil, can lead to enhanced P adsorption and low solubility of P; however, it varies with the different kinds of soil [26]. In alkaline soils, with a high concentration of electrolytes (e.g., Na and Ca), P adsorption increases through Na-P and Ca-P precipitation. In acidic soil, the formation of Al-P increases with the elevated concentration of salt [55,56]. Nasrin et al. [57] showed that the soil-salinity reduces the transformation of secondary P minerals to bioavailable P. In addition, the excessive salinity strongly effects on indigenous microbes and suppresses their activity, diversity and community dissimilarities [58], where the responsible P mobilizing microbes can be affected by salinity. Thus, the P availability for plant largely differs between saline and non-saline soil.

Effect of Salinity on P Uptake and Accumulation in the Plant
In a saline environment, the interaction and uptake of phosphorus in plant nutrition is very complex and contradictory. Champagnol, [59] showed that P addition to saline soils improved crop growth and yield in 34 of 37 crops; however, it did not affect, or increase crop salt tolerance as salinity increased from a low level to moderate and high levels, respectively. On the other hand, most of the investigations have suggested that salinity decreases the P accumulation of plant tissue, though a few results also showed either an increase in P uptake or no effect on P uptake by plants [26,60]. These contradictory results indicate to two statements: (i) the supply of P impacts plants' salt tolerance in either a positive or negative manner, and (ii) salt stress either increases, or decreases, or does not affect the P accumulation in plants. Moreover, there is some evidence that the P-deficient plants are more salt tolerant than P-sufficient plants in maize [61], and soybean [62]. However, the effect of salt stress on P accumulation and its toxicity varies depending on the plant environmental conditions, crop species, and genotype within the species, physiological developmental stages, and external salinity concentration [51].

Plant Response to Salinity and P Deficiency
The individual impact of P deficiency and salinity stress on plant development as well as physiological (metabolic and biochemical) processes has been widely studied in several species, but the study of their combined effects is limited [23]. Phosphorus is an important hetero-element found as a major component in photosynthesis and respiration related compounds (ATP, NADPH, and sugar phosphates), macromolecules (proteins, nucleic acids, and phospholipid), secondary compounds (e.g., vitamins), and it also plays a major role in nitrogen compound metabolism, signal transduction, carbohydrate transportation, carbohydrate and lipid metabolism [63,64]. Salt-induced oxidative and osmotic stress along with P deficiency, have a strong and significant influence on photosynthesis in several ways. They decrease the CO 2 availability and assimilation, inhibit RuBisco (ribulose-1, 5-bisphosphate carboxylase/oxygenase) and ATPase activity; and subsequently, reduce electron transportation, resulting in higher ROS production [19,65,66]. ROS, such as hydrogen peroxide (H 2 O 2 ), singlet oxygen ( 1 O 2 ), superoxide anion (O 2 .− ), and hydroxyl radicals (HO − ) are cytotoxic, damaging cellular organelles like DNA, proteins, lipids, and carbohydrates through an imbalance in their neutralizing system [67]. Zribi et al. [23] have shown that the combined effect of salinity and P deficiency on the Catapodium rigidum (L.) plant inhibited the photosynthetic activity through decreasing stomatal conductance, chlorophyll content, and CO 2 assimilation.
Plant evolved several adaptive responses to cope with salinity and P deficiency stress in soil. In a P deficit condition, the plant undergoes several modifications, such as physiological (changes in roots architecture like substantial production of cluster roots, lateral roots and root hair density), biochemical (secrete large amounts of organic acid, H + and phosphatase), molecular (regulation of phosphorus starvation genes like OsPTF1, AtPHR1, etc.) and microbial colonization, that lead to the solubilization and acquisition of unavailable P in soil [68][69][70]. In a saline environment, for survival, the plant develops several physiological and biochemical mechanisms, such as ionic homeostasis (maintaining Na + transportation and its compartmentalization through the regulation of transporters like the Na + /K + antiporter, H + -ATPase), biosynthesis of osmoprotectants (proline, glycine betaine, amino acids, sugars, polyols), the activation of antioxidant enzymes (e.g., catalase, superoxide dismutase, glutathione, ascorbate peroxidase), the production of antioxidant compounds (flavonoids, ascorbate, tocopherol, reduced glutathione), hormonal modulation Agronomy 2021, 11, 1552 6 of 27 (e.g., IAA, ABA, ethylene, jasmonic acid, salicylic acid), the generation of nitric oxide (NO), and microbial association [22,71,72]. However, it is usually noticed that the rhizospheric microbial community has significantly coevolved with plants in their adaptive response against both stress conditions.

Diversity of Salt-Tolerant Phosphate Solubilizing Bacteria
The rhizosphere is the greatest ecological niche of versatile microorganisms, belonging to archaea, bacteria, fungi, and even algae; some of them have the inherent capability to adapt towards a wide range of salt concentrations and stimulate plant growth through its metabolic activity and interaction. Halotolerant microbes employ two main strategies for their growth and survival under salt stress as (i) avoiding the high salt concentration in cytoplasm through the reducing uptake of salt ion (due to specialized cell membrane and wall composition), regulation of intracellular ion concentration by pumping out the ions from the cell (e.g., Na + /K + antiporters, K + /Na + ion transporter), and producing EPS that facilitate to development biofilm; and (ii) intracellular adaptation through the accumulation of osmolytes (e.g., sucrose, trehalose, and glycine betaine), protein, enzymatic adaptations, and increment energetic capacity [73,74]. Interestingly, fungi show a tendency to be more salt sensitive than bacteria, therefore bacteria/fungi ratio can be raised in saline soil [75]. In salt tolerant microbial community, ST-PSB are considered as one of the crucial PGPR which have the ability to solubilize P and K, which help for plant growth including N, as well as produce various plant growth promoting metabolites (e.g., phytohormone, siderophore, ACC deaminase, and anti-phytopathogens) under saline condition [38,41,44,45]. The diverse range of ST-PSB strains belonging to various genera (e.g., Pantoea, Burkholderia, Aerococcus, Pseudomonas , Bacillus, Ensifer, Gordonia, Acinetobacter, Arthrobacter, Providencia, Serratia, Alcaligenes, Cobetia, Microbacterium, Agrobacterium, Acromobacter, Tetrathiobacter, Aphanothece) tolerate the elevated concentration of salinity [76][77][78][79][80][81][82][83][84][85][86][87]. Zhu et al. [5] reported Kushneria sp. YCWA18, a high phosphorus-solubilizing ST-PSB isolated from the sediment of Daqiao saltern on China's eastern coast, was able to grow on a solid medium containing 20% (w/v) NaCl. An ecological survey was conducted by Johri et al. [88] and isolated 857 ST-PSBs from the root-free soil, rhizosphere, and rhizoplane of Prosopis juliflora growing in alkaline soils; and showed that PSB was highest in rhizoplane site. Several distinct ST-PSB are isolated from different rhizosphere and endophyte of the plants such as wheat [89], soyabean [76], Tamarix ramosissima [90] and even wide range of salt effected areas [78,91].

Mechanism of P Mobilization by Salt-Tolerant Phosphate Solubilizing Bacteria
Phosphate-solubilizing bacteria a number of efficient P solubilization and mineralization processes to transform from unavailable P sources to bioavailable P in soil. (Figure 2). The mobilization mechanism of P is classified into two categories based on unavailable forms of P compounds in soil: (i) inorganic phosphate (Pi) solubilization through acidification, protonation, chelation and EPS, and (ii) organic phosphate (Po) mineralization through enzymatic activity such as phosphatase, phytase, phosphonatases, and C-P lyases.

P-Solubilization through Acidification
The Pi is solubilized following the secretion of both the organic and inorganic acids by PSB. In comparison, the organic acids play a pivotal role in the solubilization process due to a greater efficacy and contribution as compared to inorganic acids [54]. In the solubilization process, the potentiality of organic acids with Pi is influenced by different factors such as (i) reducing the pH; (ii) enhancing the chelation of P-bonded cations; (iii) competing with P for adsorption sites; and (iv) forming soluble complexed ions (Ca, Al, and Fe) linked with insoluble P and thus releasing the soluble P [92]. Generally, lowering the pH in a medium is accountable to produce several organic acids by ST-PSB, which are shown in Table 1. Precisely, gluconic acid is a crucial acid in P solubilization, and it is derived from the extracellular oxidation of glucose in the periplasmic space, catalyzed by quinoprotein glucose dehydrogenase [98,99]. All the organic acids are produced in the periplasmic space by the microbial metabolism, oxidative respiration, and fermentation of different organic sources [100][101][102]. In general, the produced organic acid further reacts with insoluble Pi to generate the metal-based organic acids and release bioavailable phosphate ions ( Figure 2). In the case of the inorganic acid mechanism, Pi solubilization occurs through the production of inorganic acids by PSB, such as sulfuric acid (H 2 SO 4 ), nitric acid (HNO 3 ) and carbonic acid (H 2 CO 3 ) [54]. Basically, the inorganic acids react with Pi to release the phosphate ion and metal-based anionic compound ( Figure 2). through enzymatic activity such as phosphatase, phytase, phosphonatases, and C-P lyases.

P-Solubilization through Acidification
The Pi is solubilized following the secretion of both the organic and inorganic acids by PSB. In comparison, the organic acids play a pivotal role in the solubilization process due to a greater efficacy and contribution as compared to inorganic acids [54]. In the solubilization process, the potentiality of organic acids with Pi is influenced by different factors such as (i) reducing the pH; (ii) enhancing the chelation of P-bonded cations; (iii) competing with P for adsorption sites; and (iv) forming soluble complexed ions (Ca, Al, and Fe) linked with insoluble P and thus releasing the soluble P [92]. Generally, lowering the pH in a medium is accountable to produce several organic acids by ST-PSB, which are shown in Table 1. Table 1. List of ST-PSB and their possible mechanism for inorganic and organic phosphate solubilization and mineralization.

P-Solubilization through Protonation
The solubilization of Pi can occur through the protonation without the production of organic acid such as the excretion of protons following NH 4 + assimilation and respiratory H 2 CO 3 production by PSB [103]. In a previous study, P-solubilization by organic acids in a culture solution of Pseudomonas sp. and Penicillium aurantiogriseum was not reported while P solubilization was occurred [103]. The mineral phosphate generally reacts with the proton and produces bioavailable phosphate ions with free metal ions ( Figure 2). ST-PSB possesses a proton extrusion property through the respiration and NH 4 + assimilation; although phosphate solubilization by protons is not well known [104].

P-Solubilization through Chelation
The chelation process occurs through the combination of two or more different coordinate bonds between ligands (ions and molecules) and metal ions [105]. Organic acids (with hydroxyl or carboxyl groups), and siderophores are produced by PSB, which further bind with different cationic metals of insoluble Pi to generate the bioavailable phosphate and metal-chelating complex ( Figure 2). Interestingly, some of the PSB produced strong chelating organic acids (e.g., humic acid, gluconic acid, fulvic acid, and 2-keto gluconic acid) and were reported as highly effective in the solubilization of mineral Pi [106]. However, Park et al. [107] suggested that both protonation and chelation of gluconic acid may involve insoluble P solubilization by ST-PSB (e.g., Pseudomonas fluorescence RAF15).
Under restricted iron availability conditions, bacteria generally secrete low molecular weight and high affinity iron chelating compound, called siderophore, which further form as iron-siderophore complexes and fulfil iron demand through the uptake of it [108,109]. Siderophores increase the availability of iron through the solubilization process from precipitated iron sources in soil [110]. Iron availability in saline and sodic soil is very low due to its high precipitation with anions, where the iron phosphate is one kind of P precipitate form in soil [35,111]. Iron phosphate might be solubilized via siderophores through the chelation of iron ( Figure 2). However, the direct relationship between siderophore production and phosphate solubilization by PSB is still unknown. The role of siderophores in Pi solubilization needs to be further investigated in the future.

Solubilization by Extracellular Polymeric Substances
Microorganisms largely produce EPS (e.g., polysaccharides, proteins, and nucleic acids), which accumulates outside the microbial cells; and provides a suitable environment for nutrient entrapment, chemical reactions, and protection against environmental stresses such as salinity and drought [112,113]. Moreover, microbial EPS have the ability to bind (bioadsorption) the metals, and influence the solubilization of metal phosphate, which is present in an unavailable form in soil [114]. The affinity of metal bioadsorption mediated microbial EPS depends on abundant anionic functional groups (e.g., carboxyl and hydroxyl) [113]. The EPS produced by ST-PSB contain various anionic groups (e.g., hydroxyl, carboxyl, and amino groups), which help to bind with Na + for salt alleviation under the saline conditions [115]. Upadhyay et al. [116] reported that the EPS-producing bacterial strains exhibited the P solubilization efficiency under saline conditions. However, the correlation between the EPS and P solubilization could not be evaluated. Yi et al. [117] results showed that bacterial strains of Enterobacter sp. (EnHy-401), Arthrobacter sp. (ArHy-505), Azotobacter sp. (AzHy-510), and Enterobacter sp. (EnHy-402) could solubilize the tricalcium phosphate (TCP), where the solubilization efficiency of EnHy-401 was lower than others due to a lack of EPS production. Furthermore, this investigation also revealed that P is released during the synergetic effect of EPS and organic acid. However, EPS failed to release P, without organic acid. According to these studies, it is concluded that EPS may play an important and major role for the enhancement of phosphate solubilization; although it doesn't have a direct effect on inorganic P solubilization ( Figure 2). Therefore, the extensive and consistent research is required in the future to understand the exact role of EPS in inorganic P solubilization.

Phosphatase Activity
Phosphatase (organo-phosphoester scavenger) cleaves (dephosphorylation) the ester bond between the phosphate group and organic residue of organic phosphates, which provides P to the cell [118] (Figure 2). The release of orthophosphate from the organophosphate complex was reported in numerous ST-PSB producing the phosphatase enzyme (Table 1). According to the optimum pH, the phosphatase enzyme is classified into two groups: (i) alkaline phosphatase (maximum activity at an alkaline pH > 7) and (ii) acidic phosphatase (maximum activity at pH < 6), where both are produced by PSB depending upon external conditions [118,119].
Alkaline phosphatase consists of a homodimeric protein (metalloenzyme) and each catalytic site of the monomer contains three metal ions with five cysteine residues that are crucial to its catalytic function in optimally active alkaline pH conditions [120,121]. Prokaryotic alkaline phosphatases belong to three large families, i.e., PhoA, PhoD and PhoX, which are differentiated according to their structure, substrate specificity and activated metal ions [87]. Several alkaline phosphatase genes have been cloned and expressed from various bacterial species. The alkaline phosphatase gene (phoD) was cloned from a halotolerant cyanobacterium Aphanothece halophytica, where the expression of PhoDAp enzyme exhibited alkaline phosphatase (APase) and phosphodiesterase (APDase) activities through the activation of Ca 2+ ions [87]. Interestingly, phoD gene expression was upregulated not only by P starvation but also under salt stress conditions. Noskova et al. [83] determined a novel extracellular alkaline phosphatase/phosphodiesterase in the PhoD family, which was encoded by the genome sequence of the marine bacterium Cobetia amphilecti KMM 296 (CamPhoD), and expressed in Escherichia coli cells. The salt tolerant bimetal-based enzyme CamPhoD catalyzes the breakage of phosphate mono-and di-ester bonds in nucleotides.
Acidic phosphatase enzymes are considered to be nonspecific acid phosphatases (NSAPs). The term was originally adopted to indicate bacterial enzymes, which show optimal catalytic activity at acidic to neutral pH levels and do not exhibit marked substrate specificity [122]. NSAPs are made up of three molecular families, which belong to molecular class A, B and C [123]. From a metagenomics study, the distribution of NSAPs genes is most abundant in marine than terrestrial ecosystems, where the class C, NSAPs genes are more than other two classes A and B [124]. In the last few decades, the isolation, identification and potential biotechnological application of acid phosphatase NSAP genes gradually increased due to NSAPs gene transfer to PGPB, and were acquired to develop the phosphate-solubilizing bacterial strains using recombinant DNA technology [125].

Phytase Activity
Phytase is a subgroup of phosphatase, which has the ability to dephosphorylate of phytate in a stepwise manner to produce inositol and bioavailable phosphorus [126] (Figure 2). Phytate is the principal source of inositol and a major stored form of P contained in plant tissue, grains, seeds, and pollen [127]. A number of phytase producing ST-PSBs are shown in Table 1. Phytate is an abundant form of the soil organic phosphate pool, where the plants have the limited capacity to directly acquire the bioavailable P from phytate, because phytase constitutes less than 0.8% of the total acid phosphomonoesterase activity of root extracts; and it was not detectable as an extracellular enzyme [128]. Therefore, the transgenic plants (e.g., Arabidopsis thaliana and Trifolium subterraneum L.) were generated by genetic transformation of the phytase (phyA) gene, which was derived from Aspergillus niger. Both transgenic plants exhibit the extracellular phytase activity and a significant improvement of available P nutrition for their growth [128,129]. The highly thermostable and extremely salt tolerant phytase gene (PHY US573) was identified from Bacillus amyloliquefaciens US573, which showed phosphate solubilization activity [95].

Phosphonatases and C-P Lyase Activity
The cleavage of C-P bonds in organophosphate molecules is catalyzed through C-P lyases and phosphonatase enzymes, which are produced by a wide range of bacteria [125,130] (Figure 2). In a metagenomics study, the bacterial genome sequences revealed 44 strains out of 190 marine bacteria, which contain the phnJ gene which encodes a catalytic component of the C-P lyase enzyme complex [97].

The Role of ST-PSB in Plant Growth Promotion under Salinity
Plants have evolved complex mechanisms to deal with salinity stress where environmental adaptations and genetic traits modulate salinity tolerance in plants (halophytes and glycophytes), however, the transmission of knowledge acquired for crop improvement remains challenging [131]. The signaling networks of plant-microbe interactions that contribute to salt tolerance have been illuminated by recent developments in molecular investigations.

Ionic Homeostasis and Nutritional Balance
Ionic stress, which is caused by a high concentration of ions like Na + and Cl − , decreases the uptake of other mineral nutrients like Ca 2+ and K + , causing serious physiological and metabolic problems to plants [132,133]. The weakly voltage-dependent nonselective cation channels (NSCCs) and the high-affinity K + channel (HKT1) transport Na + across the plasma membrane into the cell, and as a result, K + /Na + ratios are reduced in the plant under high salinity conditions [134]. HKT1, which is a key factor of plant salinity tolerance in response to salt stress, can increase salt tolerance by decreasing Na + accumulation in shoot tissue, which secures leaves from Na + toxicity [134]. ST-PSB have the potential to boost K + ion absorption in plants under saline conditions, by activating the high-affinity K + transporter (HKT1), which can directly increase the greater K + /Na + ratio that favors salt tolerance [135,136]. Furthermore, ST-PSB produce EPS and VOCs (volatile organic compounds), which are considered as protectors against ion stress under saline conditions. VOCs can reduce Na + accumulation by downregulating HKT1 (K + transporter) expression in roots while upregulating it in shoots and facilitating shoot-to-root Na + recirculation [137]. The HKT1/K + transporter, which inhibits Na + inflow under salt stress in A. thaliana by downregulating HKT1 gene expression to improve salt tolerance, is also involved in VOC formation [138]. For salt stress tolerance in plants, EPS functions as a physical barrier surrounding the root where it binds the Na + and restricts Na + influx into roots, as a result, reducing the ion toxicity to the plant [74]. Mukherjee et al. [139] have proven that the extracted EPS (from Halomonas sp. Exo1) and sequestrated Na + ion in vitro; and effect on growth promotion in salt-tolerant rice seedlings. Moreover, EPS is also supposed to play a role in stabilization of biofilm formation, soil aggregation and increasing root adhering of soil (RAS) by forming a sheath around the roots of plants, which increase the water availability and nutrient (N, P, K, and Fe) acquisition from the soil [74,140]. The ability of ST-PSB to produce EPS is closely linked to the plant's ability to survive in high salt concentrations ( Figure 3).  The weakly voltage-dependent nonselective cation channels (NSCCs) and the high-affinity K + (HKT) transport sodium through the plasma membrane, allowing salt (Na + ) to enter the root. Although both of these channels can control Na + and K + influx into plant cells and some channels are more selective for K + than others. HKT1, which is a critical predictor of plant salinity tolerance in response to salt stress, can improve salt tolerance by decreasing Na + build up in shoot tissue, which protects leaves from Na + toxicity. Salt tolerant phosphate solubilizing bacteria can also help enhance the tolerance of the plant to salinity by (a) ion homeostasis in the plant (e.g., by increasing K + ion uptake via mediating the expression of an ion high-affinity K + transporter (HKT1) and VOC in plant); (b) production of exopolysaccharides (e.g., through increasing water intake and soil water holding g., production of EPS bind with Na + and decreasing Na + uptake in root, increasing water intake, soil water holding capacity and increase root adhering-soil (RAS); Volatile organic compounds (VOCs) can induce the high-affinity K + transporter (HKT1) in shoots and decrease of HKT1 in roots, restricting Na + entry into roots and promoting shoot-to-root Na + recirculation; Enhance nutrient uptake of N, P, K and Fe through nitrogen fixation, P and K solubilization, and siderophore production). (II) Hormonal modulation in plant (e.g., decrease the excessive ethylene level by ACC deaminase; regulation of ABA through cytokinin; and overall growth facilitated by IAA and GA) (III) Mitigation of osmotic and oxidative stress in plant (e.g., induction of synthesizing plant antioxidative enzymes (POD, SOD, CAT, APX, and GR) to scavenge ROS; and increasing osmolyte accumulation (Proline, glycine betaine, soluble sugar and choline, etc.) to maintain osmotic balance into plant cells) (IV) Reduced Plant Susceptibility to Pathogens (e.g., production of HCN, siderophores, salicylic acid, jasmonic acid, and lytic enzymes to control phytopathogen infections).
As mentioned above, salinity moderates the uptake/accumulation of plant nutrients including N, P, and K, which directly affect plant growth. In addition to phosphate solubilization, many strains of ST-PSB are able to supply N, K, and Fe to plant through nitrogen fixation, potassium solubilization and siderophore production; and as a result, could increase the concentration of N, K, and Fe in plant ( Table 2).

Mitigation of Osmotic and Oxidative Stress
Salinity stress leads to the formation of ROS in the plants grown under saline conditions. Plants have developed an enzymatic antioxidant defense system of the ascorbateglutathione cycle and antioxidant molecules (carotenoids, flavonoids, and other phenolics) that scavenge and detoxify ROS to protect plant tissues from oxidative damage [67]. There have been several studies on diverse ST-PSB-inoculant plants that are capable of increasing the antioxidant enzymes and non-enzymatic molecules than the non-inoculant plant ( Table 2). As an example, Ansari et al. [44] demonstrated that B. pumilus FAB10 has been shown to produce high levels of antioxidant enzymes such as APX, POD, SOD, CAT, GPX and GR in wheat (Triticum aestivum L.) under salinity stress.
The primary goal of osmoprotective compounds is to keep cells compatible with regular metabolic activities. Under salt stress, plants accumulate chemically diverse organic compatible osmolytes (e.g., such as proline, glycine betaine, sugar and polyols), which protect the structure and to maintain osmotic balance within the cell through continuous water absorption [22]. The synthesis and accumulation of organic osmolytes are varying in different plant species. However, the amino acid proline is found most commonly in taxonomically diverse plant species [158]. Moreover, proline also acts as a redox-buffering agent carrying antioxidant properties (scavenging ROS) under stress conditions [45]. The inoculation of ST-PSB in diverse crop plants (e.g., pea, maize, ground nut, etc.) enhances the production of osmoprotectants, and contributes to increased tolerance to salinity stress ( Table 2).

Hormonal Modulation to Salt Tolerance Plant
Ethylene is considered as a key regulator of the salinity stress tolerance in plants, where it modulates downstream gene expression, which can modulate salt stress responses [159]. Additionally, signaling of ethylene is critical for rapid response of plant and salinity stress resistance [160]. Stress-induced ethylene synthesis can improve plant tolerance or hasten senescence [161]. Plant adaptation to stress is controlled by ethylene at the cost of growth and development. The transcription of auxin response factors is inhibited and restricts the plant development, when ethylene levels rise in a stressful (drought and salinity) environment. IAA, produced by ST-PSB, is one of the most widely used and researched bacterial signaling molecules in plant-microbe interactions. To protect plants from a range of stressors, PGPR generates IAA and ACC deaminase. [162]. IAA accumulation causes ACC synthase genes to be transcribed, resulting in an increase in ACC concentration and the production of ethylene [162] (Figure 3). ST-PSB with ACC deaminase activity can break down some of the excess ACC into ammonia and α-ketobutyrate as a source of nitrogen and carbon, as a result lowers plant ethylene levels during environmental stress, while also allowing IAA to promote plant growth [151,153,163]. Many studies have shown that the soil ST-PSB with ACC deaminase activity confers improved stress tolerance and growth promotion in plants by decreasing ethylene level [136,156,157,[164][165][166].
Cytokinin involved in plant growth and development as well as imparts resistance to various stresses [167]. Inoculation of B. subtilis strain in Platycladus orientalis and lettuce plants resulted in increased cytokinin signaling from root to shoot, resulting in improved plant growth under stress conditions [168,169]. Gibberellins (GA) are another essential class of phytohormones that positively regulate cell division and elongation, as well as activity of meristem of the roots and leaves, as part of their role in the developmental and physiological processes of plants [170]. Under salinity stress, it is reported to increase the level of GA by ST-PSB to overcome the inhibition of plant growth. Abscisic acid (ABA), a plant stress hormone, aids plant life by regulating several developmental processes and adaptive responses to diverse stresses [74]. The overproduction of ABA hormone leads in the osmotic adjustment of the plant under salt stress by closing stomata (thereby reducing water loss), increasing ion exchange activity, and storing different proteins and osmoprotectants (proline and free amino acids) [171,172]. During the stress response, cytokinin (CKs) and ABA have antagonistic effects on transpiration, stomatal opening and photosynthesis, with CK levels decreasing and the stress response being regulated by ABA and ethylene [173]. The cytokinin producing PSB play vital role in plant salt tolerance through the inhibition of ABA [74].

Reduced Plant Susceptibility to Pathogens
In addition to disturbing the physiology and biochemical process of the plant, salinity increases the susceptibility of plant to pathogens (Figure 3). There have several mechanisms of ST-PSB that counter against the phytopathogen. Siderophore-producing rhizobacteria have been identified as potential biocontrol agents for controlling plant diseases (prevent or reduce pathogen proliferation) by depriving the iron availability to the pathogen [174]. Several ST-PSB produced siderophores. However, their direct relation with inhibition of plant pathogens still needs to be investigated (Table 2). ST-PSB can break down (hydrolysis of the (1→4)-β-glycosidic bond) the chitin, which is an integral cell wall component of fungus by the production of fungal cell walls degrading enzymes [154,175], such as chitinase, lipase, which can degrade the fungal cell-associated lipid; and protease, which can degrade the associated protein in call wall of fungus [174].The synthesis of hydrogen cyanide (HCN) by ST-PSB such as Pseudomonas aeruginosa (AMAAS57) and Pseudomonas fluorescens (BM6) may inhibit cytochrome C oxidase and other metalloenzymes, causing antifungal activity against Aspergillus niger, Aspergillus flavus, and Sclerotium rolfsii in groundnut [155,176]. In the co-evolutionary competition between plant and pathogen, plants evolved two forms of induced resistance, such as ISR (induced systemic resistance) and SAR (systemic acquired resistance) [177]. Biocontrol bacteria play a pivotal role in inhibiting plant fungal pathogens by inducing plant defense mechanisms. The biocontrol ST-PSB reduces the adverse effects of plant pathogens by activating resistance mechanisms in crops [178,179]. The salicylic acid independent of ISR involves jasmonic acid and ethylene signaling, while SAR requires salicylic acid as a signaling molecule to induce a defense system in plants [180]. To regulate biotrophic pathogens, ST-PSB can use antagonism between jasmonic acid and salicylic acid signaling pathways to increase plant colonization. In conclusion, differential beneficial role of ST-PSB have been found as a suppressor of diverse plant pathogens by producing antagonistic metabolites (siderophores, HCN, hydrolytic enzymes, etc.) and inducing immunity potentiality of crops against pathogenic stress ( Table 2).

Application of ST-PSB in Sustainable Agriculture in Saline Environments
In a sustainable agriculture practice, it is used to be maintained and preserved for longterm soil productivity and natural resources, including diverse and functional microbial populations. The plant-microbial association in rhizospheric soil is the best indicator of plant health, productivity, and soil fertility [20]. In the circumstance of salinity and P deficiency stress conditions, more particularly, the rhizospheric ST-PSB can mitigate the plant salt stress and increase phosphorus availability, resulting in improved crop yield.

Salt Alleviator and Phosphate Biofertilizer
Biofertilizer is a substance containing living or dormant cells of beneficial microbes (fungi, bacteria, and algae) with useful properties towards plant growth and development [181]. The biofertilizer formulations have been progressed over the years in different ways, such as solid carrier formulations (peat, powder, granules, etc.), liquid formulations (mineral oils, organic oils, etc.), polymer entrapped formulations (alginate, starch, chitosan, etc.), metabolite formulations, and nano-bioinoculant based formulations (gold-, silver-, lead-, copper-, and iron-nanoparticles) [182,183]. However, the liquid biofertilizer formulation is more dominated and used due to the better shelf life, low cost and easy to handle [184]. Phosphate biofertilizer is such fertilizer that meets P deficiency in the soil along with enhancing crop yield. In the global biofertilizer market, the second most dominated market is phosphate biofertilizer with 14%, after the nitrogen biofertilizer with 80% [185]. There are several beneficial endophytic and rhizospheric ST-PSB that exhibited inorganic and organic phosphate solubilization under saline and non-saline conditions, helping to diverse plants growth promotion (Table 3). Wheat (Triticum aestivum L., variety C-309) [175] Pseudomonas aeruginosa AMAAS57 and Pseudomonas aeruginosa BM6.

Achromobacter piechaudii
Increase fresh and dry weights of plant seedlings grown in the presence of up to 172 mM NaCl salt.
Both the endophytes alone or in combination promoted a greater vigor index, germination (%), plant biomass, P content compared to un-inoculated control under 160 mM NaCl.
Phyllanthus amarus [192] Bacillus altitudinis WR10 The inoculated plant Significantly improved the seed germination rate, root dry weight, and enhance P availability from organic under salinity stress (200/400 mM NaCl) and phosphorus stress.
Increased the plant growth with respect to number of branches, height, dry matter accumulation and P content of plants under sodic condition Ocimum basilicum [91] Bacillus atrophaeus S8 and Enterobacter sp. QE3 Inoculation increase germination rate and seedling growth under salinity.
Quinoa (Chenopodium quinoa wild.) [195] As an example, a potential endophytic ST-PSB, Bacillus altitudinis WR10, enhanced wheat crop productivity under low P and salinity stress by increasing available phosphate through phosphatases and phytases activity [89]. Mahdi et al. [43] have shown that inoculation of ST-PSB, Bacillus licheniformis QA1 and Enterobacter asburiae QF11, with Chenopodium quinoa plant significantly reduced salt toxicity; and increased P content in the plant compared to the non-inoculated plant. According to the previous findings, these strains can be used as a potentially suitable biofertilizer for the management of phosphate in saline affected agriculture fields.

ST-PSB as a Biocontrol Agent
Plant pathogens (biotic stress viz. viruses, fungi, nematodes, insects, and mites), cause widespread damage to the crop production, human health, and the stability of global economies [196]. However, salt-stressed plants are more susceptible to phytopathogens in saline soil-based agriculture, where both salinity and pathogens are predominant stresses limiting plant growth and productivity [197]. As previously described, the detailed possible mechanisms of several beneficial ST-PSB have been found as a suppressor of diverse plant pathogens ( Table 3). The salt, cold and pH tolerant strain, Pseudomonas putida exhibited antifungal activity that inhibiting the growth of the phyto-pathogenic fungus (e.g., Alternaria alternata and Fusarium solani) as well as salt tolerance activity in maize [186]. Similarly, a ST-PSB, Pseudomonas sp. PF17 strain also suppressed the growth of a serious native phytopathogen, Macrophomina phaseolina by 75% in saline and non-saline conditions [154]. Singh and Jha, [175] reported on a potential salt alleviator ST-PSB strain, Bacillus licheniformis HSW-164, which showed the biocontrol of diverse groups of fungus and bacteria as well as promote the growth of wheat crop grown under salinity. Therefore, the high antagonistic potential of ST-PSB against phytopathogen may offer an eco-friendly approach for long-term plant disease management by using biopesticides as an alternative to chemical pesticides in saline and non-saline soil-based agriculture.

Bioremediation
Heavy metal pollution in soil is a serious concern in food production as well as human health and the environment. It is known that PSB strains have the potentiality to detoxify and remediate various heavy metals through different strategies, such as bio-precipitation, leaching, sorption, transformation, accumulation, assimilation, metal solubilization, and to assist phytoremediation (phyto-stabilization and phyto-extraction) [198]. Desale et al. [166] have proven that Halobacillus sp., and Halomonas sp. assisted to Sesuvium portulacastrum plant for phytoremediation, however, the possible mechanism was not evaluated in this study. Moreover, various mechanisms of solubilization of phosphate by PSB, such as organic acid production, H + excretion, siderophores, and EPS contributed to bioremediation [106]. As an example, the ST-PSB strain, Halomonas sp. Exo1 efficiently produced EPS in the presence of salt and arsenic, where EPS had a major role in the sequestration of arsenic and alleviation of salt [139]. In general, the presence of salinity and heavy metal stress can lead to a remarkable reduction in productivity. Hence, the application of ST-PSB as a bioinoculant can sustainably contribute to the growth of plants under salinity and metal stress. However, there have been very limited studies on ST-PSB in bioremediation practice.

Future Prospects
Several suggestions and avenues of research would move us closer to adopting this strategy for developing environment-friendly P-biofertilizers to be used as supplements and/or alternatives to chemical P fertilizers in saline soil-based agriculture: (i) The comparative studies of interactions between plants and ST-PSB, which are currently lacking, would clarify about the mechanisms governing salinity stresses. (ii) Limited knowledge about the mechanisms by which ST-PSB-mediated P nutrition modifies the salt tolerance and P uptake of plants (specially P starvation gene expression), which should be further studied in the future. (iii) Since the ability of PSB to dissolve inorganic insoluble phosphate and mineralize organic phosphates decreases under salinity stress, in order to select and introduce the best ST-PSB, it is recommended that the screening of these bacteria be investigated in the presence of salinity and drought stress because in most studies these bacteria were screened under non-stress conditions. (iv) An assessment to select the best ST-PSB should be undertaken in poor soils because most these PSB are sensitive to environmental stresses. Under such conditions, PSB that can compete for limited resources should be selected.
(v) Since both drought and salinity stress cause osmotic stress to plants, in order to select and introduce the best PSB, it is recommended that the efficacy of these bacteria in the presence of salinity and drought stress be investigated concurrently because most studies have investigated the effect of these bacteria on alleviating either salinity stress or drought stress in plants. (vi) Since most PSB are heterotrophic and require organic matter as a source of carbon and energy, in order to improve the efficiency of these bacteria in saline soils, it is recommended to use them along with organic matter in saline soils, which are mostly poor in organic matter. (vii) Most research has been executed on culturable PSB. Since most plant-associated microorganisms including PSB are un-culturable, more research is needed on unculturable PSB in plant rhizosphere and endosphere, which will lead to an improved knowledge of the PSB community and the PSB-plant interactions. (viii) The expansion of basic science about the interactions between PSB and salinitystressed plants facilitates a better understanding of decreased salinity stresses and possibly provides better forecasts about salinity-stressed plant responses. Therefore, it is anticipated that one of the most interesting research areas in the future for agricultural scientists will be to investigate the PSB mechanisms involved in decreasing salinity stress on plants. (ix) It is known that the plants inoculated with a combination of PSB and arbuscular mycorrhizal fungi (AMF) can express synergistic effects to increase salinity-stressed plant growth indices, while maintaining safe natural resources, such as P stocks. Therefore, further research to evaluate the application and the efficacy of PSB as independent inoculations or as a co-inoculation with AMF under various environmental stresses including salinity stress on various crops fertilized with different low-solubility P sources in field conditions is necessary, where the survival of PSB and AMF, as well as how the mechanisms by which they promote plant growth, are cramped by competition with the endemic microorganisms, environmental stresses, and soil conditions (phosphorus sorption capacity, soil phosphorus status, pH, etc.).

Conclusions
Soil salinization and P deficiency are a the serious global threat in case of crop production by their adverse effects on plants and soil microbial community. To overcome this problem, our several management strategies have limitations in case of pollution, and their success rate. The use of phosphate chemical fertilizer has negative impacts on environmental issues. In this scenario, an alternative approach would be the use of phosphate biofertilizers like ST-PSB that could alleviate salt stress by producing plant growth hormones, enhancing the nutrient uptake and by acting as a biocontrol agent. These bacteria could play a major role in providing a cost-effective measure for salt stress and P management. Hence, under P deficiency conditions, ST-PSB are going to serve as efficient biofertilizers to increase the growth of the plants under salinity stress conditions. Further research is required to identify ST-PSB strains with multiple beneficial characteristics for better growth promotion of crops in sustainable saline soil-based agriculture. Data Availability Statement: The study did not report any data.