Rhizosphere phosphate-solubilizing bacteria: a natural solution for soil fertility and plant nutrition

Phosphorus (P) functions as a requisite macronutrient ranking after nitrogen for the growth of plants. It serves as a constituent of biological components, like plasma membrane, proteins and nucleic acids. Phosphorus is necessary for respiration, storage of energy and its transfer and all phosphorylation reactions. Despite the abundance of phosphorus in soil, it acts as a limiting nutrient due to its non-accessibility and low solubility potential. Phosphorus occurs in two forms: inorganic phosphorus (P_i) and organic phosphorus (P_o). P_i is derived from weathering of phosphate-containing minerals and is present as insoluble compounds of calcium, iron and aluminum (Amarasinghe, Madhusha, Munaweera, & Kottegoda, 2022). P_o originates from organic matter (biological biomass) and must be mineralized through microbial action before becoming accessible to plants. To overcome phosphorus deficiencies and make it more available in agricultural systems, huge amounts of phosphorus-based synthetic fertilizers have been applied. These synthetic fertilizers temporarily increase the concentration of soluble phosphorus in agricultural fields. However, a small amount of soluble phosphorus can be utilized by plants. In 2024, the worldwide utilization of chemical fertilizers in agriculture was around 208 Mt (million tons). Among these, phosphorus consumption was recorded as 45 Mt, indicating its critical role in plant growth (FAO, 2025). The market of chemical fertilizers was recorded at $204.73bn in 2024, and it is expected to increase to $216.52bn in 2025 (The Business Research Company, 2025). The widespread and continuous use of synthetic fertilizers has led to several environmental impacts. It can lead to the accumulation of phosphorus in soil, which can negatively impact microbial activity and overall soil health. Excessive phosphorus runoff from agricultural fields enters the water bodies, contributing to water contamination such as eutrophication, algal bloom and disturbed aquatic ecosystems. These impacts reduce crop productivity, pollute environment and diminish biodiversity. Therefore, it is essential to regulate the use of chemical fertilizers and to adopt effective and sustainable agricultural practices to minimize the impact of synthetic fertilizers used in agriculture. In response to these challenges, researchers have been exploring more sustainable options, including the use of phosphate-solubilizing microorganisms (PSMs).

The PSMs used as a biofertilizer consist of beneficial microorganisms, such as phosphate-solubilizing bacteria (PSB), mycorrhizal fungi and plant growth-promoting rhizobacteria. These microorganisms have several benefits, such as improving soil health, inhibiting the growth of pathogens and promoting long-term productivity and cost-efficiency. Most of these microorganisms are found in rhizospheric soil, nodules of root and organic matter. PSBs play a crucial role in enhancing nutrient availability to plants, supporting growth and development and have significant potential for use as biofertilizers. This review aims to provide a comprehensive analysis of soil phosphorus composition and its available and unavailable forms for plants. It emphasizes the diversity of PSBs found in rhizospheric soil, phosphate solubilization mechanisms and the various enzymes involved in this process. The review also evaluates the beneficial effects of PSBs on plant growth and development, with a focus on their potential application as biofertilizers to promote sustainable agricultural practices.

The rhizosphere is the specific area of soil that surrounds plant roots and is home to a variety of microorganisms, including algae, bacteria, fungi, nematodes, protozoa and viruses. These microorganisms influence the plant physiology and growth through neutral, beneficial or harmful interactions, with each group contributing unique ecological functions. Within the rhizosphere, plants release various compounds known as root exudates, including ions, enzymes, secondary metabolites and mucilage. These root exudates provide nutrition to soil microorganisms and simultaneously support plant health, soil fertility and microbial activity. This process enhances nutrient absorption, reduces environmental and biological stress and contributes significantly to ecosystem stability. The rhizosphere can be divided into three distinct zones: the ecto-rhizosphere (adjacent soil around roots), the endorhizosphere (internal root tissues) and the rhizoplane (root surface interface). The distribution of plant growth-promoting bacteria varies across rhizosphere zones based on proximity to root exudates. The rhizoplane contains the highest bacterial concentrations due to direct access to plant-derived signaling molecules, exudates and nutrients released by the plant roots. The endorhizosphere contains moderate levels, consisting of endophytic bacteria that have successfully colonized inside the root tissues. The ecto-rhizosphere supports lower populations because microbial access to root-derived compounds decreases with distance from the root surface (Chauhan, Saini, & Sharma, 2021). These rhizobacteria stimulate root proliferation, possess efficient mineral transport systems and contribute to organic compound breakdown and soil remediation. Growth promotion occurs through several pathways, such as production of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase to reduce stress-related ethylene, synthesis of siderophores for improved nutrient uptake and synthesis of plant growth regulators (PGRs). They protect plants from pathogens by secreting protective compounds such as exopolysaccharides (EPS), hydrogen cyanide (HCN) and antibiotics. PSMs including bacteria, fungi and actinomycetes, play crucial roles in phosphorus cycling. PSBs constitute approximately 50% of all phosphorus-solubilizing microbial communities. Common PSB genera include Bacillus, Enterobacter, Erwinia, Escherichia, Klebsiella, Paenibacillus, Proteus, Pseudomonas, Rhizobia and Serratia. These PSBs are isolated from soils, indicating their widespread distribution in plant-associated environments. The distribution of PSBs in soil exhibits considerable heterogeneity, influenced by chemical and physical properties of soil, vegetation cover, nutrient management practices and reproductive strategies (Zhu et al., 2024).

Soil serves as a natural reservoir of essential nutrients, including nitrogen, phosphorus and potassium, for the growth of plants and microbes. Among these macronutrients, phosphorus plays a fundamental role in numerous physiological and biological processes. Despite being abundant in many soil types, phosphorus availability for plants remains limited due to its highly reactive nature. In soil, phosphorus exists in two forms: organic (30–65%) and inorganic (35–70%). Both forms are largely inaccessible to plants. Pi includes mineral phosphates in which phosphate ions are chemically bound to metal ions, as well as phosphorus derived from applied fertilizers. Po is associated with microbial biomass and plant residues, and it must undergo mineralization to become available for plant uptake. Plants uptake less than 1% of total soil phosphorus. They can only absorb phosphorus as phosphate anions, specifically as dihydrogen phosphate ions (H₂PO₄^-) or hydrogen phosphate ions (HPO₄^2-). Phosphorus deficiency in plants manifests as stunted growth, delayed seed germination and premature dropping of leaves and flower buds. To address phosphorus deficiency, agricultural systems employ synthetic phosphate fertilizers. However, 70% to 90% of applied fertilizer undergoes fixation reactions with iron (Fe³⁺), aluminum (Al³⁺), and calcium (Ca²⁺) ions, forming insoluble metal phosphate complexes (Johan, Ahmed, Omar, & Hasbullah, 2021). These fixation processes substantially reduce available phosphorus concentrations in soil, resulting in poor crop growth and reduced yields. Therefore, maximizing crop productivity requires the solubilization of phosphates that have accumulated in insoluble forms within the soil matrix. Certain soil microorganisms, such as PSBs, demonstrate remarkable effectiveness at converting insoluble phosphorus into forms that plant roots can readily absorb without causing adverse effects. Agafonova, Kaparullina, Doronina, and Trotsenko (2013) isolated 14 strains of PSBs belonging to the genera Ancyclobacter, Delftia, Methylobacillus, Methylobacterium, Methylophilus and Methylovorus. Shi, Xing, Zhu, Gao, and Ying (2022) isolated organic PSBs diversity, including Bradyrhizobium, Aquabacterium, Rhizobacter and Xanthomonas. The inorganic PSBs' diversity was represented by Burkholderia, Azotobacter, Pseudomonas and Mycobacterium. Xing et al. (2021) demonstrated that diversity and functional characteristics of PSB varied significantly among different bamboo forest ecosystems. Specific bacterial strains provide distinct functions that promote plant growth. Strains of Enterobacter, Serratia, Proteus and Klebsiella produce significant amounts of the enzyme acid phosphatase. Rhizobium strains are important for the synthesis of PGRs, nitrogen fixation and phosphorus solubilization. Azospirillum functions as a plant stimulator, promoting root elongation and auxin production, while Frankia and Bacillus enhance mineral uptake capacity. Pseudomonas, Frankia and Streptomyces synthesize siderophores and small organic molecules that support plant nutrition (Hayat et al., 2010). The application of PSB in agricultural practices offers a viable alternative to conventional agrochemicals. This approach provides a cost-effective, environmentally safe and sustainable technology that supplies adequate phosphorus for proper plant maturation, ultimately resulting in enhanced crop production.

PSBs employ diverse strategies to convert insoluble phosphorus into soluble forms, which are accessible to plants. The mechanisms include production of EPS, inorganic acid, organic acid, phytohormones, siderophores and proton release. Phosphate solubilization occurs through two primary processes: the mineralization of Po and solubilization of Pi. Both processes transform insoluble phosphate compounds into a soluble form that plants can readily absorb. A brief schematic diagram summarizing PSB mechanisms is shown in Figure 1. The Po mineralization is further divided into biochemical mineralization, i.e. generation of extracellular enzymes and biological mineralization, in which phosphorus is released during substrate degradation. The Pi solubilization is based on secretion of mineral-dissolving compounds. When Pi becomes limited in soil environments, PSBs activate metabolic and genetic responses to mobilized phosphorus. Numerous genes associated with the production of Pi assimilation enzymes, as well as Po transporters, are modulated by a global regulatory mechanism termed pho (phosphate) regulon (Martín and Liras, 2021).

The PhoR_PhoB system is a two-component regulatory system in Escherichia coli, Bacillus subtilis and other bacteria that plays an important role in cellular responses to phosphate availability in soil (Park, Solhtalab, Thongsomboon, & Aristilde, 2022). The two essential proteins, PhoR (inner membrane sensor protein) and PhoB (transcriptional response regulator protein), work in coordination and regulate genes that are involved in phosphate acquisition, transport and solubilization. PhoR protein acts as a membrane-bound sensor histidine kinase that monitors phosphate concentration in soil and is activated when the level of Pi is low. When phosphate becomes limiting, PhoR undergoes a conformational change that activates its kinase activity. PhoB serves as a cytoplasmic response regulator that initiates cellular responses. The N-terminal of PhoB contains a conserved aspartate residue that accepts phosphate groups from PhoR. The C-terminal domain enables PhoB to interact with specific regulatory sequences in target gene promoters, which initiates gene expression. Another protein, PhoU (negative regulator/inhibitor), functions as an inhibitory component that restrains PhoB-mediated responses, preventing excessive activation of phosphate uptake systems when environmental phosphate levels are adequate. PhoU exerts its regulatory control through direct protein-protein interactions with both the high-affinity phosphate transport complex (PstSCAB) and the PhoR sensor kinase. The regulatory mechanism for mineralization of Po under phosphorus-depleted environments is illustrated in Figure 2(i). Under phosphate-sufficient conditions, PhoU forms stable complexes with these proteins, effectively blocking signal transduction through the PhoR-PhoB pathway. This inhibition prevents expression of phosphate-scavenging genes and maintains cellular phosphate balance. The formation of the PhoU-PstSCAB-PhoR complex regulatory network contributes to bacterial survival and ecological balance across diverse environmental conditions. When intracellular phosphate levels are adequate, PhoU binding stabilizes the inactive conformation of PhoR, preventing autophosphorylation and subsequent PhoB activation. The interaction between these three regulatory proteins allows bacteria to maintain precise control over their phosphate acquisition and utilization pathways (diCenzo et al., 2017). The regulatory mechanism for mineralization of Po phosphorus-sufficient environments is illustrated in Figure 2(ii). This advanced regulatory system illustrates how phosphorus-solubilizing bacteria have adapted to varying levels of phosphorus availability in soil environments.

Metagenomic strategies directly investigate the vast genetic variability and mechanisms that exist within soil microbiomes and even unculturable bacterial taxa. The emergence of next-generation sequencing has made it possible to identify previously uncharacterized PSBs and genes that participate in phosphate solubilization, transport and regulation. The identification of additional pho regulon has been determined by the integration of bioinformatic analysis of the genomes with transcriptomic studies (Park et al., 2022). The bioinformatics databases identify genes, indicating phosphatase action and phytase functions. Yao et al. (2018) used metagenomics and metaproteomic analysis to examine the responses of microbes to long-term phosphorus fertilization. They found that under phosphorus-deficient soil, microbes enhanced their genetic potential to utilize phosphate from phytate, nucleic acids and phospholipids.

When phosphate is organically bound, PSBs release phosphatases, phytases, lyases, etc., enzymes that serve a crucial role in facilitating the mineralization process that releases Pi.

Phosphatases or phosphomonoesterases are a group of enzymes that hydrolyze phosphorus-oxygen linkages that are present in Po compounds. These enzymes are bound to lipoprotein membranes of microbes or secreted into the extracellular environment. Approximately 50% of the microbial population performs mineralization of Po under the action of enzyme phosphatases. The major forms of extracellular phosphatases in soils are acid phosphatases, alkaline phosphatases and phytases (Hui, Mayes, & Wang, 2013; Park et al., 2022). These enzymes function according to the optimal pH of their surroundings. Acidic soil has acid phosphatases, and soil with alkaline to neutral pH is abundant with alkaline phosphatases. Phosphatases catalyze the hydrolysis of organic substances like phosphoanhydrides or phosphoesters. This process is called dephosphorylation and releases Pi that can be taken up by plant roots (Rawat, Das, Shankhdhar, & Shankhdhar, 2021).

Acid phosphatases are enzymes that release Pi after hydrolysis of phosphoesters and phosphoanhydrides. These enzymes are non-specific acid phosphatases (NSAPs) and are categorized into three groups, Nsap-A, Nsap-B and Nsap-C based on their amino acid sequence similarity. These enzymes are termed non-specific due to their wide range of substrates. The Nsap-A enzyme, acting within the cell or periplasm, was less abundant in phosphate-deficient acidic soils and Nsap-C, acting outside the cell, was more abundant (Neal et al., 2018). The Nsap-A coding gene was found in bacteria Dyella and Rhodanobacter and is known to thrive in both acidic and neutral soils while utilizing diverse carbon sources. However, the Nsap-C coding gene was observed in a variety of bacterial classes, including Alpha and Gammaproteobacteria, Sphingobacteria and Flavobacteria.

Alkaline phosphatases. Alkaline phosphatases are capable of hydrolyzing phosphoesters such as glucose-6-phosphate and adnosine triphosphate (ATP), and producing Pi (Elhaissoufi, Ghoulam, Barakat, Zeroual, & Bargaz, 2022). Among microorganisms, phoA, phoD and phoX are common gene families that encode for alkaline phosphatases (Ragot, Kertesz, & Bünemann, 2015). The bacteria can produce enzymes by gene family phoA that cleave phosphomonoester and hydrolyze both monoesters and diesters by gene family phoD and phoX, targeting different Po pools with varying cofactor requirements. PhoD and PhoX both function as extracellular enzymes that need calcium ions as a cofactor for activation, whereas PhoA is an intracellular enzyme and is activated by zinc and magnesium ions. Alkaline phosphatases have a wide substrate range and high catalytic activity. These characteristics enable microbes to use different compounds of Po under limited Pi conditions.

Phytases. Phytases are a subclass of phosphatases that catalyze the breakdown of phytate complexes (inositol hexaphosphate), a common Po compound, and release inositol and phosphate in soil. This process occurs especially during seed germination and early plant growth stages. This enzymatic activity ensures the availability of phosphorus for various metabolic processes and structural components as the seedling develops. Phytases are encoded by appA genes in various bacteria, as exemplified by Bacillus subtilis, Escherichia coli and Buttiauxella sp. (Kaur, & Reddy, 2013). Isolates of Pseudomonas and Serratia were reported to be positive for carrying out phytase activity with improved efficiency of phosphorus solubilization (Rawat et al., 2021).

Phosphonatase enzymes allow bacteria to access chemically stable forms of Po. The C-P bond is highly resistant to chemical and enzymatic hydrolysis under normal conditions; bacteria possessing this enzyme complex can access phosphorus sources. This enzyme provides a significant competitive advantage in phosphorus-depleted environments. PSBs such as Acinetobacter sp., Enterobacter sp., Burkholderia sp., Rhizobium sp., Pseudomonas sp., and Bacillus sp. break down phosphonates using phosphonatases. Enzyme phosphonatases are encoded by gene phosphonateX (phnX), which is capable of cleaving phosphonate substrate, the chemically inert and hydrolytically stable C-P bond, to yield acetaldehyde and Pi. The C-P lyase enzyme complex consists of seven genes, namely phnG, phnH, phnI, phnJ, phnK, phnL and phnM, and is encoded by various proteins that perform C-P lyase activity. Stable C-P lyase core complexes are formed from phnH, phnG, phnJ and phnI proteins, in which the arrangement of phnG and phnI proteins is at the center. This complex is then associated with another stable protein, phnK. The phnI reported nucleosidases that are capable of deglycosylation of ATP and guanosine triphosphate (GTP) to ribose 5-triphosphate. Cleavage of C-P bond to release soluble phosphates is catalyzed by phnJ (Seweryn et al., 2015). This enzymatic system represents the ultimate phosphorus scavenging capability, enabling bacteria to utilize phosphonate pools as a phosphorus source when all other phosphorus forms have been depleted.

This is achieved by producing organic acids, inorganic acids, siderophores, EPS, hydrogen sulfide (H₂S) and ammonium ions (NH₄⁺). These acids and chemical compounds lower the pH in soil and help in phosphate solubilization.

Organic acids. Secretion of low molecular weight organic acids by bacteria acidifies the microenvironment and chelates metal cations, which are bound to phosphate, thus releasing soluble phosphate ions into the soil. At lower pH, phosphate is available in the form of H₂PO₄^-. However, an increase in pH shifts the phosphate equilibrium towards less accessible forms, i.e. HPO₄^2- and HPO₄^3-. The release of organic acids leads to the acidification of both the bacterial cells and their surrounding environment and results in a decrease in proton concentration, which promotes the conversion of mineral phosphates into Pi. Some common organic acids are malic acid, gluconic acid, citric acid, alpha-ketoglutaric acid, tartaric acid, oxalic acid and lactic acid. It chelates cations that are bound to phosphate in minerals. This leads to the release of free orthophosphate ions into the soil. The enzyme responsible for gluconic acid production, glucose dehydrogenase, requires the redox cofactor pyrroloquinoline quinone (PQQ). The synthesis of PQQ involves a set of genes, usually ranging from 4 to 7, known as pqqB, pqqA, pqqD, pqqC, pqqG, pqqF and pqqE. PQQ is synthesized by various bacterial species, including Acinetobacter calcoaceticus, Enterobacter sp., Gluconobacter oxydans, Klebsiella pneumoniae, Methylobacterium extorquens, Pseudomonas sp. and Serratia sp. (Suleman, Yasmin, Rasul, Yahya, Atta, & Mirza, 2018).

Inorganic acids. Inorganic acids, including carbonic acid, nitrous acid, hydrochloric acid, sulfuric acid, nitric acid and sulfurous acid, are produced by PSBs depending on the metabolic pathways. Inorganic acid-producing bacteria are Thiobacillus, Beggiatoa, Thiothrix, Bacillus, Clostridium, Lactobacillus, Micrococcus, Neisseria, Pseudomonas and Streptococcus. These acids show direct chemical reaction by chelating cations or provide H⁺ ions to dissolve phosphate-containing minerals. In some circumstances, low levels of phosphate can also trigger the process of phosphate solubilization. H₂S operates via a separate mechanism, which produces ferrous sulfate by reacting with ferric phosphate and results in inorganic phosphate liberation (Sharma, Sayyed, Trivedi, & Gobi, 2013). PSB also produce siderophores, which bind to iron and other metals, increasing phosphate availability by reducing the formation of metal phosphate complexes. Additionally, EPS forms biofilms and creates microenvironments that promote phosphate solubilization by interacting with soil particles (Rawat et al., 2021).

Utilization of PSB inoculum as biofertilizers is a cost-effective, sustainable and non-toxic approach to resolving issues caused by chemical fertilizers. A brief schematic diagram summarizing benefits of PSB is shown in Figure 3. The microorganisms associated with the rhizosphere produce phytohormones, siderophores, antibiotics, EPS and organic acids for mineralization and solubilization of phosphates as well as inhibiting phytopathogens.

PGRs or phytohormones are small active signal molecules that enhance or inhibit plant developmental processes. The PSBs help in synthesizing plant hormones, such as Auxins, Gibberellins, Cytokinin, etc.

Auxin. Auxin regulates apical dominance and boosts the formation of roots, cell elongation and cell differentiation in plants. Auxin controls pre-harvest fruit drop, induces flowering and stimulates the development of parthenocarpic fruits. A variety of PSBs are used for auxin production, including Bacillus, Pseudomonas, Enterobacter, Burkholderia, Rhizobium and Azotobacter. Commercial production of auxin has also increased due to the wide range of microbial species used for synthesizing auxin. Raheem, Shaposhnikov, Belimov, Dodd, and Ali (2018) found that Bacillus amyloliquefaciens S-134 was capable of producing a high rate of indole acetic acid.

Gibberellins. They produce a variety of responses, like enhancement of stem elongation and leaf expansions, promoting germination and flowering. They also induce synthesis of numerous enzymes like proteases, amylases, lipases and ribonucleases for mobilizing storage reserves at the time of early growth of seedlings and seed germination. Kang et al. (2019) observed Pseudomonas sp. produces gibberellin (GA1 and GA3), which significantly enhances the biomass of lettuce and Chinese cabbage.

Cytokinin. Cytokinins are synthesized during cell division, particularly in developing shoot buds, young fruits and root apices. It has various effects on plant development, influencing cell division, leading to cell enlargement and differentiation. It reduces apical dominance and delays senescence. Bacillus cereus, Escherichia coli, Bacillus subtilis, Bacillus megaterium, etc. are reported to produce cytokinin (Egamberdieva, Wirth, Alqarawi, Abd_Allah, & Hashem, 2017).

Abscisic acid (ABA). ABA is produced by some bacterial species that can enhance plant defense against stress responses like salinity and drought. It promotes stomatal closure, stimulating synthesis of secondary metabolites, encouraging callose deposition and limiting the invasion of harmful bacteria and insects. ABA contributes to disease resistance by combining with beneficial microorganisms in the rhizosphere. This interaction induces systematic resistance, leading to the activation of defense-related genes and improving the plant immune system (Vlot et al., 2021).

PSB serve as biocontrol agents by producing specific organic acids, HCN, antibiotics and substances that protect plants from various phytopathogenic diseases. Behera et al. (2021) reported that in greenhouse pot trials, seeds treated with Bacillus subtilis and Trichoderma viridae showed a significant reduction in disease development frequency. Bacillus subtilis produces antibiotics phenazine-1-carboxylate (PCA) and Zwittermicin that target multiple pathogens (Saraf, Pandya, & Thakkar, 2014). The wheat disease and stem rot in groundnuts caused by Sclerotium rolfsii and Gaeumannomyces graminis var. tritici can be treated by the same antibiotic, PCA. Pseudomonas fluorescens secrete molecules such as 2, 4-diacetylphloroglucinol (2, 4 DAPG), which shows a 75% reduction in activity against Sclerotium rolfsii, a pathogen found in soil (Asadhi et al., 2013).

Soil health refers to the ability of soil to support the growth of living organisms and plants, regulate environmental processes and conserve biodiversity. Inoculation of PSBs in soil enables high microbial activities, reduces disease severity and makes soil structure more conducive and aerated, with improved water-holding capacity and eco-friendliness (Li et al., 2023). It also increases the rate of carbon fixation and makes other nutrients more available for plants. PSBs help to increase the growth of fibrous roots, improve soil fertility and ensure crop quality and yield (Ding, Ali, Almaroai, Eissa, & Abeed, 2021).

Plants inoculated with PSB can improve crop yield by changing plants' phosphorus acquisition strategy and distribution of phosphorus. PSBs contribute significantly to plant metabolic activities and increase the availability of phosphorus and other nutrients in the soil, resulting in improved root structure, rate of photosynthesis and increased stress tolerance against conditions like disease or drought (Billah et al., 2019). Alam et al. (2022) found that PSB utilization, along with varying levels of Pi, was found to increase quantity and quality of wheat crops. It increases wheat production up to 69%.

Management of stress plays a significant role in sustainable agricultural practices because it maintains crop yield, protects the crop from adverse environments and provides food security. PSBs can trigger unique signaling pathways and promote gene expression linked with stress responses in plants. This gene activation improves the plants' tolerance to environmental stresses. The studies demonstrated that PSBs can upregulate genes involved in the synthesis of antioxidants, heat shock responses and water uptake, which are essential for the adaptation of plants under stress conditions (Paul, Dukare, Bandeppa, Manjunatha, & Annapurna, 2017). Application of PSB such as Pseudomonas putida GAP-P45 resulted in improved soil aggregation and durability of these aggregates under different stress conditions (Ghosh, Sen, & and Mohapatra, 2018). Wang et al. (2024) isolated PSB Kushneria sp. from Daqiao Saltern on the coast of the Yellow Sea and used it for salinity-tolerant agriculture due to its ability to survive in high-salt concentrations.

The effectiveness of phosphorus solubilization and mineralization by PSB in soil is dependent on various factors including environmental factors and biological factors. Environmental factors encompass the physicochemical characteristics of soil, including humidity, salinity, soil pH, temperature and organic matter content, which have the potential to directly or indirectly influence populations of PSB. Salinity has a significant impact on PSBs in soil through various mechanisms that influence both their abundance and activity. High soil salinity induces osmotic stress, which can lead to a reduction in the density and diversity of PSB populations. The osmotic stress, along with ion toxicity, changes in pH and an overall decrease in the complexity of the soil microbial community, contribute to these effects. Nyongesa, Aloo, and Were (2025) identified that the adaptability of Citrobacter sp. SB04-3, isolated from the wild Sorghum rhizosphere, had the potential to resist high salinity conditions and suggested their utilization in challenging environments, making them promising tools for sustainable farming and ecological restoration. Idress et al. (2025) examined a sustainable method to increase phosphorus availability in high salinity soils through the application of biochar produced from sludge of sewage and Parthenium combined with PSBs. PSB can tolerate and function efficiently across different levels of alkalinity and acidity of soils (Sanchez-Gonzalez et al., 2022). Alori, Glick, and Babalola (2017) observed that soil with pH range between 6 and 7.5 is considered best for the availability of soluble phosphorus. When soil pH falls below 5.5 or rises above 7.5, phosphorus may become bound by calcium, aluminum or iron, making it inaccessible for plant uptake. Zhao et al. (2014) observed that the solubilization of phosphate by Burkholderia cepacia SCAUK0330 resulted in a decrease in pH levels. They found that at a pH of 3.12, 452 μg⋅ml^-1 of phosphorus was solubilized, whereas at pH 4.95, 154 μg⋅ml^-1 of phosphorus was solubilized. This indicates that a lower pH enhances phosphate solubilization activity. Temperature plays a critical role in the solubilization of phosphate. Numerous studies have reported that most PSBs typically thrive at temperatures ranging from 20°C to 30°C, whereas others have observed optimal PSB activity between 30°C and 40°C (Kiprotich et al., 2025). PSBs have also been reported to solubilize phosphorus in regions experiencing temperature extremes. Soil type and mineral content further play a role in shaping PSB populations. The availability of calcium ions has been correlated with greater microbial abundance of alkaline phosphatases like PhoX, PhoD and NSAPs (Garaycochea, Altier, Leoni, Neal, & Romero, 2023). Organic matter-rich soil promotes microbial activity, leading to increased microbial solubilization and mineralization of phosphorus. The soil improvers, such as biochar, manure, compost, straw, peat, sphagnum moss, biosolids, vermiculite, sulfur lime, etc., have a beneficial influence on phosphate solubilization (Choudhary, Khan, Hussain, & Ashfaq, 2021). The addition of smaller amounts of Pi to the soil-root interface of plants can enhance the mineralization and solubilization of phytic acid by microorganisms, which benefits plant phosphorus uptake and supports plant growth (Zhang et al., 2014).

Biological factors encompass the abilities of PSBs to interact with various soil microorganisms, including some of the microbial pathogens or pests, as well as surrounding vegetation. Experimental studies revealed that the number of PSBs can change significantly according to the nature of the crop and its associated rhizosphere soil. It has been discovered that the groundnut rhizosphere soils had a denser population of PSBs than cotton, sorghum and maize rhizosphere soils. Croplands tend to support a higher concentration of PSB compared to natural or undisturbed soils (Mohammadi & Sohrabi, 2012). Chamberlain et al. (2020) observed that a proper routine for alternation of crops manages nutrients, limits pathogens and weeds, improves soil fertility and increases crop productivity. Various crops can also be chemo-attractive to certain microbial communities that have the ability to produce different phosphatases. PSBs like Sphingopyxis, Asticcacaulis, Ralstonia and Cupriavidus contain phoD gene, which is more linked with sorghum rhizospheres. Other Bosea and Achromobacter species producing NSAPs are more frequently reported in maize rhizospheres. These observations indicate that plant species may select specific microbial groups based on their phosphorus-acquiring mechanisms (Neal et al., 2017). A single bacterial strain cannot match the varied and fluctuating conditions of actual soil for providing accessible phosphorus. Therefore, a microbial consortium is used to ensure complete solubilization of phosphorus. Researchers reviewed that the microbial consortium has a higher efficiency compared to the pure culture for solubilization of phosphorus (Sarmah & Sarma, 2023; Wang, Pan, Lu, & Qi, 2023). The land use alters soil microbial communities and their phosphorus cycling functions. In phosphorus-rich agricultural soils, Cyclobacteriaceae and Vicinamibacterales dominated, with some genomes containing up to five copies of the gcd gene. These organisms also possessed low-affinity phosphate transporters, suggesting they rely on solubilizing abundant phosphorus rather than competing for scarce resources. Reforestation soils were dominated by Microtrichales and Burkholderiales, which lacked phosphate solubilization genes but contained extensive high-affinity transport systems. Kumar et al. (2022) developed a consortium of environment-restoring microbes (ERM C-1) by mixing the bacterial strains Pseudomonas putida T7, Pseudomonas aeruginosa M2 and Klebsiella pneumonia M6 and fungus Aspergillus terreus TF1 with the combined functions of breaking down chlorpyrifos and enhancing plant growth. In addition to this, the consortium has the efficiency of laccase production.

The agricultural scientists are working on achieving maximum crop yields in a cost-effective, environmentally sustainable and health-conscious manner using PSBs as biofertilizer. This involves identification and characterization of genes responsible for plant growth promotion and understanding the molecular mechanisms behind phosphate solubilization. Several challenges occur in the commercialization of PSB-based products. Efforts are being made to address these challenges. To advance the field and realize the full potential of PSBs in agriculture, future research should focus on genetic and molecular studies to investigate the specific genes and molecular pathways involved in phosphate solubilization. This includes expanding our understanding of soil microbial diversity and composition. We should refine PSB-based products to enhance their efficacy, stability and ease of application in diverse agricultural settings. Conducting comprehensive field studies to validate the performance of PSB inoculants across various soil types, climatic conditions and crop species is also crucial. It is essential to generate robust data that supports the regulatory approval process for PSB-based products. Research on the detailed mechanisms and optimization of this process could result in improved strategies for increasing phosphorus availability in agricultural systems. Further research is essential to understand the specific mechanisms and regulatory pathways that control phosphorus solubilization in these bacterial genera, as well as to comprehend the ecological implications of this metabolic capability.

The integration of PSBs as biofertilizer is a promising strategy for increasing crop productivity while advancing environmentally friendly and economically sustainable agricultural systems. These microorganisms convert inaccessible forms of phosphorus into ones that plants can use, offering a sustainable and non-toxic alternative to synthetic fertilizers. They promote plant growth through various mechanisms such as increasing phosphorus accessibility, producing phytohormones, secreting organic acids and synthesizing enzymes such as phytases and phosphatases. PSBs offer other agricultural benefits, including acting as biocontrol agents against phytopathogens, preserving soil fertility and alleviating environmental pollution associated with traditional fertilization practices. Several bacterial genera, including Pseudomonas, Rhizobium and Bacillus, demonstrated highly effective phosphorus solubilization capabilities. The multifaceted benefits of PSBs make them a crucial component in modern agricultural systems. Their application addresses the dual challenges of enhancing crop yields and meeting global food demands while promoting environmental sustainability. As investigations in this area advance, the development and optimization of PSB-based biofertilizers are likely to play an increasingly significant role in the transition towards more sustainable and resilient agricultural practices. In conclusion, utilizing PSB offers a promising strategy to balance the competing needs of agricultural productivity and environmental preservation, justifying ongoing research and development activities in this domain.

The authors are thankful to the Honourable Vice-Chancellor, Head, Department of Botany, Mahatma Gandhi Central University, Bihar, for their support and encouragement.