Essential nutrients are crucial for plant growth and yield, but their deficiency can negatively impact various aspects of plant development. During salinity conditions, elevated levels of sodium Na⁺ and Cl⁻ limit the uptake of macronutrients, including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) (Guo et al., 2020). It leads to a decreased availability of essential nutrients in plants, potentially triggering leaf senescence and inhibiting overall plant growth (Kumari et al., 2022).

Potassium-solubilizing bacteria (KSB) play a pivotal role in enhancing nutrient availability for plants, particularly in saline soils. Notable among these are bacterial species, such as Pseudomonas sp. and Bacillus sp., which can make K more accessible for plant uptake by solubilizing various silicate minerals ( Supplementary Table 1 ) (Jaiswal et al., 2016; Vasanthi et al., 2018). Specifically, the PGPM strain Burkholderia cepacia SE4 has been shown to release K from soils, making it available to Cucumis sativus plants (Kang et al., 2014a). Moreover, the application of Achromobacter piechaudii to Solanum lycopersicum plants improved the uptake of K and P, while the application of salt-tolerant rhizobacteria, Bacillus aquimaris, in Triticum aestivum has been shown to enhance the uptake of K, P, and N in saline environments (Mayak et al., 2004; Upadhyay and Singh, 2015). Similarly, the application of Azospirillum lipoferum or Azotobacter chroococcum in Zea mays L under salinity enhanced K accumulation and provided salinity tolerance (Abdel Latef et al., 2020). In Glycine max seedlings subjected to salinity, inoculation with B. firmus (SW5) led to enhanced N and P accumulation and greater salinity tolerance, underscoring the significant role of B. firmus (SW5) in nutrient acquisition under stress conditions (El-Esawi et al., 2018). These findings suggest that salt-tolerant KSB can significantly enhance crop yields in saline soils.

Plant PGPMs also play a crucial role in increasing the accessibility of other essential minerals such as iron (Fe), zinc (Zn), and sulfur (S) to plants ( Supplementary Table 1 ) (Gahan and Schmalenberger, 2014; Mishra et al., 2023). Iron is an essential micronutrient for plants as it is necessary for several metalloenzymes that are crucial in processes such as respiration and photosynthesis (Kobayashi et al., 2019). Salinity causes a deficiency of Fe that impacts plant growth, development, yield, and several other biological processes, including chlorophyll biosynthesis (Kobayashi et al., 2019). It has been documented that soil microbes play critical roles in accumulating Fe in roots and in transporting Fe in different plants (Masalha et al., 2000). Under conditions of low Fe availability, both microorganisms and plants produce siderophores—small organic molecules that selectively chelate ferric ions [Fe(III)], facilitating iron uptake (Ferreira et al., 2019; Timofeeva et al., 2022). The application of endophytic streptomyces has been demonstrated to significantly enhance the growth of mung bean and rice plants, leading to a notable increase in the biomass of both roots and shoots (Rungin et al., 2012). Furthermore, salt-tolerant siderophore-producing rhizobacteria (e.g., Bacillus aryabhattai MS3) have demonstrated the ability to promote plant growth in saline soils where Fe is limited (Sultana et al., 2020, Sultana et al., 2021). Therefore, siderophore-producing rhizobacteria are recognized as highly beneficial PGPMs, enabling plants to thrive in saline soils with limited iron availability (Ferreira et al., 2019).

Zinc (Zn) is a crucial plant micronutrient, essential for their development, growth, and yield (Saleem et al., 2022). Saline, sodic, and calcareous soil often cause Zn deficiency in plants (Tavallali et al., 2009; Daneshbakhsh et al., 2013). In the saline environment, the application of Zn is known to enhance sanity tolerance and stimulate proline metabolism (Mushtaq et al., 2023). Numerous studies have shown that PGPMs, including species such as Trichoderma sp., Providencia sp., Anabaena sp., and Bacillus sp., are capable of solubilizing Zn present in the soil (Upadhayay et al., 2022), which could be used for growth developments of plants including wheat (De Santiago et al., 2011).

Sulfur is a vital macronutrient crucial for the plant development and growth (Narayan et al., 2023). P. putida has been shown to play an important role in the S cycle in the conversion of organic S to an inorganic form that plants can uptake (Kertesz and Mirleau, 2004). Application of P. putida, Pseudomonas fluorescens, and B. subtilis provided salinity tolerance in soybean (Abulfaraj and Jalal, 2021). An Enterobacter sp., SA187, is known to promote alfalfa growth and yield in field conditions (De Zélicourt et al., 2018). Under salt stress, Arabidopsis plants showed symptoms resembling S starvation (De Zélicourt et al., 2018; Andrés-Barrao et al., 2021). However, when colonized with Enterobacter sp., SA187, these plants showed enhanced uptake of S and improved sulfur metabolism. This interaction also modulated the phytohormone signaling pathway and provided salinity tolerance ( Supplementary Table 1 ) (De Zélicourt et al., 2018; Andrés-Barrao et al., 2021).

Salinity triggers osmotic stress in plants, leading them to produce various osmolytes that serve as osmoprotectants—like mannitol, inositol, trehalose, polyamines, glycine, proline, and betaine—to mitigate the severity of the salinity stress (Munns and Tester, 2008; Park et al., 2016; Van Zelm et al., 2020). In saline environments, PGPMs further support plants under osmotic stress by producing these osmoprotectants, thereby enhancing the plants’ tolerance to salinity ( Supplementary Table 1 ). For example, inoculation of PGPMs, P. fluorescens, and B. subtilis enhanced proline accumulation in cucumber plants under salinity stress compared to non-inoculated plants (Saberi-Riseh et al., 2020). Similarly, the application of salt-tolerant Stenotrophomonas maltophilia BJ01 enhanced proline accumulation in peanut plants, providing salinity tolerance (Alexander et al., 2020). Additionally, B. amyloliquefaciens NBRI-SN13 enhanced proline and total sugar accumulation in rice seedlings, in contrast to non-inoculated seedlings (Tiwari et al., 2017).

In capsicum, the application of a salt-tolerant rhizobacteria, B. fortis, improved proline accumulation and conferred salinity tolerance (Yasin et al., 2018). Chickpea plants inoculated with Azosprillum lipoferum FK1 accumulated osmolytes like betaine, glycine, proline, and soluble sugars in response to salinity (El-Esawi et al., 2019). Further studies indicated that while salt treatment alone increased soluble sugars and proline content in maize plants, inoculation with A. lipoferum or A. chroococcum significantly boosted these levels, compared to plants treated with salt alone, enhancing maize growth under salinity (Abdel Latef et al., 2020). This suggests that the production of osmolytes and other components by these two PGPMs contributed to salinity tolerance and improved maize growth.

In soybeans, inoculation of B. firmus (SW5) not only increased the accumulation of osmoprotectants like glycine betaine and proline but also enhanced root architectural traits, including root length and volume, thereby improving salinity tolerance (El-Esawi et al., 2018). Moreover, an endophytic fungus, Paecilomyces formosus, known for producing Gas, provided salinity tolerance in cucumber by enhancing the accumulation of proline and other beneficial plant traits (Khan et al., 2012).

Aquaporins, integral membrane proteins from the major intrinsic protein (MIP) superfamily, form water-selective channels across membranes that play important roles in water transport and can also transport small neutral molecules (Kapilan et al., 2018). They play vital roles in cellular water transport as members of the plasma membrane intrinsic protein (PIP) and tonoplast intrinsic protein (TIP) families (Afzal et al., 2016). Aquaporins are key contributors to plant root hydraulic conductivity (Grondin et al., 2020). Expression of aquaporins is highly regulated by drought and salinity. An aquaporin gene SpAQP1 of Sesuvium portulacastrum, a halophyte, was strongly induced in response to salt or drought treatment (Chang et al., 2016). Transgenic tobacco plants expressing SpAQP1 demonstrated enhanced salt tolerance compared to wild-type and vector control plants, underscoring the role of aquaporin genes in salinity tolerance (Chang et al., 2016). In Arabidopsis, exposure to 100 mM NaCl leads to the downregulation of PIP and TIP aquaporin genes (Boursiac et al., 2005), a response also observed in other plants like cotton and tomato (Braz et al., 2019; Jia et al., 2020), significantly impacting root hydraulic conductivity (Siefritz et al., 2002). Conversely, the application of PGPMs has been shown to upregulate the expression of aquaporin genes, enhancing plant resilience to salinity. For instance, in maize, application of Pantoea agglomerans or B. megaterium upregulated aquaporin genes, improving root hydraulic conductivity and salinity tolerance (Marulanda et al., 2010; Gond et al., 2015). Similarly, barley seedlings treated with 200 mM NaCl exhibited reduced biomass and height alongside downregulated HvPIP2;1 aquaporin gene expression (Zawoznik et al., 2011). However, inoculation with Azospirillum brasilense strain AZ39 induced HvPIP2;1 expression, mitigating biomass and height reduction (Zawoznik et al., 2011). These observations suggest that A. brasilense strain AZ39 alleviates salinity stress possibly by upregulating the HvPIP2;1 aquaporin gene, thereby enhancing root hair length, density, and improving water uptake. Furthermore, in response to salt stress, the mycorrhizae-mediated modulation of the expression of aquaporin genes has been reported in multiple plant species including Phaseolus vulgaris, Lactuca sativa, and Robinia pseudoacacia (Sharma et al., 2021). Mycorrhiza-mediated upregulation of aquaporin gene improved water status, K⁺/Na⁺ homeostasis, increased photosynthesis, enhanced expression of genes associated with ion homeostasis, including SOS1 and HKT1, and alleviated salinity tolerance in black locust (Robinia pseudoacacia) ( Supplementary Table 1 ) (Chen et al., 2017).

During salinity stress, toxic ions like Na⁺ and Cl⁻ increase in the cytosol, and excessive accumulation of these ions causes toxicity. Na⁺ not only imbalances the K⁺/Na⁺ ratio but also affects many physiological processes and functions of various proteins (Assaha et al., 2017). The SOS signaling pathway plays an important role to reduce Na⁺ inside of the cell, and that, in turn, helps maintain Na⁺ homeostasis (Park et al., 2016). Additionally, the high-affinity K⁺ transporter 1 (HKT1) contributes to Na⁺ homeostasis by removing Na⁺ from the xylem and sending Na⁺ back to the root (Kaundal et al., 2019). In rice, it has been shown that OsHKT1;4-mediated Na⁺ transport in stems plays a role in excluding Na⁺ from leaf blades during the reproductive growth stage in response to salt stress (Suzuki et al., 2016). Na⁺/H⁺ antiporters have been implicated in Na⁺ and K⁺ homeostasis and salt tolerance (Apse et al., 1999; Bassil et al., 2011). Moreover, the proton pump, AVP1, has been shown to play a role in salinity tolerance in different plants (Gaxiola et al., 2001; Lian et al., 2024).

Multiple PGPMs have been identified to induce expression of SOS pathway genes ( Supplementary Table 1 ). Notably, volatile compounds produced by rhizobacteria Alcaligenes faecalis JBCS129 have been shown to upregulate expression of Arabidopsis SOS1, HKT1, NHX1, and AVP1 under salt stress, assisting the plant in maintaining ion homeostasis during salinity stress (Bhattacharyya et al., 2015). The application of PGPM strain Glutamicibacter sp. YD01 in rice seedlings provided salinity tolerance by inducing expression of OsHKT1 significantly and maintaining ion homeostasis (Ji et al., 2020). This strain also produces ACC (1-aminocyclopropane-1-carboxylate) deaminase and IAA, further supporting plant growth under stress.

In response to salinity, maize plants showed increased Na⁺, decreased K⁺, and reduced K⁺/Na⁺ ratio. However, salt-stressed maize plants inoculated with A. lipoferum or A. chroococcum showed reduced Na⁺, enhanced K⁺ accumulation, increased K⁺/Na⁺ ratio, and improved salinity tolerance, indicating that both A. lipoferum or A. chroococcum contribute to plant ion homeostasis in response to salinity (Abdel Latef et al., 2020). Similarly, inoculating white clover plant (Trifolium repens) with A. brasilense enhanced growth under salinity and reduced Na⁺ content, enhanced K⁺ content, and increased K⁺/Na⁺ ratio, suggesting that A. brasilense treatment contributes to ion homeostasis in plants under salinity (Khalid et al., 2017). Additionally, the application of Variovorax paradoxus 5C-2 to pea plants under salinity enhanced ion homeostasis by increasing K⁺ uptake, decreasing Na⁺ accumulation, and enhancing K⁺/Na⁺ ratio, leading to enhanced growth and salinity tolerance compared to uninoculated plants (Wang et al., 2016). Under salinity stress, rice inoculated with C. albidum strain SRV4 exhibited lower Na accumulation and higher K accumulation compared to non-inoculated plants (Vimal et al., 2019). This improved tolerance to salinity indicates the positive role of C. albidum in maintaining ion homeostasis during salt stress.

Various plant enzymes, including APX, CAT, GR, POX, and SOD, exhibit antioxidant activity (Upadhyay et al., 2012). Many PGPMs have been shown to boost the activity of these antioxidant enzymes. For instance, Piriformospora indica, a root-colonizing basidiomycete fungus, promotes growth and provides resistance against mild salinity stress in barley by activating the antioxidative capacity through the glutathione–ascorbate cycle (Waller et al., 2005). Similarly, applying A. lipoferum or A. chroococcum to maize plants enhanced the activity of CAT and POX POD, showcasing their positive regulatory roles in salinity tolerance (Abdel Latef et al., 2020). In soybean seedlings, B. firmus (SW5) inoculation elevates the activity of APX, SOD, CAT, and POD, alongside reducing H₂O₂ levels, indicating its contribution to enhanced antioxidative capacity (El-Esawi et al., 2018). Inoculating okra seeds with ACC-producing Enterobacter sp. UPMR18 improved seed germination and seedling growth under salinity conditions (Habib et al., 2016). This was accompanied by heightened ROS-scavenging activity of enzymes like APX, CAT, and SOD, indicating that these ROS-scavenging enzymes play a beneficial role in enhancing salinity tolerance through the action of PGPMs ( Supplementary Table 1 ). The application of P. putida H-2–3 showed a higher activity of SOD and improved soybean plant growth under salinity and drought (Kang et al., 2014b). Additionally, a GA-producing endophytic fungus, P. formosus, aids in salinity tolerance in cucumber by accumulating antioxidants, among other beneficial traits (Khan et al., 2012). Application of Curtobacterium albidum strain SRV4 in rice under salinity showed the higher activity of antioxidant enzymes, including CAT and SOD, and provided tolerance to salinity compared to non-inoculated plants (Vimal et al., 2019).

Phytohormones play vital roles in regulating plant growth, development, and various physiological processes (Acharya and Assmann, 2009; Acharya et al., 2013; Miransari and Smith, 2014; Acharya et al., 2017). In response to salinity stress, various phytohormones, including auxins, CK, ET, and GAs, are critical in helping plant to adapt (Kaundal et al., 2021; Acharya et al., 2022a, Acharya et al., 2022b). Several PGPMs have been identified that produce and excrete hormones that plants can absorb through their roots, enhancing plant growth or regulating hormone balance to improve salinity responses (Backer et al., 2018). For instance, application of a halotolerant PGPM strain, Glutamicibacter sp. YD01, equipped with ACC deaminase, has been shown to provide salinity tolerance by reducing ET in rice seedlings ( Supplementary Table 1 ) (Ji et al., 2020). Furthermore, PGPMs are known to synthesize some phytohormones like auxin, CK, ET, GA, and SA, modulating physiological activity through molecular responses (Orozco-Mosqueda et al., 2023). Additionally, many organic compounds produced by PGPMs are known to influence plant physiological activities, underlining the significant role these microorganisms play in enhancing plant health and stress resilience. The next section explores how PGPMs influence the regulation of various phytohormones.

Biofilms are complex and structured communities of microorganisms, primarily bacteria that adhere to surfaces and are encased in a self-produced matrix of extracellular polymeric substances (EPS), comprising polysaccharides, proteins, nucleic acids, and other molecules (Di Martino, 2018). Biofilm-producing microorganisms gain a survival advantage in unfavorable conditions, including saline soils, where increased osmotic pressure could otherwise lead to cell death through cytoplasmic lysis. The ability to form biofilms equips these microorganisms with a protective mechanism against saline environments and other abiotic stresses, effectively serving as barriers that enable them to withstand and thrive under harsh conditions (Yin et al., 2019).

Halotolerant PGPMs thrive in saline environments, establishing themselves around the root zone, and promoting plant growth and development (Ahemad and Kibret, 2014). They produce various beneficial chemicals and growth regulators in the rhizosphere. Among these, certain halotolerant PGPM strains have been discovered to enhance salinity stress tolerance in plants. For example, two halotolerant biofilm-forming PGPM strains, AP6 and PB5, affiliated with B. licheniformis and P. plecoglossicida, respectively, were found to produce IAA and ACC deaminase (Yasmeen et al., 2020). These strains contributed to salinity tolerance and led to better growth and yield of sunflower plants than the non-inoculated plants. It demonstrates the multifaceted benefits of biofilm formation, including the production of IAA and ACC deaminase, which contribute to improved plant growth, productivity, and salinity tolerance (Yasmeen et al., 2020). Furthermore, wheat seedlings inoculated with exopolysaccharide-producing bacteria have been shown to stimulate growth and provide salinity tolerance by restricting Na⁺ influx (Ashraf et al., 2004). An exopolysaccharide-producing PGPM strain, C. albidum strain SRV4, provided tolerance to rice (Vimal et al., 2019).

PGPMs can be highly context-dependent, varying across different soil types and climates (Martínez-Viveros et al., 2010). One of the primary challenges in utilizing PGPMs is ensuring their survival and persistence in the soil environment. Soil conditions, such as temperature, pH, and the presence of competing microorganisms, can impact the viability of these microbes (Martínez-Viveros et al., 2010), making it difficult for farmers to predict and ensure positive outcomes from PGPM application, which, in turn, slows their widespread adoption in agriculture.

While some PGPMs exhibit broad-spectrum benefits, many work optimally with specific plant species or crop cultivars (Dhawi and Hess, 2017; Pratush et al., 2018; Ma et al., 2020). Identifying the most effective strain for each crop–soil combination requires extensive testing, complicating large-scale implementation. Additionally, natural crop variability within a species further challenges finding a “one-size-fits-all” solution.

The development of PGPMs is based on screening assays in a laboratory setting, which measure specific PGPM activities such as IAA production, calcium phosphate solubilization, and siderophore production (Ganesh et al., 2022). However, the presence of these characteristics in microorganisms does not always guarantee effective PGPM function under field conditions. Conversely, microbes lacking these in vitro properties might possess alternative mechanisms for promoting plant growth, which are less well-understood. Because of this knowledge gap, such microbes risk being overlooked and discarded during the early stages of laboratory screening, potentially missing out on effective PGPM candidates (Cardinale et al., 2015).

PGPMs are living entities with specific viability requirements, and owing to their structural and cellular composition, they have a relatively short shelf life (Arriel-Elias et al., 2018). Maintaining their effectiveness throughout production, storage, and application can be expensive, complex, and challenging, especially for small-scale farmers.

Regulatory systems frequently lag scientific progress. Concerns about unintended environmental impacts and the complex nature of microbial communities can create regulatory hurdles, stalling commercialization efforts (Leggett et al., 2011). Additionally, the lack of consistent standardization in strain identification, characterization, and quality control for agricultural applications impedes broad adoption and undermines farmer confidence.

A clear economic benefit demonstrated for farmers is crucial for widespread adoption. The microbes’ application method must align with the farmer’s equipment and agricultural practices. Factors like upfront costs, application complexity, and reliable performance data compared to conventional methods need careful consideration. Farmers commonly perceive PGPM formulations as costlier and less effective than chemical alternatives, which needs to be addressed.

Crop producers usually depend solely on farming for their livelihood and sustenance, with little to no extra financial runway between two crops. With such financial constraints, farmers are unwilling to use nontraditional measures compared to tried-and-tested methods. This can pose a big hurdle in the widespread adoption of PGPMs.

More extensive work needs to be done by agricultural researchers and farmers to adopt PGPM formulations on a broader scale (Parnell et al., 2016). Educating farmers about the long-term benefits and building trust in PGPM technology is essential. Farmer education and awareness play a crucial role. Shifting from traditional practices to effectively utilizing PGPMs requires knowledge and training, which may be limited to certain regions. Addressing these challenges through continued research, improved formulations, streamlined regulations, and effective farmer education is crucial to unlocking the full potential of PGPMs and transforming agriculture toward a more sustainable future.