The rhizosphere is the area in soil that immediately surrounds the roots of plants [36,37]. The rhizosphere contains countless species and a vast diversity of microorganisms [36,38]. One important component of the rhizosphere is plant mucilage. Mucilage is excreted by plant root tissue and can serve as a carbon source for microorganisms. The amount and composition of the mucilage can have a large impact on the bacterial species that live in the rhizosphere [37,39,40]. By excreting mucilage, plants attract beneficial microbes to the rhizosphere and benefit from microbial ability to break down sugars, fix nitrogen, suppress pathogens, etc. Mucilage can also play a role in attracting and supplying sugars for halophilic bacteria. These halophilic rhizobacteria could then, in turn, aid plants in saline soils. Bacteria found to be associated with the roots and that stimulate plant growth are termed plant-growth-promoting rhizobacteria (PGPR), and along with those that become established within plant tissue as endophytes are more broadly termed PGPB (plant-growth-promoting bacteria).

Species of bacteria and fungi that live within plant tissues without causing harm to the plant are defined as bacterial and fungal endophytes, respectively. There has been increasing interest in the role of endophytes in plant adaptation to different adverse environmental conditions in a variety of ways, as outlined in Figure 1 [41,42]. It has been well-established that microorganisms associated with plants thriving under harsh environmental niches play a crucial role in their adaptations to these suboptimal conditions [43]. The key mechanisms involved in such microbial-mediated adaptations to different stresses include modulation of phytohormones biosynthesis (auxins, cytokinins, gibberellins, abscisic acid, ACC deaminase (1-aminocyclopropane-1-carboxylate deaminase), brassinosteroids, and ethylene), accumulation of osmoprotectants (betaines, proline, and soluble sugars), upregulation of different defense genes, and production of secondary metabolites [43,44].

Plant hormones are chemical messengers that regulate various physiological processes in plants, including plant growth, development, and response to environmental stimuli such as light, temperature, and stress [45,46]. These hormones are master regulators, enabling plants to adapt and thrive in changing environmental conditions [47,48]. The diversity of phytohormones mirrors the multifaceted challenges that plants encounter in their environments. Each type of hormone is specialized in governing specific physiological responses, yet their interplay creates a sophisticated web of regulations that facilitate seamless coordination between different cellular and developmental processes. Moreover, plants often exhibit pleiotropy, where a single phytohormone can have widespread effects, influencing various aspects of plant life, while multiple hormones collaborate to finetune a particular biological event. This intricately woven system of phytohormonal control empowers plants to maintain homeostasis and optimize their growth in response to internal and external cues. In response to environmental changes, such as light, temperature, water availability, or the presence of pests and pathogens, plants dynamically adjust their phytohormone profiles. Studies have shown that in the presence of high concentrations of salt, production of auxin is severely limited. Plants grown in salty soils had insufficient auxin levels and suffered from stunted growth [48]. Different bacterial strains have been shown to induce hormone production and supplement those produced by the plant through the function of microRNAs (miRNAs) [47,49]. If a plant has insufficient hormone levels, the growth and development of the plant, as well as the fruit, can suffer and yield can be significantly decreased.

Indole-3-acetic acid (IAA) is a plant hormone that regulates the production of new root and shoot tissues. It has been shown that IAA produced by bacteria can induce adventitious shoot growth [44,47,50,51,52]. High salinity induces the utilization of 1-aminocyclopropane-1-carboxylic acid (ACC). ACC is a precursor for ethylene, a plant hormone that mediates a wide range of essential plant responses. However, at elevated levels, ethylene has a deleterious effect on root and shoot elongation, leaf expansion, and overall plant health [52]. ACC deaminase is an enzyme that breaks down ACC, preventing the production of ethylene, and it can alleviate the stress response in plants. ACC deaminase-producing bacteria can help aid the plant when it is under salt stress, and even help promote plant growth and antioxidant production [52], as has been shown for the French bean [53].

Microorganisms are known to produce over 20,000 different secondary metabolites [54]. These metabolites can affect the survival and performance of other organisms. Not only do endophytes produce secondary metabolites beneficial to plant growth and disease response but endophytes also produce novel biomolecules and plant growth promotors [55]. Because of these beneficial effects, the utilization of these endophytes holds potential to improve plant growth, particularly under adverse environmental conditions such as on marginal lands, and it may confer resilience to climate change.

There are a considerable number of Bacillus strains that have been identified as having general PGPB activity (reviewed in [56]). To date, there has been some limited analysis of possible functions as ST-PGPB for some, which is briefly summarized here. One recent report suggests that a soil Bacillus strain is recruited to coastal halophytes by exudates from plant roots, but this work does not include plant growth stimulation studies [57]. There is also an interesting recent publication on combinations of a Bacillus strain with other bacteria including a Pseudomonas strain to promote plant growth [58]. Other work has shown the promise of a combination of bacterial and mycorrhizal fungal applications in enhancing soil fertility and rice production due to enhanced mineral uptake from the soil [59].

The microbial community found to be associated with halophytes growing in saline soils represents a rich source of ST-PGPB (salt-tolerant, halophilic, rhizobacteria, or endophytes) [60,61,62,63,64,65,66,67,68,69,70,71,72,73,74]. Improvement of soil health and potential in bioremediation have also been reported [48,61]. A combination of strains may provide synergistic benefits. For example, one recent study showed the value of using a consortium of multiple halophilic strains isolated from halophytes to improve the growth of several crops under saline conditions [75]. Other studies have shown beneficial effects of ST-PGPB in reducing salt stress and improving yield in rice [76,77]. PGPB strains that have ACC deaminase and ROS scavenging activity have been shown to contribute to amelioration of salinity stress [78]. These reports provide a strong foundation for the use of halophiles isolated from the rhizosphere or roots of halophytes as inocula to stimulate the growth of salt-sensitive crops [79,80]. Recently, an Actinomycete was found to have antifungal activity and alleviate salt stress in tomato [81]. Another Actinomycete was reported to promote the growth of a halophyte using irrigation with seawater. Some biofilm-forming strains of Bacillus have been shown to enhance the growth of maize in saline conditions [82,83]. Another example is Bacillus subtilis (GB03), which was identified as a rhizosphere bacterium that stimulates growth of white clover under saline conditions [84], and which has since been renamed Bacillus amyloliquifaciens GB03 [85,86]. This strain produces volatile compounds that enhance plant photosynthetic capacity and chlorophyll content and induces an elevation of endogenous sugar content and suppression of abscisic acid (ABA)-induced RNA transcripts. Some of these strains also aid in dealing with other types of abiotic stress [87]. B. amyloliquifaciences GB03 has been shown to stimulate tall fescue growth under nitrogen limitation by altering regulation of phytohormones and nutrient homeostasis [88,89]. Other effects include alterations of plant gene expression, including upregulated expression of the HKT1 sodium transporter gene in shoots and downregulated expression in roots, which results in lower sodium accumulation throughout the plant [84,90].

In addition to PGPR isolated from soil around plant roots, endophytes (bacteria growing within plant tissues) have also been identified that stimulate plant growth [80,91,92]. Mechanisms by which endophytes enhance plant growth are thought to include enhanced nutrient acquisition and changes in host plant gene expression. For example, ACC (1-aminocyclopropane-1-carboxylase) deaminase is a bacterial enzyme found in many endophytes that stimulates nutrient acquisition and plant growth by reducing the amount of ACC converted to ethylene, a known inhibitor of plant growth, in response to salt, drought, and other environmental stresses [80,93,94]. Burkholderia phytofirmans is an endophyte that alters plant gene expression to enhance growth of six of the eight cultivars of switchgrass that were tested [91]. Inoculation with this strain was found to induce wide-spread changes in gene expression in the plant host, including altered expression of some transcription factors that are known to regulate the expression of plant stress factor genes [95]. Other bacterial endophytes (species of Sphingomonas, Pantoea, Bacillus, and Enterobacter) have been identified that enhance the salt tolerance of hybrid elephant grass [69], likely because of enhanced nutrient acquisition and/or gene expression changes [96].

Salt stress is one of the largest abiotic factors that can impact growth of an organism, including bacteria, which are also susceptible to osmotic stress. Halophilic bacteria have multiple mechanisms to counteract the osmotic stress of saline environments. There are two main types of adaptation mechanisms that halophiles use to prevent desiccation in the presence of salt: accumulation of water-soluble organic compounds in the cytoplasm and controlling the flux of inorganic ions. The main way that bacteria control the flux of inorganic ions is by exporting K⁺ ions to offset the influx of Na²⁺ ions. In addition, many halophiles utilize accumulation of water-soluble organic compounds (ectoine, hydroxyectoine, betaine, and choline) to offset the osmotic stress of highly saline environments. The accumulation of these compounds, or osmolytes, helps to draw water into the bacterial cell, preventing desiccation of the cell [97]. Another strategy that is employed by a wide variety of halophiles is controlling the flux of inorganic ions in the cell. If a cell has an influx of inorganic ions, this can lead to desiccation of the cell and eventual death of the organism. One method to prevent this influx of ions, typically sodium ions, is to actively pump intracellular potassium outside of the cell. This potassium typically comes in the form of KCl, and the export of KCl helps to offset the influx of NaCl from the saline environment [97].

It appears that in addition to the mechanisms used by PGPBs in general, there may be different mechanisms for PGPB stimulation of plant growth in saline conditions depending on the plant and bacterial species involved. The mechanism(s) by which halophilic bacteria stimulate plant growth may involve production of volatile compounds or other signals that stimulate expression of genes to enhance growth via increased photosynthesis [98] and other processes, including increased expression of plant membrane ion transport proteins. Other mechanisms that have been proposed include, but may not be limited to, the following ([67], Figure 1 and Table 1):

• Some microbes produce biofilm/exopolysaccharides in the rhizosphere that trap water and nutrients and decrease plant uptake of sodium ions from the soil.
• Some microbes inhibit growth of fungi and plant pathogens and/or select a certain microbial community in the rhizosphere.
• Enhance plant access to nutrients.
• Some microbes function as phytostimulators to produce ABA, IAA, and other plant hormones that stimulate shoot formation and plant growth by enhancing expression of specific plant genes.
• Microbes can function as biofertilizers to produce nutrients or improve nitrogen fixation for the plant and/or enhance photosynthesis.
• Solubilization and translocation of ions such as phosphate and iron (by siderophores) to the plant. Phosphate and iron are critical to the plant but are often in low abundance or present only in inorganic forms that must be solubilized for use by plants ([82,83], Table 1).

While much of the published work has focused on Bacillus species or other Gram-positive bacteria, some Gram-negative bacteria also have been shown to have PGPB potential. Two that have been shown to stimulate plant growth of salt-sensitive plants in salty soil are Kushneria and Halomonas (Table 1). Kushneria is a genus of bacteria in the Halomonadaceae family and comprises halophiles. The genus Kushneria was formed in 2009 when Halomonas marisflavi along with two other Halomonas strains were moved into this novel genus [116]. One of the most notable features of Kushneria is its ability to grow in high salt concentrations. Studies have shown that some species of Kushneria can grow in up to 25% NaCl concentration, which is higher than the salt concentration of seawater. This makes the genus an interesting subject of research for its potential use in bioremediation of saline soils, as well as in the production of salt-tolerant crops. Strains of Kushneria have been isolated from a variety of different salty environments, including a solar saltern, the leaves of black mangroves, sea water, salt mines, cured meats, and salt fermented foods [117,118,119,120,121,122,123]. Many species in this genus are adapted to hypersaline environments, and different strains have been isolated from the rhizosphere as well as the endosphere of halophytes [119,124]. These bacteria exhibit the ability to produce a variety of osmolytes, bioactive compounds (including betaine and ectoine that help protect from stress), and plant growth hormones [119,124]. Certain Kushneria species have been found to promote plant growth and act as biofertilizers or may function in phytoremediation, especially in saline agricultural soils [125]. A study by Parida and Das [126] revealed that Kushneria sp. NRCC 31,399 enhanced the growth of rice plants under saline conditions, indicating its potential use as a bioinoculant for salt-affected agricultural lands. Kushneria species are known for their ability to accumulate compatible solutes, which are small organic molecules that help the bacteria to cope with osmotic stress. These solutes have potential applications in various industries. In a study by Kulkarni et al. [127], Kushneria sp. GSB1 was found to produce the compatible solutes ectoine and hydroxyectoine in considerable amounts, highlighting the biotechnological potential of this genus in the production of these valuable compounds. Studies have shown that some species of Kushneria produce compounds that exhibit antimicrobial activity against various pathogenic bacteria and fungi. This suggests that Kushneria may have potential applications in the development of novel antibiotics and antifungal agents. Kushneria strains have been isolated from both the endosphere and the rhizosphere of plants and many exhibit the ability to produce a variety of plant hormones (Table 1). These examples demonstrate the diverse and promising biotechnological applications of Kushneria species. Continued research and exploration of this salt-tolerant bacterial genus may uncover even more practical uses in the future.

Kushneria and Halomonas bacteria have been isolated from a halophyte, Salicornia, in Tunisia, and some have been shown to have plant growth promotion activity [63]. A Kushneria marisflavi isolate in combination with Pseudomonas stutzeri was found to reduce salinity-stress-induced damage in lettuce and barley [101]. Despite the potential applications of Kushneria, much of its biology remains poorly understood. Further research is needed to fully elucidate the metabolic capabilities and genetic makeup of the genus. This would not only provide insights into the biology of Kushneria but also pave the way for the development of new biotechnological applications of the genus.

Halomonas bacteria are able to grow in high-salt conditions and at high pH values. Halomonas can also resist contamination by other microbes, due to its ability to grow in highly saline and alkali conditions [91]. Halomonas spp. have been isolated from the endosphere of different plants, shrubs, and trees. They have been identified as Gram-negative, aerobic with yellow pigmentation, and are rod shaped [128]. Strains of Halomonas have been isolated from a variety of highly saline environments including salt marshes, the endosphere of halophytes, salt-cured meats, and fermented foods [125,128,129,130,131]. Bacteria from this genus have been found that produce a wide variety of diverse biochemicals and exopolysaccharides (EPSs) [129,130]. Table 1 includes other Halomonas strains that have been reported.

Some Halomonas species isolated from plants have been shown to have potential as PGPB [132,133] or in phytoremediation to improve soils [125] (Table 1). A collection of halotolerant microbes isolated from Salicornia ramiosissima [132] contains some isolates with PGPB activity. Two Halomonas and two Bacillus strains were identified from the rhizosphere of quinoa that have different activities as PGPB, and it was shown that these strains exhibit beneficial traits that are salt-regulated [133].

A consortium of five Halomonas strains was shown to improve salt tolerance of rice [106], suggesting that combinations of two or more strains may provide synergism. In wheat, inoculation with Halomonas sp. 3H led to an increase in chlorophylls, carotenoids, sugars, and phenolics [108]. Halomonas sp. MAN5 improved root growth of Sesuvium portulcastrum [110]. In separate studies, several other Halomonas strains have been shown to enhance salt tolerance in purple basil [111], maize [112], chickpea [131], and sunflower [114]. One Halomonas strain, H. ventosae JPT10, was tested with multiple plants and enhanced salt tolerance in foxtail millet, soybean, tomato, wheat, and maize [115], indicating that some strains may serve as PGPB for multiple crop species.