In plants, ABA signaling and MAP kinases are involved in the production and accumulation of osmolytes
[101][53]. The rapid activation of MAPKs such as MAPK3, 4, 6 was frequently observed in response to salt stress
[102][54]. For instance, the salt-induced activation of MKK4 regulates the activity of MPK3, which increases the expression of stress-responsive genes such as
NCED3 and
RD29A [103][55]. The protein kinase SnRK2 family can be activated in both an ABA-dependent and -independent way. SnRK2.2, SnRK2.3, and SnRK2.6 are important for transducing the ABA signal, activating AREB/ABF TFs (ABA-responsive element binding factors), and regulating the downstream responsive gene
[104,105,106][56][57][58].
43.4.2. Antioxidant Activity
Many adverse environmental factors can alter the equilibrium between ROS production and scavenging activity
[107][59]. The content of reactive oxygen species (ROS) increases dramatically during salt stress, mainly due to the disruption of electron transport chains (ETC) during photoinhibition and/or a decrease in water potential
[108][60]. ROS comprise different compounds such as O
2•
−, H
2O
2,
1O
2, HO
2•
−, OH•, ROOH, ROO•, and RO•, which react spontaneously with organic molecules and cause membrane lipid peroxidation, protein oxidation, enzyme inhibition, and DNA and RNA damage
[109][61]. In order to detoxify ROS, plants have developed antioxidant systems including antioxidant enzymes and non-enzymatic compounds. Antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), ascorbate peroxidase (APX), and glutathione reductase (GR), and the accumulation of non-enzymatic antioxidant compounds (carotenoids, flavonoids and other phenolics, proline) are positively correlated with a tolerance to salinity in plants
[49][39].
MAPK cascades act as important signaling pathways in responding to oxidative stress and regulating ROS homeostasis. For example, the activity of scavenging enzymes is controlled by the MEKK1-MKK1/MKK2-MPK4 cascade to maintain ROS homeostasis. In
Arabidopsis, two other proteins, MPK3 and MPK6, phosphorylate and activate HEAT SHOCK FACTOR A4A (HSFA4A), controlling ROS homeostasis and positively regulating the salt stress response
[110][62]. Increasing evidence suggests an active role for PGPR in modulating the antioxidant defense system of plants by increasing the activity of various antioxidant enzymes under stress conditions
[111,112,113][63][64][65].
43.4.3. Ion Homeostasis
Regardless of their nature, neither glycophytes nor halophytes can tolerate high concentrations of Na
+ in their cytoplasm. The maintenance of ion homeostasis through Na
+ ion compartmentalization is critical for growth during salt stress
[45][36]. The regulation of Na
+ uptake and transport in salt-stressed plants has been interpreted in the context of maintaining high K
+/Na
+ ratios in the cytoplasm. Because Na
+ and K
+ possess very similar physical and chemical properties, the two ions compete for many key metabolic processes in the cytoplasm. In fact, Na
+ inhibits the enzymatic activity of many enzymes that require K
+ to function. Since more than 50 different cytoplasmic enzymes are activated by K
+, the disruption of their activity leads to a dramatic effect on cellular metabolism
[117][66]. Na
+ control mechanisms involve Na
+ exclusion from roots, Na
+ long-distance transport, and Na
+ compartmentation
[118][67]. Three main proton pumps have been identified as being related to salt stress tolerance in plants: (i) vacuolar-proton phosphatase which generates a proton gradient by using energy from pyrophosphate (Ppi); (ii) plasma membrane H
+/ATPase; and (iii) vacuolar H
+/ATPase which couples H
+ transport and ATP hydrolysis
[119][68]. The salt overly sensitive (SOS) stress-signaling pathway is considered the first mechanism used by plants to exclude Na
+, and increasing evidence demonstrates its role in ion homeostasis and salt tolerance. The SOS signaling pathway is composed of three proteins: SOS3 (Ca
2+-binding protein), SOS2 (serine/threonine kinase), and SOS1 (Na
+/H
+ antiporter). High salinity triggers an increase in cytosolic Ca
2+ concentration, inducing the Ca
2+-mediated activation of SOS3. The interaction between SOS2 and the SOS3 protein results in the activation of kinase and the release of the SOS3/SOS2 complex into the cytosol, where it phosphorylates SOS1, activating its transport function
[48,120][38][69]. SOS1 is required to extract excess Na
+ from cells (i.e., into the rhizosphere through root epidermal cells, or into the xylem through parenchyma cells of the xylem), and thus reduce ionic stress
[121,122][70][71].
The
Bacillus megaterium ZS-3 endophytic strain induces systemic tolerance to salt stress in
Arabidopsis thaliana through the regulation of different processes, including photosynthesis, osmotic adjustment, and ion homeostasis.
B. megaterium ZS-3 increases the K
+/Na
+ ratio by directly limiting the accumulation of Na
+, rather than by increasing the K
+ content, in both the presence and absence of salt. Furthermore,
B. megaterium ZS-3 determines the down-regulation of
HKT1 and the up-regulation of
NHX1 and
AVP1 (a vacuolar H
+-pyrophosphatase) pumping H
+ into vesicles, which acidify to generate a H
+ gradient across the membrane.
43.4.4. Hormonal Modulation
Plants have developed several strategies to cope with salt stress by optimizing the balance between growth and stress responses, and plenty of evidence suggests a critical role for phytohormones. Abscisic acid (ABA), salicylic acid (SA), and jasmonic acid (JA) are considered stress hormones, while auxins (IAA), cytokinins (CKs), and gibberellin (GB) are considered growth promoters
[125][72]. Most studies on hormone modulation induced by PGPR are about ABA and its induced pathways. Indeed, among all phytohormones ABA is the main factor in the regulation of resistance to abiotic stresses in plants, which coordinates a variety of functions. Its activity depends on its concentration in the plant. At a normal level, ABA regulates various physiological processes such as stomatal opening, embryo morphogenesis, seed development, dormancy, and the synthesis of storage proteins and lipids
[126[73][74],
127], while at high concentrations, ABA inhibits plant growth. Under conditions of abiotic stress, such as drought and salt stresses, ABA biosynthesis is strongly induced, leading to an increase in its content in the plant, and determining stomatal closure and a change in gene expression, which may favor plant adaptation and survival
[128][75].
The importance of ABA in this phenomenon is represented by the fact that in
Arabidopsis seedlings, the genes modulated by ABA comprise about 10% of the genome, evenly divided between induced and repressed genes, i.e., two to six times more than those modulated by other plant hormones
[131][76]. Most ABA-modulated genes encode proteins involved in stress tolerance, such as dehydrins and enzymes that detoxify ROS, as well as those involved in osmolyte metabolism, several transporters, transcription factors, protein kinases and phosphatases, and enzymes involved in phospholipid signaling
[132][77]. Furthermore, ABA biosynthesis is also regulated by its end products since ABA negatively regulates its own accumulation by activating its catabolic enzymes
[133][78].
During salt stress, many transcription factors (TFs) such as MYC, MYB, bZIP, MADS, and BHLB play a fundamental role in the response to stress. TFs generally act as key negative or positive regulators of gene expression. For instance, the regulation of MYB TFs in response to salt stress involves ABA. When plants grow under high salinity conditions, the ABA content increases significantly, initiating a cascade of salt stress response signals in the ABA-dependent pathway that leads to the up- or down-regulation of downstream response genes.
Ethylene is a key gaseous phytohormone with a wide range of biological activities that can affect plant growth and development. At high concentrations, it induces defoliation and other cellular processes that may lead to an inhibition of root and stem growth and premature senescence, reducing crop performance
[136,137][79][80]. Under normal conditions, ethylene is produced, starting from 1-aminocyclopropane-1-carboxylate (ACC) through the action of the ACC synthetase enzyme. As a response to exposure to various environmental stresses such as drought, salt stress, cold, infections, heavy metals, and flooding, plants increase the production of ACC, resulting in a rise in ethylene concentration
[31][81]. Auxins, such as IAA, control a wide range of processes in plant development and growth
[138][82]. By increasing both the surface area and the length of the roots, IAA allows plants to have greater access to soil nutrients
[139][83].
PGPR can modulate plant hormones to alleviate salt stress symptoms. Most PGPR can produce IAA, thus stimulating the development of secondary roots. Furthermore, by increasing the absorption of nutrients and water, IAA alleviates the effects of some abiotic stresses such as salinity and drought
[142][84]. The production of the ACC-deaminase enzyme is another important physiological trait of PGPR that facilitates plant growth
[143,144][85][86]. Indeed, under stressful conditions, when the level of ethylene in the plant might reach inhibitory levels, this enzyme supports plant growth by degrading ACC
[38,145,146][26][87][88].
43.4.5. Nutrient Uptake
A high Na
+ concentration induces nutritional disorders that reduce the activity of many essential nutrients in the soil, making them less available to plants. Salinity reduces the uptake and translocation of nitrogen (N), potassium (K
+), phosphorous (P), calcium (Ca
2+), and iron (Fe)
[18,158,159][7][89][90]. Micronutrients may be less important for plants compared to N, K
+, Ca
2+, and P in terms of resistance to salinity
[160][91]. The ability to fix nitrogen is widespread among prokaryotes
[161][92]. An estimated 80% of the biological nitrogen fixation comes from symbiotic association, while the rest is provided by free-living (diazotrophs) or associative systems
[162][93]. Non-symbiotic nitrogen-fixing rhizobacteria mainly belong to the
Azoarcus,
Azotobacter,
Acetobacter,
Azospirillum,
Burkholderia,
Diazotrophicus,
Enterobacter,
Gluconacetobacter, and
Pseudomonas genera
[136,163][79][94]. The inoculation of biological N
2-fixing PGPR in crops promotes plant growth, reduces the need for pesticides, and restores nitrogen levels in agricultural soil under adverse environmental conditions such as salinity
[164][95]. After nitrogen, P is the most important element in plant nutrition. It is involved in almost all major metabolic processes, such as signal transduction, respiration, photosynthesis, macromolecular biosynthesis, and energy transfer
[165][96].
Fe is a key component of various metabolic pathways. More than 100 metabolic enzymes require iron as a cofactor and are essential for many plant processes, including photosynthesis, electron transport, oxidative phosphorylation, and hormone production
[169][97]. Although it is the fourth most abundant element on earth, this huge amount of iron is not bioavailable for plants since free Fe(II) is rapidly oxidized to Fe(III), which is not assimilable due to its low solubility
[170][98]. PGPR can produce siderophores, whose main function is to convert insoluble Fe to a form accessible to microorganisms
[171][99]. Siderophores are also known to provide plants with iron and promote their growth
[172][100].
Zn is one of the micronutrients required at a small concentration (5–100 mg kg
−1) and is a co-factor and a metal activator of many enzymes
[173][101]. Zn deficiency affects membrane integrity, and the synthesis of carbohydrates, auxins, and nucleotides
[174][102].
Following the inoculation of
Kosaconia radicincitans KR-17 in
Raphanus sativus L. grown under saline conditions, Shahid et al.
[80][103] found the content of N, P, K
+, Ca
2+, Mg, Zn, Fe, Cu, and Na
+ to rise maximally compared to non-inoculated plants. In a study on
C. quinoa, the inoculation of the
Pseudomonas sp. M30-35 strain mitigated salt stress, maintaining P content and homeostasis
[81][104]. The application of
Priestia endophytica SK1 isolated from rhizospheric soil of fenugreek in
Trigonella foenum-graecum induced nodule formation and was found to raise the content of N and P under 100 mM NaCl compared to control plants
[82][105]. The inoculation of
Glutamicibacter sp. and
Pseudomonas sp. in
Suaeda fruticose resulted in a reduction in Na
+ and Cl
− in the shoots of stressed plants. The inoculation of
Pseudomonas sp. increased the K
+ and Ca
2+ content and improved the activities of urease, ß-glucosidase, and dehydrogenase compared to their corresponding non-inoculated stressed plants
[83][106]. Comparative transcriptomic analyses between wheat roots inoculated with
Arthrobacter nitroguajacolicus or not, revealed that under salt stress, the inoculation induces the up-regulation of various genes related to metal binding and involved in iron acquisition
[84][107]
43.4.6. Biofilm Formation
Biofilms are extracellular matrices composed of proteins, nucleic acids, lipids, exopolysaccharides (EPS), and embedded microorganisms, which allow rhizosphere bacteria to adhere to the surface of plant roots
[176][108]. The biofilms produced by PGPR also protect plants from stress conditions such as drought and salinity, as their components can co-ordinately function as osmoprotectors
[177][109]. EPS is a natural mixture of high-molecular-weight polymers released by bacteria into the environment to mitigate physiological stress and plant environmental stresses such as salinity
[178][110].
43.4.7. Volatile Organic Compounds (VOCs)
Volatile organic compounds (VOCs) are usually a mixture of metabolites produced by microorganisms during primary or secondary metabolism, characterized by their low molecular weight (>300 Da), lipophilic nature, and low boiling point. VOCs can travel from the point of production through soils, liquid, and atmosphere, which makes them ideal chemical messengers for intra- and inter-organismic communication
[179,180][111][112]. The production of VOCs by microorganisms depends on the microbial species and can be affected by many environmental factors such as pH, temperature, humidity, etc.
[181][113].
PGPR synthesize a broad range of VOCs, including aliphatic aldehydes, esters, alcohols, organic acids, ethers, ketones, sulfur-containing compounds, and hydrocarbons. Mainly known to be involved in biocontrol activity against plant phytopathogens or the stimulation of plant defense mechanisms, VOCs produced by PGPR also affect plant growth and abiotic stress tolerance
[182][114].
54. Conclusions
In conclusion, PGPR have been shown to be effective in improving plant growth under salinity. Thanks to these bacteria’s ability to grow under salt stress conditions and their effects on plant growth, they are a very interesting, sustainable solution for improving crop productivity in saline soils. Although some PGPR have been associated with the modulation of genes belonging to different signal transduction pathways such as
MYC,
DREB2, and
WRKY, extensive research remains to be conducted to understand the signaling pathways involved in plant and PGPR cross-talk, inducing the plant’s salt tolerance response.