Since salts are also plant nutrients, excess salt in the soil imposes competition during the absorption, transfer, or distribution of nutrients, leading to an unbalance in the plant ionic composition, and thus affecting the physiological characteristics of the plant (Rabie and Almadini, 2005; Farissi et al., 2011; Munns and Tester, 2008). Studies in saline environments have indicated that an abundance of Na⁺ and Cl⁻ ions hinder the solubility, mobility and absorption of nutrients, for instance, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), copper (Cu), and zinc (Zn) in plants (Zai et al., 2021; Hasegawa et al., 2000; Evelin et al., 2019). Na⁺ can enter the roots through non-selective cation channels (NSCCs) in the plasma membrane, increasing Na⁺ content in roots and shoots, and leading to unfavourable Ca²⁺/Na⁺ and K⁺/Na⁺ ratios (Abdel-Fattah and Asrar, 2012; Van Zelm et al., 2020; Pedranzani et al., 2016). Since Na⁺ competes with K⁺ and Ca²⁺ for membrane transfer sites, maintaining a high ratio of Ca²⁺/Na⁺ and K⁺/Na⁺ in the cytosol is crucial for increasing plants’ tolerance to salinity and enhancing enzymatic processes (Evelin et al., 2019; Ahanger et al., 2015; Samaddar et al., 2019). A reduction in the ratio of K⁺/Na⁺ in the cytosol causes a disturbance in stomatal movement, turgor maintenance, photosynthesis, activity of enzymes, and protein synthesis (Benito et al., 2014; Maathuis and Amtmann, 1999). In contrast, a decrease in the ratio of Ca²⁺/Na⁺ causes photosynthetic tissue destruction, disruption of Ca²⁺ signalling pathways, and a reduction in hydraulic conductivity (Ait-El-Mokhtar et al., 2020) ( Figures 1 , 2 ).

N is an essential macronutrient that plants absorb it as nitrate (NO₃-), and ammonium (NH₄ ⁺) ions, whereas salinity interferes with their absorption by immobilizing them (Frechilla et al., 2001; Munns and Tester, 2008). In the saline environment, the NH₄ ⁺ uptake is challenged by Na⁺, whereas the NO₃- uptake is inhibited by Cl⁻ in the membrane, which causes the low flux of NH₄ ⁺ and NO₃- from soil to roots, leading to a reduction in nitrate reductase activity (NR) in plants (Hoff et al., 1992; Evelin et al., 2019). P is another nutrient whose absorption by roots is interfered with salinity because of its precipitation with other cations, for example, Zn2⁺, Mg2⁺, and Ca2⁺ (Bano and Fatima, 2009; Evelin et al., 2019). The decrease in P content as an essential macronutrient in plants causes the stoppage of growth and premature death of leaves (Giri et al., 2003; Taiz et al., 2015). Furthermore, salt in the soil renders water unavailable to plant roots, resulting in a decline in turgor pressure in plant cells, cell dehydration, and water stress (Füzy et al., 2008; Zou et al., 2013; Ait-El-Mokhtar et al., 2020) ( Figure 2 ).

A plant’s root system absorbs water and nutrients, and it also comes in direct contact with a salty environment, so it contributes greatly in protecting the plant from salinity stress (Rana et al., 2019; Evelin et al., 2019). There is a decrease in growth of roots with increasing salinity of soil, and this is the first response of plants to excessive soil salts (Amanifar et al., 2019; Greenway and Munns, 1980). When salt is present in the rhizosphere, primary roots grow slower because salt inhibits cell division and root epidermal cells elongation, while lateral roots grow (Bernstein et al., 2013; Jung and McCouch, 2013). Furthermore, there is evidence that the toxic effects of salinity on root growth and development are related to the inhibition of endogenous phytohormones levels, such as Indole-3-acetic acid (IAA), and indole-3-butyric acid (IBA) (Khan et al., 2015; Egamberdieva, 2009; Egamberdieva et al., 2016).

Photosynthesis, as a primary metabolism’s key process, is highly sensitive to salinity (Jia et al., 2019; Porcel et al., 2012), and salinity of the soil inhibits plant photosynthetic ability through its effect on leaf surface area, photosystem II efficiency, stomatal conductance, and content of chlorophyll (Farissi et al., 2011; Miransari et al., 2008). Salinity directly damages the complete ultrastructure of photosynthetic organelles such as chloroplasts and also reduces photosynthetic pigment content (total chlorophyll, carotenoids, Chl a and Chl b) (Liang et al., 2021; Akram and Ashraf, 2011; Ait-El-Mokhtar et al., 2020). Chloroplasts are the most susceptible cellular organelles to salt, and in high salinity conditions, grana and thylakoids begin to disintegrate, and vanish as a result of concentration of cations changes in chloroplasts, membrane damage and swelling of thylakoids, which consequently affects the efficiency of light energy utilization (Kaya et al., 2009; Peng et al., 2019). According to studies, salinity suppressed enzymes involved in the synthesis of photosynthetic pigments, including chlorophyll synthetase, and increased chlorophyll degradation enzyme activity, reducing chlorophyll and content of photosynthetic pigments (Wang et al., 2019a; Zai et al., 2021; Tsunekawa et al., 2009). In addition, under salinity stress, chlorophyll levels can decrease due to low mineral absorption, particularly magnesium (Giri and Mukerji, 2004). However, photosynthetic pigment synthesis, and efficiency of photosynthesis in saline environments strongly depends on the plant species and even the genotypes (Bistgani et al., 2019; Khan et al., 2015) ( Figure 1 ).

In addition to directly damaging photosynthetic machinery, salt stress can also damage the photosystem II (PSII) reaction center, interfere with electron transfer from PSII to photosystem I (PSI), and ultimately decrease photosynthesis (Evelin et al., 2013; Liu and Shi, 2010; Jia et al., 2019). Furthermore, the researchers found that under salinity stress potential photochemical efficiency (Fv/Fo), maximum quantum efficiency of PSII (Fv/Fm), and photochemical quenching coefficient (qP) decreased while non-photochemical quenching (NPQ) increased (Liang et al., 2021; Sheng et al., 2008). Osmotic stress caused by salinity reduces the water content in shoots and leaves, thus closing the stomata and reducing CO₂ availability (Gao et al., 2020; Duarte et al., 2013; Zu et al., 2005). In the absence of CO₂ absorption, excess electrons are accumulated in the thylakoid membranes (disruption of electron transfer between PSII and PSI), resulting in PSII degradation and damage to other photosynthetic apparatus components (Gao et al., 2020; Chang et al., 2018).

Salt-induced stress leads plants to produce an excessive amount of reactive oxygen species (ROS), which causes oxidative degradation to components of cells (Amanifar et al., 2019; Parihar et al., 2020; Boorboori and Zhang, 2022). Oxidative stress process in plants occurs when multiple metabolic pathways are disrupted, and high-speed and energy electron transfers to molecular oxygen (Gill and Tuteja, 2010; Miller et al., 2010). Oxidative damage caused by ROS to plants includes hydroxyl radicals (·OH), superoxide radicals (O₂ ^•−), hydrogen peroxide (H₂O₂), and singlet oxygen (1O₂) (Abd_Allah et al., 2015; Gao et al., 2020). ROS buildup disrupts important cellular structures, including organelles, membrane lipids, nucleic acids, proteins, macromolecules (especially enzymes), leading to a reduction in nutrient absorption and transfer, disruption of respiratory and photosynthesis systems, and ultimately limiting growth and development of plants (Porcel et al., 2016; Janah et al., 2021b; Abeer et al., 2014) ( Figures 1 , 3 ).

Lipid peroxidation changes cell membrane’s selective permeability, and eventually causes membrane leakage, and membrane integrity loss (Abeer et al., 2014; Huang et al., 2010). Maintaining the integrity of the cell membrane is essential for plants to cope with salinity stress, since the damaged cell membrane loses its biological function and affects plant’s natural metabolism (Stevens et al., 2006; Jabeen et al., 2014). According to Nat et al., cell membranes of halophytes can be damaged by peroxidation of lipids when salt concentrations are above 300 mM for a week or more (Nath et al., 2016). Malondialdehyde content (MDA) as lipid peroxidation’s final product, in combination with relative electrolyte leakage (REL) is used as an oxidative damage index, and to identify cell membrane damage extent (Adolfsson et al., 2017; Pedranzani et al., 2016).

Plants contain a very flexible system for adjusting their morpho-physiological, molecular, biochemical and metabolic mechanisms for survival in salinity environments (Porcel et al., 2012; Evelin et al., 2019). Plants enhance their resistance against stress salinity by raising the accumulation of compatible osmolytes, regulating water absorption, maintaining the endogenous levels of growth regulators, increasing chlorophyll synthesis, regulating antioxidant molecules, and compartmentalizing toxic ions in vacuoles (Bistgani et al., 2019; Abbaspour et al., 2021; Munns and Tester, 2008). Additionally, plants regulate the rate of leaf transpiration, stomatal conductance, and photosynthesis rate by adjusting the stomata aperture, leading to greater photosynthesis efficiency (Kaya et al., 2009; Elhindi et al., 2017; Liang et al., 2021).

Osmotic regulation is another defense mechanism that helps plants control osmotic and ionic toxic impacts through the expression of genes involved in nutrient transport and partitioning, accumulation of solutes, and aquaporins (Li et al., 2020; Apse and Blumwald, 2007; Van Zelm et al., 2020). Salt Overly Sensitive 1 (SOS1) is a Na⁺/H⁺ antiporter in plasma membrane, which has an essential function in maintaining ions homeostasis and managing Na⁺ and K⁺ transport in plasma membrane and tonoplast (Li et al., 2020; Gao et al., 2020). The study conducted on Suaeda salsa showed that at 400mM NaCl concentration, Na⁺ was actively removed from the cytoplasm into the rhizosphere, while the expression of SOS1 was the highest in the roots (Gao et al., 2020). Furthermore, vacuolar Na⁺/H⁺ antiporters (NHXs) are responsible for the sequestration of Na⁺ into vacuoles, which is driven through the proton motive force generated by H⁺-PPase and H⁺-ATPase (VHA) (Diao et al., 2021; Gao et al., 2020). Moreover, The Stelar K⁺ outward rectifier channel (SKOR) is a K⁺ channel in plants that transports K⁺ from the roots to aerial parts (Li et al., 2023). SKOR identify as a transport protein with a role in loading K⁺ to the xylem, and if it was disrupted K⁺ content in shoots was significantly reduced, while K⁺ content in roots was not affected (Abbaspour et al., 2021; Sharma et al., 2013). Moreover, plasma membrane intrinsic proteins (PIPs), a plant’s aquaporin subfamily, was found to be the primary water absorption and transport channels in plant cells and can mediate the plant cell water loss under the stress of salt (Abbaspour et al., 2021).

Increasing the production of ROS can be harmful to cells, but they also regulate numerous fundamental plant processes, including salt stress response (Chen et al., 2020). Nevertheless, plants have evolved detoxification mechanisms to combat oxidative damage, including the induction of secondary metabolite production and a wide range of enzymatic and non-enzymatic antioxidants (Behdad et al., 2020; Toscano et al., 2019; Sharma et al., 2012). The antioxidant enzymes that reduce salinity-induced ROS include catalase (CAT), peroxidase (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR), and polyphenol oxidases (PPO) (Janah et al., 2021a; Waszczak et al., 2018; Apel and Hirt, 2004). In contrast, non-enzymatic antioxidants that destroy ROS include glutathione (GSH), ascorbic acid (AsA), carotenoids, flavonoids, and α-tocopherol (Foyer and Noctor, 2011; Chaparzadeh et al., 2004).

Salt stress stimulates phenolic acid, phenolic compounds, and flavonoids accumulation in plants as stress-resistance mechanism (Boughalleb et al., 2020; Duc et al., 2021; Liang et al., 2021). A change in the accumulation of phenolic compounds may be related to changes in other metabolic processes, such as phenolic acids, fatty acids, organic acids, amino acids (Saleh et al., 2020), and sugars, which leads to an improvement in the defense system of the plant, ROS reduction, and osmotic regulation (Abeer et al., 2014; Arzani and Ashraf, 2016; Saxena et al., 2015). Further, the increase in total phenolic acids is mainly due to the slope of quinic, protocatechuic, and gallic acids, followed by epicatechin, catechin, and quercetin-3-O-galactoside (Boughalleb et al., 2020). Additionally, plants are able tolerating salt by osmolytes accumulation in their cell cytoplasm for osmotic regulation, including polyamines (spermine, spermidine, and putrescine), proline, glycine, organic acids, total suspended solids (maltose, dextrin, sucrose, and glucose), α-amino nitrogen, and betaine (Evelin et al., 2013, 2019; Dodd and Pérez-Alfocea, 2012; Chen and Murata, 2011; Yang and Guo, 2018).

The mutual interaction between AMF, and salt-stressed plants enhances the selective absorption of some elements (Ca, K), limits the absorption of some other elements (Na⁺), increases the efficiency of water consumption, and promotes host plant growth (Hammer et al., 2011; de Varennes and Goss, 2007; Santander et al., 2017). In saline environments, plant root and soil colonization by AMF can improve the rhizosphere condition of the soil through strengthening the absorption of organic carbon in the soil, increasing N, P and K pools, improving the content of organic matter, adjusting pH of the soil, and preventing soil erosion (Zai et al., 2021; Chang et al., 2014; El Kinany et al., 2019; Aliasgharzadeh et al., 2001). Moreover, AMF improves water and nutrient absorption for host plants by forming a wide hyphal network and spreading myciniums outside the rhizosphere (Ortiz et al., 2015; Lin et al., 2016; Yan et al., 2016) ( Figure 4 ; Table 2 ).

Colonization of AMF also affects the concentrations and characteristics of polyamines and organic acids in plants (Sheng et al., 2011). Polyamines by increasing nutrient and water absorption, while organic acids by reducing the electrical conductivity of the soil and increasing the availability of N, P, and K in the soil, helping maintain ionic homeostasis in plant cells (Sheng et al., 2011; Evelin et al., 2019). However, different AMF species have varying abilities to obtain and provide nutrients to salt-stressed plants (Taylor et al., 2015) ( Table 2 ).

Adding AMF to plants stressed by salt enhances their growth through allowing them to increase water and nutrient absorption (Pedranzani et al., 2016; Parihar et al., 2020). Researchers have found that AMF in saline environments increases biomass production, shoot and root dry weight, number of branches, plant height, seedling diameter, leaf area and plant yield (Qiu et al., 2007; Zhao et al., 2015; Tao et al., 2012; Parihar et al., 2020). There are several plants whose biomass has increased under saline conditions due to AMF, including Chrysanthemum morifolium, Elaeagnus angustifolia, Gossypium hirsutum, Medicago sativa, Phoenix dactylifera, Zelkova serrata, Oryza sativa, Verbena officinalis, and Trigonella foenum-graecum (Jia et al., 2019; Duc et al., 2021; Evelin et al., 2019). Furthermore, AMF improves plant growth in saline environments by increasing endogenous production of growth regulators like IBA and IAA (Waqas et al., 2012; Abd_Allah et al., 2015). Research conducted by Guo et al. on Malus domestica Borkh exposed to salt, Funneliformis mosseae inoculation resulted in a positive regulation of genes involved in IAA-responsive proteins in aerial parts (TRINITY_DN10259_c0_g1), and roots (TRINITY_ DN17372_c0_g1) (Gao et al., 2020). Furthermore, AMF colonization can enhance the plant’s ability to explore water and food through rising the length and conductivity of the roots, and ultimately improve the plant’s adaptation to salinity (Chandrasekaran et al., 2019; Kumar et al., 2010).

Plant roots inoculated with AMF when stressed by salt helps to enhance nitrogen absorption, and studies have shown that up to 25% of plant nitrogen is supplied by AMF hyphae (Evelin et al., 2019; Younesi et al., 2013). The improvement of nitrate uptake by symbiotic plants with AMF could be due to maintaining membrane stability and increasing Nitrate reductase (NR) (Evelin et al., 2019). In plants stressed by salt, coexistence of AMF increases the expression of ammonium transporters (AMT1.1, and AMT1.2) and nitrate transporters (NRT1.1، and NAR2.2) (Fileccia et al., 2017; Saia et al., 2015). Meanwhile, under salinity conditions, phosphorus absorption by plants is greatly reduced, while AMF symbiosis is considered as an effective biological method to increase P absorption by plants and stimulate their growth (Romero-Munar et al., 2019; Shokri and Maadi, 2009; Ghorchiani et al., 2018). The findings of a study on Zelkova serrata seedlings grown in saline environments indicated that root inoculation with Funneliformis mosseae resulted in a noticeable enhance in P levels in root and leaf (Jia et al., 2019). The rise in P uptake in AMF-inoculated plants depends on several factors, including 1) increasing the P availability in soil due to alkaline and acid phosphatases secreted by hyphae, 2) P’s continuous movement into the root, because AMF is able to accumulate large amounts of absorbed P to the root, 3) maintaining the intrinsic concentration of phosphate (Pi) through forming polyphosphates within the hyphae, and 4) AMF is able to absorb P at a lower threshold because of high-affinity phosphate transporter genes expression (GmosPT, GiPT, and GvPT) (Vassilev et al., 2012; Abdel-Fattah and Asrar, 2012; Jia et al., 2019). Furthermore, P’s effective absorption by mycorrhizal plants under salinity stress conditions leads to the following benefits: 1) the selective absorption of ions and thus reducing the adverse effects of salinity, 2) partitioning of toxic ions in vacuoles, 3) reduction of ion leakage, and 4) maintaining cell membrane integrity (Bothe, 2012; Cantrell and Linderman, 2001; Evelin et al., 2019) ( Table 2 ).

The researchers observed that the coexistence of AMF rises the absorption of K⁺ in salt-exposed plants, and plant use K⁺ uptake as a coping mechanism to salt stress (Giri et al., 2007; Amanifar et al., 2019). Experiments have demonstrated that inoculating salt-exposed roots with AMF upregulates SKOR, resulting in passive secretion of K⁺ in the xylem flow, ultimately increasing K⁺ accumulation in the aerial organs and enhancing K⁺/Na⁺ ratios (Wang et al., 2021; Evelin et al., 2019). A significant increase in K⁺ concentration in the roots and leaves of mycorrhizal seedlings, in addition to preventing disturbances in cellular enzymatic processes and inhibiting protein synthesis, enhances stomatal conductance, enhancing water requirement for transpiration (Garg and Bhandari, 2016; Sheng et al., 2008; Van Zelm et al., 2020). Additionally, the results of several studies indicate AMF helps to overcome salinity-induced deficiencies of Mg2⁺, and Ca2⁺ by an observable rise in the content of these ions in roots and leaves (Navarro et al., 2014; Evelin et al., 2012; Van Zelm et al., 2020) ( Table 2 ).

It seems that, in addition to preventing Na⁺ absorption, AMF can reduce salinity’s effects on plants through limiting its transfer to plant aerial parts and also diluting Na⁺ (Janah et al., 2021b; Borde et al., 2011; Talaat and Shawky, 2011). Vesicles in the AMF can store ions such as Na⁺ and Cl⁻ under salt stress, and increase plant adaptation to saline conditions by inhibiting their uptake through roots (Ait-El-Mokhtar et al., 2020; Miransari, 2010). When salt is present, AMF participates in the selective ion absorption such as Mg, N, P, Ca, and K, and reduces the absorption of Na⁺ (Paul and Sinha, 2017; Goussi et al., 2018). Furthermore, mycorrhizal plants can control Na⁺ transport to the upper parts of plant, as well as regulate the internal concentration of Na⁺ (Evelin et al., 2019; Santander et al., 2019). Mycorrhizal plants also reduce the accumulation of Na⁺ in leaves and aerial parts by sequestering Na⁺ in root vacuoles or taking it out of the cytosol (Evelin et al., 2019; Chen et al., 2017). Moreover, AMF assist the host plant in collecting Na⁺ from the xylem and diverting it from the tissues responsible for photosynthesis to the roots (Evelin et al., 2012). Further, there is a possibility that the reduction of ionic toxicity in salinity conditions is due to the dilution effect, since AMF boosts growth and biomass by improving the nutritional status of plants, leading to Na⁺ and Cl⁻ dilution (Talaat and Shawky, 2011; Amanifar et al., 2019).

According to previous studies, plants inoculated with AMF up-regulate aquaporin genes (such as PIP), H⁺-ATPase genes (such as VHA-B), and Na+/H+ antiporters genes (such as NHX1 and SOS1), and the function of these genes mediates sodium flow and water potential in tissues of plants (Diao et al., 2021; Pedranzani et al., 2016; Evelin et al., 2019; Munns, 2005). Various plant species express these genes differently, in such a way that the expression of NHX is regulated by AMF symbiosis in rice under salt conditions, while there is no significant difference for Robinia pseudoacacia (Diao et al., 2021). Meanwhile, AMF coexistence increased the expression of SOS1 in rice shoots and roots of Robinia pseudoacacia, but decreased this gene’s expression in root of rice (Chen et al., 2017). In addition, Several studies have shown AMF coexistence in saline environments increases the expression of PIP1;1, PIP2;1, and PIP1;3 in Robinia pseudoacacia, while it decreases the expression of PIP1 in tomato roots (Diao et al., 2021; Ouziad et al., 2006).

AMF’s extensive hyphae allow higher hydraulic conductivity, and water absorption capacity even in low water potential and improve soil water availability (Medina and Azcón, 2010; Sheng et al., 2008). In addition, plant root inoculation by AMF in saline environments enhances relative water content (RWC), shoot water content and water use efficiency (WUE) (Al-Khaliel, 2010; Kapoor et al., 2008; Sheng et al., 2008). AMF also rises leaf relative water content (LRWC), and leaf water potential (LWP) in plants grown in saline environments, possibly because it improves water absorption capacity and hydraulic conductivity (Kapoor et al., 2008; Chandrasekaran et al., 2019). The improvement of WUE resulting from root inoculation by AMF resulting in a boost in gas exchange capacity, stomatal conductance and subsequent transpiration in plants stressed by salinity (Elhindi et al., 2017; Sheng et al., 2008; Chandrasekaran et al., 2019).

Studies in the past have stated AMF-inoculated plants have shown a higher photosynthetic capacity stressed by salinity (Liang et al., 2021; Abdel Latef and Chaoxing, 2014). AMF can increase photosynthesis of plants suffering from salinity stress by improving transpiration rate, stomatal conductance, water status of leaves, strengthen of photosynthesis machinery, pigment content, photochemistry and non-photochemistry of PSII, carbon uptake and transfer to mycorrhizae (Zai et al., 2021; Cho et al., 2006; Ait-El-Mokhtar et al., 2020). The coexistence of AMF in salty environments affects age-related changes (ARCs) in metabolome of leaves and partially prevents leaf aging and leads to the better metabolites accumulation (Shtark et al., 2019). In addition, AMF treatment protects leaf cells of saline-grown plants by preventing cell wall detachment, reducing plasma membrane damage, inhibiting chloroplasts vanish, stopping thylakoid destruction, and maintaining chloroplast structure (Liang et al., 2021; Evelin et al., 2013). Less damage to chloroplast structure caused by AMF inoculation might be as a result of a higher osmolyte concentration (sugars, betaine, proline, glycine) and polyamines, and larger and more plastoglobules (higher concentration of alpha-tocopherol) in plants (Evelin et al., 2013; Talaat and Shawky, 2014). Osmolyte accumulation in mycorrhizal plants protects CO₂ fixing enzymes [rubisco activase (Rca), and PSII pigment protein complexes ribulose-1,5-bisphosphate (RuBisCO)] (Talaat and Shawky, 2014). Plastoglobules, as tocopherol synthesis sites, also can play a protective role in membrane of thylakoids and proteins (Austin et al., 2006). However, tocopherol probably protects PSII by preventing photooxidation of membrane lipids (Liang et al., 2021). AMF leads to the upregulation of the expression of chloroplast-related genes (RppsbA, and RppsbD), which gives the plant a higher PSII efficiency and then boosts the photosynthetic capacity under salinity stress (Chen et al., 2017). Furthermore, AMF treatment improves photosynthetic capacity by increasing N absorption, because N is considered to be a major component of Rubisco enzymes (Ahanger et al., 2014) ( Figure 4 ; Table 2 ).

AMF inoculation of plant roots reduces the adverse effects of salinity on chlorophyll content and photosynthetic pigments by removing toxic Na⁺ and inhibiting its transfer to aerial organs (Abeer et al., 2014; Chandrasekaran et al., 2019; Khalil et al., 2011). AMF can also promote chlorophyll production and accelerate photosynthetic activity by improving the chlorophyll synthetase enzyme activity (Wright et al., 1998; Liang et al., 2021). The study conducted on Echinacea angustifolia has shown that the inoculation of plant roots by Rhizophagus irregularis under salinity stress increased the chlorophyll content of leaves 2 to 3 times more than the treatments without AMF (Liang et al., 2021). Liang et al. also demonstrated that AMF treatment gave a much greater increase in Chl a content compared to Chl b and carotenoids, such that the AMF-induced improvement in Chl a was 25%, however, it was less than 20% for Chl b (Liang et al., 2021). The results of the studies conducted on Sesbania sesban, Solanum lycopersicum, Zelkova serrata, Panicum turgidum, Arundo donax, and Ocimum basilicum grown in saline environments also indicated that the inoculation of roots by AMF species increases the photosynthetic pigments and chlorophyll content (Elhindi et al., 2017; Liang et al., 2021; Pollastri et al., 2018; Wang et al., 2019b). Additionally, research has shown that the mediating AMF’s effect in saline environments on leaf surface, photosynthesis rate, total chlorophyll, Chl a and Chl b in C3 plants is more than C4 plants (Chandrasekaran et al., 2019). Since there is a reciprocal relationship between Soil-Plant Analysis Development (SPAD), leaf nitrogen content, and total chlorophyll, research has shown that colonization of AMF in saline conditions increases SPAD (Campanelli et al., 2013; Porcel et al., 2015; Wu et al., 2007). Moreover, AMF application enhances the absorption of Mg2⁺ by plants in saline environments, and since the Mg2⁺ ion is the chlorophyll molecule’s central ion, it boosts the content of chlorophyll in the inoculated plants (Rao and Chaitanya, 2016; Jia et al., 2019). Mg2⁺ is also necessary for the appropriate function of several enzymes such as glutathione synthase, protein kinases, ATPases, carboxylases, phosphatases, and RNA polymerases (Shaul, 2002).

It is well known that fixation of photosynthetic CO₂ is essential for rapid plant growth process, and is extremely sensitive to changes in the environment, including salt stress (Liang and Shi, 2021). Research has indicated that a higher content of photosynthetic pigments is the basis of better photosynthetic gas exchange, and since plant-AMF symbiosis increases chlorophyll content and photosynthetic pigments, thus also improving gas exchange and carbon fixation (Takai et al., 2010; Elhindi et al., 2017). The response of mycorrhizal plants under salinity stress can be complicated in relation to gas exchange, so that the results of studies have shown that although CO₂ exchange increases, however, plants face a decrease in intercellular CO₂ concentration by increasing photosynthesis, which is due to the effective use of CO₂ with the coexistence of AMF (Sheng et al., 2008; Chandrasekaran et al., 2019). Further, the researchers’ results have shown that the coexistence of plants with AMF in saline environments reduces stomatal conductance and transpiration rate (Liang and Shi, 2021). Research also has shown that although C3 plants show higher photosynthesis rate and transpiration rate than C4 plants, however, C4 plants have a higher amplitude in stomatal conductance, WUE, and RWC (Pinto et al., 2014; Chandrasekaran et al., 2019). Additionally, there is a significant relationship among stomatal conductance and photosynthetic capacity (A_max) in C4 plants inoculated with AMF in salty conditions, so that increasing stomatal conductance indicates the enhancement of A_max (Chandrasekaran et al., 2019).

Using AMF in saline environments can improve the ability of plants to dissipate excessive energy through increasing the regulation of energy splitting among photochemical and non-photochemical events and protect the photosynthetic apparatus from excessive light (Sheng et al., 2008; Jia et al., 2019). Increasing photochemical efficiency in plants improves CO₂ fixation, photosynthetic activities, Rubisco activities, water status of plants, and stomatal conductance (Gao et al., 2020; Porcel et al., 2015; Navarro et al., 2014). Additionally, AMF balances the absorption and use of light energy to obtain photoprotection and reduces salt stress damage to PSII by decreasing the values ​​of ФPSII (actual PSII efficiency), and qP (photochemical quenching) (Zribi et al., 2009; Chen et al., 2017; Hanachi et al., 2014; Ait-El-Mokhtar et al., 2019). When the salinity is high, AMF inoculation’s positive effects on the level of PSII photoinhibition decreases, which can be due to the host plant ionic imbalance and ultimately disrupting the normal cellular function (Jia et al., 2019). Moreover, several studies have found increasing in non-photochemical quenching (NPQ) in leaves of AMF-inoculated plants in salt-stressed conditions, which is an energy dissipation mechanism that provides protection for the photosynthetic apparatus from excess light under salinity (Jia et al., 2019; Zribi et al., 2009). However, AMF’s effects in saline environments on NPQ was different, so that it increased in Zea mays leaves, while it decreased in Cucumis melo leaves and did not show any change in Robinia pseudoacacia leaves (Jia et al., 2019). AMF also up-regulates the synthesizing genes expression of abscisic acid 8′-hydroxylase, capsanthin/capsorubicin synthase and NAD(P)H-ubiquinone oxidoreductase to enhance the photoprotection mechanisms of chloroplasts (Begum et al., 2019) ( Table 2 ).

AMF reduces ROS, oxidative damage and cell leakage in salt-exposed plants by enhancing the enzymatic, and non-enzymatic antioxidant defense system, Phytohormone synthesis, solute accumulation, stimulation of synthesis of osmolytes, and protection of membrane lipids (Parihar et al., 2020; Bistgani et al., 2019; Begum et al., 2019). In addition, the inoculation of plants grown in saline environments by AMF reduces oxidative stress markers (MDA, and H₂O₂), and lipid peroxidation (Ait-El-Mokhtar et al., 2020; Gao et al., 2020). Some studies have shown that the content of H₂O₂ and MDA with AMF inoculation is variable in different plant organs, such that the leaves have a lower content of H₂O₂ and MDA while the roots have a higher level of them, which is due to more Na⁺ accumulation in the roots (Chen et al., 2017, 2020; Sewelam et al., 2016) ( Table 2 ).

During saline conditions, symbiosis with AMF reduces the threshold concentration of ROS required to cause oxidative degradation by increasing antioxidants activities such as APX, CAT, SOD, POD, GSH, monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), POX, glutathione reductase (GR), and AsA (Bose et al., 2014; Shahvali et al., 2020; Ait-El-Mokhtar et al., 2020; Apel and Hirt, 2004; Samaddar et al., 2019). In their study on Robinia pseudoacacia stressed by salt, Chen et al. showed that root inoculation by Rhizophagus irregularis caused antioxidant enzyme gene expression (RpMn-SOD, RpCu/Zn-SOD, RpAPX2, RpAPX1, RpGR), which resulted in the reduction of H₂O₂, MDA, and REL in the leaves (Chen et al., 2020). Coexistence with AMF through increasing the secondary metabolites accumulation in tissues of plants causes morphophysiological and hormonal changes in host plants, including removal of ROS, and production of antioxidant (Pan et al., 2020; de Lazzari Almeida et al., 2018; Evelin et al., 2013). Furthermore, according to studies, AMF not only increases secondary metabolite production and accumulation in medicinal plants under salt stress, but also improves their medicinal value (Zeng et al., 2013; Amanifar et al., 2019) ( Figure 5 ; Table 2 ).

Phenols are one of the bioactive compounds that increases in medicinal plants organs by AMF inoculating (Kapoor et al., 2017). Among the polyphenols that rise under the influence of AMF are quercetin-3-arabinoside, luteolin, 4-O-caffeoylquinic acid, 4,5-dicaffeoylquinic acid, and protocatechuic (Aseel et al., 2019). However, in a salt-salt stressed mycorrhizal plant, conflicting observations have been published about phenolic compounds accumulation (Duc et al., 2021). Although research conducted on Echinacea angustifolia showed that mycorrhization increased flavonoids’ total content, the research performed on lettuce leaves identified that the content of phenol decreased significantly (Liang et al., 2021; Santander et al., 2019). Nevertheless, according to Duke et al., these differences are due to the fact that the majority of prior research investigated polyphenol profiles only at a harvest, while polyphenol content varies with plant age (Duc et al., 2021). Additionally, researchers have found that stress intensity affects both increasing and decreasing trends in phenolic compounds in plants colonized by AMF, and the best performance of AMF is in moderate salt stress (Duc et al., 2021; Amanifar and Toghranegar, 2020). Moreover, changes in the metabolism of carbohydrates and primary metabolites, which can be caused by the improvement of nutrients, water absorption and photosynthesis in colonized plants, enhance Phenolic compounds (Arzani and Ashraf, 2016; Pedone-Bonfim et al., 2018).

The synthesis mechanisms of proline, as a proteinogenic amino acid, can be improved by AMF colonization and enhance tolerance of plants to salt (Abd_Allah et al., 2015; Ahanger et al., 2013). Proline reduces the risk of salt stress by preventing free radical damage, maintaining osmotic balance, protein degradation induced by stress, redox enzyme stabilization, protecting membrane integrity, improving cell water retention, and increasing K concentration in the cell (Duc et al., 2021; Lin et al., 2016; Reddy et al., 2015; Ait-El-Mokhtar et al., 2020; Parihar et al., 2020). Plants’ increased proline content can be attributed to factors, such as 1) inactivation of proline dehydrogenase (catalyzes the breakdown of proline), 2) a greater glutamate dehydrogenase enzyme activity (involved in glutamate synthesis, a precursor of proline), 3) a higher level of Pyrroline-5-carboxylate synthase (P5CS), and 4) increasing in P5CS gene expression (Abo-Doma et al., 2011; Garg and Baher, 2013). Nevertheless, studies on the effects of AMF on the concentration of proline in plants under salinity have been contradictory, although several researches have informed higher content of proline in mycorrhizal plants, while others have shown lower proline in plants (Tuo et al., 2015; Elhindi et al., 2017; Shekoofeh et al., 2012; Yooyongwech et al., 2016). Since the proline molecule is regarded as a stress marker, the reduction of proline synthesis in mycorrhizal plants possibly as a result of AMF-mediated stress reduction (Duc et al., 2021; Evelin et al., 2019).

Organic acids are essential osmolytes in the vacuoles of plants, and AMF colonization plays an important involvement in regulating their concentrations and metabolism, increasing tolerance to salinity in plants (Guo et al., 2010; Evelin et al., 2019). A number of organic acids are increased in mycorrhizal plants stressed by salt, including oxalic, malic, fumaric, citric, acetic acids (Sheng et al., 2011). Moreover, plants with AMF symbiosis accumulate more soluble carbohydrates that reduce the harm resulting from excessive salt exposure, through stabilizing the structure and activities of protein complexes, membrane integrity, maintaining the activity of mature leaves, and balancing energy transfer (Gao et al., 2020; Dodd and Pérez-Alfocea, 2012; Diao et al., 2021). Trehalose (α-D-glucopyranosyl-1,1-α-D-glucopyranoside) as a non-reducing disaccharide controls carbohydrate metabolism, and AMF symbiosis can increase this osmolyte accumulation in plants in saline environments (Lunn et al., 2014; Chang et al., 2014). The higher concentration of trehalose in mycorrhizal plants can be assigned to the increased Trehalose-6-phosphate phosphatase (TPP), and Trehalose-6-phosphate synthase (TPS) activities (enzymes responsible for trehalose biosynthesis) by AMF and the decreased TRE activity (trehalose-degrading enzymes, including Trehalase, and Trehalose phosphorylase) (Garg and Bhandari, 2016).

On the other hand, studies have shown that in roots inoculated with AMF, glycyrrhizin biosynthesis is rised in salinity-stressed conditions, that reduces oxidative stress (Orujei et al., 2013; Amanifar et al., 2019). Further, the observed increase in the concentration of glycyrrhizin in mycorrhizal plants might be due to the rise in enzyme molecule number, as well as the boost in the production of terpenoid biosynthesis precursors (Amanifar et al., 2019). A rise in P uptake by mycorrhizal plants results in a rise in the biosynthesis of terpenoids through enhancing the production of pyrophosphate compounds such as dimethylallyl diphosphate (DMAPP), and isopentenyl pyrophosphate (IPP), which are key precursor molecules for the biosynthesis of terpenoids (Xie et al., 2018). In addition, P contribution in the formation of other precursors of terpenoids through MVA (mevalonic acid) [NADPH (Nicotinamide adenine dinucleotide phosphate), ATP (Adenosine triphosphate), and acetyl-CoA (Acetyl coenzyme A)], and MEP (phosphoglyceraldehyde and pyruvate) pathways is crucial (Kapoor et al., 2017). As the major component of glycyrrhizin biosynthesis, farnesyl pyrophosphate (FPP) is synthesized with IPP and DMAPP condensation sequentially (Kapoor et al., 2017) ( Table 2 ).