The plant material (Table 1) consists of seeds of the two best performing hybrid tomato varieties (Solanumlycopersicum L.) identified during work under in vitro conditions, namely the Lady Nema andMongal varieties [13] . The seeds were supplied by the company Tropica Sem-Senegal (Technisem Novalliance Group, 2019), located in Dakar city. The characteristics of the seeds and the storage conditions are similar to those used during the in vitro study of salt stress [13] .

To evaluate the impact of fungal inoculation on the growth and development of tomato plants under salt stress, three strains of arbuscular mycorrhizal fungi were used. They come from the collection of the Joint Microbiology Laboratory (LCM, IRD/ISRA/UCAD**) of the ISRA-IRD research center in Dakar Bel-Air (Senegal). These are Claroideoglomus etunicatum,Rhizophagus fasciculatus and Funneliformis mosseae [14] , whose old names were respectively Glomusetunicatum, Glomusfasciculatum and Glomus mosseae. The references of the strains are specified in Table 2.

To obtain enough inoculum, each pure AMF strain was propagated under a potted shadehouse using a mycotrophic plant, maize (Zeamays L.), in Sangalkam soil sterilized at 120˚C for 2 hrs. The composition and the physico-chemical characteristics of the Sangalkam soil are listed in Table 3. After 3 months of cultivation, the roots and the culture substrate were collected to evaluate the density of the spores on the one hand [15] and, on the other hand, the root colonization rate for each AMF strain [16] [17] . The maize roots colonized by each AMF strain were then cut into fragments of about 1 cm and mixed with the culture medium containing spores and hyphae, to constitute the fungal inoculum.

The soil taken from Sangalkam (Table 3), located 50 km from Dakar (approximate GPS position: Latitude, 14˚78'11"N, Longitude, −17˚22'78"W), served as a substrate for the greenhouse study of the effect of mycorrhizal fungi on the salt stress tolerance of tomato plants. The soil was taken three weeks before the experiments, from a horizon between 10 and 20 cm deep. It was sterilized in an oven at 120˚C for 96 hours to eliminate all native microflora.

In semi-arid regions, mycorrhizal symbiosis plays a major role in the development of crops and their tolerance to abiotic stresses, particularly for tomatoes. AMF have been associated with a wide range of plants and different soil salinity levels [1] [7] . Indeed, AMF have the ability to associate with the roots of plants, thereby improving their hydromineral nutrition [10] . There are two ways of root colonization of a plant by an AMF. Primary colonization from pre-symbiotic fungal hyphae from a spore and secondary colonization from the extra-root mycelium of a fungus that has already colonized the root system of a host plant. Once the hyphae are in the root cells, they differentiate into highly branched arbuscules occupying most of the cell volume and forming an extensive surface for nutrient exchanges [25] . They also grow out of the root forming a branching mycelium that explores the soil for the absorption of mineral nutrients and water. However, high salt levels in soil can reduce the colonization capacity and germination of AMF spores [7] .

According to the present study, the frequencies and intensities of mycorrhization, as well as the relative mycorrhizal dependency varied significantly (P < 2 × 10⁻¹⁶) with increasing NaCl concentrations. The decrease in the frequency of mycorrhization varies from 86% to 92% and that of the intensity of mycorrhization from 75% to 93% at [NaCl 210 mM]. A decrease in root infection by AMF of 51% was recorded in tomatoes grown under saline conditions (7500 ppm) [8] . Plants inoculated with C.etunicatum andR.fasciculatus revealed the best frequencies and intensities of mycorrhization in Lady Nema andMongal varieties, respectively. These mycorrhization parameters are more considerable with the plants of the Lady Nema variety compared to those of the Mongal variety. Indeed, plants of the Lady Nema variety inoculated with C.etunicatum recorded mycorrhization frequencies of 94.03%, 94.01%, 54.01% and 13.05% and those of the Mongal variety inoculated with the R.fasciculatus strain recorded 90.24%, 91.69%, 50.36% and 10.86%, respectively at [0, 70, 140 and 210 mM of NaCl]. Plants of the Lady Nema variety inoculated with C.etunicatum also recorded the best mycorrhization intensities with 39.59%, 40.47%, 21.33% and 9.92% while those of the Mongal variety inoculated with the R.fasciculatus strain obtained mycorrhization intensities of 35.82%, 35.14%, 14.86% and 5.71% in the same saline conditions. However, a lower reduction of 14% and 27% in the frequency of mycorrhization in tomato inoculated with F.mosseae, is noted respectively under medium (4.7 dS·m⁻¹) and high salt stress (7.4 dS·m⁻¹) [10] . Small reductions in the frequency of mycorrhization (Funneliformismosseae) of 24.6% and 21.9% at 4.9 dS·m⁻¹ and 35% and 41% at 7.1 dS·m⁻¹ were obtained, respectively in two tomato varieties Pello (tolerant) and Marriha(susceptible) [6] . Mycorrhization frequencies twice lower than those recorded in our study (control: 55% and at [NaCl 100 mM]: 27%) were also observed in tomato plants inoculated with Funneliformismosseae [11] . Degrees of relative mycorrhizal dependency are classified as follows: excessive (RMD > 75%), high (50 < RMD < 75%), medium (25 < RMD < 50%), marginal (RMD < 25%), and independent (RMD ≤ 0%) [26] . The mycorrhizal dependencies recorded in our experiments show positive and negative values relating to biomass gains or losses in mycorrhizal plants. However, without saline constraint, the dependencies are all negative at all inoculation conditions, i.e., the plants are independent of mycorrhization. In addition, the maximum dependency is observed at [NaC140 mM l] in plants of the Mongalvariety inoculated with R.fasciculatus (43.95%), followed by those of the Lady Nema variety inoculated with F.mosseae (35.62%). Thus, in the presence of saline constraint, the RMD is accentuated because the plants of the two varieties need more and more to associate with the AMF which results in an average dependency. However, plants of the Lady Nema variety inoculated with R.fasciculatus at [140 and 210 mM of NaCl] recorded RMDs of −43.09% and −42.17%, respectively. In addition, those inoculated with F.mosseae at [NaCl 210 mM] also recorded a RMD of −6.21%. Thus, with these RMD less than 0, these plants always remain indifferent to mycorrhization. In the Mongal variety, plants inoculated with F.mosseae at [NaCl 210 mM] are also indifferent to mycorrhization with a RMD of −131.25%. Mycorrhizal dependency (MD) expresses the contribution of AMF in stimulating plant growth compared to the same non-mycorrhizal plants. Mycotrophic plants such as tomato have a coarse root system which determines their reliance on symbiosis [12] . Some species such as carrot and onion have a strong mycorrhizal dependency, while others such as sorghum and maize have an intermediate mycorrhizal dependency. Finally, a last category including wheat, rice and tomato show a low dependency on mycorrhizal colonization [27] . This is in contradiction with our results which revealed an intermediate mycorrhizal dependency of the tomato varieties studied. For a given soil and plant, there is a maximum quantity of extractable phosphorus above which the response of the plant to mycorrhization is zero, at least as far as plant growth is concerned [28] . Several factors can influence the MD of the plants, including the plant species used, the structure of the root system, the fungal symbiont and the content of the soil in assimilable phosphorus. Under unfavorable mineral nutrition conditions, the two partners in the symbiosis can compete for carbonaceous substrates, resulting in a depressive effect of the fungus on the juvenile growth of plants [5] .

The plant factors that are at the origin of the differences observed in the aptitude for endomycorrhizal colonization seem to be linked to the morphological and physiological characteristics of the plants [12] . Our results showed that tomato still had the ability to establish symbiotic relationships with AMF even under high saline concentration. A similar conclusion was made by [29] . Several authors have also shown that salinity reduces mycorrhizal colonization by inhibiting spore germination and hyphal growth or by reducing the spread of mycorrhizal colonization and the number of arbuscules [30] . The negative effect of soil salinity on mycorrhizal root infection could also be due to the accumulation of H₂O₂ in the roots of mycorrhizal plants, which could induce the degradation of arbuscules [31] .

The direct effects of soil salinity on plant growth may involve three distinct physiological pathways: firstly, plants subject to the low osmotic potentials of saline soil are at risk of physiological drought because they must maintain lower internal osmotic potentials to prevent water movement from roots to soil [3] . Secondly, the toxic effects of specific ions such as Na⁺ and Cl⁻, disseminated in saline soils, disrupt the structure of enzymes and therefore their function, damage cellular organelles and disrupt photosynthesis, respiration and protein synthesis [32] . Finally, salinity also leads to a nutrient imbalance in the plant caused by a change in the absorption and/or transport of nutrients to the aerial part, which leads to ionic deficiencies [33] . The application of AMF could improve the physico-chemical and biological characteristics of the soil [34] . Furthermore, the spread of AMF hyphae beyond the root zone provides the nutrients necessary for plant development [25] . Thus, several studies have demonstrated that several species of AMF are involved in promoting plant resistance to salinity [31] . Our results revealed a significant decrease (P < 2 × 10⁻¹⁶) in survival rates which increases with high NaCl levels and treatment duration with better results in inoculated plants compared to controls. Several authors have reported that inoculated plants grow better than un-inoculated ones under salt stress [32] . Thus, the survival rate of the plants in this study is greater (>40%) on the plants inoculated with C.etunicatum for theLady Nema variety and R.fasciculatus for the Mongal variety. A one hundred percent survival rate is obtained in a variety of tomato inoculated with F.mosseae in a salt stress situation (ECse = 7 dS·m⁻¹) [29] . The effects of salinity observed would result from a disturbance of the water and nutrient supply as well as toxicity following a strong accumulation of salt in the leaves [35] . Indeed, following an increase in the osmotic potential in the culture substrate, the cell turgor would be based on the accumulation of ions in the vacuole, which alters the cell organelles causing the appearance of leaf necrosis and, finally, the death of the cell plant. To adapt to salt stress, the plant can avoid damage by reducing growth [33] ; this is the most common effect of abiotic stresses on plant physiology. Indeed, this delay in development allows the plant to accumulate energy and resources to combat stress before the imbalance between the interior and exterior of the organism increases to a threshold where the damage will be irreversible [35] . Plant protection by mycorrhization against salt stress has also been reported to result from increased and/or better selection in nutrient uptake, accumulation of osmoregulatory compounds, high stomatal conductance, an increase in photosynthetic activity or even a limitation of leaf dehydration [33] .

The negative impact of salinity on plant development is reflected in the fresh and dry weight of plants [30] . Thus, in the present study, a clear and significant decrease in the fresh and dry weight of the aerial and root parts is observed with the increase in NaCl content. However, the inoculated plants showed better development compared to the non-inoculated ones. Indeed, an increase in fresh root weight is observed at [NaCl 70 mM] varieties for plants inoculated with R.fasciculatus (76% for Lady Nema) and C.etunicatum (48% and 41% respectively for Lady Nema andMongal varieties. At [NaCl 70 mM], the dry weight of plants of the Lady Nema variety increased by 99% (C.etunicatum) and 123% (R.fasciculatus) while that of the plants of the Mongal variety increased by 99% (C.etunicatum). Similar results under saline conditions were obtained in tomato plants inoculated with F.mosseae [36] and with different AMF strains including R.fasciculatus and F.mosseae [29] . Several other authors have reported the beneficial effect of AMF on the dry weight of plants under osmotic and ionic stress conditions [30] . Maintaining water homeostasis is essential to mitigate the impact of salinity on plant growth and crop production. Under stress conditions, inoculation with AMF often improves the water status of plants due to the presence of extra root hyphae which absorb more water. AMF colonization increases active solute transport as a mechanism that allows water to continue to flow through plant roots [37] . However, a reduction in the aerial dry weight of 62% to 83% and in the root dry weight of 50% to 58% in the presence of [NaCl 210 mM] is observed on the mycorrhizal plants of the Lady Nema variety compared to non-stressed plants. With regard to the Mongal variety, the reduction in aerial dry weight varies between 46% and 90% and that of root dry weight between 58% and 79%, always in the presence of [NaCl 210 mM]. Reductions in aerial (50%) and root (60%) dry weights were observed in tomatoes under salt stress conditions [38] . However, the aerial dry weights are greater than those of the roots in all conditions for both varieties, which suggest that the AMF would promote better development of the aerial part versus the root part and that salinity has a greater impact on this root part which is the part in direct contact with harmful ions. Reduced plant growth under saline stress is attributed to reduced water and nutrient availability due to excessive NaCl levels in the growth medium, but also by imbalance and specific ion toxicity leading to down-regulation of other metabolic activities [39] . The reduction in biomass can be caused by several modifications, the main ones being: 1) reduction in the thickness of the epidermis, mesophyll, intercellular spaces and reduction in cell size [40] ; 2) reduction in the number of chloroplasts, leaf area, and stomatal density [41] ; 3) structural damage to cell walls, dilation and reduction in the number of mitochondrial ridges; 4) damage caused by stress-induced increase in ethylene [42] . It may also be due to ionic toxicity caused by the accumulation of Na⁺ and Cl⁻ ions [32] . The heterogeneity of the results with the different AMF strains could be explained by the fact that the AMF diverge functionally [15] ; different species show heterogeneity in performance in hyphal growth, fungal uptake and symbiotic nutrient transport [5] .

The effectiveness of a plant in controlling the consumption, distribution and compartmentalization of Na⁺ is decisive for its tolerance to salinity [4] . Soil salinity significantly reduces the concentration of beneficial elements making it difficult for nutrients such as P and K⁺ to be absorbed [39] . This leads to nutritional disorders that AMF could improve [37] .

Under our experimental conditions, the mineral nutrition of tomato plants was found to be dependent on the variety, the strain of mycorrhizal fungus inoculated and also on the level of salinity applied. Indeed, with regard to the two varieties tested, the foliar contents of potassium and phosphorus decreased with salinity while that of sodium increased. Similar results were obtained on several tomato genotypes [38] . At the root level, the K⁺ content decreased with salinity while the Na⁺ content increased significantly. The inoculated plants garner higher K⁺ and P contents than those recorded on the non-inoculated plants. In addition, Na⁺ contents are higher in non-inoculated plants. Numerous studies have indicated that AMF contribute to plant growth via enhancing the uptake of mineral nutrients, especially immobile (P) ones from the soil [37] . This may be due to increased availability and/or transport (absorption/translocation) of these minerals by AMF hyphae. Mycorrhizal fungi have a regulatory effect on the translocation of Na⁺ at the level of the aerial part, which makes it possible to maintain a lower Na⁺ aerial part/root part ratio in the tissues of inoculated plants than in the tissues of non-inoculated ones [7] . Regarding the potassium contents, they decreased by more than 65% at [NaCl 210 mM] for the Lady Nema and Mongal varieties. The best results are obtained with the inoculation of C.etunicatum in the plants of the Lady Nema variety, followed by the inoculation with R.fasciculatus in the plants of the Mongal variety. Tomato plants inoculated with Glomusintraradices (Rhizophagusintraradices) obtained a 90% reduction in K⁺ levels [2] . These results are similar to the values obtained in our work. However, the increase in Na⁺ contents (200%) is far lower than that obtained in our work. As for mycorrhizal tomato plants with F.mosseae and grown under saline stress, a greater drop in phosphorus and potassium levels (+86%) was recorded [6] . However, the increase in sodium of those (155% to 249%) is far less significant than that obtained in our experiments. We noted a positive correlation between leaf potassium and phosphorus contents, but also a negative correlation between potassium, phosphorus, and sodium contents. Similar results were obtained in tomato plants mycorrhized with F.mosseae [10] . At the root level, a high K⁺ content is accompanied by a low Na⁺ content and vice versa. Similar results in a salinity situation were also reported for tomato plants inoculated with R.intraradices [2] . Increasing NaCl concentrations also negatively impacted the K⁺/Na⁺ selectivity ratio with drastic decrease rates. However, this ratio is greater at the level of the leaves compared to the roots. Meanwhile, at [140 and 210 mM of NaCl], this ratio is greater at the level of the roots of plants of the Lady Nema variety inoculated with C.etunicatum. For the Mongal variety, this ratio is in favor of the roots for the plants inoculated with R.fasciculatus at [NaCl 210 mM]. This testifies to the respective effects of these AMF in mineral nutrition by promoting significant absorption of K⁺. Therefore, higher values of these ratios protect photosynthetic tissues by suppressing Na⁺ influx and improving hydraulic conductivity and cell signaling pathway [43] . It could also be attributed to the enhancement of the affinity of K⁺ transporters or to the regulation of K⁺, Na⁺ transporters and H⁺ pumps, which generate the force necessary to transport these elements under saline conditions [40] . Under salt stress, AM symbiosis can increase K⁺ uptake and reduce Na⁺ translocation to stems and leaves [36] . The importance of maintaining appropriate K⁺/Na⁺ ratios for metabolic functioning has become widely accepted as an indicator of NaCl sensitivity [44] . In a study carried out on 60 species, including the tomato, it was reported that AMF could induce a marked increase in leaf K⁺/Na⁺ ratio by 28% and root ratio by 115% [45] . They also recorded a small increase in Na⁺ (roots) and K⁺ (leaves) content of 18%. AMF vesicle vacuoles can store different ions such as sodium and chloride under salt stress [46. In our studies, AMF influenced the increase in potassium and phosphorus levels more than the reduction in sodium levels. In addition to balancing the quantities of Na⁺ and Cl⁻ by storing them in their vacuole [46] , AMF induce an increase in K⁺ contents which also contribute to greater root hydraulic conductivity and consequently an improvement in water availability under osmotic stress [37] . Under normal conditions, plant cytosol contains high concentrations of K⁺ and low concentrations of Na⁺, which imparts a negative electrical potential to cells [47] . At high salt concentrations, there could be a competition between Na⁺ and K⁺ ions for the occupation of entry sites in the root plasma membrane which affects K⁺-dependent metabolic processes and also disrupts the integrity of root membranes [7] . Thus, membrane transporters exhibit low affinity to K⁺ and high affinity to Na⁺ from the environment outside the cytosol. The potassium ion is vital for the plant, as it is necessary for osmotic balance, plays a role in the opening and closing of stomata and is a key factor in protein biosynthesis [37] . The impact of mycorrhizal symbiosis on potassium uptake demonstrated that its accumulation in plants was linked to an improvement in their tolerance to salinity [37] . Unlike many other mineral nutrients, phosphorus is very poorly mobile in soils. Under the action of root removal, areas of impoverishment are quickly created around the roots. A small proportion (generally less than 1%) is immediately available to plants, which have difficulty acquiring this element when their needs are great [48] . Indeed, phosphorus is an essential element for the life of the plant. This compound enters into the synthesis of many molecules such as ATP, monophosphate nucleotides, phospholipids and certain enzymes and co-enzymes [49] . Thus, plants have developed various strategies to increase their ability to absorb phosphorus or its availability in soils [26] . Alongside the strategies allowing the plant to directly take up phosphorus from the soil, the most common method of taking up phosphorus consists of the so-called “mycorrhizal” route, via the extra-root mycelium of the AMF [48] . To access soil phosphorus pools inaccessible to plants, AMF would be able to hydrolyze organic phosphorus into inorganic phosphorus to make it available in the soil to the plant or even transfer it directly to the host plant [3] in exchange for carbohydrates from the plant and transferred to the MAC through the mycorrhizal interface [25] . The reduction in phosphorus uptake in saline soils has been attributed to the precipitation of H 2 PO 4 − with Ca²⁺ ions in the soil [26] . Studies have revealed that the symbiosis of plants with arbuscular fungi induces the expression of specific P transporters [50] .

Salinity decreases the osmotic potential of the soil solution and therefore reduces water uptake by the roots. Cellular turgidity is reduced which leads to a phenomenon of plasmolysis. Some plants regulate their internal osmotic pressure by synthesizing osmoprotectors, mainly amino acids such as proline. When present in high concentrations, organic solutes are called “osmolytes” because they maintain high osmotic pressure for important osmotic functions. This term is synonymous with the term “compatible solutes”, because they are compatible with enzymatic activities. The level of proline is determined by the balance between its biosynthesis and its catabolism [51] . Proline synthesis is an adaptive action taken by plants and its content is correlated with tolerance to harmful salts [52] . Indeed, a significant increase (P < 2 × 10⁻¹⁶) in the proline content of the plants is observed at all the inoculation conditions with the increase in the NaCl concentration. The inoculated plants stored higher levels of proline than the un-inoculated ones. Thus, for the Lady Nema variety, plants inoculated with C.etunicatum have the highest increase in proline content (131%). For the Mongal variety, inoculation with R.fasciculatus gave the greatest increase (132%). Several authors have recorded a significant increase in proline content of up to 500% in tomato plants inoculated with AMF [8] . In this present study, under salt stress conditions, tomato plants recorded increased proline levels, which were significantly increased by the application of AMF. Studies have also revealed an increase in proline levels in tomato plants grown under saline conditions thanks to inoculation with F.mosseae [10] . Increasing proline levels may be a defensive response to improve plant tolerance to salt stress by maintaining osmotic balance [52] and mitigating free radical damage [31] . In the presence of NaCl, the colonization of the roots by AMF could set up mechanisms allowing the launching of a better synthesis of proline and, therefore, a better tolerance of the plant to salt stress [39] . Proline plays several major roles during saline stress. It acts as a metal chelator, a signaling molecule, an active anti-oxidant and a defense molecule that helps buffer cellular redox potential and scavenges free radicals [51] . An increase in the proline of tissues has also been considered an incidental consequence of NaCl exposure in less tolerant plants [44] .