The Roles of Arbuscular Mycorrhizal Fungi in Inﬂuencing Plant Nutrients, Photosynthesis, and Metabolites of Cereal Crops—A Review

: Arbuscular mycorrhizal (AM) fungi are one of the important microbiota involved in a relationship with plant roots in which the plants and fungi both share and exchange nutrients and shelter. Cereal crops are the most essential sources of carbohydrates, dietary protein, and vitamin B for humans, and they supply the most fundamental diets. AM fungi are introduced as the optimal approach for real agricultural systems for increasing growth and productivity. According to a study from the previous decade, AM fungi were shown to promote crop growth and production, particularly in cereal crops. The AM fungi symbiosis provides a pleasant environment for microorganisms in the root and soil system, which promotes plant nutrition and water availability. AM fungi increase nutrient uptake and assimilation and also increase photosynthetic activity, which is directly associated with plant growth. Furthermore, AM fungi increase the primary and secondary metabolites, as well as soluble proteins and carbohydrates, in cereals crops. AM fungi have been shown to improve plant biomass, yield, and productivity in cereal crops. Additionally, the use of AM fungi enhances plants’ stress tolerance against various environmental stresses. In this review, we integrate the recent ﬁndings regarding the effects of AM fungi application on soil, root systems, nutrient availability and uptake, photosynthesis, metabolites, plant growth, and productivity. Furthermore, a large number of studies have been reviewed, and several limitations and research gaps have been identiﬁed that must be addressed in future studies.


Introduction
Arbuscular mycorrhizal (AM) fungi are one of the most significant microbiota, which are involved in a symbiotic relationship with the roots of plants [1]. Such symbiotic relationships have been reported in several cereal crops, including wheat (Triticum aestivum maize (Zea mays) and rice (Oryza sativa), and show very positive effects on the growth, yield, and productivity of the host plants [2][3][4][5]. However, the effect of AM fungi on plant growth is also dependent on the plant and AM fungi species, including the soil type and plant growth stage. To date, more than 255 AM fungi species have been identified, including Glomus etunicatum, Funneliformis mosseae, and Rhizophagus irregularis, and they are being used as a commercial AM inoculum for main crops in advanced agricultural systems [6][7][8][9]. Many of these are utilized for a variety of crops, including wheat, maize, and rice, and have either positive, neutral, or even negative effects during pot and field experiments. Nitrate (NO 3 − ) and ammonium (NH 4 + ) are used more often in order to produce foods with higher yields, and they alter the structure of the soil and the plant community while also disturbing the function of microorganisms [1,10]. AM fungi may provide some plants with a better environment and improve nutrient uptake even in soil with a low nutrient

AM Fungi and Mineral Nutrition
AM fungi provide nutrient access to the host plants that are otherwise unavailable, resulting higher nutrient uptakes [54]. AM fungi provide higher nutrient availability and promote plant nutrient uptake to the host plant via the mycorrhizal uptake pathway, as shown in (Figure 1). The wheat plants inoculated with F. mosseae and R. intraradices showed a 2.5 times higher Zn uptake compared with non-inoculated plants (Table 2). Similarly, as reported by Pellegrino et al. [55] and Schweiger and Jakobsen [56], AM fungi considerably contributed to overall P uptake and soil fertility through the mycorrhizal uptake pathway in field-grown winter wheat. It is commonly acknowledged that the mycorrhizal uptake pathway is the one that allows plants to acquire phosphorus more quickly and effectively than the root uptake pathway. The N and P uptake in organic field wheat was increased by about 2.3 times by Glomus species as compared to the traditional system in the Canadian prairie [57]. However, it is also hypothesized that AM fungi could produce non-beneficial effects on wheat in the field depending on the P status in the soil, including the AM fungi status in the soil community [58]. Maize plants inoculated with R. irregularis had longer roots and higher P absorption under alkaline conditions; this is because AM fungi facilitate N and P uptake [38]. The AM fungus Glomus intraradices boosted the Zn and P content in the shoots of maize after 45 and 75 days after inoculation, respectively [59]. During the grain filling stage, the contents of N and P in maize roots and shoots in inoculated plants were considerably higher, which suggests that AM colonization mediates more translocation of N and P during the grain filling stage of host plants [60]. Inoculation of AM fungus with bacteria, i.e., G. mosseae and Rhizobium, increased maize N uptake by AM colonization and nodulation [61]. Thus, it is suggested that both AM fungi and bacteria can play a positive role in maize's nutrient uptake and availability by mycorrhization and nodulation.
In rice, the mixed AM fungi increased N uptake in the roots and P concentration in the shoots, and resulted in higher growth and yield [62]. He and Dijkstra [63] reported that AM fungi significantly increased N and P concentrations in the roots of rice. In another study, the total 15 N uptake of rice was higher in inoculated rice as compared to the control [64]. The AM fungus G. intraradices enhanced the P concentration of rice and increased grain yield and straw biomass by reducing the negative effect of heavy metals under arsenic (As) conditions [65]. Further, it is explained that this is because of the 'dilution effect' lowering the As concentrations in the grains due to the higher growth in AM-inoculated plants. In sorghum (sorghum bicolor), the AM fungus Glomus versiforme increased N, P, and K uptake, but there was no effect on carbon: nitrogen: phosphorus (C: N: P) stoichiometry [66]. In comparison to non-inoculated plants, sorghum plants inoculated with G. mosseae showed a higher 59 Fe content in shoots under low-nutritional soil conditions; this is due to C 4 crops being considered more responsive toward AM colonization than C 3 plants [67]. G. etunicatum considerably enhanced P, N, sulfur (S), and molybdenum (Mo) concentrations in the both roots and shoots of sorghum [68]. AM fungi R. irregularis inoculation reduces the growth of sorghum at an early stage, but increases P, Zn, and Fe concentrations in grains and the harvest indices in the mature stage [69]. It may be that AM fungi are more efficient and beneficial at the mature stage as compared to the seedling stage. In oat (Avena sativa) plants, the four mixed AM fungi, i.e., F. mosseae, Gigaspora spp., Claroideoglomus spp., and R. irregularis, were applied as seed coats under fungicides and increased Ca, B, and Cu contents in the roots, but had no significant effect on the P concentration of the host plant [70]. Mixed AM fungi increased P concentrations in shoots of oat plants and increased nutritional levels in sand beach soil [71]. Similarly, the inoculation of nine mixed-AM species increased the growth of oat plants by enhancing N and P uptake in a Mediterranean environment, suggesting that inoculation of AM fungi is a good option for oat crops in a Mediterranean climate to improve agro-ecosystem sustainability [72].
wheat was increased by about 2.3 times by Glomus species as compared to the traditional system in the Canadian prairie [57]. However, it is also hypothesized that AM fungi could produce non-beneficial effects on wheat in the field depending on the P status in the soil, including the AM fungi status in the soil community [58]. Maize plants inoculated with R. irregularis had longer roots and higher P absorption under alkaline conditions; this is because AM fungi facilitate N and P uptake [38]. The AM fungus Glomus intraradices boosted the Zn and P content in the shoots of maize after 45 and 75 days after inoculation, respectively [59]. During the grain filling stage, the contents of N and P in maize roots and shoots in inoculated plants were considerably higher, which suggests that AM colonization mediates more translocation of N and P during the grain filling stage of host plants [60]. Inoculation of AM fungus with bacteria, i.e., G. mosseae and Rhizobium, increased maize N uptake by AM colonization and nodulation [61]. Thus, it is suggested that both AM fungi and bacteria can play a positive role in maize's nutrient uptake and availability by mycorrhization and nodulation.  In barley (Hordeum vulgare), inoculation with the AM fungus G. mosseae decreased cadmium (Cd) and cobalt (Co) uptake under conditions of heavy metal (Cd, Co, and Pb)polluted soil, demonstrating that AM colonization has an alleviating effect on barley under heavy metal conditions [43]. Another study found that R. irregularis boosted Zn uptake in barley plants as compared to non-inoculated plants [73] because AM fungi explore the soil volume beyond the nutrient depletion zone of the roots. AM fungi are in a better position to supply a plant with inorganic soil nutrients such as Zn 2+ in higher amounts than found in the plants. Furthermore, another study indicates that the addition of humic acid may improve the performance of AM fungi, as a result, AM fungi may increase zinc absorption and improving plant zinc nutrition indirectly [74]. Under salinity conditions, the AM fungus G. intraradices promoted P, Fe, and Zn uptake in barley plants, while it inhibited the uptake of sodium (Na) in the host plants [75]. In buckwheat (Fagopyrum esculentum), the total N and P absorptions were positively affected by the mixed AM fungi under inorganic and organic P applications [49]. It has been stated that the mixed AM fungi might have a more positive impact on plants; such an example was found when millet (Pennisetum glaucum) plants were treated with mixed AM fungi, which led to higher P, K, Ca, Mg, and Zn absorptions in roots as compared to the controls [76]. Similarly, millet plants inoculated with Emericella rugulosa had higher nutrient levels, especially P in the roots and shoots, and this may be because E. rugulosa produces phosphatases and phytase, which mobilize P and enhance millet plants' growth and productivity [77]. The application of three AM fungi, i.e., G. mosseae, G. fasciculatum, and G. decipiens, enhanced millet plants' growth and glomalin-related soil protein (GRSP) under barren soil conditions [78], suggesting that AM fungi contribute to heavy metal sequestration in polluted soils and sediments in semi-arid environments.

AM Fungi and Photosynthetic Activity of Cereal Crops
AM fungi inoculation has been reported to influence photosynthetic parameters, i.e., chlorophyll-a (Chl-a), chlorophyll-b (Chl-b), carotenoids (Caro), and the SPAD value. AM fungi increase photosynthetic activity, regulate stomatal conductivity, and enhance tolerance against various stress conditions, resulting in increased plant growth (Figure 2). AM fungi were reported to increase the photosynthesis rate in wheat, maize, and rice by facilitating the uptake of N and P from the soil. Beltrano and Ronco [33] and Peterson et al. [91] reported that wheat inoculated with AM fungi showed higher chlorophyll contents as compared to non-inoculated plants under drought stress conditions. Further, the AM fungus F. mosseae increased photosynthetic activity and mitigated the negative effects of environmental stressors on wheat growing in salty soil [31]. AM fungi were reported to protect the water splitting complex followed by enhancing the primary photochemistry of photosystem-II (PSII) under high temperatures. The leaf area and photosynthetic rate (Pn) of maize plants were positively influenced by two AM fungi species, G. mosseae and G. intraradices [37]. Compared to non-inoculated plants, G. etunicatum improved maize chlorophyll content and Pn by up to 29% [36]. This is because AM fungi positively influence the P content of maize, which, in turn, influences a variety of physiological processes, including photosynthesis. Similarly, the AM fungus Glomus tortuosum improved physiological metabolisms by increasing the chlorophyll content, light energy utilization efficiency, gas exchange, and rubisco activity of maize under salinity stress conditions [92]. This suggest that AM fungi could mitigate the growth limitations caused by salinity stress and hence play a very important role in promoting the photosynthetic capacity of maize under salt stress.
AM fungi inoculation increased photosynthesis as well as soluble carbohydrates in rice plants; this is because AM fungi improved the efficiency of PSII photochemistry as compared to the non-inoculated plants [93]. AM fungi were observed to enhance photosynthetic activities in barley plants [94]. According to Achatz et al. [95], enhanced AM colonization leads to early plant development and higher phosphate supply, resulting in a very positive influence on the photosynthesis and stomatal conductivity of barley plants. An additional study found that AM fungi had a favorable influence on rubisco and PSII in barley, and thus improved its tolerance against salt and drought stress [96,97]. AM fungi favorably interact with plants and other microorganism to increase plants' N uptake, resulting in increased photosynthesis rates and growth in various plants including sorghum [98]. Under drought stress conditions, AM fungi boosted agro-physiological characteristics, including photosynthesis and grain production, in sorghum plants by supplying more water and increasing water use efficiency [99]. This might be because AM fungi improved the roots' capacity to absorb soil moisture, which kept the stomata open in the leaves and increased dry matter production. Water conductivity improvement may also be due to an increase in root surface area for water and P absorption, as well as improved osmoregulation provided by AM fungi and rhizobacteria in the soil. In the context of quinoa plants inoculated with AM fungi (G. mosseae and Glomus tortuosum), the results showed a considerable increase in chlorophyll content, as well as PSII photochemical efficiency [100]. AM fungi assisted N uptake in quinoa, which is associated with high enzymatic activity leading to a higher chlorophyll content and photosynthetic activity [100]. When millet plants were inoculated with Glomus fasciculatum under salt conditions, the total chlorophyll content and plant growth were significantly increased due to positive changes in antioxidant enzymatic activities [101]. Additionally, the role of AM fungi in cereal crops regarding photosynthesis has been thoroughly researched; in the context of AM fungi, some plants, such as buckwheat, quinoa, and millet, required further exploration. Nonetheless, a thorough study and understanding of the deep mechanisms of photosynthesis are require further investigation.
acteristics, including photosynthesis and grain production, in sorghum plants by supplying more water and increasing water use efficiency [99]. This might be because AM fungi improved the roots' capacity to absorb soil moisture, which kept the stomata open in the leaves and increased dry matter production. Water conductivity improvement may also be due to an increase in root surface area for water and P absorption, as well as improved osmoregulation provided by AM fungi and rhizobacteria in the soil. In the context of quinoa plants inoculated with AM fungi (G. mosseae and Glomus tortuosum), the results showed a considerable increase in chlorophyll content, as well as PSII photochemical efficiency [100]. AM fungi assisted N uptake in quinoa, which is associated with high enzymatic activity leading to a higher chlorophyll content and photosynthetic activity [100]. When millet plants were inoculated with Glomus fasciculatum under salt conditions, the total chlorophyll content and plant growth were significantly increased due to positive changes in antioxidant enzymatic activities [101]. Additionally, the role of AM fungi in cereal crops regarding photosynthesis has been thoroughly researched; in the context of AM fungi, some plants, such as buckwheat, quinoa, and millet, required further exploration. Nonetheless, a thorough study and understanding of the deep mechanisms of photosynthesis are require further investigation. The figure shows that higher AM colonization increases the leaf area and photosynthetic rate by improving nutrient uptake. AM colonization stimulates plant hormones such as GA and ABA and facilitates anti-oxidant activities, resulting in higher plant growth and improved plant tolerance against biotic and abiotic stresses. Further, the figure shows that AM fungi reduce water loss, enhance root surface area, improve osmoregulation, and regulate stomata directly and indirectly under harsh conditions.

AM Fungi with Primary and Secondary Metabolites
AM fungi are essential for the beneficial enhancement of primary and secondary metabolites, which are required for plant development, enzymatic activities, and other processes, including stress tolerance (Figure 3). AM fungi increase water and nutrient absorption, recalibrate the metabolic pathways of plants, and affect the concentration of primary

AM Fungi with Primary and Secondary Metabolites
AM fungi are essential for the beneficial enhancement of primary and secondary metabolites, which are required for plant development, enzymatic activities, and other processes, including stress tolerance (Figure 3). AM fungi increase water and nutrient absorption, recalibrate the metabolic pathways of plants, and affect the concentration of primary and secondary metabolites [1]. The studies by Pepe et al. [20] and Ryan et al. [21] showed that some essential phytochemicals (phenolic acids, flavones, and phytic acid) belonging to secondary metabolites were higher in cereal plants under AM fungi inoculation. The concentrations of phytochemicals such as alkaloids, terpenoids, flavonoids, and organic compounds were also reported to be higher in AM-inoculated plants [22][23][24]. AM fungi also increase the photosynthetic rate by activating the primary and secondary metabolomic biosynthesis pathways via changes in phytohormonal concentrations and creating signaling molecules [60]. According to Bowles et al. [102], AM fungi improved nutrient uptake, especially of N and P, which is associated with increased enzymatic activity, leading to increased synthesis of primary and secondary metabolites. Talaat and Shawky [103] reported that AM fungi had a significant impact on the Membrane Stability Index (MSI), photochemical reactions during photosynthesis, carbohydrates, and soluble protein in two wheat cultivars (cv. Sids 1 and cv. Giza 168). Another study found that AM fungi increased proline biosynthesis and 9-cis-epoxycarotenoid dioxygenase (nced) activity in wheat, increased protein synthesis, and sped up the metabolic processes of the plants, resulting in higher anti-oxidative reactions and immune responses, which lead to increased tolerance against abiotic stresses in plants [104]. These results suggested that AM fungi may potentially have a role in the synthesis of essential plant hormones such as abscisic acid. The AM fungus F. mosseae alleviated the negative effect of heavy metals, promoted lipid production, and increased carbohydrate concentrations in wheat, which showed higher Performance Index (PI) values under Cd conditions [105,106]. By enhancing the amino acid biosynthesis pathway, AM fungi in combination with plant growth-promoting rhizobacteria (PGPR) improved sugar, starch, and galactose metabolism in wheat and barley plants [72]. Similarly, AM fungi improved soil fertility and increased tolerance against different environmental stresses, which have positive effects on both the quantity and quality of the secondary metabolites [16]. G. intraradices promoted the concentration of glucose and glyceraldehyde 3-phosphate, while G. mosseae, G. intraradices, and G. rosea increased the levels of cyclohexanone and its derivatives, which lead to higher secondary metabolites (Table 3). Higher AM colonization was reported to increase flavonoid, isoflavonoid, phenolic, triterpenoid, cinnamic acid amide, and Apo-carotenoid accumulation in the roots and shoots of barley [107]. and secondary metabolites [1]. The studies by Pepe et al. [20] and Ryan et al. [21] showed that some essential phytochemicals (phenolic acids, flavones, and phytic acid) belonging to secondary metabolites were higher in cereal plants under AM fungi inoculation. The concentrations of phytochemicals such as alkaloids, terpenoids, flavonoids, and organic compounds were also reported to be higher in AM-inoculated plants [22][23][24]. AM fungi also increase the photosynthetic rate by activating the primary and secondary metabolomic biosynthesis pathways via changes in phytohormonal concentrations and creating signaling molecules [60]. According to Bowles et al. [102], AM fungi improved nutrient uptake, especially of N and P, which is associated with increased enzymatic activity, leading to increased synthesis of primary and secondary metabolites. Talaat and Shawky [103] reported that AM fungi had a significant impact on the Membrane Stability Index (MSI), photochemical reactions during photosynthesis, carbohydrates, and soluble protein in two wheat cultivars (cv. Sids 1 and cv. Giza 168). Another study found that AM fungi increased proline biosynthesis and 9-cis-epoxycarotenoid dioxygenase (nced) activity in wheat, increased protein synthesis, and sped up the metabolic processes of the plants, resulting in higher anti-oxidative reactions and immune responses, which lead to increased tolerance against abiotic stresses in plants [104]. These results suggested that AM fungi may potentially have a role in the synthesis of essential plant hormones such as abscisic acid. The AM fungus F. mosseae alleviated the negative effect of heavy metals, promoted lipid production, and increased carbohydrate concentrations in wheat, which showed higher Performance Index (PI) values under Cd conditions [105,106]. By enhancing the amino acid biosynthesis pathway, AM fungi in combination with plant growthpromoting rhizobacteria (PGPR) improved sugar, starch, and galactose metabolism in wheat and barley plants [72]. Similarly, AM fungi improved soil fertility and increased tolerance against different environmental stresses, which have positive effects on both the quantity and quality of the secondary metabolites [16]. G. intraradices promoted the concentration of glucose and glyceraldehyde 3-phosphate, while G. mosseae, G. intraradices, and G. rosea increased the levels of cyclohexanone and its derivatives, which lead to higher secondary metabolites (Table 3). Higher AM colonization was reported to increase flavonoid, iso-flavonoid, phenolic, triterpenoid, cinnamic acid amide, and Apo-carotenoid accumulation in the roots and shoots of barley [107].  AM fungi were reported to mediate the production of organic acids such as oxalic, malonic, fumaric, malic, citric, and T-aconitic in maize roots through the seed coating [108].
AM fungi significantly increased antioxidant enzymes and antioxidant activity in oat leaves, and increased tolerance against abiotic stress conditions [109]. In another study, AM fungi colonization effectively counteracted sulfur dioxide (SO 2 )-induced alterations, increased the photosynthetic rate, maintained Superoxide dismutase (SOD) activity, and increased catalase (CAT) activity in oats under salt stress condition [110]. Plants cultivated in Arabian medium crude oil (ACO) with the AM fungus, i.e., Glomus intraradices, had a considerably improved degradation of ACO, but there were no variations in proline, total phenolic, nitrate reductase levels, or plant-gas exchange [111].
AM fungi stimulated plant metabolism and homeostasis by increasing dehydrogenase (DH) activity in oat, which is involved in the synthesis of nicotinamide adenine dinucleotide phosphate (NADPH) and nicotinamide adenine dinucleotide (NADH); both are involved in plant homeostasis against different stresses [112]. When sorghum plants were exposed to varying levels of chromium (Cr) stress, the plant tolerance increased due to higher antioxidant enzyme activity and primary metabolites under AM fungi inoculation [113]. Similarly, in another study, AM fungi were discovered to protect plants from oxidative stress by boosting antioxidant enzymatic activity in sorghum [114]. Furthermore, under Fe deficiency, AM fungi improved the enzymatic pathways that inhibit oxidation and enhanced secondary metabolites in sorghum [115,116]. Two AM fungi, i.e., G. mosseae and G. tortuosum, increased soluble sugar and proline content, as well as enzymatic activity, in quinoa, but decreased these when high N was applied. It is suggested that AM fungi could perform better under medium-N application [51]. In quinoa plants, the concentrations of natural lipid fatty acids (NLFAs) and phospholipid fatty acids (PLFAs) were higher than in wheat, millet, and sorghum in bulk soil under AM inoculation [117]. The soluble sugar, proline, and flavonoid contents were found to be higher with the AM fungus R. intraradices in millet plants [118]. In another experiment, it was shown that F. mosseae increased the primary metabolomics, including fatty acids and enzymatic activities, of millet plants in contrast to non-inoculated plants [119]. In the same report, another AM fungus, R. intraradices, was shown to increase the concentration of benzenoids, phenols, and flavonoids in millet, suggesting that AM fungi facilitate the production of secondary metabolites. In most cases, AM symbioses promote the accumulation of primary and secondary metabolites in many plants, including cereals, which have a positive effect on plant growth and development. To expand understanding in this area, studies with many other plants should be conducted; in the case of cereal, the use of nitrogen-fixing bacteria in combination with AM fungi aiming to optimize the synthesis of bioactive chemicals should also be investigated.   Under drought stress conditions [120] _ indicates that no result is available in the study.

The Roles of AM Fungi in Grain Yield and the Quality of Cereal Crops
AM fungi G. intraradices and G. mosseae with Trichoderma atroviride improved wheat grain productivity by 32.1 and 8.3%, respectively. AM fungi alter micronutrient availability and absorption of N, P, and other micronutrients at grain filling stage, resulting higher protein concentration and enhanced cereal crop quality and quantity [138]. In two wheat cultivars, i.e., Brookton and Krichauf, AM plants produced a lower grain yield per plant without the addition of P, while the addition of P positively affected grain yield, and the cultivar Brookton was more efficient in the case of grain weight as compared to Krichauf in highly calcareous soil [139]. Generally, it is concluded that AM fungal colonization increases P absorption by the AM pathway, which improves resource accumulation and promotes flowering initiation [140]. This influence may, in turn, contribute to increased reproductive success since early flowering leads to a higher number of flowers (and therefore grain) [74]. In maize, G. intraradices increased the silage yield, as well as the green and dry matter yield, under both irrigated and non-irrigated conditions; AM fungi strengthen the plants' ability to increase their tolerance to biotic and abiotic stress conditions under high-and low-humidity conditions [141]. Similarly, AM fungi and Azotobacter chrocoocum facilitated N and P uptake during the fruiting stage and enhanced the cob weight, length, grain weight, and starch content of maize [142]. Rice plants treated with F. mosseae, Acaulospora laevis, and Gigaspora margarita improved spikelet fertility (%), and rice productivity by 125% and 143%, respectively, under salt treatments [143]. At the mature stage, the inoculated rice allocated more nutrients to reproductive parts as compared to the non-inoculated plants and had 18.4% increase in grain weight. However, AM fungi inoculation had no significant effect on the contribution of C or N translocation to seeds, which may be the result of the simultaneous increase in sink potential under inoculation. Furthermore, the 100-seed weight was also reduced by 4.0%, while the panicle number was increased by 26.1%; as a result, the grain yield was increased by 28.2% in rice compared to the control treatment [39]. Barley plants treated with R. irregularis had improved grain yields and biochemical characteristics of seeds due to the boosting of Zn concentrations and starch content in two cultivars [29]. Similarly, the use of five AM fungi, Pacispora franciscana, F. mosseae, F. geosporum, R. irregularis, and Glomus tenebrosum, boosted barley kernel weight and grain production by 90% and 68%, respectively. The levels of K, Cu, Fe, Zn, and Ca in grain were likewise significantly greater in inoculated plants, whereas the Na content in grain was lower [144]. The sorghum cultivars (Ajabsido-MNO9-7018, Macia-PRO9110-4319, and Sureno (Latin American; PRO9110-4317)) produced 285 % more grain/plants under AM inoculation since it has been observed that AM colonization allocates available nutrients to grain rather than vegetative biomass (harvest index) [145]. Furthermore, G. mosseae increased the grain yield by 27% and significantly improved the agronomic characteristics of sorghum plants under drought stress condition [101]. AM fungus R. irregularis was applied to sorghum plants resulting in the accumulation of higher amounts of P, Zn, and Fe in the grain, which resulted in higher yield and harvest indices as compared to noninoculated plants [73]. The plant growth and yield of oat plants increased when barley and rye (Secale cereal) crops were recycled with AM fungi in the cropping system, suggesting that a low-input cropping system with the application of AM fungi could increase oat yield and productivity [146]. In another study, after 45 days of treatment with mixed-AM fungi, oat plant 'variety Pepita' performed better than oat 'variety Supernova' in terms of growth and yield production in a field experiment held in Southern Chile [147]. In combination with Azotobacter chroococcum and Trichoderma harzianum, the AM fungus Acaulophora laevis showed 95% higher colonization and increased herbage yield of Buckwheat plants, demonstrating that AM fungi are environmentally friendly with other microorganisms and facilitate higher yields and productivity [48]. In the same study, large flower clusters, herbage yields, the number of seeds per cyme, the maximum harvest index, and seed yields were higher in buckwheat under AM fungi application. In quinoa, the addition of organic compost inoculated with mixed-AM fungi led to increases in dry weight, grain production, and nutritional quality under drought stress [148]. Furthermore, quinoa treated with G. mosseae had increased plant shoot growth, but it had no significant effect on yield [51]. Two AM fungi, i.e., Rhizophagus fasciculatus and Ambispora leptoticha, in combination with PGPR bacteria (Pseudomonas) increased grain yields more effectively than single inoculation, suggesting that the application of AM fungi and PGPR bacteria together could boost millet plant growth and productivity [149]. When mixed commercial AM fungi were treated with SWSC (solid waste-based soil conditioner), AM colonization rates rose up to 81%, resulting in higher millet growth and yield compared to the control [150]. Moreover, the hyphae of the AM fungus C. etunicatum extended throughout a 12 cm soil layer, absorbing more nutrients and resulting in higher growth, revealing that the AM fungus has the potential to boost millet yield and productivity [151]. The majority of the research has claimed that AM fungi have good impacts on grain yield and quality, while only few studies have shown that AM fungi have a detrimental effect on grain yield and quality. In addition, the roles of many AM fungus spores are unknown; as a result, this is an opportunity for researchers to learn more about the impact of various and distinct AM fungal spores on cereal crops.

Conclusions
Arbuscular mycorrhizal (AM) fungi have many roles in various developmental processes in plants, especially in cereals. AM fungi have an effect on nutrient availability and uptake, increase the photosynthetic rate, improve antioxidant activities, and increase tolerance against environmental stress. Moreover, AM fungi have a positive impact on both the primary and secondary metabolites and also increase growth, yield, and productivity. AM fungi are utilized in greenhouses and on a smaller scale in agricultural systems, but it is possible that they may also be used on a larger scale. However, there are a higher number of AM fungi spores and the function of each group or each spore in different crops is still unknown. More studies are required to investigate the role and function of AM fungi spores in numerous plants, particularly cereal crops. In addition, the molecular processes and genetic aspects of each species should be researched in order to have a better understanding of the distinctions in the functions that are present in different AM fungi species. Plants such as oat and barley plants should be more thoroughly investigated under AM conditions to provide more knowledge about the effects of AM fungi on these plants since there has not been much study on these plants in relation to AM symbiosis.