Current developments in arbuscular mycorrhizal fungi research and its role in salinity stress alleviation: a biotechnological perspective.

Arbuscular mycorrhizal fungi (AMF) form widespread symbiotic associations with 80% of known land plants. They play a major role in plant nutrition, growth, water absorption, nutrient cycling and protection from pathogens, and as a result, contribute to ecosystem processes. Salinity stress conditions undoubtedly limit plant productivity and, therefore, the role of AMF as a biological tool for improving plant salt stress tolerance, is gaining economic importance worldwide. However, this approach requires a better understanding of how plants and AMF intimately interact with each other in saline environments and how this interaction leads to physiological changes in plants. This knowledge is important to develop sustainable strategies for successful utilization of AMF to improve plant health under a variety of stress conditions. Recent advances in the field of molecular biology, "omics" technology and advanced microscopy can provide new insight about these mechanisms of interaction between AMF and plants, as well as other microbes. This review mainly discusses the effect of salinity on AMF and plants, and role of AMF in alleviation of salinity stress including insight on methods for AMF identification. The focus remains on latest advancements in mycorrhizal research that can potentially offer an integrative understanding of the role of AMF in salinity tolerance and sustainable crop production.


Introduction
Salinity is one of the most prevalent agricultural problems in the arid and semiarid regions of the world, affecting approximately 1 billion ha of land (Abdel Latef & Chaoxing, 2010). Estimations indicate that increased salinization of arable land will result in 30% land loss within the next 25 years, and up to 50% within the next 40 years Wang et al., 2003). Besides having deleterious effects on the physicochemical properties of soil and its microbiological processes, increased salinity also seriously affects plant growth and productivity. Presence of excessive salts in soil results in stunted growth and limited productivity of plants by reducing water uptake, inhibiting metabolic processes, affecting nutrient composition, osmotic balance and hydraulic conductivity (Huang et al., 2010). Many reactive oxygen species (ROS) are also generated in response to salt stress, which causes oxidative damage to lipids, proteins and nucleic acids thereby changing the selective permeability of the cell membranes, eventually causing them to leak (Abdel Latef & Chaoxing, 2010;Huang et al., 2010). A significant research effort has been focused on developing salt-tolerant crops; however, there has been limited success as only a few genetic traits of salt tolerance have been identified (Schubert et al., 2009). Besides the use of salt tolerant crops or their development through genetic engineering, an alternative strategy is to apply biofertilizers that seems to be a more environment friendly and economically viable option (Evelin et al., 2009). Arbuscular mycorrhizal fungi (AMF) are among the most abundant organisms on earth; a very conservative estimate suggests that they represent 5-10% of global soil microbial biomass (Lanfranco & Young, 2012). These symbiotic associations are commonly described as the result of co-evolution events between fungi and plants and also considered as a plant strategy for growth under a variety of stress conditions (Bandou et al., 2006;Bonfante & Genre, 2008). AMF associate with the roots of higher plants and are found in 480% of plant species (Parniske, 2008). They establish an intimate association with the roots of most land plants (Gutjahr & Paszkowski, 2013), with the fungi supplying mineral nutrients from the soil while acquiring carbon compounds from the photosynthetic host (Lanfranco & Young, 2012). In this association, both partners profit from the relationship; AMF improve the host plants nutrient status, influencing mineral nutrition, water uptake, growth and disease resistance, whereas in exchange, the host plant provides organic carbon and the substrate necessary for fungal growth and reproduction (Smith & Read, 2008). AMF play an important part in nutrient cycling with the help of their mycelium in absorbing soil nutrients and providing them to the plant, although their role in carbon flux is less well defined. Recent work by different workers has established that AMF can effect plant fitness (Hoffmann et al., 2011;Wilson et al., 2009), productivity and biodiversity (Wilson et al. 2009). Pellegrino & Bedini (2014) reported that the application of foreign AMF Funneliformis mosseae and Rhizophagus irregularis showed great potential in biofertilization of crops and biofortification of foods. Two review articles published previously (Evelin et al., 2009;Porcel et al., 2012) have explored the physiological mechanisms by which AMF improve plant salinity tolerance. Therefore, in this article, we summarize the most important mechanisms by which mycorrhizal colonization in salt-affected plants alleviates salt stress and also lists the latest significant contributions by researchers on the role of AMF in improving different crop performance under salt stress. We also discuss the method of detection of AMF, latest developments in AMF research and focus on approaches that can provide an integrative understanding of the role of AMF in salinity tolerance and sustainable crop production in detail.
Mechanism of AMF root colonization and methods for detection of AMF is given in Supplementary material.

Salinity stress: impact on crop plants
The percentage of agriculture land affected by salinity is continuously increasing, owing to both natural causes and agricultural practices such as irrigation (Munns & Tester, 2008). Of the total 14 billion ha of land available on earth, about 1 billion ha are natural saline soils. It is estimated that more than 50% of global arable land could be salinized by the year 2050 . Salinity reduces crop yield by inhibiting seed germination and also by altering the physiology and anatomy of the plants (Karan & Subudhi, 2012). Salt stress inhibits plant growth and development through different means, including perturbation of various metabolic activities and triggering programmed cell death, leading to reduced crop yield or total crop failure.
Most of the crops, commonly used for food production are sensitive to salinity stress (Flowers & Colmer, 2008) and vary in their response to salt stress tolerance. Among cereals, rice (Oryza sativa) is the most sensitive, while barley (Hordeum vulgare) is regarded as the most tolerant. Bread wheat (Triticum aestivum) is comparatively more tolerant than durum wheat (Triticum turgidum ssp. durum). The dicotyledonous species are more tolerant than monocotyledonous species. Cereal crop plants (rice, wheat and barley) are the most extensively studied for understanding the physiological and molecular basis of salt tolerance (Karan & Subudhi, 2012).
Plants respond to salinity stress through the complex interactions of many genes, proteins and metabolite/signaling pathways (Ashraf, 2009;Jamil et al., 2011). Plants deal with these stresses by adapting different strategies (Galvan-Ampudia & Testerink, 2011;Kumar et al., 2012); for example, reducing salt uptake or increasing salt exclusion, enhanced K + /Na + ratio, tissue tolerance, closure of stomata, synthesis of osmolytes, water use efficiency (WUE), up-regulation of antioxidant system for protection against ROS, early flowering and vigorous growth to dilute the salt concentration in plant tissue (Colmer et al., 2006;Türkan & Demiral, 2009). Salinity stress also causes hyper-osmolarity that triggers the mitogen activated protein kinase cascade (Kiegerl et al., 2000), that results in induction of transcription factors followed by increased synthesis of osmolytes, osmoprotectants and detoxifying enzymes (Vinocur & Altman, 2005).
Recently, the use of AMF to alleviate soil stresses on plant growth has received increased attention (Daei et al., 2009;Kumar et al., 2009Kumar et al., , 2010Miransari, 2010) and their role is discussed in the following section.

Impact of salinity on AMF
Arbuscular mycorrhizal colonization in plant roots is influenced by various factors when plants grow under salinity stress conditions. Many researchers reported reduction in colonization under salinity in several plant species such as tomato (Hajiboland et al., 2010), lotus (Sannazzaro et al., 2006), acacia (Giri et al., 2003(Giri et al., , 2007 and Jatropha curcas (Kumar et al., 2010). This decline in colonization could be due to the negative effect of salinity on the germination of spores (Hirrel, 1981), inhibited growth of hyphae and hyphal proliferation in soil (McMillen et al., 1998) and reducing the number of arbuscules (Pfetffer & Bloss, 2006), or alternatively, indirectly by the effect on host plants.
Studies on effect of soil salinity on spore production by AMF are rare and need more detailed investigation. Spore formation and germination in saline soils might depend on complex physiological and ecological parameters and also on the genotypes of the plants and AMF. The presence of relatively high spore numbers (mean of 100/10 g soil) in the severely saline soils (EC $162 dS m À1 ) have been reported previously (Aliasgharzadeh et al., 2001;Bhaskaran & Selvaraj, 1997) and contrasted with other studies where the average density of spores in saline areas (EC445 dS m À1 ) was reported to be low (Barrow & Aaltonen, 2001;Hirrel, 1981;Kim & Weber, 1985). In some cases, higher spore production may be due to induction of sporulation by salinity stress. If soil salinity affects spore germination, hyphal growth and colonization, then total spore production is likely to be reduced in saline soil relative to non-saline soils. Aliasgharzadeh et al. (2001) reported that inhibition of spore germination may also cause spore accumulation in saline soil. Asghari & Cavagnaro (2010) reported detrimental effects of soil salinity on spore germination and hyphal growth, and have been identified as being the most important reason for the absence of colonization in halophytes. It is extremely difficult to distinguish between direct and plantmediated effects on AMF biology because of their obligate biotrophic life style. Previous studies on the effects of salinity stress on spore germination indicated inhibition of spore germination under NaCl concentrations (Hirrel, 1981;Juniper & Abbott, 2006). However, other studies showed that low water potentials delayed germination rather than preventing germination (Hirrel, 1981) and germ tube length was reported to be reduced by decreased water potential (Koske & Halvorson, 1981). Juniper & Abbott (2006) revealed that propagules of different AMF from saline and non-saline soils differ in their ability to germinate and grow in the presence of NaCl in soil solution. Several studies indicated that the formation of AMF colonization may be reduced by increasing soil salinity (Giri et al., 2007;Hajiboland et al., 2010;Huang et al., 2010;Juniper & Abbott, 2006;Sheng et al., 2008). Mechanism of AM root colonization is presented in Fig. 1.

AMF: salinity stress alleviation
Plants can respond to salinity stress by modifying themselves at morphological, anatomical and cellular levels, which allow the plant to avoid the stress or to increase its tolerance. These modifications are of vital significance for some plant species, but are not found as a common response in all plants (Jamil et al., 2011). Plants, in addition to the inherent adaptation against these stresses, also show association with many rhizospheric soil microorganisms that can alleviate the stress effect. Salinity affects plant growth and survival by disrupting several physiological mechanisms such as photosynthetic efficiency, membrane integrity and water status (Evelin et al., 2009). There is evidence that AMF, besides using the abovementioned mechanisms, can alleviate salt stress by improving the physiological processes in plants. However, salinity not only adversely affects the host plant but also the growth of AMF by hampering colonization, spore germination and hyphal elongation as discussed previously (Kumar et al., 2010;Porcel et al., 2012;Sheng et al., 2008). Contrary to some reports, increase in sporulation and colonization has also been observed (Aliasgharzadeh et al., 2001).
Mycorrhizal association with the plants have been shown to increase plant growth and yield, water status, enhance accumulation of nutrients, influence plants hormones and affect various physiological and biochemical properties of plants in saline and alkaline soil conditions (Abdel Latef & Chaoxing, 2010;Bandou et al., 2006;Evelin et al., 2009;Kashyap et al., 2004;Kumar et al., 2010;Murkute et al., 2006). Several mechanisms have been proposed to explain the protection of plants from the adverse effect of salt stress. Improvement in the nutrient status of colonized plants can be attributed not only to the uptake of nutrients via the mycorrhizal pathway, but also to indirect effects carried out by morphological and physiological changes in roots due to colonization. Improved salt tolerance of mycorrhizal plants can be mainly related to the changes in physiological processes, for example, increased carbon dioxide exchangeable rate, transpiration, stomatal conductance and WUE, enhanced uptake of nutrients with low mobility, such as P, Zn and Cu (Giri & Mukerji, 2004;Giri et al., 2007;Ruiz-Lozano et al., 2002).
Several studies reveal that arbuscular mycorrhizal inoculated plants maintained a relatively higher water content compared with non-inoculated plants (Colla et al., 2008;Evelin et al., 2009;Jahromi et al., 2008;Sheng et al., 2008), which is facilitated by the improved hydraulic conductivity of the root at low water potential (Kapoor et al., 2008). This improved root conductance is associated with changes in the morphogenetic characters of roots. AMF decrease the meristem activity of root apices and thus lead to an increase in the formation of adventitious roots and these AMFmediated modifications in root morphology may assist in maintaining nutrient uptake and water balance in the host plant under salinity stress. Sukumar et al. (2013) recently reviewed that changes in root systems in response to AM leading to an increased root branching and root system volume. Giri et al. (2003) reported increased water uptake per unit root length in colonized plants under stress conditions. All these parameters, improved by mycorrhizal colonization, enable host plants to use water more efficiently and allow them to maintain a lower intercellular carbon dioxide concentration. As a consequence, the gas exchange capacity increases in the mycorrhizal plants (Evelin et al., 2009).
Several examples of salinity stress alleviation by AMF have been reported, and these are briefly summarized in Table 1. To some extent, these fungi have also been considered as bio-ameliorators of saline soils (Giri et al., 2007). The following subsections, will discuss the role of AMF during salt stress on growth and biochemical response in plants. Giri et al. (2007) reported that soil salinity decreased root and shoot dry weights in Acacia and there was a significant positive effect of mycorrhizal inoculation on plant growth regardless of salt concentrations. Inoculating olive plantlets with Glomus mosseae, Glomus intraradices or Glomus claroideum increased plant growth and the ability to acquire N, P and K from non-saline as well as saline media (Porras-Soriano et al., 2009). Abdel Latef & Chaoxing (2010) reported that tomato in salinity stress reduced the dry matter of roots, stems, leaves, leaf area and total biomass significantly in comparison with the control treatment. They also reported accumulation of dry matter in shoots, over root biomass, which might be due to the allocation of a greater proportion of carbohydrates to the shoot than root tissues in mycorrhizal plants (Abdel Latef and Chaoxing, 2010;Shokri & Maadi, 2009 observed that pearl millet inoculated with AMF showed a significant increase in leaf number, shoot and root length at a moderate level of salinity stress.

Solute accumulation
The main impact of NaCl stress on plant tissues is marked in terms of loss of intracellular water and osmotic damage. Osmotic stresses are associated with inhibition of cell growth by restricting wall extension and cellular expansion, leading to reduced plant growth. Production of free amino acids is important osmolytes contributing to osmotic adjustment in plants (Hajlaoui et al., 2010). Osmotic adjustment enables plants to maintain higher turgor pressure under salinity stress (Munns & Tester, 2008). Some studies showed that increasing external salt concentration induces free amino acids accumulation in the leaves and roots (Hajlaoui et al., 2010;Neto et al., 2009;Sheng et al., 2008) but to a lesser extent in arbuscular mycorrhizal plants (Sheng et al., 2011). Root colonization by AMF can induce major changes in the relative abundance of the major groups of organic solutes (Sheng et al., 2011), such as modifying the composition of carbohydrates (Augé et al., 1987) and inducing accumulation of specific osmolytes such as proline (Ruiz-Lozano et al., 1995), thus facilitating osmotic adjustment. Mycorrhizal colonization in plants raised the leaf concentrations of soluble sugars, reducing sugars, soluble protein, total organic acids, fumaric acid, acetic acid, oxalic acid, malic acid and citric acid, and decreased the concentrations of total free amino acids, proline, formic acid and succinic acid. These results suggest that the symbiosis affects the metabolic regulation of organic acids under saline conditions, involving enzymes from the tricarboxylic acid cycle, glyoxylate cycle or glycolysis, thus improving the energetic status of the plant and helping to mitigate the stress (Sheng et al., 2011). Proline is an osmoprotectant that is synthesized by plants in response to stress, and thereby helps in maintaining the osmotic balance of the cell to alleviate the abiotic stress effect. It also plays key role in scavenging free radicals, stabilizing subcellular structures and buffering cellular redox potential under stresses (Ashraf, 2009). Previous studies have reported a higher proline concentration in arbuscular mycorrhized plants in comparison with non-AMF plants at different salinity levels Kumar et al., 2010;Talaat & Shawky, 2011). In contrast, other studies have reported that AMF plants accumulated significantly lower proline than non-AMF plants at various salinity levels (Borde et al., 2010;Jahromi et al., 2008;Rabie & Almadini, 2005;Sheng et al., 2011). Another important solute, glycine betaine (N,N,N-trimethylglycine betaine), also stabilizes the structure and activity of enzymes and protein complexes and maintains the integrity of membranes against the damaging effects of excessive salt (Rhodes et al., 2004). Increased accumulation of betaines in AMF-inoculated plants has been reported (Duke et al., 1986).
Soluble sugar accumulation for lowering osmotic potential is a common physiological response during salt stress. Al-Garni (2006) reported higher concentrations of sugars in Phragmites australis plants colonized by Glomus fasciculatum, than the non-inoculated plants. The concentration of soluble and reducing sugars decreased when salinity increased in both AMF-inoculated or non-inoculated maize plants, but at the same NaCl level, the AMF symbiosis favored sugar accumulation (Sheng et al., 2011). Many authors have reported the positive correlation between sugar accumulation and mycorrhization (Evelin et al., 2009;Feng et al., 2002;Ocón et al., 2007). The accumulation of high levels of sugars in AMF plants may be the result of an increase in photosynthetic capacity (Sheng et al., 2008;Wu et al., 2010). Accumulation of soluble sugars to adjust the osmotic potential of plants during salt stress constitutes an important plant protection mechanism (Evelin et al., 2009). Increased total carbohydrate content was positively correlated with mycorrhization of P. australis plants colonized by G. fasciculatum (Al-Garni, 2006), soybean plants colonized by G. intraradices (Porcel & Ruiz-Lozano, 2004) and J. curcas by AMF consortia (Kumar et al., 2010).
The regulation of organic acid metabolism in plants also plays a key role in salinity tolerance (Guo et al., 2009). AMF inoculation increases the accumulation of organic acids in maize leaves (Sheng et al., 2011). However, the effect of AMF symbiosis was different depending on organic acids. Concentrations of oxalic, acetic, fumaric, malic and citric acids increased, while formic and succinic acid levels decreased although lactic acid concentration was not significantly affected (Sheng et al., 2011). Furthermore, leakage of organic acids into the rhizosphere of AMF plants resulted in a decrease in soil pH, soil EC, organic carbon, and an increase in soil availability of N, P and K (Usha et al., 2004).
Other than organic acids, involvement of three main polyamines from plants such as putrescine, spermidine and spermine are supposed to play a role in plant responses to a variety of environmental stresses such as salinity, high osmolarity, hypoxia and oxidative stress.
Beneficial effects of AMF symbiosis under salinity may be due to alterations in basal energetic metabolism (Giri & Mukerji, 2004;Rabie & Almadini, 2005). Thus, changes in plant metabolism have also been reported in mycorrhizal (G. mosseae) maize plants grown at different salt levels (Sheng et al., 2011). Evelin et al. (2009) reported that maintenance of the photosynthetic apparatus via the accumulation of protective molecules and osmolytes and/or the upregulation of antioxidant metabolism by AMF suggests interesting biotechnological opportunities for improving agricultural productivity (Choudhary et al., 2011).

Chlorophyll concentration
Salinity stress decreases photosynthesis, often through affecting photosynthetic enzymes, chlorophyll, carotenoids and chloroplast structure (Chaves et al., 2009;Sheng et al., 2008). Reduction of chlorophyll content in leaves is common in response to increased salt stress. This may also be due to the synthesis of nitrogen compounds, such as proline, during stress, which consumes large amounts of nitrogen ), suppression of specific enzymes that are responsible for the synthesis of photosynthetic pigments, reduction in the uptake of minerals (e.g. Mg) needed for chlorophyll biosynthesis (Evelin et al., 2009), and also increased activity of cell degrading chlorophyllase (Younis et al., 2008). The increase of photosynthetic ability in mycorrhizal seedlings under salt stress could result from a lower intercellular CO 2 concentration (Sheng et al., 2008). In the presence of AMF, the antagonistic effect of Na on Mg uptake is counterbalanced and suppressed (Giri et al., 2003). Huang et al. (2011) showed higher chlorophyll contents in the leaves of mycorrhizal plants, confirming that the symbiosis plays a key role in modifying photosynthetic and metabolic activities. Campanelli et al. (2012) confirmed that the photosynthetic rate and chlorophyll content were higher in mycorrhizal plants than in non-mycorrhizal plants, and similar observations were also recorded in previous studies (Abdel Latef & Chaoxing, 2010;Borde et al., 2010;Busquets et al., 2010).
Several previous reports showed higher chlorophyll content in leaves of AMF plants under saline conditions (Abdel Latef & Chaoxing, 2010;Colla et al., 2008;Kaya et al., 2009). Wu et al. (2010) demonstrated a significant reduction in the net assimilation rate, transpiration and stomatal conductance in both AMF and non-AMF citrus plants under salt-stress. However, AMF alleviated the salt stress effect in citrus plants by maintaining higher net assimilation rate, transpiration and stomatal conductance than the non-AMF seedlings. AMF-inoculated plants under salt stress reached levels of photosynthetic capacity higher than those of non-stressed plants, showing that in this respect, mycorrhization is capable of fully counterbalancing stress (Zuccarini & Okurowska, 2008).

Mineral nutrition
The cycling of macro and micronutrients in an ecosystem is influenced by multiple interactions involving soil microbial populations (Barea et al., 2005;Dames & Ridsdale, 2012). AMF have the potential to improve the sustainability of agroecosystems through their fundamental roles in the efficiency of both nutrient cycling and soil aggregation processes. Mycorrhizas increase the uptake of the nutrients to plant roots mainly by diffusion, particularly in dry soil when diffusion rates are most limited (Marschner et al., 1997). Phosphate is one of the important elements driving the symbiosis (Maeda et al., 2006). Plant roots acquire P as inorganic orthophosphate (Pi) through different pathways, presence of Pi transporters in plants can be the major routes for P uptake (Balzergue et al., 2011). Because of phosphate's low mobility and reduced availability in soil (Schachtman et al., 1998), plants have developed a variety of ways to cope with phosphate limitation. The most common one is the formation of symbiosis with AMF (Karandashov & Bucher, 2005). AMF colonized plants rely exclusively on the symbiotic route for phosphate acquisition (Smith et al., 2003). Plant phosphate status and AMF development are known to be linked, because high phosphate levels in the soil reduce fungal colonization within the root (Breuillin et al., 2010), as well as carbon transfer toward the AMF (Olsson et al., 2010). Importantly, inactivation of the symbiotic plant phosphate transporters leads to fungal growth arrest (Javot et al., 2007;Maeda et al., 2006). In this regard, Kiers et al. (2011) have recently shown that more cooperative fungi, in terms of plant growth responses, cost of carbon per unit of P transferred, and resource hoarding strategies, are allocated more carbon. Helber et al. (2011) reported high-affinity Monosaccharide Transporter2 (MST2) from Glomus sp. that functions at several symbiotic root locations. Sugars in plant cell wall can efficiently outcompete the Glc uptake capacity of MST2, proposing that they can serve as alternative carbon sources. The expression of MST2 closely correlates with that of the mycorrhiza-specific Phosphate Transporter4 (PT4). These outcomes highlight the symbiotic role of MST2 and support the hypothesis that the exchange of carbon for phosphate is tightly linked (Helber et al., 2011). The improvement of plant P uptake by AMF has been reported and was considered one of the main explanations for amelioration of stress in salt-affected AMF-colonized plants .
Several mechanisms have been suggested by which the AMF plant can enhance growth and nutrient uptake (Miransari, 2010). The extra-radical hyphae assist plant roots to connect to the surrounding soil and increase the soil volume exploited by host plants and also enable plants to survive in nutrient-and/or water-depleted zones (Marschner et al., 1997). Thus, in lownutrient conditions, AMF-colonized roots may have a greater uptake of relatively immobile macro and micronutrients (Azcón et al., 2003). Additionally, AMF roots often have not only increased length but also modified root architecture. Improved growth of AMF plants has been largely attributed to mycorrhizal-mediated P acquisition, since the addition of P fertilizer has similar effects in the absence of AMF. P has lowest mobility in soil, and thus often limits plant growth, particularly when soil water potential and P diffusion rate is lowered in dry or saline soils. It also tends to be precipitated by ions like Ca 2+ , Mg 2+ and Zn 2+ (Abdel Latef & Chaoxing, 2010;Evelin et al., 2009;Giri et al., 2007;Plenchette & Duponnois, 2005).
Saline conditions modify the uptake of mineral nutrients and nutrient balance (Giri et al., 2007). Na and Cl concentrations were lower in mycorrhizal plants than that in nonmycorrhizal plants. Some researchers have hypothesized that lower Na and Cl concentrations in plant tissues may be due to the capacity of the fungus to retain these ions in intra-radical fungal hyphae or to compartmentalize them in the root cell vacuoles (Al-Karaki, 2006). In Acacia nilotica plants, total accumulation of P, Zn and Cu was higher in AMF than in non-AMF plants under both control and medium salt stress conditions (Giri et al., 2007). AMF enhance the uptake of various nutrients in plants during salinity stress (Table 2).
Salinity interferes with nitrogen (N) acquisition and utilization by influencing different stages of N metabolism, such as, NO À 3 uptake, reduction and protein synthesis (Evelin et al., 2009). AMF enhance better assimilation of N by the host plants. However, the exact mechanism behind N uptake and its transfer from the fungus to the host plant is still not very clear (Evelin et al., 2009;Govindarajulu et al., 2005). Hajiboland et al. (2010) observed lower calcium uptake in salt-affected non-AMF tomato plants, although AMF colonization significantly increased Ca uptake and the Ca:Na ratio of both leaves and roots. Hammer et al. (2011) reported that G. intraradices can selectively take up elements such as K + , Mg 2+ and Ca 2+ while avoiding Na + to keep internal K + :Na + and Ca 2+ :Na + ratios within narrow limits, despite concentration changes of several orders of magnitude in the growth environment. These selective mechanisms for ion uptake could partially alleviate salinity stress in host plants by improving their nutrition (Hammer et al., 2011). Campanelli et al. (2012) showed that AMF-mediated growth could be the result of an improvement in nitrogen status, as suggested by the SPAD (Chlorophyll Meter) values monitored during a glasshouse trial. The SPAD values were closely linked to leaf chlorophyll content and leaf N content (Esfahani et al., 2008;Netto et al., 2005). Mycorrhizal fungi had a positive influence in helping to maintain the potassium content at all salinity exposure levels (Campanelli et al., 2012). Potassium is important for water regulation, stomatal behavior and cell expansion (Evelin et al., 2009;Kaya et al., 2009); it also activates a range of enzymes, and Na cannot substitute it in this role (Giri et al., 2007). The positive influence of AMF on maintaining nutrient status and growth of plants grown under salt stress conditions could be regarded as a strategy for salinity tolerance in plants, particularly in nutrient poor soils.

Antioxidants production
Salinity stress induces oxidative stress in plants and resulted in generation of ROS such as superoxide radical (O À 2 ), hydrogen peroxide (H 2 O 2 ), hydroxyl radical ( OH) and singlet oxygen ( 1 O 2 ). The activated oxygen species thus generated can cause damage to plant cell structure and function if not controlled by protective mechanisms. Plant cells use several mechanisms to minimize the oxidative damage caused by ROS (Ahmad et al., 2010;Ashraf, 2009;Jamil et al., 2011). The induction of ROS-scavenging enzymes, such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX) is the most common mechanism for detoxifying the ROS generated during stress conditions. Apart from the salinityinduced antioxidative response, AMF per se could also induce activation of antioxidant defense enzymes. It has been shown by various authors that AMF symbiosis might enhance the activity of such antioxidant enzymes (CAT, POD, APX and SOD) and thus protect the plant from oxidative damage during salt stress (Evelin et al., 2009;Hajiboland et al., 2010;He et al., 2007). Table 3 shows the changes in the activity of various antioxidants by AMF-inoculation during salt stress. Borde et al. (2011) reported enhanced SOD activity in AMF-inoculated pearl millet grown under salinity, as compared with non-AMF plants, supporting the view that increased antioxidative enzyme production could be involved in the beneficial effects of AMF colonization on the    (Hajiboland et al., 2010;He et al., 2007;Huang et al., 2010;Kohler et al., 2010).

Phyto-hormone signaling
Phyto-hormones including cytokinins, ethylene, abscisic acid (ABA), auxin, jasmonic acid and salicylic acid can also act as signal molecules during the process of AMF symbiosis (Gutjahr & Paszkowski, 2009;Miransari et al., 2012). Understanding of such signaling and communicating molecules between AMF and the host plant can be useful for a more precise characterization of the AMF-plant symbiosis. Miransari et al. (2012) reviewed some of the most recent findings regarding the signaling effects of plant hormones on mycorrhizal fungal symbiosis. It is suggested that enhanced P uptake by AMF plants can increase cytokinins production in the roots. Increased concentrations of cytokinins may enhance shoot growth and not root growth (Hause et al., 2007;Miransari et al., 2012). Plant hormones affect P uptake by changing root growth and sugar signaling (Rouached et al., 2010). This may indicate that plant hormones can affect the uptake of nutrients by altering plant physiology and morphology. The response of plants to P deficiency is regulated by soil P levels and mycorrhizal symbiosis through alteration in the expression of the related genes. The binding of transcription factors to the promotion of the P responsive genes can affect the plant's response to P stress. Such effects are influenced by the related P pathways, allocation and homeostasis regulation in plant cells (Sobkowiak et al., 2012). This may also further elucidate the role of cytokinins during P stress in plant. Mukherjee & Ané (2011) investigated the effects of exudates produced by the germinating spores of G. intraradices and ethylene on the process of symbiosis in monocots and eudicots. The exudates induced morphological and physiological (activation of symbiotic genes) changes in the host plant roots. However, ethylene inhibits the process of mycorrhizal symbiosis by adversely affecting the effects of the exudates on the activation of symbiotic association (Miransari et al., 2012). Strigolactones, released by plant roots, can regulate auxin transport in the plant by affecting auxin carriers (Koltai et al., 2010) or by affecting the secondary messengers, which are active in auxin signaling (Brewer et al., 2009;Ferguson & Beveridge, 2009). This suggests that auxin may indirectly affect the symbiotic level as it is influenced by strigolactones. Hence, the role of Indole Acetic Acid (IAA) in mycorrhizal symbiosis has been suggested (Ludwig-Müller, 2010, 2011. The role of jasmonic acid and salicylic acid to improve plant systematic acquired resistance is also well documented. The level of jasmonates increased in the mycorrhizal roots of plants including Cucumis sativus (Vierheilig & Piche, 2002), Medicago truncatula (Stumpe et al., 2005) and Glycine max (Meixner et al., 2005). Research has also indicated that AMF roots contain high amounts of jasmonic acid in comparison with non-AMF roots (Hause et al., 2007;Meixner et al., 2005). Shekoofeh et al. (2012) evaluated the role of AMF (G. mosseae and G. intraradices) and salicylic acid (0.2 mM) Figure 1. Diagrammatic representation of mechanism of AM root colonization. DOI: 10.3109/07388551.2014.899964 in improving the salinity tolerance of green basil (Ocimum basilicum L.). They observed that mycorrhization and salicylic acid improved plant tolerance to salinity by decreasing lipid peroxidation and increasing proline, proteins, reduced sugars and K in aerial parts of basil. Dodd & Pérez-Alfocea (2012), in their recent review, mentioned the role of soil microbes in improving the crop salt tolerance by altering phyto-hormonal root-shoot signaling. Production of ABA is essential for AM colonization in plant roots and it controls the formation of arbuscules. In contrast, some mycorrhizal fungi accumulate substantial quantities of ABA (Esch et al., 1994), which may be responsible for increased root ABA concentrations of some mycorrhizal plants (Danneberg et al., 1993;Jahromi et al., 2008). Martín-Rodríguez et al. (2011) concluded that ABA had a direct promotive role in arbuscule formation, while ethylene negatively regulated fungal development. Studies demonstrated that different plant hormones can also act as signal molecules and positively or adversely affect AMF symbiosis (Miransari et al., 2012). Plant hormones may control the level and hence specificity of mycorrhizal symbiosis but detailed investigation is required to unravel this mechanism.

Latest development in AM fungal research
The advancement of metagenomics and next generation sequencing technologies has characterized all-known AMF into a single clade, the phylum Glomeromycota, a sister group of Ascomycota and Basidiomycota on the basis of ribosomal RNA phylogeny (Krüger et al., 2012;Lanfranco & Young, 2012). However, phylogenetic reconstruction based on mitochondrial or protein coding nuclear sequence placed the phylum near to Mucorales. Furthermore, ribosomal RNA gene analysis has resulted in changes of AM fungal nomenclature. For example, the frequently studied G. intraradices DAOM197198, which was recently re-identified as Glomus irregulare (Stockinger et al., 2009) has now become R. irregularis. Meanwhile, G. mosseae becomes Funneliformis, and G. claroideum and Glomus etunicatum become Claroideoglomus. Lanfranco & Young (2012) discussed the genomic advancement of AM fungal research at various fronts such as genetic processes of AM fungal colonization, genome-wide transcriptomic analysis, and new insight into nutrient exchange. However, Fitter et al. (2011) discussed nutritional exchange in the AM symbiosis with respect to sustainable agriculture in great detail. Lee & Young (2009) demonstrated the feasibility of obtaining a high-quality sequence from both the nucleus and mitochondria after whole genome amplification of DNA from AM spores. On the basis of complete mitochondrial sequencing, the phylum Glomeromycota shows typical fungal features and is related to Mucormycotina. Deciphering the genome of G. intraradices and other AMF will address many questions and be a starting point to understand the relevance of genetic diversity for the evolution and ecology of AMF. Tisserant et al. (2012) produced first genome-wide analysis of the transcriptome of G. intraradices DAOM 197198 and transcript profiling on symbiotic and asymbiotic fungal structures based on oligoarray and pyrosequencing data, which provided novel insights into the molecular basis of symbiosis-associated traits. Helber et al. (2011) provided the first demonstration of host-induced gene silencing in AMF. In addition, they conducted detailed characterization of a Glomus high-affinity monosaccharide transporter with a broad substrate spectrum that was active at multiple symbiotic interfaces. The latest breakthrough related to the unraveling of signal molecules Myc factors as lipochitooligosaccharides that are closely related to nodulating factors in rhizobia, Maillet et al. (2011) has opened new avenues to address early communication and functional aspects of AM symbiosis. The Myc factor triggers the expression of symbiosisrelated genes such as ENOD11 (Kosuta et al., 2003).
The latest advances in the gene-based study contribute to our understanding of mechanisms involved in the establishment of AMF at every level (Ercolin & Reinhardt, 2011). The complete sequencing of an AM fungal genome will advance our knowledge, and aid in the application of AMF to improve plant productivity under salinity stress and nutrient-limited soil conditions.

Conclusions
In this review, we have shown that AMF play a key role in salinity tolerance by increasing host plant nutrition, and also by improving osmotic adjustment. Successful commercialization of AMF to improve crop growth and yield in saline soil needs further detailed investigation, particularly participation of cation proton antiporters, cyclic nucleotide-gated channels and the isolation of AM fungal salt-tolerant strains for development of a salt-tolerant inoculum. In recent years, our knowledge has improved considerably due to application of genomic technologies. However, several key questions still remain, specifically regarding how AMF invade plant tissues, suppress host defense, exchange nutrients, affect plant hormone signaling and the ecology of AMF in natural environments. Another desirable future goal will be to enhance the potential of AM fungal technology for sustainable agricultural and environmental management by identifying the key molecular players that are required for AM development and function.