*4.1.2 AMF-induced changes in expression of aquaporin genes, transcriptional profiles*

The negative water potential in dried soils exerts the problem for plants to obtain adequate water amount, a process where aquaporins (AQPs) get involved in [64]. AQPs belonging to the large major intrinsic protein family of transmembrane proteins functioning as water channels are crucial in osmoregulation [64]. On top of that, their regulation of transcellular movement of many molecules such as small alcohols, boron, and osmolytes has been reported [65]. In AMF, the first AQP gene *GintAQP1* of *Rhizophagus irregularis* was cloned, with evidence of a compensatory mechanism between *GintAQP1* expressions and the host aquaporins under drought stress [66]. Furthermore, two AQP genes *GintAQPF1* and *GintAQPF2* present in *Rhizophagus irregularis* were upregulated under osmotic stress, assisting the fungus survival and contributing to the host plant tolerance to water stress [67, 68]. Upregulation of *RiAQPF2* in *Rhizophagus irregularis* was also found under water deficit [10], suggesting its putative involvement in host plant tolerance in response to drought.

On the plant side, AMF could induce changes in the expression of various AQP genes in the host in order to strengthen root hydraulic conductivity and host tolerance under water-stress conditions in several plants, such as maize [69–71], tomato [10, 11], black locust [72], trifoliate orange [73], olive [74], and *Populus x canadensis* plants [75]. AM-induced alterations in expression of plant AQPs could depend on stress duration as the observation in maize plants [69]. Under short-term water deficit, the AM symbiosis upregulated ten AQP genes with diverse aquaporin classes in roots inoculated with *Rhizophagus intraradices*, stimulating more water uptake in the host [69]. By contrast, under sustained water-stress conditions, AM-mediated downregulation of 6 different AQP genes was found, restricting plant water loss [69]. Intriguingly, drought-sensitive cultivars may gain higher physiological benefit from AM inoculation than drought-tolerant cultivars [71]. Downregulation of genes *TIP1;1, TIP2;3, PIP1;1, PIP1;3, PIP1;4, PIP1;6, PIP2;2*, and *PIP2;4* whereas only upregulation of *TIP4;1* were observed in drought-sensitive cultivar colonized by *Rhizophagus irregularis*, supporting the decrease in water loss in host plants subjected to drought stress [71]. Recent research also revealed a significant shift in the transcriptional regulation profiles with AQP genes as potential targets in mycorrhizal roots, in comparison to non-AM ones during a water stress event, which may influence some key metabolic pathways linked with drought response [76]. In parallel, it has been proposed that during drought stress a controlled mechanism mediated by the presence of arbuscules at cortical cells in roots fine-tuned the gene expression regulation in the host plant [76].

In general, fungal and plant AQPs work together in mycorrhizal plants under water restrictions. The simultaneous induction of both fungal and plant AQP genes together with differential regulation of drought-responsive genes in host plant indicates that AMF mediate colonized plant responses to drought stress.

#### *4.1.3 Increased photosynthetic efficiency*

Numerous reports illustrate that AMF could increase photosynthetic activity or protect the photosynthetic apparatus under water stress conditions [77, 78]. In fact, AM colonization considerably influences the stomatal behavior in the leaves of host

*Drought - Detection and Solutions*

**4.1 Direct benefits of AM symbiosis for host plants under water deficit**

*4.1.1 Improved water and nutrient uptake through the hyphal network of AMF*

highlighting the role of the symbiosis in the water status of host plants.

exposed to both water-stress levels, while this upregulation was in

Apart from that, AM colonization enhances the rate of nitrogen

by fungal hyphae [56]. Several NO3

(N)-assimilation of plants under drought [55] as a result of the direct uptake of

transporters in AMF [57, 58] while mycorrhiza-inducible ammonium transporters in some plants have been identified [59, 60]; therefore, AMF considerably contribute to the total N uptake of the host. Increased N nutrient could promote protein synthesis and higher levels of compatible osmolytes in stressed AM plants. Other studies also confirmed that inadequacy of necessary macro- and micro-nutrients could be alleviated in mycorrhizal plants under water deficit [61, 62]. Hydraulic conductivity of colonized roots was enhanced to absorb more N, P, and K, leading

−

and NH4

+

transporters and metal

activation of fungal aquaporins [54].

*F. mosseae*-infected plants subjected to severe water stress. A role of *PT4* genes in root tips, creating a connection among root branching, Pi-signaling mechanisms, and Pi-perception has been proposed [51]. In addition, on the fungal side, *R. irregularis PT* gene was up-regulated under moderate drought conditions [53]. Phosphate is taken up by mycorrhizal phosphate transporters and assimilated to polyphosphate translocated toward the plant. This process is facilitated by the

The widespread extraradical mycelia also enhance the absorption of mineral nutrients in soils, which is more critical for host plants under water-stress conditions where nutrient mobility is limited. As soon as external hyphae transport water to the host, mineral nutrients also follow the water flow to the plant from the soil-root interface [50]. AM colonization is well known to improve phosphorus (P) nutrient into the host plants particularly under low-nutrient conditions, increasing stress tolerance in plants. Interestingly, plants possess a symbiotic inorganic phosphate (Pi) uptake pathway, and AM symbiosis has been proved to specifically induce the expression of genes encoding plant Pi transporters to enhance P acquisition, for instance, *LjPT4* in *Lotus japonicus* and *MtPT4* in *Medicago truncatula* [51], recently *LbPT3, LbPT4,* and *LbPT5* in *Lycium barbarum* [52]. Under water restrictions (moderate and severe), different expressions of five tomato *PT* genes (*LePT1-LePT5*) in the absence/presence of *Rhizophagus irregularis* or *F. mosseae* were observed [53]. *LePT4* was overexpressed in *R. irregularis*-colonized plants

is a superior water allocation mediated by the fungal hyphal network, facilitating the colonized root access to water in a lower soil water potential [45]. Indeed, the host root system is extended by widespread extraradical mycelia, enabling colonized roots to reach more water and nutrient pools unavailable to uncolonized roots. Fungal hyphae diameters (3–7 μm) are much smaller than those of fine root hairs (5–20 μm); nevertheless, hyphal densities are ten-hundred times higher than root densities [46]. Hence, the absorption surface of mycorrhizal roots is improved substantially. It is calculated that the rate of water transport from external hyphae to the root ranged from 0.1 [47] to 0.76 μl H2O h−1 per hyphal infection point [48], which is adequate to alter plant water relations [47]. Lettuce plants pretreated by *Rhizophagus irregularis*, *Funneliformis mosseae*, *Funneliformis coronatum* (formerly *Glomus coronatum*), and *Claroideoglomus claroideum* (*G. claroideum*) obtained 3–4.75 ml H2O plant−1 day−1 higher than uncolonized plants, which might be related to the amount of extraradical mycelium and root colonization frequency [45]. Furthermore, AMF contribute approximately 20% to total plant water uptake [49],

An important benefit of AM colonization to the host plant under drought stress

**174**

NO3 − or NH4 + plants, determining the water vapor efflux, CO2 gas exchange, and thus photosynthetic activity [79]. Stomatal conductance changed by AM inoculation is closely connected to leaf water potential and relative water content in host plants. Under water restrictions, the first response of plants is stomatal closure to limit water loss through transpiration. Additionally, reduction of CO2 uptake and carbon assimilation whereas favoring photorespiration may occur in plants [80]. Upregulation of *LeEPFL9* involved in the regulation of stomatal development together with greater stomatal density was found in tomato plants colonized by *R. irregularis* [10]. Inoculation of *Septoglomus deserticola* or *S. constrictum* sustained stomatal opening in host plants under drought conditions, substantially contributing to the carbon assimilation [11]. Improvement of stomatal conductance (gs) in mycorrhizal castor bean [78], black locust [72], and strawberry [81] plants exposed to water stress has been detected.

One of the widely known benefits of mycorrhizal inoculation is the improvement of host water status under drought stress. Leaf water potential (LWP) and relative water content (RWC) of plants were substantially higher in the presence of mycorrhiza [11, 81]. Several studies illustrated a higher water use efficiency or intrinsic water use efficiency in AM plants during water stress [10, 81, 82]. It is believed that photosynthetic activity correlates with chlorophyll content and stomatal conductance, which have been enhanced by AMF. Drought stress changes photosynthetic pigments and damages chloroplasts. Nonetheless, AM inoculation alleviates the damage of these parameters caused by the stress [77]. *Rhizophagus irregularis*-colonized castor bean plants subjected to water restriction increased contents of chlorophyll a (by 26%), b (30%), carotenoid (by 28.5%), and total chlorophyll (25.5%) in comparison to counterparts of non-AM plants [78]. These increases in AM plants may be attributed to the improved nutrient uptake, particularly N and Mg that are structural components of chlorophyll.

Mycorrhizal colonization has been found to alleviate the adverse impacts of drought stress on photochemical efficiency and photosystem II (PSII) reaction center [77, 83]. Under water deficit, application of AMF promoted a higher maximum quantum efficiency of PSII (Fv/Fm) [11], greater photosynthetic efficiency [84], transpiration rate, and net photosynthesis rate (PN) [10, 81]. Although mycorrhizal plants usually have higher photosynthetic capacities, environmental factors such as high atmospheric drought or low radiation can decide the beneficial effects of mycorrhiza on photosynthesis [85].

#### *4.1.4 Phytohormonal changes*

Phytohormones not only modulate a plethora of events during plant development but also are essential signaling molecules for interaction between plants and AMF [86]. Changes in plant hormone homeostasis also affect plant tolerance against abiotic stresses [87, 88]. During mycorrhization, changes in levels of several plant hormones have been reported [86], hence may contribute to the improved host plant tolerance to subsequent stresses.

Abscisic acid (ABA) is the most fundamental stress hormonal signal, modulating transpiration rate, root hydraulic conductivity, and aquaporin expression [89]. The concentration of ABA is heightened in plant tissues under drought stress to induce stomatal closure for reduction of water loss and activate different stressresponsive genes, increasing plant tolerance to drought [90]. A lower ABA concentration was found in roots and leaves of mycorrhizal plants versus nonmycorrhizal plants under drought stress [9, 10, 91]. Downregulation of *SlNCED* gene, a critical ABA biosynthetic gene, in *Septoglomus constrictum*-infected roots under water stress concurred with the greater gs and higher water status of tomato plants, indicating a higher stress tolerance in colonized plants compared to uninoculated plants [11].

**177**

*Benefits of Arbuscular Mycorrhizal Fungi Application to Crop Production under Water Scarcity*

Nonetheless, an increase in ABA concentration in trifoliate orange plants colonized by *F. mosseae* was also observed under drought stress [73]. The reason for this

The role of jasmonate (JA) in water uptake and transport, exerting influence on stomatal conductance, root hydraulic conductance, and regulating the expression and abundance of aquaporins in tomato plants has been revealed [91]. Tomato plants defective in JA synthesis altered the AM impacts on the host plant, interfering phytohormones and expression of AM-induced aquaporin genes. The content of JA and its precursors was higher in leaves of *Digitaria eriantha* plants infected by *Rhizophagus irregularis* under water deficit, relative to noninfected plants, which could enhance plant tolerance to the stress [92]. Likewise, mycorrhizal inoculation substantially increased methyl jasmonate (MeJA) in trifoliate orange plants exposed to drought stress [93]. Under water-stress conditions, significantly higher expression levels of JA-biosynthetic gene *SlLOXD* in roots and leaves of colonized tomato plants were detected, supporting plant response to drought stress by triggering a

Strigolactones (SLs), as phytohormones, not only modulate the coordinated development of plants exposed to nutrient shortages but are also host detection signals for AM establishment in the host plant [94]. Upregulation of the SL-biosynthesis gene *SlCCD7* together with a greater content of SLs was found in *Rhizophagus irregularis*-inoculated tomato roots subjected to water-stress conditions, correlated with the increase in AM colonization rate [9]. The stimulated production of SLs promoting symbiosis establishment as a strategy of plants to cope

Auxin is a key regulator in root-hair initiation, growth, and developmental processes

Although important roles of phytohormones are irrefragable in plant responses to water stress, little attention has been paid to them in mycorrhizal plants. Previous

In response to drought stress, plants accumulate compatible solute compounds or osmolytes functioning for osmotic adjustment to maintain a favorable gradient for water uptake [98]. Osmotic adjustment is essential for water influx, turgor maintenance, sustaining physiological activity in plants such as stomatal opening, photosynthesis, cellular expansion, and growth during the stress [98]. Compatible solutes include a variety of sugars, proline, glycine betaine, polyamines, and organic acids such as oxalate and malate [99]. Interestingly, discrepant observations in osmolyte accumulation have been reported in a wide range of mycorrhizal plants

studies have just revealed changes in concentrations and expression of genes encoding biosynthesis of few hormones in colonized plants during drought stress;

[95, 96]. In a recent study, an increased content of indole-3-acetic acid (IAA) which is the dominant naturally occurring auxin was found in mycorrhizal tomato plants exposed to drought [91]. Similarly, stimulation of biosynthesis and transport of IAA in roots of trifoliate orange under water restrictions were demonstrated [97]. Under drought conditions, AM colonization overexpressed *PtYUC3* and *PtYUC8* involved in IAA biosynthesis, and downregulated auxin efflux carriers (*PtPIN1* and *PtPIN3*), while up-regulated auxin-species influx carriers (*PtABCB19* and *PtLAX2*) in roots, leading to significantly higher IAA accumulation in mycorrhizal roots versus non-AM roots [97]. Together with higher IAA, colonized trifoliate orange plants showed a significant increase in MeJA, nitric oxide, and calmodulin in roots, supporting greater root adapta-

tion of morphology as a crucial strategy for drought adaptation [93].

thereby, further research is required to understand it.

*4.1.5 Osmotic adjustment*

[10, 83, 100, 101].

*DOI: http://dx.doi.org/10.5772/intechopen.86595*

LOXD-mediated pathway [10, 11].

with drought stress has been proposed.

remains poorly understood, which requires further research.

#### *Benefits of Arbuscular Mycorrhizal Fungi Application to Crop Production under Water Scarcity DOI: http://dx.doi.org/10.5772/intechopen.86595*

Nonetheless, an increase in ABA concentration in trifoliate orange plants colonized by *F. mosseae* was also observed under drought stress [73]. The reason for this remains poorly understood, which requires further research.

The role of jasmonate (JA) in water uptake and transport, exerting influence on stomatal conductance, root hydraulic conductance, and regulating the expression and abundance of aquaporins in tomato plants has been revealed [91]. Tomato plants defective in JA synthesis altered the AM impacts on the host plant, interfering phytohormones and expression of AM-induced aquaporin genes. The content of JA and its precursors was higher in leaves of *Digitaria eriantha* plants infected by *Rhizophagus irregularis* under water deficit, relative to noninfected plants, which could enhance plant tolerance to the stress [92]. Likewise, mycorrhizal inoculation substantially increased methyl jasmonate (MeJA) in trifoliate orange plants exposed to drought stress [93]. Under water-stress conditions, significantly higher expression levels of JA-biosynthetic gene *SlLOXD* in roots and leaves of colonized tomato plants were detected, supporting plant response to drought stress by triggering a LOXD-mediated pathway [10, 11].

Strigolactones (SLs), as phytohormones, not only modulate the coordinated development of plants exposed to nutrient shortages but are also host detection signals for AM establishment in the host plant [94]. Upregulation of the SL-biosynthesis gene *SlCCD7* together with a greater content of SLs was found in *Rhizophagus irregularis*-inoculated tomato roots subjected to water-stress conditions, correlated with the increase in AM colonization rate [9]. The stimulated production of SLs promoting symbiosis establishment as a strategy of plants to cope with drought stress has been proposed.

Auxin is a key regulator in root-hair initiation, growth, and developmental processes [95, 96]. In a recent study, an increased content of indole-3-acetic acid (IAA) which is the dominant naturally occurring auxin was found in mycorrhizal tomato plants exposed to drought [91]. Similarly, stimulation of biosynthesis and transport of IAA in roots of trifoliate orange under water restrictions were demonstrated [97]. Under drought conditions, AM colonization overexpressed *PtYUC3* and *PtYUC8* involved in IAA biosynthesis, and downregulated auxin efflux carriers (*PtPIN1* and *PtPIN3*), while up-regulated auxin-species influx carriers (*PtABCB19* and *PtLAX2*) in roots, leading to significantly higher IAA accumulation in mycorrhizal roots versus non-AM roots [97]. Together with higher IAA, colonized trifoliate orange plants showed a significant increase in MeJA, nitric oxide, and calmodulin in roots, supporting greater root adaptation of morphology as a crucial strategy for drought adaptation [93].

Although important roles of phytohormones are irrefragable in plant responses to water stress, little attention has been paid to them in mycorrhizal plants. Previous studies have just revealed changes in concentrations and expression of genes encoding biosynthesis of few hormones in colonized plants during drought stress; thereby, further research is required to understand it.

#### *4.1.5 Osmotic adjustment*

In response to drought stress, plants accumulate compatible solute compounds or osmolytes functioning for osmotic adjustment to maintain a favorable gradient for water uptake [98]. Osmotic adjustment is essential for water influx, turgor maintenance, sustaining physiological activity in plants such as stomatal opening, photosynthesis, cellular expansion, and growth during the stress [98]. Compatible solutes include a variety of sugars, proline, glycine betaine, polyamines, and organic acids such as oxalate and malate [99]. Interestingly, discrepant observations in osmolyte accumulation have been reported in a wide range of mycorrhizal plants [10, 83, 100, 101].

*Drought - Detection and Solutions*

plants, determining the water vapor efflux, CO2 gas exchange, and thus photosynthetic activity [79]. Stomatal conductance changed by AM inoculation is closely connected to leaf water potential and relative water content in host plants. Under water restrictions, the first response of plants is stomatal closure to limit water loss through transpiration. Additionally, reduction of CO2 uptake and carbon assimilation whereas

favoring photorespiration may occur in plants [80]. Upregulation of *LeEPFL9* involved in the regulation of stomatal development together with greater stomatal density was found in tomato plants colonized by *R. irregularis* [10]. Inoculation of *Septoglomus deserticola* or *S. constrictum* sustained stomatal opening in host plants under drought conditions, substantially contributing to the carbon assimilation [11]. Improvement of stomatal conductance (gs) in mycorrhizal castor bean [78], black locust [72], and strawberry [81] plants exposed to water stress has been detected. One of the widely known benefits of mycorrhizal inoculation is the improvement of host water status under drought stress. Leaf water potential (LWP) and relative water content (RWC) of plants were substantially higher in the presence of mycorrhiza [11, 81]. Several studies illustrated a higher water use efficiency or intrinsic water use efficiency in AM plants during water stress [10, 81, 82]. It is believed that photosynthetic activity correlates with chlorophyll content and stomatal conductance, which have been enhanced by AMF. Drought stress changes photosynthetic pigments and damages chloroplasts. Nonetheless, AM inoculation alleviates the damage of these parameters caused by the stress [77]. *Rhizophagus irregularis*-colonized castor bean plants subjected to water restriction increased contents of chlorophyll a (by 26%), b (30%), carotenoid (by 28.5%), and total chlorophyll (25.5%) in comparison to counterparts of non-AM plants [78]. These increases in AM plants may be attributed to the improved nutrient uptake, particu-

larly N and Mg that are structural components of chlorophyll.

mycorrhiza on photosynthesis [85].

host plant tolerance to subsequent stresses.

*4.1.4 Phytohormonal changes*

Mycorrhizal colonization has been found to alleviate the adverse impacts of drought stress on photochemical efficiency and photosystem II (PSII) reaction center [77, 83]. Under water deficit, application of AMF promoted a higher maximum quantum efficiency of PSII (Fv/Fm) [11], greater photosynthetic efficiency [84], transpiration rate, and net photosynthesis rate (PN) [10, 81]. Although mycorrhizal plants usually have higher photosynthetic capacities, environmental factors such as high atmospheric drought or low radiation can decide the beneficial effects of

Phytohormones not only modulate a plethora of events during plant development but also are essential signaling molecules for interaction between plants and AMF [86]. Changes in plant hormone homeostasis also affect plant tolerance against abiotic stresses [87, 88]. During mycorrhization, changes in levels of several plant hormones have been reported [86], hence may contribute to the improved

Abscisic acid (ABA) is the most fundamental stress hormonal signal, modulating transpiration rate, root hydraulic conductivity, and aquaporin expression [89]. The concentration of ABA is heightened in plant tissues under drought stress to induce stomatal closure for reduction of water loss and activate different stressresponsive genes, increasing plant tolerance to drought [90]. A lower ABA concentration was found in roots and leaves of mycorrhizal plants versus nonmycorrhizal plants under drought stress [9, 10, 91]. Downregulation of *SlNCED* gene, a critical ABA biosynthetic gene, in *Septoglomus constrictum*-infected roots under water stress concurred with the greater gs and higher water status of tomato plants, indicating a higher stress tolerance in colonized plants compared to uninoculated plants [11].

**176**

Proline, an amino acid, plays a crucial role in osmoregulation and acts as an efficient scavenger of reactive oxygen species (ROS) [102] (discussed in Section 4.1.7). Enhanced drought tolerance with a higher proline concentration in mycorrhizal plants has been shown in many studies [10, 78, 100]; nevertheless, opposite results have also been reported [81, 83]. Inoculation of either *F. mosseae* or *Paraglomus occultum* in trifoliate orange plants substantially reduced leaf proline content but improved the host plant growth under water deficit [103]. These results suggest that AMF strongly altered leaf proline metabolism through regulating proline-metabolized enzymes, which is important for osmotic adjustment of the host plants.

Sugars are osmoprotectants, which contribute up to 50% of osmotic potential in plants [104, 105]. In general, under water stress, the higher accumulation of total soluble sugars offers a defense mechanism in mycorrhizal plants such as watermelon [100] and flax [106]. Concentrations of sucrose, glucose, and fructose were significantly heightened in leaves of mycorrhizal trifoliate orange seedlings exposed to drought, which could function as osmolytes to stabilize and protect structures and macromolecules in plants from the stress, therefore improving host plant tolerance [103]. AMF-mediated increases in leaf sugar metabolism by modulating sugarmetabolized enzymes notably contribute to the osmotic adjustment of colonized plants. However, contrast observations have been shown in olive trees [101] and maize [107] colonized by AMF, which may be due to the fact that host plants suffer less stress. Noticeably, under severe drought inoculation with *Rhizophagus clarus* significantly reduced soluble sugars in leaves of strawberry plants, but this parameter was remarkably enhanced in roots in response to mild and severe water stress [81]. These changes together with an improved water status and plant biomass suggest different strategies for the enhanced water status triggered by AMF in roots and leaves of strawberry.

In summary, increased accumulation of compatible solutes in AM-inoculated plants in exposure to water deficit is supposed to protect plants from the stress and curtail the plant osmotic potential, whereas the lower osmolyte accumulation in host plants is thought to be due to colonized plants successfully gaining drought mitigation.

#### *4.1.6 Enhanced plant tolerance to oxidative stress*

One of the consequences of water stress is the overproduction of reactive oxygen species (ROS) such as hydroxyl radicals (˙OH), superoxide radicals (O2˙ – ), singlet oxygen (1 O2), and hydrogen peroxide (H2O2) mainly in chloroplasts and mitochondria. The excessive ROS results in unbalanced cellular homeostasis and then oxidative stress, damaging membrane lipids, proteins, and nucleic acids and even causing the death of cells [108]. To cope with oxidative stress, plants have evolved ROS scavengers in both nonenzymatic and enzymatic defense systems. Nonenzymatic antioxidants comprise phenolic compounds, glutathione, ascorbic acid, alkaloids, carotenoids, and tocopherol [109], which not only play a direct role in ROS removal but also serve as a substrate for the antioxidant enzymes in scavenging ROS [110]. Under water deficit, AMF ameliorate oxidative damage through augmented production of phenolic compounds and secondary metabolites detoxifying ROS in various plants [111–113]. AM inoculation also significantly increased the concentrations of anthocyanins and carotenoids [106] and ascorbic acid [82, 106] in plants in exposure to water constraints.

Another important ROS scavenger system is enzymatic antioxidants which could be enhanced in mycorrhizal plants including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), guaiacol peroxidase, ascorbate peroxidase (APX), glutathione reductase (GR), dehydroascorbate reductase (DHAR), and

**179**

ological stress response in host plants.

**5. Altered plant growth, yield, and quality**

It is often found that AM symbiosis can improve plant growth in numerous plants, such as lettuce [9], tomato [9, 11], strawberry [81], maize [128], black locust [72], digitgrass (*Digitaria eriantha*), a source of forage [92], and damask rose [129].

*Benefits of Arbuscular Mycorrhizal Fungi Application to Crop Production under Water Scarcity*

monodehydroascorbate reductase (MDHAR) [110]. The AM symbiosis has been reported to improve plant protection against oxidative stress by decreasing the level of lipid peroxidation (MDA) and H2O2 accumulation by strengthening significantly antioxidative enzymes SOD, POD, and CAT in roots and leaves under mild and severe drought [11, 81]. SOD and CAT are the most important ROS scavenging enzymes among the enzymatic antioxidants. These enzymes together with the cooperative enzymes (GR, MDHAR, DHAR, and APX) in the ascorbate-glutathione (ASA-GSH) cycle play pivotal roles in controlling overproduced ROS to maintain cellular homeostasis [114–116]. Remarkable increases in SOD, CAT, GR, APX, and

MDHAR at transcription and enzymatic level correlated with lower O2˙

MDA have been revealed in drought-stressed mycorrhizal plants versus counterparts of non-AM plants, improving host protection against oxidative damage [101]. Higher nonenzymatic and enzymatic antioxidants in colonized plants help for the rapid and efficient elimination of excess ROS. Nevertheless, discrepant results, no change or decrease in ROS scavengers, have also been demonstrated [70, 117]. Results are not entirely consistent with all reports because of different ages of host plants [118] and/or the specific combination of mycorrhizal strains and plant species, even cultivars [11] (as discussed in Section 3) or successful drought mitigation

**4.2 Indirect benefits of AM symbiosis for host plants under water deficit**

The hyphal network of AMF is believed to improve soil water retention properties in the mycorrhizosphere through its physical, biological, and chemical influences. It has been reported that AMF produce polysaccharides, glomalin, mucilages, and hydrophobins that act to bind soil particles, leading to soil aggregation with enhanced water-holding capacity in soil [119]. Glomalin, a stable glycoprotein, highly persists in the soil, defined as glomalin-related soil protein (GRSP) [120]. The higher amounts of GRSP in the soil, the more enhanced capacity of water retention was found since soil aggregation increased protection of C-rich debris from the decomposition of soil microbes [120, 121]. Indeed, fungal hyphae coated by GRSP sharp a hydrophobic layer into the aggregate surface, hence decreasing water loss within soil aggregates [122]. When the fungal hyphae form branching structures with glomalin, they physically stick micro-aggregates with macro-aggregates [119]. The physical interaction of external hyphae on soil particles forms stable aggregates [123] in general and under water deficit [124]. Moreover, mycorrhizosphere also influences soil aggregation through alterations in the soil microbial food web, habitats for soil microbes, and biological activities in the host rhizosphere, which could result in an enhancement in microaggregate soil structure [125]. Thus, soils possess well-structured property in the presence of AMF, maintaining relatively higher available water than poorly structured soils without mycorrhizal presence under water stress [126]. Notably, in artificial substrates, an enhancement in water retention and water transport within substrates inoculated with AMF was observed under severe drought, suggesting that host plants perceive less stress at the root surface as reducing substrate moisture [127]. Hence, AMF postponed the physi-

−

, H2O2, and

*DOI: http://dx.doi.org/10.5772/intechopen.86595*

in colonized plants.

#### *Benefits of Arbuscular Mycorrhizal Fungi Application to Crop Production under Water Scarcity DOI: http://dx.doi.org/10.5772/intechopen.86595*

monodehydroascorbate reductase (MDHAR) [110]. The AM symbiosis has been reported to improve plant protection against oxidative stress by decreasing the level of lipid peroxidation (MDA) and H2O2 accumulation by strengthening significantly antioxidative enzymes SOD, POD, and CAT in roots and leaves under mild and severe drought [11, 81]. SOD and CAT are the most important ROS scavenging enzymes among the enzymatic antioxidants. These enzymes together with the cooperative enzymes (GR, MDHAR, DHAR, and APX) in the ascorbate-glutathione (ASA-GSH) cycle play pivotal roles in controlling overproduced ROS to maintain cellular homeostasis [114–116]. Remarkable increases in SOD, CAT, GR, APX, and MDHAR at transcription and enzymatic level correlated with lower O2˙ − , H2O2, and MDA have been revealed in drought-stressed mycorrhizal plants versus counterparts of non-AM plants, improving host protection against oxidative damage [101].

Higher nonenzymatic and enzymatic antioxidants in colonized plants help for the rapid and efficient elimination of excess ROS. Nevertheless, discrepant results, no change or decrease in ROS scavengers, have also been demonstrated [70, 117]. Results are not entirely consistent with all reports because of different ages of host plants [118] and/or the specific combination of mycorrhizal strains and plant species, even cultivars [11] (as discussed in Section 3) or successful drought mitigation in colonized plants.

### **4.2 Indirect benefits of AM symbiosis for host plants under water deficit**

The hyphal network of AMF is believed to improve soil water retention properties in the mycorrhizosphere through its physical, biological, and chemical influences. It has been reported that AMF produce polysaccharides, glomalin, mucilages, and hydrophobins that act to bind soil particles, leading to soil aggregation with enhanced water-holding capacity in soil [119]. Glomalin, a stable glycoprotein, highly persists in the soil, defined as glomalin-related soil protein (GRSP) [120]. The higher amounts of GRSP in the soil, the more enhanced capacity of water retention was found since soil aggregation increased protection of C-rich debris from the decomposition of soil microbes [120, 121]. Indeed, fungal hyphae coated by GRSP sharp a hydrophobic layer into the aggregate surface, hence decreasing water loss within soil aggregates [122]. When the fungal hyphae form branching structures with glomalin, they physically stick micro-aggregates with macro-aggregates [119]. The physical interaction of external hyphae on soil particles forms stable aggregates [123] in general and under water deficit [124]. Moreover, mycorrhizosphere also influences soil aggregation through alterations in the soil microbial food web, habitats for soil microbes, and biological activities in the host rhizosphere, which could result in an enhancement in microaggregate soil structure [125]. Thus, soils possess well-structured property in the presence of AMF, maintaining relatively higher available water than poorly structured soils without mycorrhizal presence under water stress [126]. Notably, in artificial substrates, an enhancement in water retention and water transport within substrates inoculated with AMF was observed under severe drought, suggesting that host plants perceive less stress at the root surface as reducing substrate moisture [127]. Hence, AMF postponed the physiological stress response in host plants.
