**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*

An important benefit of AM colonization to the host plant under drought stress 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], highlighting the role of the symbiosis in the water status of host plants.

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 exposed to both water-stress levels, while this upregulation was in *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 activation of fungal aquaporins [54].

Apart from that, AM colonization enhances the rate of nitrogen (N)-assimilation of plants under drought [55] as a result of the direct uptake of NO3 − or NH4 + by fungal hyphae [56]. Several NO3 − and NH4 + transporters and metal 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

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

to a higher protein concentration in host plants under drought stress [63]. Thus, more vigorous uptake of water and nutrients may provide adequate necessary

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

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

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.

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

substances for better growth of mycorrhizal plants under such stress.

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

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

expression regulation in the host plant [76].

*4.1.3 Increased photosynthetic efficiency*

*profiles*

to drought.

to a higher protein concentration in host plants under drought stress [63]. Thus, more vigorous uptake of water and nutrients may provide adequate necessary substances for better growth of mycorrhizal plants under such stress.
