**2. General features of AMF**

AMF are obligate root symbionts inhabiting almost all terrestrial ecosystems. They can form a symbiotic association with around 80% of vascular plants and with approximately 90% of agricultural plants [12]. In this mutual association, the fungus receives 10–20% of total photosynthates [13] and lipids [14] from the host plant, whereas the plant is enhanced through uptake of water and mineral nutrient by the mycorrhizal partner [12]. AMF are the most common fungi in soils and represent 9–55% of the soil microbe biomass and 5–36% of the total soil biomass [15]. These fungi play a vital role in agricultural ecosystems, since they can improve plant nutrient, water status, and plant growth [12], enhance survival rate and development of seedlings, crop uniformity, and reproductive capacity [16], decrease the input of P and N fertilizer, and increase resistance or tolerance to environmental adversities [8, 17].

Currently, AMF are classified as a member of phylum Glomeromycota including four orders (Archaeosporales, Diversisporales, Glomerales, and Paraglomerales), with 11 families, 25 genera, and nearly 250 species [18]. However, data based on next-generation sequencing of root samples [19] and recent results [20] suggest that its number may be an order of magnitude higher. Spores of AMF which are the major survival units of AMF have multi-nucleate, heterokaryotic structures [21], and are formed singly, in clusters or sporocarps in the soil, and within root tissue in some mycorrhiza species as well (**Figure 1A–C**). The development of AM symbiosis starts with signaling taking place before physical contact between the plant and the fungus. Both partners produce molecular signals triggering preparative responses in the other [22]. The mycorrhization process can be divided into distinct steps, consisting of germinating spores, hyphae differentiation, appressorium formation, penetration of the host root, intraradical hyphae formation, intercellular growth along with developed external mycelium (extraradical hyphae), and arbuscule formation, subsequently exchanging nutrients and carbohydrates between the host and fungus [23].

The primary structures of AMF consist of coenocytic hyphae with unlimited growth in the rhizosphere called external hyphae, which penetrate the cortex

**171**

**Figure 1.**

*representing 20 μm.*

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

layer of roots and form different organs. The extraradical hyphae merely in some species of Diversisporales [18] producing auxiliary cells could have functions in reproduction or nutrition and storage [24]. Mycelium outside the roots absorb mineral nutrients and water and subsequently transport them to the host plant via intraradical hyphae (**Figure 1C, D**) growing inside root cells [6]. Hyphae growing within roots form either the Paris-type or the Arum-type. The Paris-type is featured by intracellular mycelium development to shape coils, whereas the Arum-type is characterized by intercellular hyphae growth forming arbuscules [12] (**Figure 1D**), thereby establishing the nutrient exchange sites between AMF and the host plant [25]. Vesicles containing high quantity of lipids and glycogen are formed from intraradical hyphae at intercalary position (their terminal) in the root, functioning

*Tomato roots without (A) and with (B–D) staining showing AM fungal structures. The presence of arbuscular mycorrhiza (AM) structures (arbuscules, vesicles, intraradical hyphae, and spore) was assessed by means of an Olympus BX51 light microscope with Nomarski interference contrast optics, using an objective of 40×. Scale Bar* 

as nutrient storage, and propagules [23] but not all AMF produce vesicles.

persed mycelium with densities from 1 to 2 m cm−3.

**3. Variable crop responses to AMF**

AMF species isolates differ in the ability to spread mycelia, the viability, structure, and possibility of anastomosis [26, 27]. Taxonomic variation in mycelium structure among AMF families was also observed [28]. *Gigasporaceae* are prone to possess vigorous, thickly aggregated mycelium with densities from 6 to 9 m cm−3, while *Acaulosporaceae* and *Glomeraceae* show a tendency to maintain thinly dis-

Although a majority of plants are responsive to AMF, plant species in families *Amaranthaceae, Brassicaceae, Caryophyllaceae, Chenopodiaceae, Cyperaceae,* 

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

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

#### **Figure 1.**

*Drought - Detection and Solutions*

**2. General features of AMF**

adversities [8, 17].

Notably, under natural conditions, plants frequently interact with microbes, which directly mediate plant responses to environmental adversities. Some microbeplant interactions lead to a mitigation of stress-related damages and improvement of plant tolerance to stressful conditions [5]. As a crucial element of soils, microbes are an integral part of the agricultural ecosystem. Arbuscular mycorrhizal fungi (AMF) are ubiquitous soil microorganisms, which can form a symbiotic association with most terrestrial plants. These beneficial microbes have been proved to offer an array of benefits to host plants [6]. During mycorrhization, besides significant improvement of plant nutritional status, AMF can enhance plant performance and tolerance against several stresses, particularly drought stress [7]. The exploitation of AMF is considered as one of the most efficient practices to increase plant tolerance to environmental stresses [8]. Previous studies illustrate the substantial contribution of AM symbiosis to improved stress plant tolerance to water deficit by various mycorrhizal benefits such as strengthened water and nutrient uptake, alterations in host physiology, for example, photosynthesis, osmotic adjustment, phytohormones, and more efficient antioxidative systems [9–11]. This chapter presents the current knowledge on AMF application to crop production under water deficit. Variable benefits of AMF are also discussed to explain the reason why positive outcomes of AM colonization are not always the case. Finally, challenges of the fungal symbiont

application are highlighted for practical use in crop production.

AMF are obligate root symbionts inhabiting almost all terrestrial ecosystems. They can form a symbiotic association with around 80% of vascular plants and with approximately 90% of agricultural plants [12]. In this mutual association, the fungus receives 10–20% of total photosynthates [13] and lipids [14] from the host plant, whereas the plant is enhanced through uptake of water and mineral nutrient by the mycorrhizal partner [12]. AMF are the most common fungi in soils and represent 9–55% of the soil microbe biomass and 5–36% of the total soil biomass [15]. These fungi play a vital role in agricultural ecosystems, since they can improve plant nutrient, water status, and plant growth [12], enhance survival rate and development of seedlings, crop uniformity, and reproductive capacity [16], decrease the input of P and N fertilizer, and increase resistance or tolerance to environmental

Currently, AMF are classified as a member of phylum Glomeromycota including four orders (Archaeosporales, Diversisporales, Glomerales, and Paraglomerales), with 11 families, 25 genera, and nearly 250 species [18]. However, data based on next-generation sequencing of root samples [19] and recent results [20] suggest that its number may be an order of magnitude higher. Spores of AMF which are the major survival units of AMF have multi-nucleate, heterokaryotic structures [21], and are formed singly, in clusters or sporocarps in the soil, and within root tissue in some mycorrhiza species as well (**Figure 1A–C**). The development of AM symbiosis starts with signaling taking place before physical contact between the plant and the fungus. Both partners produce molecular signals triggering preparative responses in the other [22]. The mycorrhization process can be divided into distinct steps, consisting of germinating spores, hyphae differentiation, appressorium formation, penetration of the host root, intraradical hyphae formation, intercellular growth along with developed external mycelium (extraradical hyphae), and arbuscule formation, subsequently exchanging nutrients and carbohydrates between the host and fungus [23].

The primary structures of AMF consist of coenocytic hyphae with unlimited growth in the rhizosphere called external hyphae, which penetrate the cortex

**170**

*Tomato roots without (A) and with (B–D) staining showing AM fungal structures. The presence of arbuscular mycorrhiza (AM) structures (arbuscules, vesicles, intraradical hyphae, and spore) was assessed by means of an Olympus BX51 light microscope with Nomarski interference contrast optics, using an objective of 40×. Scale Bar representing 20 μm.*

layer of roots and form different organs. The extraradical hyphae merely in some species of Diversisporales [18] producing auxiliary cells could have functions in reproduction or nutrition and storage [24]. Mycelium outside the roots absorb mineral nutrients and water and subsequently transport them to the host plant via intraradical hyphae (**Figure 1C, D**) growing inside root cells [6]. Hyphae growing within roots form either the Paris-type or the Arum-type. The Paris-type is featured by intracellular mycelium development to shape coils, whereas the Arum-type is characterized by intercellular hyphae growth forming arbuscules [12] (**Figure 1D**), thereby establishing the nutrient exchange sites between AMF and the host plant [25]. Vesicles containing high quantity of lipids and glycogen are formed from intraradical hyphae at intercalary position (their terminal) in the root, functioning as nutrient storage, and propagules [23] but not all AMF produce vesicles.

AMF species isolates differ in the ability to spread mycelia, the viability, structure, and possibility of anastomosis [26, 27]. Taxonomic variation in mycelium structure among AMF families was also observed [28]. *Gigasporaceae* are prone to possess vigorous, thickly aggregated mycelium with densities from 6 to 9 m cm−3, while *Acaulosporaceae* and *Glomeraceae* show a tendency to maintain thinly dispersed mycelium with densities from 1 to 2 m cm−3.

#### **3. Variable crop responses to AMF**

Although a majority of plants are responsive to AMF, plant species in families *Amaranthaceae, Brassicaceae, Caryophyllaceae, Chenopodiaceae, Cyperaceae,*  *Juncaceae,* and *Urticaceae* are rarely or never colonized by the symbiotic fungus [29]. How AMF evaluate the AM host and nonhost status of plant species is not well known. The current hypothesis proposes that nonmycorrhizal plant species lost orthologs of important putative genes, required for symbioses [30], and/or cannot synthesize or degrade strigolactones, essential signals for symbiosis establishment [31], and/or their root exudates constitute antifungal products [29]. Under certain conditions, some nonhost species develop rudimentary AM phenotypes described by Cosme et al. [30] giving a more in-depth explanation of this question.

Utilization of AMF has become an appealing tool for sustainable agriculture due to the positive attributes of mycorrhizal symbiosis. Nevertheless, the opposite or neutral influence of AMF has also been found [32]. The obligate biotrophic life cycle of AMF which relies on photosynthates supplied by a nurturing autotrophic host is the key point; therefore, choosing the right partner (target plant) is crucial. Even though this widespread symbiont is thought to be a generalist due to low host specificity, each AMF species highly varies in the responsiveness to the host plant. Hence, the variable benefits of AM symbiosis exist among mycorrhiza species [10, 33]. The interaction between the host plant and AMF could range from mutualism to parasitism in which colonized plants exhibit a decrease in growth [34] owing to the carbon drainage in the host inflicted by the fungus [35]. Many factors that can affect the AM benefits to target plants include host plant genotypes, AMF species, and environmental conditions. Dissimilar plant responses to different AMF species under environmental adversities have been observed [11, 36]. Fascinatingly, AM benefits for plant fitness augment with adversity, supporting the concept of AM colonization as a 'health insurance' for host plants, in which the beneficial effects of AMF become more obvious under stressful environments [36]. Metabolites differentially accumulated in roots colonized by different fungal symbionts (*Rhizophagus irregularis*, *Funneliformis mosseae*, and *Claroideoglomus etunicatum*) under abiotic stresses, which may underlie their enhanced stress tolerance in host plants [36]. Cultivar differences in response to mycorrhizas have been reported in many crops such as tomato [37], pepper [38], wheat [39], maize [40], and some other crops [41]. For chickpea, only three of thirteen varieties with different genotypes and phenotypes were more positively responsive to AM mixed inoculation with *Diversispora eburnea*, *Claroideoglomus etunicatum*, and *Glomus* sp. [42]. More recently, twenty geographically different barrel clover (*Medicago truncatula*) accessions showed differences in their growth, stomatal conductance (gs), and AM colonization in response to *Funneliformis mosseae* treatment [43]. Also, root hydraulic conductivity, expression of the mycorrhiza-induced phosphate transporter gene (*MtPT4*), and five aquaporin genes (*MtAQP1*, *MtPIP1, MtPIP2, MtNIP1,* and *MtNIP4*) vary with mycorrhizal treatment during further analysis of five accessions. In the case of wheat, old accessions have been shown to be more responsive to AMF than new ones [39].

Selection and breeding programs generally tend to maximize plant performance and crop yield under high-input production systems, which could cause the loss of genes, phytochemicals, and/or other plant traits which are necessary for the establishment of efficient symbioses. Modern cultivars could absorb phosphate without the AM assistance in soils with high phosphorus availability, decreasing the degree of AM dependence. As a consequence, AMF are less responsive to new lines. Recent research has proved that domestication decreased AM benefits for domesticated crops in exposure to high P supply [44]. However, in maize, which is highly mycorrhizal-dependent, modern breeding programs do not necessarily result in the less mycorrhizal colonization. Replicated field experiments with 225 genotypes consisting of hybrids, inbred lines, and landraces originating from different locations were conducted for two consecutive years to explore the variation in

**173**

*strigolactones.*

**Figure 2.**

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

mycorrhizal colonization [40]. The findings showed that AM colonization differed profoundly and continuously among genotypes, with substantially greater values in

It is well known that AMF offer indispensable advantages to the host plant subjected to water shortage, with two major strategies that mycorrhizal plants use to deal with water deficit: drought mitigation and drought tolerance. Drought mitigation strategy is involved in indirect AM benefits and enhanced water uptake through the extensive hyphae network, enabling host plants to suffer less stress than non-AM plants, whereas drought tolerance includes a combination of direct AM benefits that improve plant's innate ability to cope with the stress (**Figure 2**).

*Strategies of mycorrhizal plants to cope with water scarcity, that is, drought mitigation and drought tolerance. Multiple benefits/mechanisms could be simultaneously induced by arbuscular mycorrhizal fungi in the host plant exposed to water deficit. The blue arrows show increase/up-regulation, whereas the orange arrows indicate decrease/down-regulation, relative to control non-mycorrhizal plants. Italic words indicate genes. ABA, abscisic acid; AQP, aquaporin; Car, carotenoids; Chla, chlorophyll a; Chlb, chlorophyll b; Fv/Fm, maximum quantum efficiency of PSII; gs, stomatal conductance; IAA, indole-3-acetic acid; iWUE, intrinsic water use efficiency; JAs, jasmonates; LWP, leaf water potential; MDA, malondialdehyde; MeJA, methyl jasmonate; PN, net photosynthesis rate; ROS, reactive oxygen species; RWC, relative water content; SLs,* 

**4. Mechanisms of AMF mitigate drought stress in host plants**

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

modern hybrids than old landraces and inbred lines.

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

mycorrhizal colonization [40]. The findings showed that AM colonization differed profoundly and continuously among genotypes, with substantially greater values in modern hybrids than old landraces and inbred lines.
