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

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**).

#### **Figure 2.**

*Drought - Detection and Solutions*

*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

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

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AMF than new ones [39].

*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, strigolactones.*
