Introduction

Anthropogenic activities, such as desertification, imprudent utilization of water and industrialization lead to pollution and consequently cause climate changes (Perera & Nadeu, 2022). Climate change is multi-faceted, and includes changing concentrations of greenhouse gases in the atmosphere, rising temperatures, changes in precipitation patterns, and increasing frequency of extreme weather events (e.g., Sisco et al., 2017).

Plant growth is controlled directly by plant water stress and only indirectly by soil water stress. Plant water stress depends on the relative rates of water absorption and water loss rather than on soil water supply alone. Therefore, it is not safe to assume that a given degree of soil water stress always will be accompanied by an equivalent degree of plant water stress. Plant water stress should be measured direct- ly in research on the effects of water supply on plant growth and plant processes if the results are to be interpreted correctly. In this review, we focused on the effects of water stress on plant developmental processes (e.g., phytohormones production), and on the phytohormones-induced water stress tolerance in mycorrhizal olive plants. Drought is one of the most important factors that limits crop production worldwide (Qi et al., 2022). For example, approximately 67% of crop losses were reported in the

U.S.A. due to drought stress during the last 50 years. Severe drought conditions de- termine increasing and forcing up food prices. On the other hand, world population is increasing day by day and may reach over 9.8 billion in 2050 (United Nations, 2017). Fulfilling food requirements of a burgeoning population, the agriculture food must be increased up to 70% (Sah et al., 2016). We need to develop technologies and policies regarding climate change and drought stress, efficient water use, control of the growing human population, and production of drought-tolerant crops.

In response to water stress conditions, plants have evolved biochemical mech- anisms to maintain constant cellular water potential and/or relative water content (Soltys-Kalina et al., 2016). These processes are controlled by numerous phytohor- mones which are the basic mediators to tolerate or avoid the negative effects of water stress. This review mainly focuses on the abscisic acid-induced drought tolerance in mycorrhizal herbaceous and olive plants.

Phytohormones

Phytohormones are the key regulators of plant growth and developmental processes as well as crucial for biotic and abiotic stress response throughout their life cycle (Sah et al., 2016; Ullah et al., 2017). In addition to various other phytohormones, abscisic acid (ABA) is considered the main hormone which intensifies water stress tolerance in plants through various morpho-physiological and molecular processes including stomata regulation, root development, and initiation of ABA-dependent pathways (Aslam et al., 2022). Phytohormones perform as chemical messengers in response to various abiotic stresses, like water stress. After stress signal perception, phytohormones release which activate various plant physiological and developmental processes including stomatal closure, root growth stimulation, and accumulation of osmolytes to avoid water stress conditions (Fahad et al., 2016).

ABA is a weak acid that was first isolated as an abscission-accelerating substance (Lee et al., 2021). ABA, which is not directly involved in the process of abscission, is a major stress hormone that participates in various crucial physiological processes during the plant life cycle, including stress responses, development and reproduction (Dong et al., 2015). ABA is synthesized in the plastids and cytoplasm and derived from zeaxanthin, a plant pigment. A cluster of NCED (9-cis-epoxycarotenoid diox- ygenase) genes are thought to be involved in ABA synthesis (Huang et al., 2015). There are other phytohormones (e.g., jasmonic acid, salicylic acid, ethylene, aux- ins, gibberellins, cytokinins, and brassinosteroids) that congregate the challenges of water stress (Chen et al., 2017; Liu & Zhang, 2017; Lo et al., 2017). However, these hormones are usually cross talk with each other to increase the survival of plants under water stress conditions (Verma et al., 2016). On the other hand, the transgenic approach is currently the most accepted technique to engineer the genes responsible for the synthesis of phytohormones in response to water stress.

Osmotic stress promotes the synthesis of ABA, which activate gene expression and adaptive physiological changes (Rowe et al., 2016). ABA levels increase during stress conditions and decrease when stress is relieved. In addition, ABA concen- tration is higher in the younger than in the mature leaves. As leaves develop and mature, the ABA concentration declines and older leaves export ABA to younger ones. At the same time, the sensitivity of stomata to ABA increases throughout leaf development (Haworth et al., 2018). Upon stress arrival, signal perception occurs through the plasma membrane, ABA synthesis is initiated, which occurs mostly in the plastids, with the exception of xanthoxin conversion to ABA, which takes place in the cytoplasm (Ma et al., 2018). Generally, ABA synthesis occurs in the roots. It is then transported via vascular tissues, and it shows responses in a variety of cells, such as guard cells and others (Postiglione & Muday, 2020). ABA must be translocated from the cells where it is synthesized via intercellular transport into the neighboring tissues (Silva, 2020). As a weak acid, ABA is a charged anion (i.e., ABA-) in the cytoplasm (pH 7). In the more acidic cell wall (pH 5.5), some is left un- charged (ABAH). This presumably enhances the movement of ABA into but not out the cells, while cell-to-cell ABA transport is enhanced by transporters [i.e., plasma membrane-bound ATP-binding cassette (ABC) transporters, which hydrolyze ATP to transport it, and a family of low-affinity nitrate transporters]. ABA perception and signal transduction are mediated by two pathways, which are ABA-dependent and ABA-independent. Water stress predominantly induces gene expression through the initiation of an ABA-independent pathway (Soma et al., 2021). These pathways have been reviewed in detail (Kuromori et al., 2018).

ABA-dependent signaling pathways play a critical role in stress-responsive gene

expression during various stresses, especially osmotic stress. ABA receptors are im- portant elements for ABA signal transduction. Various receptors have been identified in different sub-cellular compartments, including the plasma membrane, nucleus, cytosol, and chloroplast envelope. ABA content is low under normal conditions, and SnRK2 protein kinase activity is inhibited by PP2C phosphatases, which leads to dephosphorylation. The cellular ABA level increases when plants are exposed to water stress, and ABA then binds to PYRABACTIN RESISTANCE1/PYR1- LIKE/ REGULATORY COMPONENTS OF ABA RECEPTORS (i.e., PYR/PYL/RCARs,

respectively) which in turn bind and inactive protein phosphatases 2C (PP2Cs, Ullah et al., 2017). Thus, PYR/PYL/RCARs are necessary for ABA responses, and various genes encode these receptors. The sucrose nonfermenting 1-related protein kinase 2 (SnRK2s) is auto activated when it dissociates from PP2Cs. The SnRK2s are protein kinases that promote ABA responses (Yang et al., 2019); SnRK2.2, SnRK2.3, and SnRK2.6 have been proven to be involved in ABA response (Hasan et al., 2022). In addition, there are 76 PP2Cs in Arabidopsis but only clade A participates in ABA signaling (Wang et al., 2023). Even more, ABA-dependent signaling pathway has several branches controlled by various transcription factors including MYB, MYC and NAC. In addition, ABA-responsive elements (ABREs) are also involved in ABA signaling (de Ollas & Dodd, 2016). Calcium-dependent protein kinases (CDPKs) also participate in ABA signaling (Shi et al., 2018). Two CDPKs (i.e., CPK4 AND CPK11) have been reported to be involved in the regulation of ABA signaling in Arabidopsis (Shi et al., 2018). Activated SnRK2s phosphorylate downstream targets and trigger ABA-induced physiological and molecular responses including germina- tion, stomata regulation, root development and photosynthesis (Wang et al., 2018). ABA regulates many stress-related genes to enhance water stress tolerance in plants (Shen et al., 2017).

Arbuscular mycorrhizal fungi

AMF belong to the phylum Glomeromycota, encompassing ten out of eleven fam- ilies: Acaulosporaceae, Ambisporaceae, Archaesporacea, Claroidoglomeraceae, Di- versisporaceae, Gigasporaceae, Glomeraceae, Pacisporaceae, Paraglomeraceae, and Sacculosporaceae (http://www.amf-phylogeny.com/, accessed on 1 June 2023). Given their status of obligate biotrophs, the AMF life cycle cannot be completed in the absence of host plants. It starts with an asymbiotic phase, during which spores germinate in response to physical factors such as moisture, temperature and pH, producing hyphae with a limited lifespan (Giovannini et al., 2020). In the presence of root exudates from host plants, a differential hyphal morphogenesis occurs, with germling hyphae reorienting the direction of elongation and initiating a differential branching pattern (Giovannini et al., 2020): this pre-symbiotic phase is followed by physical contact between AMF hyphae and host roots, with the differentiation of appressoria, which give rise to hyphae growing intercellularly within the root cortex, eventually penetrating in root cells and producing highly branched hyphal tree-like structures similar to haustoria, the arbuscules. The extraradical hyphae of AMF of one plant root system forage for the soil nutrients and induce the root colonization of the nearby plants, which leads to the formation of common mycorrhizal networks that interconnect roots (Busso & Busso, 2022). Inoculation with AMF can increase the root length, surface area and volume of seedlings in nutrient-limited karstic soils. Mycorrhizal symbioses can secrete glomalin to help promoting soil aggregates for water and nutrients storage, through an extended hyphae to absorb water and nutri- ents from long distances. AMF can boost rhizosphere soil enzyme activities, and may help to drive carbon sequestration. AMF also improve plant growth by advancing soil quality through influencing its structure and texture. As a result, AMF and CMNs benefit plants through improving soil quality and enhancing morphological (e.g., hyphal length, tillering, number of stolons per individual), physiological (e.g., water use efficiency) and productive (e.g., fresh and dry shoot and root weights) traits (Busso & Busso, 2022).

As obligate biotrophs, AMF receive photosynthetic products and lipids from

root cells (Giovannini et al., 2020). When addressing the regulation of such an in- timate plant-microbe interaction as AMF, plant hormones are primer targets. The colonization of a plant root by AMF often improves growth and stress tolerance of the whole plant, besides the most important feature of the mutualistic symbiosis (Begum, 2019). In particular, AMF can alleviate water stress conditions. Indeed, a number of cases have been reported where the beneficial effects of AMF on plant performance became apparent under water stress conditions (Chen et al., 2020). Al- though reports are not unanimous on this point, the beneficial consequences of AMF colonization regarding water stress seem to contribute not only towards a better mineral nutrition of plants, but encompass a more direct improvement of the plant water status. In addition, reduced levels of ABA and, accordingly, improved pho- tosynthetic parameters have been described for above-ground parts of AMF plants (Ren et al., 2019). Given this context, it makes sense that root ABA is necessary for a sustained colonization by AMF. This might ensure that roots become colonized particularly strongly when it is most needed (i.e., under water stress conditions, when ABA levels are high).

AMF produce changes in isoprenoids content in leaves of some plant species, favoring the production of essential instead of nonessential isoprenoids, especially in water stress conditions or after jasmonic acid applications (Karunanantham et al., 2022). The increasing production of essential isoprenoids has been associated with the increasing demand and biosynthesis of compounds derived from carotenoids such as ABA and strigolactones in mycorrhizal plants under water-deficit conditions (Goyal et al., 2020). Contrasting effects have been observed for ABA concentrations in mycorrhizal plants exposed to water stress conditions, which reflects the com- plexity of interactions. In some plant species, the beneficial effect of symbiosis with AMF under stress conditions has been attributed to increases of transpiration and water absorption by roots (Busso & Busso, 2022) and it is associated with lower concentrations of ABA (Torres et al., 2018). However, in some plant species, water stress has significantly increased the ABA content, with or without mycorrhizal colonization (Ouledali et al., 2019). It has been determined that, in water-deficit stress conditions, the symbiosis with AMF regulates the ABA content (Liang et al., 2021). In fact, in some plant species, ABA increases the susceptibility of plants to AMF colonization and appears to play an important role in the development and functionality of arbuscules (Chareesri et al., 2020).

Strigolactones correspond to a new class of plant hormones that regulate the

architecture and reproductive development plants (Zwanenburg et al., 2016), but the function for which they were initially recognized for is the intermediation ca- pacity in the mycorrhizal symbiosis process, where they act as a molecular signal in unfavorable conditions for plants (Goyal et al., 2020). One of the possible roles of ABA comes from its regulating effect on the strigolactone concentrations (Visentin et al., 2020).

The biochemical and molecular responses to AMF colonization in host plants also include the activation of the jasmonic acid signal pathway (Bahadur et al., 2019). Particularly, jasmonic acid appears to be involved in the stress signal in mycorrhizal plants through an increase of ABA concentration (Wang et al., 2020). Gibberellic acids modulate arbuscule formation in a dose-dependent manner (Liao et al., 2018). The narrow relationship found between ABA and AMF colonization in water stress conditions, and the functionality of the arbuscules suggests that the association between AMF and some plants could be one strategy to deal with a water stress, probably regulated by the interaction of ABA with other hormones.

There are involved a complex sequence of events and intracellular modifica- tions in the fungal penetration and establishment in the host roots (Carotenuto et al., 2019). Compatibility between plant roots and AMF implies a clear and selec- tive recognition by the plant host that distinguishes the beneficial effects of the AMF. The elucidation of the recognition mechanisms and molecules involved in the AMF interaction is the key to understanding the phenomenon of compatibility. Biochemical and morphogenetic events mediated by phytohormones during AMF formation have been suggested in this context, and certain roles for phytohormones have been suggested in AMF (Liao et al., 2018). In the colonization of a plant root by AMF, that not only improves growth and stress tolerance of the whole plant, there have been implicated alterations in the homeostasis of plant hormones. There are a large number of publications showing that the levels of plant hormones like ABA, strigolactones, gibberellic acids, and auxin actually change upon the establishment of AMF (Liao et al., 2018; Bahadur et al., 2019).

Phytohormones-induced water stress tolerance in mycorrhizal herbaceous and olive (olea europaea) plants

Some plant species evolved to avoid water stress whereas others adopted water stress tolerance strategies, like the olive (Kokkotos et al., 2020). In this respect, root coloni- zation by AMF plays a crucial role in the response of plants to water stress (Bahadur et al., 2019). It allows an integrative water stress response combining avoidance and tolerance strategies (Rapparini & Peñuelas, 2014).

A key factor in the response of olive trees to water stress is the presence of AMF in the soil (Calvo-Polanco et al., 2016). Under constraining conditions, mycorrhizas improve the resilience of plants against environmental stresses like water stress, AMF also improve the growth of olive trees and their resistance to transplantation stress (Bizos et al., 2020). Crucial factors to reach high productivity of olive trees is the enhancement of soil fertility and the fulfillment of its water requirements (Mer- wad et al., 2015). This plant species is known for its strong tendency for alternate bearing, with higher yields produced every second year. This tendency is enhanced by low fertility, high temperatures and water stress (Kour et al., 2018). Ennajeh et al. (2006) reported various morphological and physiological traits involved in the plant resistance to water stress in the leaves of the olive cultivars ‘Meski’ (water stress sensitive) and ‘Chemalali’ (water stress tolerant). The relative water content (RWC), net photosynthesis rate and stomatal conductance decrease in the leaves of both cultivars by withholding irrigation. Since this decrease was greater in ‘Meski’ than in ‘Chemalali’, the former cultivar was more sensitive to water stress. Leaf tri- chome and stomatal densities were higher in ‘Chemlali’ than in ‘Meski’. However, the desiccation curves were comparable when leaves of both cultivars were kept in the dark. This implied that the two varieties had similar leaf cuticle transpiration. Leaves of both cultivars accumulated soluble sugars and proline under severe water stress, possible for osmotic adjustment. Both cultivars showed the same levels of RWC at which soluble sugar concentrations started to increase and decline. However, more soluble sugars were accumulated by the leaves of the ’Meski’ variety. Lower soluble sugar and higher proline concentrations were determined on detached than attached leaves of ‘Chemlali’. The authors suggested that it was possible that solu- ble sugars were diverted to proline synthesis in response to injury. The RWC levels at which proline accumulation started and ended were higher in ´Meski´ than in

´Chemlali´. In the study of Ennajeh et al. (2006), high leaf trichome and stomatal

densities and effective osmotic adjustment with proline may explain the greater water stress tolerance in ‘Chemlali’ than in ‘Meski’.

The role of mycorrhizas in plants’ response to water stress has been well doc- umented in olive trees (Calvo Polanco et al., 2016). The contribution of AMF to plants’ tolerance of water stress is partly due to the larger volume of soil explored by roots and the extra-radical hyphae (Zhang et al., 2016), and more efficient sto- matal regulation by controlling the ABA metabolism (Torres et al., 2018). Bizos et al. (2020) reported that the AMF species Rhizophagus irregularis and Glomus mosseae stimulated the root growth improving the resistance of olive plants to environmental and transplantation stresses. However, Bompadre et al. (2014) inoculated olive plants with two strains of the AMF Rhizophagus irregularis (GC2 and GA5), which have dif- ferent strategies of colonization in vitro and in soil conditions. The GC2 strain had a high density of external mycelium, slow growing at the beginning of in vitro culture which increase with the proportion of mycelium ramification, and few number of big spores (160.52±19.8 cm2; 87.4±0.4 µm) (Silvani, 2011). Its spores and mycelium are limited to the vicinity of the roots where the colonization takes place. In contrast, GA5 presents little external mycelium at the beginning of culture but then increases its density forming a mycelium little branched, has a higher growth rate, and its spores are smaller and more abundant than GC2 (293.4±81.8 cm2; 70.8±0.5 µm) (Silvani, 2011). In olive seedlings, AMF increase growth, nutrients uptake, and root hydraulic conductivity (Calvo-Polanco et al., 2016). They also help to reduce damage due to soil dryness by the activation of antioxidant defenses (Sepahvand et al., 2021). Under field conditions prevalent in semi-arid olive-growing areas in southwestern Argentina there might be fairly long periods of water stress interrupted by sporadic torrential rains of spring and summer.

A wide variety of biotic and abiotic stresses needs to be overcome by plant

species which affect their growth, development and production. Within the ma- jor factors that will determine those parameters is water stress. Within the Phyto- geographical Province of the southern ‘’Caldenal’’, in southwestern Argentina, are common, large oscillations in total annual rainfall and water stress episodes (Peláez et al., 2021). Olive orchards in this region includes water stress-tolerant cultivars. This is because they have morpho-anatomical adaptations and physiological and biochemical defense mechanisms (Ennajeh et al., 2006; Khabou et al., 2014). These olive cultivars differ in their water stress tolerance which can be exploited to improve their performance under water stress (Ennajeh et al., 2009). Mycorrhization has been evaluated as a promising biological approach to mitigate the impact of water stress on the productivity of olive trees. Several studies have shown that mycorrhiza have a large beneficial effect on several aspects of plant physiology like root growth and morphology (Jabborova et al., 2021). Chandrasekaran (2022) reported that mycorrhi- zal inoculated plants were less affected by water deficiency than those non-inoculated through developing more root biomass. Their results highlight the importance of the production of mycorrhizal plants to alleviate the field water stress mainly in arid and semiarid areas.

By modulating metabolic and physiological processes, plants adapt to their

environment. They may interact with some organisms through symbiotic associa- tions to allow this adaptation. The AMF symbiosis is beneficial for the plant under water stress conditions. AMF may influence the hormonal balance of the plant to determine an integrated response to water stress. For example, the olive trees form beneficial associations with AMF (Ouledali et al., 2018). ABA is a major phytohor- mone that can be induced by AMF colonization in water stress-stressed olive plants (Ouledali et al., 2019). The plant´s resistant to water stress is reinforced by AMF (Xu et al., 2018).

The key phytohormone modeling the stomata behavior under water stress con- ditions is ABA (Ouledali et al., 2019). However, Ouledali et al. (2019) reported that in AMF-inoculated plants of the olive cultivar ‘Zarrazi’, the ABA amount increased under moderate water stress and even further under water relief, but decreased under severe water stress. However, stomatal closure was not correlated with leaf ABA con- tent in the study of Ouledali et al. (2019). The results of these authors showed that ABA is not the key factor controlling the stomatal closure in these AMF-inoculated olive plants under water stress conditions. They indicated that other AMF-related factors are involved in the control of stomata regulation in mycorrhizal olive plants exposed to severe water stress conditions, allowing a suitable stomata behavior.

In AMF-inoculated olive plants of the cultivar ‘Zarrazi’, water stress caused a decrease in conductance and leaf transpiration rate. Guard cells respond to various stimuli including plant hormones and elicitors (Agurla et al., 2017). The major plant hormone inducing stomatal closure is ABA (Ouledali et al., 2019). On the other hand, Hu et al. (2013) indicated that exogenous application of a synthetic cytokinin on Poa pratensis L. promoted stomatal reopening following drought-induced closure, leading to an enhanced stomatal conductance. Ren et al. (2019) reported that higher stomatal conductance rates are associated with lower ABA content in the leaves in mycorrhizal plants. In turn, Schmidt et al. (2017) reported that the arbuscular my- corrhiza fungus Rhizophagus intraradices increased the concentrations of numerous citokinins in the leaves of the C₄ perennial grass Miscanthus × giganteus J.M. Greef & Deuter. AMF-symbiosis can thus influence the production of ABA and cytokinins in roots exposed to dying soil to regulate stomatal opening and limit water loss (Hu et al., 2013; Schmidt et al., 2017; Ren et al., 2019). AMF-inoculated plants can reg- ulate better their ABA level than AMF-non-inoculated plants (Ruiz-Lozano et al., 2009). This results in a suitable balance between leaf transpiration and root water uptake during water stress. AMF-symbiosis could modulate the stomatal behavior through the regulation of the 14-3-3 genes in the ABA signaling pathway to improve water stress tolerance (Xu et al., 2018). Therefore, the results of Ouledali et al. (2019) suggest that AMF-inoculation of the olive cultivar ‘Zarrazi’ helped somehow to maintain transpiration and stomatal conductance in plants of that cultivar under water stress conditions.

When water stress was moderate, and in the leaves of the AMF-inoculated

olive plants of the cultivar ‘Zarrazi’, stomata closure was well correlated with ABA accumulation. Nevertheless, AMF-inoculated olive plants closed their stomata de- spite the lower leaf ABA concentrations under severe water stress (Ouledali et al., 2019). These authors suggested the eventual presence of an AMF-dependent signal replacing ABA in olive when dehydration is severe. The lower ABA concentration may be caused by a (1) negative feed-back exerted by the AMF-induced signal on the ABA-biosynthesis pathway, or (2) competition for a shared common precursor between ABA and the signal. Parwez et al. (2022) reported the role of ABA in regu- lation of different physiological mechanisms through integration of environmental cues via its positive and negative crosstalks with other phytohormones (like aux- in, gibberellin, cytokinin, ethylene, salicylic acid, brassinosteroids, jasmonic acid, strigolactones, and melatonin). Munemasa et al. (2019) showed that the ethylene signaling inhibits jasmonate signaling as well as ABA signaling in guard cells of Arabidopsis thaliana, and revealed the signaling crosstalk mechanism. Both an eth- ylene precursor 1-aminocyclopropane-1-carboxylic acid, and an ethylene-releasing compound ethephon, induced transient stomatal closure, and also inhibited methyl jasmonate-induced stomatal closure as well as ABA-induced stomatal closure. While the ethylene inhibition of methyl jasmonate-induced stomatal closure was abolished in the ethylene-insensitive mutant etr1-1, the methyl jasmonate-induced stomatal closure was impaired in the ethylene-overproducing mutant eto1-1 of Arabidopsis (Munemasa et al., 2019).

Canales et al. (2021) explored the strategy set up by two oat cultivars to cope

with drought based on root morphological, anatomical, physiological and molecular studies. These authors found a dramatic and rapid ABA increase in the susceptible genotype resulting in a tight and rapid reduction of stomatal conductance; despite this, leaf water potential decreased concomitantly due to a decrease in the root hy- draulic conductivity. Contrarily, the resistant genotype, showed a mild and slow increase in ABA that allowed maintaining transpiration longer. They linked this response to an increase in root hydraulic conductance through an increase in total root length, in the length of the thinnest roots and a rise in root conductivity. Ren et al. (2019) reported that under water-stressed conditions, inoculation of Zea mays with Funneliformis mosseae greatly reduced leaf ABA content, and postponed the decline in photosynthetic rate, stomatal conductance and osmotic adjustment. Ouledali et al. (2019) informed that leaf transpiration rate and stomatal conductance became very low in the mycorrhizal olive cultivar Zarrazi, and reached zero in non-mycorrhizal plants, when dehydration became severe. Therefore, in this water stress tolerant cultivar, stomatal regulation was efficient (Ouledali et al., 2018).

AMF symbiosis induces strigolactone biosynthesis under water stress and im-

proves water stress tolerance in lettuce and tomato (Ruiz-Lozano et al., 2016). Both being derives from carotenoids, strigolactones share their biosynthetic precursors with ABA (Stauder et al., 2018). ABA functions at multiple levels to regulate the symbiosis with AMF (Martín-Rodríguez et al., 2016). Liu et al. (2015) proposed that a transcriptionally regulated, early strigolactones decrease under osmotic stress is needed (but not sufficient) to allow the physiological increase of ABA in roots. Vis- entin et al. (2016) in tomato showed that genetic strigolactone depletion may affect ABA concentration and stomatal sensitivity to ABA. Thus, stomatal regulation under water stress in olive could partially be modulated by an AMF-dependent interplay between ABA and strigolactones.

Formenti & Rasmann (2019) informed that the mechanisms of AM fungal-me- diated increased resistance include a manipulation of the hormonal cascades, such as the systemic activation of jasmonic acid dependent defenses. AMF colonization of trifoliate orange (Poncirus trifoliata) by Funneliformis mosseae notably regulated the changes in root phytohormone levels under drought conditions (Liu et al., 2017). This was by increasing the indole-3-acetic acid, ABA, methyl jasmonate, zeatin ribo- side, and brassinosteroids concentrations (Liu et al., 2017). These results concluded that AMF enhanced drought tolerance in trifoliate orange through modulation of root phytohormones. Herrera-Medina et al. (2007) demonstrated that ABA contrib- uted to the susceptibility of tomato to infection by AM fungi, and that it seems to play an important role in the development of the complete arbuscule and its func- tionality. The narrow relationship found between ABA and AMF colonization under water stress conditions, and the functionality of the arbuscules, suggests that the association between AMF and some plants could be one strategy to deal with water stress, probably regulated by the interaction of ABA with other hormones. Liao et al. (2018) informed a new component of the ABA pathway, the Protein Phosphatase 2A (PP2A) holoenzyme subunit, PP2AB’1. They reported that the positive effect of ABA on the AMF colonization, requires a Protein Phosphatase 2A (PP2A) holoen- zyme subunit, PP2AB’1. Thus, ABA signaling appears to play an important positive role in AMF colonization, and this function is predominantly via the promotion of the PP2A haloenzyme complex.

Ouledali et al. (2018) either inoculated or not one-year-old olive plants with

AMF and exposed these plants to a 40-day-water stress period. They showed that mycorrhizal plants were less affected by water stress than non-mycorrhizal plants proving the involvement of the AMF in the alleviation of the water stress impact on olive tree. The turgor potential in mycorrhizal plants exhibited positive values during the whole treatment period, while it was negative in non-mycorrhizal plants mainly under severe stress intensity. Moreover, the stomatal function was less affect- ed by water stress in mycorrhizal than in non-mycorrhizal plants. The maximum of mycorrhizas relative water stress alleviation rate (RDAR) was estimated to be 40% for relative water content, 36% for the osmotic potential, 86% for turgor pres- sure, 16% for stomatal conductance and 27% for leaf transpiration rate. The osmotic adjustment by proline was earlier in mycorrhizal than in non-mycorrhizal plants. The inoculation with AMF also improved the mineral uptake of K, N, Zn and Fe. After 40 days of water stress, mycorrhizal olive plants survived but non-mycorrhizal plants did not. Thus, inoculation of young olive trees with the AMF improved their resilience to drought.

Conclusion and future considerations

The tolerance of plants to water stress is increased by AMF. However, the symbiosis under environmental stresses have caused a complexity and diversity of responses. In turn, this have prevented the establishment of a clear mechanism and it appears to have different modulations according to the species of AMF, the plant species, and the conditions under which the symbiosis is set. The plant susceptibility to a water stress, and the effect that this one has on its physiology and metabolism because of the presence of an additional sink during a stress condition, cannot always be supported by the plant. This is another important component in the complexity of this relationship. Therefore, it is likely that where the symbiosis quickly generates a benefit for the plant, the symbiotic association is advantageous, only under expo- sure to a mild or moderate water stress. For many mycorrhizal plants, it has been confirmed an increase in water stress tolerance as a consequence of multiple-level controlled mechanisms. This indicates that a more holistic approach is required to understand this phenomenon.

To cope with water stress, novel and dynamic approaches are currently desirable; thereby, engineering genes responsible for phytohormones synthesis could be a great approach to maintain and improve the productivity.

So far little progress has been made at the molecular level to understand the mechanisms of this miraculous organism which include the involvement of aquapo- rins, some binding protein genes (BiPs), mitogen-activated protein kinase (MAPK) pathway genes, and stress responsive genes like proline dehydrogenase, invertase, proline synthetase, etc. However, with the advancement of new age molecular tech- niques it seems possible that the unravelling of this mystery is not far away.