Arbuscular mycorrhizal fungi as integrative modulators of plant tolerance to drought, salinity, and heavy metal stress: mechanistic insights and future directions

Abstract

Climate change and anthropogenic pressures have intensified abiotic stresses such as drought, salinity, and heavy metal (HM) contamination, severely impairing plant growth and productivity. Arbuscular mycorrhizal fungi (AMF), through their symbiotic association with plant roots, offer a promising biological strategy to enhance plant resilience under these stresses. This review synthesizes recent advances in understanding the physiological, biochemical, and molecular mechanisms by which AMF confer stress tolerance. Key mechanisms include modulation of aquaporin expression for water homeostasis, regulation of abscisic acid (ABA) and mitogen-activated protein kinase (MAPK) signaling pathways, enhancement of antioxidant defenses, and fine-tuning of osmolyte metabolism such as proline. Under salinity, AMF improves ion homeostasis by regulating SOS1 and NHX transporters and enhancing K⁺/Na⁺ discrimination. In HM-contaminated environments, AMF facilitate metal immobilization, chelation via phytochelatins and metallothioneins, and vacuolar sequestration, thereby reducing oxidative damage. The review also highlights AMF-mediated transcriptional reprogramming involving 14-3-3 proteins and stress-responsive transcription factors (e.g., WRKY, MYB, bHLH). By integrating rhizospheric interactions with intracellular signaling, AMF emerge as multifaceted modulators of plant stress physiology.

This review delineates key gaps in current understanding and outlines strategic directions for harnessing AMF in sustainable agriculture under complex abiotic stress scenarios. By integrating mechanistic insights across drought, salinity, and heavy metal stress, it emphasizes the convergence of AMF-mediated signaling pathways and cross-tolerance mechanisms that underpin plant resilience.

1. Introduction

Climate change and escalating anthropogenic activities are intensifying the severity and co-occurrence of major abiotic stresses, including drought, salinity, and heavy metal (HM) contamination.1, 2 These environmental constraints pose a significant and growing threat to global food security by disrupting fundamental plant physiological processes, such as water and ion homeostasis, and inducing widespread oxidative damage, ultimately leading to substantial yield losses.3, 4 The development of sustainable and effective strategies to enhance crop resilience against these multifaceted stresses is therefore a critical priority for modern agriculture.

In this context, the symbiotic association between plants and Arbuscular Mycorrhizal Fungi (AMF) represents a powerful, nature-based solution5, 6. AMF form mutualistic relationships with the roots of approximately 80 % of terrestrial plant species, extending the plant’s effective root system through an extensive network of extraradical hyphae⁷. This symbiosis is well-established for its role in enhancing nutrient acquisition, particularly phosphorus, but its capacity to confer tolerance to a wide range of abiotic stresses has become a major focus of recent research⁵. The mechanisms underlying AMF-mediated stress tolerance are complex and highly integrated, involving a sophisticated molecular dialogue that results in profound changes to the host plant’s physiology, biochemistry, and gene expression.⁸

Recent studies have revealed that AMF do not simply alleviate stress through enhanced nutrient or water uptake; rather, they act as complex modulators of the plant’s internal stress response machinery. Under drought, AMF fine-tune water relations by regulating the expression of aquaporins (AQPs) and promoting osmotic adjustment through the accumulation of compatible solutes like proline9, 10. In saline environments, the fungi play a crucial role in maintaining cellular ion homeostasis by regulating the transport and compartmentalization of toxic ions such as Na⁺, often by influencing key transporters like salt overly sensitive 1 (SOS1), a plasma membrane Na+/H + antiporter (NHX)11, 12. Furthermore, in soils contaminated with heavy metals, AMF employ a dual strategy of metal immobilization in the rhizosphere and internal detoxification mechanisms, including chelation via phytochelatins (PCs) and metallothioneins (MTs), to reduce metal-induced oxidative stress.13, 14

Crucially, the plant’s response to these distinct stresses is not entirely separate. A common thread across AMF-mediated tolerance to drought, salinity, and HM stress is the convergence on core signaling pathways and defense mechanisms15, 16. These include the modulation of key phytohormone signaling, such as the abscisic acid (ABA) pathway,¹⁷ the activation of conserved mitogen-activated protein kinase (MAPK) cascades, and the significant reinforcement of the plant’s antioxidant defense system to maintain redox homeostasis18, 19. This suggests that AMF-induced resilience is underpinned by a coordinated, cross-tolerance mechanism that primes the plant to better withstand multiple environmental challenges simultaneously.⁷ While the individual mechanisms for each stress have been extensively studied, a comprehensive synthesis that integrates these distinct molecular and physiological responses to highlight the common, AMF-regulated hubs of cross-tolerance is still needed (Fig. 1. Table 1).

Fig. 1.

AMF effect on plant growth promoting and resisting stress. Including regulation of elements N: Nitrogen, P: Phosphorus, Zn: Zinc, GAs: Gibberellins, SLs: Strigolactones, impact on plant growth, and the mechanism of ABA: Abscisic Acid, PCs: Phytochelatins, Pro: Proline to deal with abiotic stress. IRM: intraradical mycelium, ERM: extraradical mycelium. (Figure generated using Biorender (https://www.biorender.com/).

Table 1.

An overview of AMF-assisted plant growth in drought, salinity, and heavy metals stress.

Stresses	plant species	AMF	Mechanism	References
Drought	S. melongena	G. intraradices	increased fruit production, improved crop growth, dry biomass, and high-water use efficiency	143
	C. melo L	G. intraradices and Glomus spp	increased chlorophyll content and improved physiological parameters and photosynthesis.	144
	Medicago truncatula	R. irregularis	Increase Pi uptake and plant growth	145
	Triticum aestivum	R. intraradices F. mosseae F. geosporum	optimized photochemistry, increased chlorophyll concentration, lowered relative water content, and boosted PSI photochemical activity.	146
	Zea mays L	G. versiforme	Increased levels of compatible solutes like proline, sugars, and free amino acids, as well as improved mineral uptake and assimilation, increased antioxidant system activity	147
	Poncirus trifoliata	F. mosseae, Paraglomus occultum	increased hyphal length, hyphal water absorption rate, and potential water content of leaves	148
Salinity	S. lycopersicum	R. irregularis	Increased the fresh weight of roots and shoots as well as the number of leaves	149
	Cucumis sativus	G. intraradices	Increased antioxidant enzyme levels	150
	Glycine max L. Merrill	C. etunicatum R. irregularis F. mosseae	Decreased generation of hydrogen peroxide and lipid peroxidation. Enhanced plant growth and symbiotic efficacy by elevating endogenous auxin concentrations, facilitating superior root systems and nutrient uptake under saline stress.	151
	Cucumis sativus L	C. etunicatum R. Irregularis F. mosseae	Increasing biomass, pigment synthesis, antioxidant enzyme activity (including superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase), and ascorbic acid content Rising Jasmonic acid, Salicylic acid, and several essential mineral elements (K, Ca, Mg, Zn, Fe, Mn, and Cu) while decreasing the uptake of harmful ions such as Na+	151
	Phoenix dactylifera L	Sclerocystis sp Acaulospora sp	increase host resistance by salt root colonization, increasing H2O2 accumulation in AMF roots, which may eventually lead to arbuscular degradation	152
	Pisum Sativum L	R. intraradices, F. mosseae, R. fasciculatum Gigaspora sp	Increased nutrient uptake, accumulation of compatible osmolytes, and decreased electrolyte leakage in the cell	152
	Zea mays	Kocuria rhizophila	Regulating plant hormone levels (IAA and ABA) and improving nutrient acquisition, increased transcript levels of antioxidant genes (ZmGR1 and ZmAPX1), and salt tolerance genes (ZmNHX1, ZmNHX2, ZmNHX3, ZmWRKY58, and ZmDREB2A).	153
Heavy metals	Zea mays	Rhizophagus fasciculatus, R. intraradices, F. mosseae, G. aggregatum	Cd, Cr, Ni, and Pb Phytoextraction improves the soil enzyme activity like dehydrogenase, β-Glucosidase, acid, and alkaline phosphatase.	154
	Cynodon dactylon (bermudagrass)	F. mosseae, Diversispora spurcum	Pb, Zn, and Cd AMF changed the heavy metals content and accumulation characteristics in the plant	155
	Glycine max (soybean)	R. irregularis	Cd AMF colonization did not affect Cd accumulation and translocation in HX3 and HN89 plants.	156
	Medicago sativa	G. aggregatum, G. intraradices, G. elunicatum, G. versiforme	Cd Reduction in cadmium uptake of alfalfa grown in Cd polluted so	157
	Sunflowers	lomus mosseae, G. intraradices	Cr, Mn, Ni, Cu, Zn, Al, Pb, Co, Mo, Fe, and Si, Combined Glomus mosseae and sludge amendments resulted in the highest metal uptakes and glomalin contents	158
	Hordeum vulgare L	F. mosseae, R. aggregatus, C. etunicatum, R. intraradices	AMFs and lignin biochar (LBC) reduced bioavailable Pb	159
	Zea mays L.	G. intraradices	AMF inoculation or biochar alone can boost maize growth while lowering Cd uptake. AMF inoculation was more effective at alleviating Cd stress and promoting maize growth. Biochar was more effective at increasing soil alkalinity and immobilizing Cd. AMF inoculation and biochar worked synergistically to reduce Cd phytotoxicity.	160

This review aims to synthesize the most recent advances in understanding the molecular and physiological basis of AMF-mediated tolerance to three major abiotic stresses: drought, salinity, and heavy metal contamination. We will systematically delineate the specific mechanisms employed by AMF under each stress condition, focusing on the regulation of water transport (AQPs), ion homeostasis (SOS1, NHX), and detoxification (PCs, MTs). Furthermore, we will integrate these findings to identify the central, AMF-co-opted signaling hubs, such as the ABA and MAPK pathways, and the role of 14-3-3 proteins, that confer a broad-spectrum, cross-tolerance phenotype. Finally, we will highlight key knowledge gaps and propose strategic directions for future research to effectively harness AMF in developing climate-resilient and sustainable agricultural systems.

2. Core mechanisms of AMF-mediated stress mitigation

2.1. Aquaporin regulation and water homeostasis

AQPs are membrane-embedded proteins that facilitate the transport of water and small solutes across cellular membranes²⁰. They are especially abundant in actively dividing or metabolically active cells, including plant roots, where they play a crucial role in optimizing water uptake to support growth and development.²¹ AQPs are classified into several subfamilies based on their sequence homology and subcellular localization: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin-26-like intrinsic proteins (NIPs), small basic intrinsic proteins (SIPs), X-intrinsic proteins (XIPs), GlpF-like intrinsic proteins (GIPs), and hybrid intrinsic proteins (HIPs).²² While AMF-mediated aquaporin regulation varies across host species, these differences reflect strategic adaptations to stress severity and plant physiology. For instance, upregulation of AQPs in maize and rice under moderate drought enhances water uptake and transport, supporting growth.23, 24 Conversely, downregulation in species like trifoliate orange under severe drought minimizes water loss and ROS influx.²⁵

Under DS, AMF modulate AQP expression in host plants through two distinct strategies. In some cases, AMF downregulate AQP genes, thereby reducing membrane permeability and promoting water retention within cells an adaptive response to severe water deficit²⁶. In other contexts, AMF enhance AQP expression, facilitating improved water permeability, uptake, and transport to support physiological functions under moderate drought.

A molecular study identified and cloned six AQP genes GintAQP1, GintAQPF1, GintAQPF2, RcAQP1, RcAQP2, and RcAQP3 from the AMF species Glomus intraradices and Rhizophagus clarus²⁷. Notably, AMF can modulate host plant AQP expression in diverse ways: upregulating AQPs to enhance water uptake, downregulating them to limit water loss and oxidative stress, or maintaining stable expression levels.²⁷ This modulation of AQPs is not exclusive to drought; the maintenance of cellular water status and turgor is equally vital for osmotic adjustment under salinity and for mitigating secondary oxidative stress induced by HMs.

2.2. ABA biosynthesis and drought stress regulation

Abscisic acid (ABA) is a pivotal phytohormone that orchestrates plant responses to DS by modulating signaling networks, gene expression, and physiological adaptations.²⁸ Elevated ABA levels under drought conditions enhance antioxidant enzyme activity, regulate ROS metabolism, and suppress malondialdehyde (MDA) accumulation, thereby mitigating oxidative damage.29, 30

In higher plants, ABA biosynthesis follows an indirect pathway originating from carotenoids.³¹ Key enzymes include zeaxanthin epoxidase (ZEP), which converts zeaxanthin to violaxanthin; 9-cis-epoxycarotenoid dioxygenase (NCED), which cleaves violaxanthin and neoxanthin to produce xanthoxin; and abscisic aldehyde oxidase (AAO), which catalyzes the final step converting xanthoxin to ABA³². Among AAO isoforms, AAO4 is particularly important due to its high expression in drought-stressed tissues.³²

ABA-mediated drought responses include reduced leaf elongation, stomatal closure, decreased leaf area, and improved water-use efficiency, all of which contribute to limiting water loss33, 34. For example, exogenous ABA application has been shown to increase Pro accumulation in apple leaves, enhancing oxidative stress resistance,³⁵ and improve tomato growth and water uptake under drought conditions.³⁶

Recent evidence highlights that ABA accumulation under drought stress is tightly linked to the activation of antioxidant enzymes and Pro biosynthesis, forming a protective network against ROS and MDA damage (Fig. 2). Transcriptomic studies in Brassica napus revealed that drought stress and biostimulants such as γ-polyglutamic acid (γ-PGA) synergistically upregulate ABA biosynthetic genes, including BnZEP, BnNCED3, and BnAAO4, leading to enhanced ABA accumulation and improved drought tolerance.37, 38 Notably, γ-PGA not only amplifies ABA biosynthesis under stress but also primes ABA pathways under non-stress conditions, suggesting its potential as a pre-conditioning agent.³⁹ The influence of AMF on ABA biosynthesis varies depending on developmental stage and stress conditions. In B. napus, AMF combined with γ-PGA enhances ABA signaling and drought resilience, whereas in orchids, AMF downregulates ABA to facilitate symbiosis during germination.⁴⁰ The AMF-mediated modulation of ABA, while central to drought response, also plays a foundational role in cross-tolerance by priming the plant’s general stress response.

Fig. 2.

The diagram illustrates the role of AMF in promoting proline biosynthesis in plants. Proline is synthesized in the cytoplasm and chloroplasts via pathways involving enzymes such as pyrroline-5-carboxylate synthase (P5CS) and pyrroline-5-carboxylate reductase (P5CR), which convert glutamate into proline. Degradation of proline occurs in mitochondria through proline dehydrogenase (PDH) and ornithine aminotransferase (OAT), yielding intermediates like ketoglutarate (KG). The proline transporter (ProT) facilitates the movement of proline between organelles. Additionally, the figure highlights γ-polyglutamic acid (γ-PGA) as a factor enhancing drought resilience in Brassica napus. This process involves upregulating the expression of genes BnNCED3, BnZEP, and BnAAO4, which stimulate the biosynthesis of abscisic acid (ABA). Solid arrows denote activation or promotion, while dashed arrows indicate inhibitory effects (Figure generated using Biorender (https://www.biorender.com/).

2.3. Proline biosynthesis and AMF regulation

Proline (Pro) is a multifunctional amino acid that accumulates in plants under DS, contributing to osmotic adjustment, ROS scavenging, and stabilization of proteins and membranes.⁴¹ Its biosynthesis occurs via two main pathways: the glutamate pathway, involving pyrroline-5-carboxylate synthetase (P5CS), pyrroline-5-carboxylate reductase (P5CR), and the ornithine pathway, catalyzed by ornithine δ-aminotransferase (δ-OAT).⁴² In kenaf (Hibiscus cannabinus), AMF inoculation enhanced drought tolerance by increasing Pro content and regulating AQP genes such as HcPIP1;2 and HcPIP2;7.⁴³ Similarly, In walnut (Juglans regia), AMF symbiosis altered root metabolite profiles DS, including elevated Pro levels, indicating a metabolic shift toward stress resilience⁴⁴. Furthermore, Soybean plants inoculated with AMF showed improved physiological traits and upregulation of polyamine biosynthesis genes, which are closely linked to Pro metabolism.⁴⁵ Additionally, In rice, AMF enhanced Pro accumulation under low-temperature stress by decreasing glutamate and ornithine concentrations and upregulating OsP5CS2, OsOAT, and OsProDH1.⁴⁶

The variation in Pro accumulation across species inoculated with AMF suggests species-specific metabolic reprogramming. In kenaf and walnut, elevated Pro levels correlate with enhanced AQP expression and antioxidant activity, while in rice, AMF modulates precursor availability and biosynthetic gene expression. These differences underscore the need for integrative studies linking metabolomics with transcriptomics to decode AMF-induced osmotic adjustment strategies. The overall pathway and AMF points of control are summarized in (Fig. 2). The ProT transporter facilitates Pro redistribution across organelles, ensuring dynamic osmotic regulation under stress.⁴⁷ AMF modulates this pathway by upregulating Pro biosynthesis genes and enhancing Pro accumulation, as observed in species like H. cannabinus, J. regia, and Oryza sativa43, 44, 46. This modulation is often accompanied by improved AQP expression and antioxidant activity, contributing to enhanced drought tolerance.48, 49 ABA not only regulates stomatal closure and water-use efficiency but also promotes Pro accumulation, linking hormonal signaling with osmolyte metabolism.²⁸ Together, these processes underscore the multifaceted role of AMF in coordinating metabolic, hormonal, and transport pathways to enhance plant resilience under drought stress.

2.4. MAPK signaling and AMF interaction

The mitogen-activated protein kinase (MAPK) cascade is a conserved signaling pathway in plants that transduces environmental stress signals into cellular responses.⁵⁰ It consists of three modules, MAPKKK, MAPKK, and MAPK, and is involved in regulating ROS detoxification, hormone signaling, and gene expression under drought stress.50, 51

Studies have demonstrated that AMF modulates MAPK gene expression in both their fungal mycelium and plant hosts during drought stress.17, 52 This dual regulation facilitates a molecular dialogue that enhances stress resilience. In soybean, coordinated expression of MAPK genes from both the host and the AMF (G. intraradices) was observed under drought conditions, suggesting that MAPK signaling operates synergistically across the symbiotic partners to regulate drought responses.⁵³ Similarly, in Eucalyptus grandis, AMF colonization significantly upregulated 18 MAPK cascade genes, which correlated with enhanced antioxidant enzyme activities superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) and reduced accumulation of ROS, including hydrogen peroxide and superoxide radicals.¹⁷ Additionally, In Bombax ceiba, AMF inoculation downregulated ROS-generating genes such as respiratory burst oxidase homologs (Rboh), while simultaneously upregulating a suite of antioxidant enzymes (SOD, CAT, APX, GPX, GR, MDAR, DHAR), resulting in improved ROS detoxification and reduced oxidative damage.⁵⁴ These physiological changes are accompanied by improved osmotic adjustment, as evidenced by increased Pro and soluble sugar accumulation in AMF-treated plants. In soybean and apple (Malus hupehensis), the symbiosis led to higher Pro levels and better water retention, contributing to enhanced drought tolerance through osmotic balance and membrane stability.⁵³ AMF-enhanced antioxidant system is the plant’s primary defense against the oxidative burst triggered by all abiotic stresses. This system is crucial not only for drought but also for mitigating the secondary oxidative damage caused by excessive Na⁺ accumulation under salinity (Section 4) and the generation of free radicals by HMs (Section 5). The convergence on redox homeostasis underscores a key principle of AMF-mediated cross-tolerance.

Moreover, Arabidopsis thaliana in DS has been shown to activate the MAPK signaling cascade, specifically through the phosphorylation of AtMKK1, which subsequently activates AtMPK4. This signaling pathway plays an essential role in regulating the accumulation of H₂O₂ during drought conditions by modulating the activity of CAT a key antioxidant enzyme involved in ROS detoxification.55, 56 The coordinated MAPK signaling observed under drought represents a core component of the AMF-plant molecular dialogue. This pathway is a critical point of convergence, as it is also known to regulate the expression of key genes for ion homeostasis in salinity and the activation of phytochelatin synthesis for HM detoxification (See section 6 and Fig. 6), demonstrating its role as a central hub for cross-tolerance.

Fig. 6.

Integrated signaling network linking stress-specific pathways with the AMF colonization module and ABA-MAPK-AQP hub. AMF colonization begins with Strigolactone (SLs) secretion by roots, stimulating fungal hyphal branching. AMF releases Mycorrhizal Lipo-Chitooligosaccharides (Myc-LCOs) and Chitooligosaccharides (COs), perceived by Lysin Motif Receptor-Like Kinases (LysM-RLKs) such as Chitin Elicitor Receptor Kinase 1 (CERK1), Mycorrhizal Receptor 1 (MYR1), and LysM Kinase 8 (LYK8), and trigger nuclear Ca2⁺ oscillations via Doesn't Make Infections 1(DMI1). These signals are decoded by Calcium and Calmodulin-Dependent Protein Kinase (CCaMK) and Cytokinin Response Factor-Like Protein (CYCLOPS) activating GRAS transcription factors (Required for Arbuscule Morphogenesis1(RAM1), Reduced Arbuscule Development 1(RAD1)) for arbuscule development. DELLA proteins (amino acid motif) integrate gibberellin signaling, while AMF effectors (e.g., rhizophagus irregularis nuclear localized effector 1 (RiNLE1)) suppress plant immunity by inhibiting histone H2B monoubiquitination. Exposure to heavy metals triggers reactive oxygen species (ROS) and Ca²⁺ signals, activating MAPK Kinase Kinase (MAPKKK), MAPK Kinase (MAPKK), MAPK (MPK3/MPK6). (MAPKKK /MAPKK/MAPK). These kinases regulate detoxification genes such as phytochelatin synthase (PCS) and metallothioneins, and control transporters (Heavy Metal ATPase (HMA), zinc and iron-like protein (ZIP)) for vacuolar sequestration. MAPKs also crosstalk with ABA signaling to stabilize antioxidant defenses. High salinity activates Calcineurin B-Like Proteins (CBL) and CBL-Interacting Protein Kinases (CIPK), initiating the Salt Overly Sensitive (SOS3-SOS2-SOS1) for Na⁺ efflux for xylem Na⁺ retrieval. ABA and MAPKs co-regulate ion transporters and Aquaporins (AQPs), while stress-responsive genes such as Late Embryogenesis Abundant (LEA) proteins and Calcium-Dependent Protein Kinases (CDPKs) maintain osmotic balance. Water deficit elevates ABA, which binds Pyrabactin Resistance/PYR1-Like/Regulatory Component of ABA Receptors (PYR/PYL/RCAR), inhibiting Type 2C Protein Phosphatases (PP2Cs) and activating Sucrose Non-Fermenting 1-Related Protein Kinases 2 (SnRK2s), notably Open Stomata 1 (OST1) which can lead to the phosphorylation of ABA-Responsive Element Binding Factor (ABF) and ABA-Responsive Element Binding Protein (AREB). Figure generated using Biorender (https://www.biorender.com/).

3. AMF and drought stress

3.1. Role of 14-3-3 family protein and H⁺-ATPase activity genes

Recent research has shown that genes from the 14-3-3 protein family, particularly PcGRF10 and PcGRF11.⁴⁸ While these proteins are well-known for their role in regulating the plasma membrane H⁺-ATPase to drive ion transport under salinity as detailed in (Section 4), their function under drought is equally significant. in P. cathayana when inoculated with AMF under DS led to increased expression of these genes, which was positively correlated with enhanced antioxidant enzyme activities and osmotic regulation.⁴⁸ Another study by Han et al. (2022b) further demonstrated that AMF and phosphorus supplementation synergistically improved drought resistance by modulating ROS homeostasis, phosphorus metabolism, and 14-3-3 gene expression, with PcGRF10 and PcGRF11 again showing strong associations with stress mitigation responses⁵⁷. These findings highlight the essential regulatory function of 14-3-3 proteins in AMF-induced drought tolerance. The proteins function as molecular hubs, regulating signaling cascades and physiological responses that assist plants in managing water scarcity circumstances⁴⁸.

In parallel, AMF such as F. mosseae and R. irregularis have been shown to modulate plant responses to salinity and DS through both fungal and plant 14-3-3 gene expression.⁵⁸ Specifically, Fm201 and Ri14-3-3, fungal 14-3-3 genes, are upregulated in mycorrhizal roots and hyphae following NaCl treatment, indicating their involvement in plant-fungal crosslink during salt stress adaptation.⁵⁸

Furthermore, AMF regulates H⁺-ATPase activity and influences gene expression during DS through specific pathways that enhance plant resilience and recovery under adverse conditions.⁴⁸ By activating these adaptive pathways, AMF enables plants to withstand water scarcity more effectively, emphasizing their pivotal role in enhancing drought resilience and ensuring survival through diverse mechanisms, as outlined below:

• 1-Promotes H⁺-ATPase activity: Under DS, the activity of H + -ATPase, an enzyme that pumps protons across membranes, is significantly enhanced by AMF such as F. mosseae.59, 60 This has been observed in trifoliate orange (P. trifoliata) plants, where drought treatment increases H+ -ATPase activity in both leaves and roots, and inoculation with AMF further strengthens this increase.⁵⁹
• 2-Enhance gene expression and auxin regulation under DS: AMF inoculation with F. mosseae significantly increased PtAHA2 expression (encodes a plasma membrane H⁺-ATPase) in both leaves and roots under DS, PtAHA2 expression was more pronounced, especially in AMF-inoculated plants,⁵⁹ it’s also enhanced H⁺-ATPase activity correlated with higher ammonium content in leaves and roots and lower soil pH, improving nutrient solubility and uptake.⁵⁹ These changes are linked to auxin synthesis and transport, modulated by AMF.⁶¹ Interestingly, Genes involved in auxin biosynthesis (e.g., PtYUC3, PtYUC8) and transport (e.g., PtABCB19, PtLAX2) were upregulated, while auxin efflux carriers (PtPIN1, PtPIN3) were downregulated.⁶¹

3.2. AMF controls transcription factors linked to drought tolerance

AMF enhances plant drought tolerance not only through improved water and nutrient acquisition but also by reprogramming transcriptional networks that govern stress signaling and adaptation.62, 63 Recent transcriptomic studies demonstrate that AMF colonization modulates key transcription factor (TF) families bHLH, MYB, ERF, WRKY, NAC, and heat shock factors (Hsfs) which orchestrate drought-responsive gene expression.64, 65, 66 Furthermore, RNA-seq study in maize (Z. mays) inoculated with F. mosseae under drought stress reported 189 differentially expressed genes (DEGs) enriched in Ca²⁺ signaling and transcription factor regulation.⁶⁴ Notably, co-expression analysis revealed strong positive correlations between the transcription factor WHIRLY1 and the calcium sensor gene CBL11. WHIRLY1 is a transcription factor involved in regulating stress-responsive genes related to drought tolerance,⁶⁷ while CBL11 is a calcineurin B-like protein that acts as a calcium sensor decoding Ca²⁺ signals and mediating downstream responses.⁶⁸ This regulatory interaction suggests that WHIRLY1 influences calcium signaling pathways via CBL11 to enhance drought stress adaptation in maize with F. mosseae symbiosis.⁶⁴ This study highlighted that AMF symbiosis modulates expression of Ca2⁺ signaling components such as CBLs, CIPKs, MAPKs, and CDPKs, along with transcription factors like bHLH, ERF, and MYB families that collectively improve drought tolerance.⁶⁴

AMF also influences ABA-responsive TFs (e.g., AREB/ABF) and heat shock TFs, enhancing stomatal regulation and oxidative stress mitigation. In walnut, inoculation with Diversispora spurca under DS significantly upregulated JrHsf03, JrHsf22, and JrHsf24, correlating with higher antioxidant enzyme activities and reduced ROS damage.⁶⁵ This demonstrates AMF’s role in activating Hsf-centered ROS homeostasis during drought. Additionally, Transcriptomic profiling of AMF-inoculated plants under combined stresses revealed induction of WRKY72, WRKY53, WRKY6, WRKY26, MYB86, and bHLH112, highlighting AMF’s ability to regulate multiple TF families involved in osmotic adjustment, antioxidant defense, and hormone signaling.⁶⁶

TFs regulated by AMF control downstream drought-responsive genes such as aquaporins (PIPs), LEA proteins, and osmolyte biosynthesis enzymes. For example, AMF inoculation in maize upregulated ZmPIP2,4, a plasma membrane aquaporin critical for water transport and drought tolerance, illustrating how AMF-TF networks connect to functional drought gene.⁶⁴ Details key transcription factors modulated by AMF under drought stress, including their roles in hormone signaling, aquaporin regulation, and MAPK cascade activation depicted in (Table 2).

Table 2.

The transcription factors influenced by AMF during drought stress.

Physiological responses	Plants	AMF	Effect on gene expression	References
Plant hormones	P. trifoliata	F. mosseae	Regulate genes involved in Indoleacetic acid (IAA) biosynthesis PtYUC3, PtYUC8 PtABCB19, PtLAX2.	161
	Malus pumila	R. irregularis	Regulate strigolactone (SL) synthesis genes MdIAA24	162
	S. lycopersicum	R. intraradices	Regulate genes responsible for the Abscisic acid (ABA) signalling pathway, TFT2, and TFT3.	163
Transporter proteins	G. max	G. intraradices G. mosseae	Genes involved in Aquaporins expression, GmPIP2	164
	Poncirus trifoliata	F. mosseae	Upregulate genes involved in Aquaporins expression, PtTIP1;2, PtTIP2;1, PtTIP4;1, PtTIP5;1, Downregulate aquaporins genes expression PtTIP1, 1, PtTIP2;2	165
Drought tolerance	Soybean	G. intraradices	MAPK cascade genes improve the resistance of mycorrhizal soybeans to drought stress.	17
	Poncirus trifoliata	G. mosseae	Upregulated genes expression PtTIP1;2, PtTIP1;3, PtTIP4;1 which is Significantly enhanced leaf relative water content and plant growth performance	162

3.3. Biochemical mechanisms of AMF-assisted drought stress tolerance

AMF enhances drought resilience in plants by modulating biochemical pathways that mitigate oxidative stress and maintain cellular homeostasis. Two primary mechanisms underline the reduced oxidative damage in AMF-colonized plants:

• 1.Fungal-Mediated water transport: AMF hyphae extend beyond the root depletion zone, absorbing water from soil micropores inaccessible to roots and delivering it to the host plant⁶⁹. This improves plant hydration, alleviates drought-induced water deficit, and stabilizes physiological processes such as photosynthesis and stomatal conductance.⁷⁰ A compartmentalized experiment using ¹⁸O-labeled water demonstrated that AMF hyphae directly contributed 12.3 % of total transpiration water under high soil moisture and 17.0 % under low soil moisture in alfalfa⁷¹. Another study using isotopic mixing models and fluorescent dyes showed that AMF hyphae can transport water across air gaps, accounting for up to 34.6 % of transpired water in host plants.⁶⁹
• 2.Antioxidant defense: Under DS, plants experience oxidative damage due to the excessive accumulation of ROS, which harm proteins, nucleic acids, and lipids⁷². To counteract this, plants activate both enzymatic and non-enzymatic antioxidant defense mechanisms that help maintain cellular integrity and redox balance73, 74. A recent meta-analysis confirmed that AMF inoculation under drought conditions increased SOD activity by 32 %, CAT by 23 %, and POD by 28 %, while reducing H₂O₂ accumulation by 20 %.⁷⁵ These findings underscore the importance of strategic AMF management in arid and semi-arid agroecosystems, including rhizosphere optimization in nurseries and field-scale AMF inoculation, to enhance crop resilience and survival under water-limited conditions.

Integrated drought responses across water transport, osmolytes, ROS detoxification, and photosynthesis are schematized in (Fig. 3). It integrated key pathways such as AQP regulation, osmolyte biosynthesis, antioxidant defense, and hormonal signaling, which are discussed across various sections of the article. AMF improves water acquisition by expanding the root-soil interface through extraradical hyphae, facilitating water uptake from micropores inaccessible to roots.70, 76 This fungal-mediated water transport contributes significantly to transpiration, especially under drought conditions. At the molecular level, AMF upregulate AQP genes such as PIP1 and PIP2, enhancing water permeability and maintaining cellular hydration.²⁶ Simultaneously, AMF stimulates the biosynthesis of osmolytes like Pro and soluble sugars, which stabilize proteins and membranes under osmotic stress.³⁹ The diagram also explores the AMF-induced activation of antioxidant enzymes such as POD, which mitigate ROS accumulation and oxidative damage, discussed in (Section 2.4)⁷⁵. Moreover, AMF influence photosynthetic efficiency by stabilizing photosystems and regulating intercellular CO₂ concentration, contributing to sustained growth under drought stress.⁷⁷ While AMF-mediated drought tolerance relies heavily on AQP regulation and ABA signaling, these same pathways along with osmolyte accumulation also support AMF’s role in salinity adaptation, where ionic homeostasis becomes critical.

Fig. 3.

A diagram of several pathways facilitated by AMF to enhance growth and yield under drought stress. AMF enhances water-holding ability, increases root exudation, expands water surface area, promotes lateral root development, and activates molecular mechanisms such as P5CS, SPS, PIP1, and PIP2. It also increases physiochemical mechanisms involving ABA, POD, and ROS, substantially increasing drought stress tolerance. Furthermore, AMF enhances hormonal interactions osmolyte accumulation and induces signaling transduction phenolic accumulation, maintains ionic homeostasis, optimizes the efficiency of PS-I and PS-II, regulates intercellular CO₂ concentration, and augments gene expression, thereby improving drought stress tolerance and promoting plant growth and yield under drought conditions. Figure generated using Biorender (https://www.biorender.com/).

4. AMF and salinity stress

Global salinization currently affecting substantial fractions of irrigated and arable lands reduces plant growth by combining osmotic stress with Na⁺/Cl⁻ toxicity and nutrient imbalances⁷⁸. AMF mitigates these constraints through a suite of mechanisms that translate into significant biomass gains across diverse hosts, AMF taxa, and salinity levels, with effect sizes increasing at higher salinity and over longer exposure (>4 weeks).⁷⁸

Recent syntheses conclude that AMF-inoculated plants under salt stress show increased K⁺/Na⁺ ratios, better osmotic adjustment (e.g., Pro, glycine betaine, soluble sugars), improved photosynthesis, and higher water-use efficiency compared with non-mycorrhizal controls⁷⁹. A classic meta-analysis spanning 43 studies showed AMF increase total, shoot, and root biomass and uptake of P, N, and K while reducing Na accumulation establishing a generalizable growth and nutrition benefit under salinity.⁸⁰

Additionally, AMF hyphae exude glomalin-related soil proteins (GRSP) that stabilize soil aggregates, improve structure and water retention, and can bind cations properties that indirectly improve plant performance in saline substrates.⁸¹ Salinity and osmotic stress can even stimulate glomalin production in AMF in vitro, potentially reinforcing aggregate stability in stressed soils⁸². At the signaling and transport levels, AMF interactions influence plant AQPs and polyamines, modulating water flow and redox balance in a context-dependent manner; AMF can upregulate or downregulate host AQPs to optimize water acquisition versus conservation under stress,¹⁰ these mechanisms justify AMF’s role as “bio-ameliorators” for salinity in agroecosystems.⁷⁹

4.1. AMF-driven nutrient and water uptake under salt stress

AMF-mediated ion homeostasis under salinity is fundamentally driven by the plasma membrane H⁺-ATPase, which generates the electrochemical gradient necessary for the function of transporters like SOS1 and NHX,⁸³ (discussed in Section 6). This H⁺-ATPase activity is tightly regulated by 14-3-3 proteins, which, as noted in the context of drought (Section 3.1), also modulate key enzymes in the antioxidant defense system, thus linking ion and redox homeostasis across stresses. Salinity stress causes ionic imbalances, mainly through the excessive accumulation of sodium ions (Na⁺), which disrupt plant cellular functions and nutrient uptake⁷⁸. Study in maize demonstrated that co-inoculation of AMF and spore-associated bacteria (SAB) under salinity stress improved K⁺/Na⁺ ratios in roots.⁸⁴ This was achieved through upregulation of ion transporter genes such as ZmAKT2, ZmSOS1, and ZmSKOR, which are involved in K⁺ uptake and Na⁺ exclusion.⁸⁴

Furthermore, Evelin et al. (2019) reviewed the physiological and molecular mechanisms by which AMF mitigates salinity stress.⁸⁵ They highlighted that AMF enhances nutrient uptake, regulate Na⁺ extrusion, and promote K⁺ acquisition, thereby maintaining favorable K⁺/Na⁺ ratios. Moreover, salinity stress disrupts plant cellular function primarily through excessive accumulation of Na⁺, which interferes with nutrient uptake and water balance. Recent advances in biophysical and genetic research have shed light on the distinct spatial functions of ion transporters involved in systemic Na⁺ regulation, with particular emphasis on the SOS1 Na⁺/H⁺ antiporter (as discussed in Section 6 & Fig. 6)86, 87. SOS1 operates in outer root tissues to limit cytosolic Na⁺ and restricts its translocation to shoots. Under specific conditions, SOS1 expression in the central vascular region promotes Na⁺ loading into the xylem, enabling dynamic regulation of Na⁺ distribution within the plant.⁸⁶ In addition to vacuolar sequestration, OsSOS1, a plasma membrane Na⁺/H⁺ antiporter, plays a central role in rice’s response to salt stress by mediating Na⁺ efflux from the cytosol and facilitating Na⁺ loading into the xylem⁸⁸. This mechanism helps prevent Na⁺ accumulation in photosynthetic tissues and contributes to long-distance Na⁺ transport.⁸⁸

In Robinia pseudoacacia roots under salinity stress, AMF symbiosis facilitates the exclusion of Na⁺ from root cells, unloading of Na⁺ from the xylem, and enhanced translocation of K⁺ to the shoots.⁸⁹ These beneficial physiological effects are closely associated with the upregulation of key ion transporter genes in roots, including RpSOS1, RpSKOR (a K⁺ outward-rectifying channel facilitating K⁺ loading into the xylem), and RpHKT1 (a transporter implicated in Na⁺ retrieval from the xylem), which collectively mediate ion homeostasis under salt stress.⁸⁹ Co-inoculation with AMF and salt-tolerant rhizobia notably enhances the expression of these genes, thereby improving the K⁺/Na⁺ ratio and overall salt tolerance in Robinia seedlings.⁹⁰

4.2. AMF-driven osmotic balance under salinity

With regard to the organic acids (OAs) such as citric, malic, fumaric, and oxalic acids are key intermediates of the tricarboxylic acid (TCA) cycle and play multiple roles in plant stress physiology⁹¹. Under salt stress, plants accumulate OAs in vacuoles to maintain osmotic balance, regulate cytosolic pH, and chelate excess cations like Na⁺, thereby reducing ionic toxicity.⁹² AMF colonization alters the concentration and composition of OAs in host plants. In maize, AMF-inoculated plants under salt stress showed increased levels of acetic, citric, fumaric, malic, and oxalic acids, while formic and succinic acids decreased compared to non-mycorrhizal plants lactic acid remained unchanged.⁹³ Furthermore, direct OA production by AMF has been reported by AMF itself to secrete low-molecular-weight organic acids (LMWOAs) such as citrate and oxalate, which facilitate nutrient mobilization (e.g., phosphorus) and may indirectly influence host OA metabolism.⁹⁴ However, AMF enhance plant resilience to salinity by improving ion homeostasis, water uptake, and organic acid-mediated osmotic adjustment, yet the precise molecular and metabolic mechanisms remain unclear. Bridging these gaps through integrated multi-omics and field validation is essential for developing sustainable AMF-based strategies for saline agriculture.

4.3. AMF enhance photosynthesis process under salinity stress

Similar to drought, salinity stress triggers oxidative imbalance and ion toxicity. AMF-mediated mechanisms such as ROS detoxification, 14-3-3 protein regulation, and vacuolar sequestration provide a unifying framework for cross-tolerance.³ Salinity stress impairs photosynthesis by inducing stomatal closure, reducing CO₂ assimilation, and damaging photosystem II (PSII) (demonstrated in Fig. 3), leading to photoinhibition and oxidative stress95, 96. AMF inoculation mitigates these effects by improving stomatal conductance, transpiration rate, and net photosynthetic rate (Pn), while maintaining higher chlorophyll content and relative water content.⁹⁷

Recent meta-analyses confirm that AMF enhance PSII efficiency by increasing the actual quantum yield (ΦPSII), electron transport rate (ETR), and photochemical quenching, while reducing non-photochemical quenching (NPQ), thereby optimizing energy utilization under saline conditions.98, 99 Studies in woody species (Populus spp.) and high-value crops (Pistacia vera) demonstrate that AMF not only preserve PSII photochemistry but also maintain physiological homeostasis under severe salinity, highlighting their potential as a sustainable bio-amelioration strategy for saline soils.97, 99 Key physiological contrasts between mycorrhizal and non‑mycorrhizal plants under salinity are illustrated in (Fig. 4). In non-mycorrhizal plants, salinity stress leads to excessive ROS accumulation, chlorophyll degradation, impaired photosynthesis, and reduced water and nutrient uptake, resulting in cellular dehydration and growth inhibition.100, 101 Additionally, AMF activates MAPK signaling cascades and ABA pathways (discussed in Section 6), which regulate transcription factors (e.g., WRKY, MYB) that drive osmolyte biosynthesis, including Pro, trehalose, and sucrose.¹⁰² In combination, these interconnected pathways underline the systemic function of AMF in managing water relations, ion transport, osmolyte metabolism, and redox homeostasis under saline circumstances.

Fig. 4.

Illustrates the role of AMF symbiosis in alleviating salinity stress. Key enzymes involved in this process include pyroline-5-carboxylate synthase (P5CS), trehalose-6-phosphate synthase (TPS), sucrose phosphate synthase (SPS), and sucrose synthase (SS). Under conditions of salinity stress, increased levels of sodium (Na⁺) and chloride (Cl⁻) ions in the soil lower the water potential compared to that of plant cells, limiting water absorption and leading to cellular dehydration. To combat these effects, plants boost the synthesis of osmolytes such as proline, trehalose, polyamines, and sucrose, stabilizing cellular hydration and safeguarding against stress-induced damage. Additionally, mitogen-activated protein kinase (MAPK) cascades, comprising MAPKKK, MAPKK, and MAPK, function as osmotic sensors, transmitting stress signals and coordinating adaptive responses to restore osmotic balance. Furthermore, the figure contrasts mechanisms in plants with AMFs with those without them. Figure generated using Biorender (https://www.biorender.com/).

5. AMF and heavy metal stress

Remediating HMs using AMF involves altering mycorrhizal plants' root architecture and rhizosphere. Numerous plants exhibit significant potential for enhancing HM stress tolerance associated with AMF.103, 104 An excessive buildup of HMs in plant tissues can result in toxicity, damage the integrity of cell membranes, impede photosynthesis and the intake of nutrients, affect morpho-physiological and biological processes, and eventually interfere with plant growth¹⁰⁵. Rapidly expanding hyphae, capable of thriving in adverse metal toxicity levels and challenging environmental situations, promote the establishment of AMF symbiosis with host plants¹⁰⁶. AMF can elicit alterations in host plants necessary to adapt to the hazardous conditions encountered during symbiotic interactions,¹⁰⁷ It also indirectly enhances plant tolerance by promoting the absorption of water and mineral nutrients, augmenting shoot biomass, inducing alterations in root architecture, and mitigating oxidative stress from heavy metal exposure.¹⁰⁸

The remediation process is accelerated by increasing shoot biomass production, and AMF plays a vital role in accumulating biomass in the presence of HM toxicity. When subjected to Zn toxicity, AMF inoculated with R. irregularis from maize plants produced more biomass than non-mycorrhizal plants.¹⁰⁹ The cellular detoxification framework and mycorrhizal metal transport routes are depicted in (Fig. 5). The primary line of defense against HMs within the plant cell centers on glutathione (GSH) metabolism and vacuolar sequestration.¹¹⁰ HMs entering the cell (e.g., Cd²⁺, Cu²⁺, As³⁺) trigger the synthesis of GSH from its precursors (cysteine, glutamate, glycine). The enzyme PCS then catalyzes the polymerization of GSH into PCs.¹¹⁰ These PCs act as potent chelators, binding HMs to form HM-PC complexes.

Fig. 5.

Illustrates the mitigation of heavy metal (HM) stress through AMF by employing chelating agents. The figure depicts the uptake of heavy metals facilitated by zinc and iron-like protein (ZIP) transporters and other ion channels. Upon entering the cytosol, HM ions activate the enzyme phytochelatin synthase (PCS), which catalyzes the formation of phytochelatins (PCs) from glutathione (GSH). GSH is synthesized from glutamate (Glu) and cysteine (Cys) through the transfer of γ-glutamyl-cysteinyl (γ Glu-Cys) conjugates, a process mediated by glutathione synthetase (GS). PCs then bind to cytosolic HM ions, forming heavy metal-phytochelatin (HM-PC) complexes that are transported into the vacuole via ATP-binding cassettes (ABC), P1B ATPase (P1B), and various transporters, including heavy metal ATPases (HMA) and natural resistance-associated macrophage proteins (NRAMP). Moreover, HM ions contribute to generating reactive oxygen species (ROS), such as superoxide radicals (O²⁻). These radicals are dissimulated by superoxide dismutase (SOD), resulting in the production of hydrogen peroxide (H₂O₂), which is subsequently decomposed into water (H₂O) by ascorbate peroxidase (APX) and catalase (CAT). Additional enzymes involved in this process include peroxidase (POD), glutathione peroxidase (GPX), and glutathione reductase (GR). Additionally, the mycorrhizal pathway encompasses several metal transporters that facilitate the transfer of metals from the extraradical mycelium (ERM) to the intraradical mycelium (IRM) and ultimately to the plant root at the symbiotic interface. These transporters include a fungal copper transporter, iron permease, manganese transporter, and zinc transporter. Figure generated using Biorender (https://www.biorender.com/).

This chelation is crucial for neutralizing the reactive and toxic forms of metals. Subsequent sequestration involves transporting these HM-PC complexes from the cytosol into the vacuole.¹¹⁰ This energy-dependent process is facilitated by ATP-Binding Cassette (ABC) transporters located on the vacuolar membrane, consuming ATP to power the translocation. Alternative fates for complexes, such as the formation of NH₄-PC complexes or degradation, may also occur.¹¹¹ Additionally, specific HM ATPases (HMAs), often located on the plasma membrane or internal membranes, actively pump free metal: ions either out of the cell or into compartments, further reducing cytosolic HM concentrations.¹¹² Ion channels and transporters on the cell wall and plasma membrane represent the initial points of HM entry or exclusion.¹¹³ AMFs can reduce heavy metals in different ways, involving:

• 1.AMFs promote biomass accumulation and mitigate HM stress in plants: By forming symbiotic associations with host plants, AMF modulates HM dynamics, often sequestering metals in root tissues and restricting their translocation to aerial parts.¹¹⁴ This “metal immobilization” strategy reduces HM bioavailability in shoots, thereby mitigating phytotoxicity and improving shoot biomass.¹¹⁵ For instance, in wheat (T. aestivum) AMF particularly (G. mosseae) colonization significantly reduced Cd accumulation in grains while increasing Cd retention in roots, suggesting a protective role against edible tissue contamination.¹¹⁶ Similarly, Z. mays colonized by F. mosseae and Diversispora spp. in HM-contaminated soils exhibited enhanced root uptake of arsenic (As), Cd, Zn, and Pb, with limited transfer to shoots, underscoring AMF’s role in metal compartmentalization.¹¹⁷ Concurrently, AMF upregulates plant genes encoding metal transporters (e.g., ZIP, NRAMP) and PCs which bind and detoxify HMs in root vacuoles¹¹⁸. This dual-action soil immobilization and cellular sequestration minimize oxidative stress in shoots while promoting root biomass, which acts as an HM sink.¹¹⁹ Additionally, studies also highlight AMF-mediated modulation of soil pH and organic acid exudation, further immobilizing HMs like Cd and Pb in less bioavailable forms.120, 121 Notably, AMF species exhibit functional diversity in HM management. For instance, R. irregularis enhances the phytostabilization of Cd in legumes, while Claroideoglomus etunicatum improves Zn tolerance in hyperaccumulators like Sedum alfredii.122, 123
• 2.Fungal metal transporters and their regulation in AM symbiosis: Beyond plant transporters, AMF possess their own suites of metal transporters that operate across extraradical mycelium (ERM), intraradical mycelium (IRM), and the symbiotic interface. Genome-wide analysis in R. irregularis indicates expanded families for Cu transporters (CTR/Ctr-like), Fe uptake modules (Fet3-Ftr1 permease-oxidase system; FTR), and Zn transporters, among others, with stage-specific up-regulation in mycorrhizal roots consistent with active metal handling during symbiosis.¹²⁴ Putative NRAMP and CDF/HMA-like transporters likely contribute to intracellular trafficking and sequestration.115, 124 Together, these fungal systems can buffer external metal loads, coordinate micronutrient delivery, and indirectly shape host detoxification demands (e.g., PCs/MTs and vacuolar flux), thereby complementing plant ABC/HMA/NRAMP activity depicted in (Fig. 5).
• 3.AMF-Facilitated nutrient absorption under HMs stress: The enhanced nutrient uptake by host plants facilitated by AMF is a well-established benefit of this symbiotic relationship. A recent pot experiment investigated the role of C. etunicatum under Cd and lanthanum (La) stress¹²⁵. The results demonstrated that AMF significantly increased plant biomass and improved nutrient acquisition, with N, P, and K uptake rising by 20.1 % to 76.8 % in maize tissues. Additionally, AMF reduced the toxic effects of HMs by limiting their accumulation in plant organs, highlighting their protective role under HM stress.¹²⁵ Across drought, salinity, and HM stress, AMF-mediated tolerance converges on three core strategies: (i) maintaining water and ion homeostasis, (ii) reinforcing antioxidant defenses, and (iii) reprogramming signaling networks via hubs such as 14-3-3 proteins and MAPK cascades. These shared pathways highlight AMF’s potential to confer cross-tolerance under combined stress scenarios.

HMs exposure inevitably leads to the generation of reactive ROS, resulting in oxidative stress. Plant tolerance in this context relies heavily on the AMF-enhanced antioxidant defense system detailed in (Section 2.5), which is universally activated across all stress types. Furthermore, the upstream signaling for this defense, involving the ABA and MAPK pathways (2.2, 2.4), is co-opted to regulate the expression of antioxidant enzymes and chelation agents like PCs and MTs, thereby linking the molecular response to HMs with the general stress response.

6. Integrated ABA-MAPK signaling, and aquaporin regulation in stress adaptation

AMF establish a conserved mutualistic relationship with most terrestrial plants common symbiosis signaling pathway (CSSP). Under phosphate-deficient conditions, plant roots secrete strigolactones that stimulate AMF spore germination and hyphal branching, while AMF release Myc factors lipo-chitooligosaccharides (Myc-LCOs) and short-chain chitooligosaccharides (CO4/CO5) to initiate symbiotic signaling.¹²⁶ These signals are perceived by plant LysM receptor-like kinases such as CERK1, MYR1, and LYK8, which form receptor complexes to distinguish symbiotic signals from pathogenic chitin fragments, thereby activating SYMRK (also known as DMI2) to initiate downstream signaling.¹²⁶ SYMRK activation triggers nuclear calcium oscillations via ion channels DMI1(Doesn't Make Infections 1), CASTOR, and POLLUX located on the nuclear envelope, which are decoded by CCaMK and its partner CYCLOPS to translate calcium signatures into transcriptional responses.¹²⁷ This leads to transcriptional reprogramming through GRAS family transcription factors such as RAM1 and RAD1, which activate genes required for arbuscule development, while DELLA proteins interact with the CCaMK-CYCLOPS complex to stabilize symbiotic progression.¹²⁷

To facilitate colonization, AMF secrete nuclear-targeted effectors like RiNLE1 that suppress plant immunity by inhibiting histone H2B monoubiquitination, reducing defense gene expression and promoting fungal accommodation¹²⁸ Ultimately, arbuscules form within root cortical cells to enable nutrient exchange plants absorb phosphate via PT4 transporters on the periarbuscular membrane, while supplying carbohydrates and lipids to the fungus and AMF further enhance plant stress tolerance through ABA accumulation, MAPK activation, and AQP regulation.¹²⁸

The ABA signaling pathway initiates with ABA perception by PYR/PYL/RCAR receptors, which inhibit Type 2C protein phosphatases (PP2Cs), releasing sucrose non-fermenting 1-related protein kinases 2 (SnRK2s) to activate via autophosphorylation. Activated SnRK2s phosphorylate ABA-responsive element binding factors (ABFs), which translocate to the nucleus and regulate ABA-responsive genes, orchestrating stress adaptation¹²⁹. In parallel, MAPK cascades (discussed in 2.4) comprising MAPKKKs, MAPKKs, and MAPK serve as key transducers of environmental and symbiotic signals¹²⁹. These cascades are activated by drought, salinity, heavy metal exposure, and notably AMF colonization, which primes host plants for enhanced stress tolerance by triggering MAPK modules and modulating hormonal signaling130, 131. AMF not only activate MAPKs but also amplify ABA signaling, creating a synergistic effect that strengthens stress resilience.¹³²

A critical downstream target of this integrated network is AQPs, membrane channels essential for water transport and hydraulic conductance. ABA upregulates AQP gene expression particularly PIP-type aquaporins such as HcPIP1;2 and HcPIP2;7 under drought stress in AMF-colonized plants discussed in (section 2.3)10, 23. This regulation occurs via ABA-activated SnRK2s and ABF-mediated transcription. Concurrently, MAPKs such as MPK3 and MPK6 can phosphorylate transcription factors that bind AQP promoters, adding an additional layer of control.¹³²

Beyond AQPs, ABA-MAPK integration regulates a broad spectrum of stress-responsive genes. Under salinity stress, AMF-colonized plants exhibit enhanced expression of genes involved in the SOS pathway, CDPKs, and late embryogenesis abundant (LEA) proteins, which collectively maintain ion homeostasis, osmoregulation, and cellular protection¹³¹. Similarly, MAPKs such as MPK3 and MPK6 modulate genes associated with HM detoxification, chelation, and transport, reducing metal accumulation and toxicity.113, 133 This multilayered regulation underscores the central role of ABA-MAPK signaling in orchestrating complex transcriptional networks that enable plants to withstand diverse abiotic stresses while benefiting from symbiotic. This integrated signaling network is illustrated in (Fig. 6), which links stress-specific pathways with the AMF colonization module and the ABA-MAPK-AQP hub, highlighting key molecular players and their interactions under abiotic stress conditions.

7. Current challenges and future directions

Despite the significant potential of AMF in enhancing plant stress resilience, several critical challenges must be addressed to enable their effective integration into agricultural systems:

• 1.Species-specific compatibility and functional variability: AMF symbiosis demonstrates significant specificity regarding host genotype and fungal strain, resulting in diverse colonization efficiency and stress alleviation effects. However, the lack of standardized screening protocols for determining optimal AMF-host pairings constitutes a substantial impediment to practical implementation.¹³⁴
• 2.Limited field validation and environmental complexity: Most insights into AMF-mediated stress tolerance derive from controlled experiments, which fail to capture the heterogeneity of field conditions. Soil type, native microbial communities, and climatic variability significantly influence AMF performance. Long-term, multi-location field trials are essential to validate laboratory findings and ensure ecological robustness.¹³⁵
• 3.Microbiome interactions and community-level dynamics: AMF operates within a broader soil microbiome, where interactions with other beneficial microbes (e.g., rhizobia, endophytes) can be synergistic or antagonistic.¹³⁶ Deciphering these community-level dynamics is critical for designing effective microbial consortia that enhances plant resilience under stress.
• 4.Formulation stability and commercial scalability: Developing AMF inoculants with consistent efficacy, extended shelf-life, and adaptability across diverse cropping systems remains a technological challenge. Optimization of carrier materials, spore viability, and application strategies is necessary to achieve large-scale deployment.¹³⁷
• 5.Real-world implementation, soil pH & fertility, and integration with sustainable systems: Soil pH and fertility shape AMF colonization and community structure. Nutrient additions can shift pH and alter colonization rates, spore density, and dominant taxa; high available P often suppresses root colonization, while moderate P can sustain AMF benefits in the field.¹³⁸ Long-term P management, N forms, and crops phenology interact with pH to restructure AMF communities and function, necessitating site-specific fertility plans integration strategies resulting in align AMF with reduced tillage and cover crops (off-season hosts), seed coating or in-furrow inoculation at root-zone placement, co-formulation with biochar or PGPB, and moderate P programs to avoid colonization suppression.¹³⁹ These practices fit well within low-input/regenerative systems and can be adapted to high-input systems with careful nutrient and pesticide stewardship.
• 6.Integration under combined abiotic stresses: Emerging case studies illustrate AMF’s potential under multi-stress conditions. For example, AMF combined with biochar improved sweet pepper resilience under simultaneous drought and salinity by enhancing photosynthetic pigments and antioxidant defenses¹⁴⁰. Similarly, maize under calcium-saline stress plus drought exhibited improved photosynthesis and nutrient stoichiometry with AMF inoculation¹⁴¹. However, trade-offs exist; coastal dune grass inoculated with native AMF showed higher survival under mild salinity but reduced performance under drought, underscoring the need for environment-specific AMF consortia.¹⁴² Therefore, Integration of AMF under combined abiotic stresses addresses challenges such as multi-stress tolerance, nutrient imbalance, oxidative damage, photosynthetic decline, soil degradation, and the need for stress-specific AMF consortia. Future research should employ integrated omics approaches transcriptomics, proteomics, and metabolomics to unravel these complexity of the multiple stresses interactions.

To translate these insights into practice, we propose a research roadmap: (1) develop standardized AMF-host screening protocols, (2) conduct long-term field trials under multi-stress conditions, (3) integrate AMF with beneficial microbial consortia, (4) advance formulation technologies for scalable inoculants, and (5) develop guidelines for pH adjustment and moderate P fertilization to sustain AMF colonization without suppression. These steps will unlock AMF’s full potential in climate-smart agriculture.

8. Conclusion

AMFs have emerged as pivotal biological allies in enhancing plant resilience against the escalating threats of drought, salinity, and heavy metal, contamination. This review consolidates mechanistic insights across physiological, biochemical, and molecular domains, revealing how AMF orchestrates a multifaceted defense strategy. Through modulation of AQPs, ABA and MAPK signaling, antioxidant systems, and ion transporters, AMF not only improve water and nutrient uptake but also fine-tune stress-responsive gene expression and redox homeostasis. Their role in regulating transcription factors, osmolyte biosynthesis, and vacuolar sequestration underscores their systemic influence on plant stress physiology.

Importantly, AMF-mediated tolerance is not confined to isolated stress conditions; evidence suggests a convergence of mechanisms that confer cross-tolerance under combined stress scenarios. This positions AMF as integrative modulators capable of synchronizing rhizospheric interactions with intracellular signaling networks. The review also highlights the ecological and agronomic relevance of AMF, including their contributions to soil structure, nutrient cycling, and sustainable crop production.

Emerging combined‑stress studies reinforce the integrative model. For instance, in maize facing calcium‑salinity plus drought, AMF improved growth, photosynthesis, and C:N:P stoichiometry primarily under water deficit, indicating context‑dependent synergies under compound stress. In coastal dune grass, native AMF enhanced survival at low salinity but reduced survival under drought, highlighting potential trade‑offs that demand environment‑specific consortia and management. These cases underscore the need for predictive rules linking AMF taxa, host genotype, and stress matrices, resolved through field‑based multi‑omics.

As climate change intensifies the frequency and severity of abiotic stresses, leveraging AMF-based strategies offers a promising avenue for resilient agriculture. However, translating these insights into field-ready applications demands a deeper understanding of AMF diversity, host specificity, and multi-stress integration. Future research should prioritize multi-omics approaches, long-term field trials, and the development of robust AMF formulations to unlock their full potential in climate-smart farming systems.

CRediT authorship contribution statement

Mutaz Mohammed Abdallah: Writing – review & editing, Writing – original draft, Software. Chenmei Suo: Writing – original draft, Validation, Investigation. Youxin Cui: Writing – review & editing, Validation, Conceptualization. Rana Hissan Ullah: Writing – review & editing, Writing – original draft. Hoang Hong Nhung: Writing – original draft, Validation, Software. Lixin Li: Validation, Supervision, Conceptualization. Changli Liu: Visualization, Supervision.

Informed consent

All the authors have given their consent for publication of this manuscript by Journal of Genetic Engineering and Biotechnology.

Ethics approval

Not applicable.

Funding

The authors gratefully acknowledged the funding supported by the Natural Science Foundation of Heilongjiang Province [Grant Number ZHLJZR230150002].

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Not applicable.