Plants are pivotal for many ecosystems and thus essential for the survival of virtually all living organisms. They are not only a source of food for humans and animals and the main point of entrance of solar energy and organic carbon in ecosystems, but they also play a critical role in regulating the Earth’s climate and sustaining the planet’s biodiversity. However, plants are constantly under threat from various biotic and abiotic stresses, such as pests, diseases and environmental factors, like drought and salinity ^[1][2][3][1,2,3]. Under the current context of anthropogenic global warming, forest and cultivated plants must adapt to the novel conditions or become extinct ^[4][5][4,5]. To overcome these challenges, plants have evolved complex mechanisms that involve a wide range of signaling pathways and interactions with other organisms, including microbes.

The significance of arbuscular mycorrhizal fungi (AMF) and strigolactones (SLs) in plant–microbe interactions lies in their ability to positively influence plant growth, development and overall health ^[6][7][6,7]. AMF establish mutualistic associations with plants, colonizing their root systems and providing various benefits [8]. These benefits include enhanced nutrient acquisition, such as an increased availability of phosphorus and micronutrients, improved water uptake and protection against biotic and abiotic stresses [2]. AMF can also induce systemic resistance in plants, making them more resistant to pathogens and pests and abiotic stresses ^[9][10][9,10]. On the other hand, SLs, a class of plant hormones, regulate several critical processes in plants [11]. They are involved in shaping root architecture, promoting the development of lateral roots and stimulating the establishment of beneficial associations with symbiotic microbes, such as AMF ^[7][11][12][7,11,12]. SLs also play a role in plant defense responses, including the activation of systemic defense mechanisms against pathogens and the induction of plant secondary metabolites [13].

Recent studies have shown that the synergistic interaction between AMF and SLs can have a significant impact on plant–microbe interactions and plant development. The combined effects of these two components result in improved plant growth, an increased resistance to biotic and abiotic stresses and enhanced nutrient uptake ^[6][7][14][6,7,14]. Therefore, understanding the interplay between these two components is crucial for developing sustainable agricultural practices and improving crop yields.

Mechanistically, AMF have been found to influence the production and release of SLs by plants [15]. They can stimulate the synthesis and secretion of SLs, which act as signaling molecules to attract beneficial microbes and promote symbiotic interactions [16]. In turn, SLs can modulate the colonization and establishment of AMF within the plant root system, facilitating their beneficial effects ^[6][7][14][6,7,14]. The significance of this synergistic interaction becomes evident in its potential to improve crop productivity, nutrient utilization and plant resistance in the face of environmental challenges ^[6][14][6,14]. By harnessing the combined effects of AMF and SLs, agricultural practices can be optimized to enhance nutrient acquisition efficiency, leading to increased crop yields.

2. Overview of Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhizal fungi extend their hyphae into the soil, exploring a larger volume and accessing nutrients inaccessible to the root ^[17][18][17,18]. In addition, AMF spore dynamics are found at higher densities in rhizosphere soil [19]. They can solubilize nutrients from solid soil particles and organic matter, making them available for plant uptake [20]. Furthermore, AMF release enzymes that break down complex organic compounds, releasing nutrients for plant uptake [21]. They can convert inorganic forms of macronutrients, such as phosphorus, into molecules that plants can assimilate [22]. This promotes efficient nutrient utilization by plants. Furthermore, AMF can affect the synthesis, release and signaling pathways of growth-promoting phytohormones, such as SLs, auxins and cytokinins in plants, leading to enhanced plant growth and development ^[15][23][15,23]. AMF can induce systemic resistance in plants, preparing them for pathogen and pest attacks. For example, AMF activate plant defense mechanisms by triggering the expression of defense-related genes [24]. This leads to the production of defense compounds, such as pathogenesis-related proteins and antimicrobial peptides that protect plants against pathogens and pests [25]. Furthermore, AMF prime the plant’s immune system, enabling a more rapid and effective defense response upon pathogen or pest attacks [25]. This priming improves the plant’s ability to recognize and respond to subsequent challenges, increasing its overall disease resistance.

2.1. Molecular Signaling

AMF are beneficial soil microorganisms that form symbiotic relationships with plants, enhancing nutrient uptake and contributing to the health and sustainability of terrestrial ecosystems [2]. In the early stages of AMF symbiosis, molecular signals are exchanged between the plant and the fungus ^[26][27][26,27]. The plant releases signaling molecules, such as SLs, into the soil in response to nutrient stress [28]. AMF hyphae in the soil sense these signals and initiate a molecular response ^[7][29][7,29]. This includes the expression of genes related to hyphal growth and colonization. The growth and branching of AMF hyphae towards plant roots are regulated by various signaling pathways ^[17][18][17,18]. These pathways involve receptor proteins on the fungal hyphae that recognize specific plant signals. The interaction between plant root cells and AMF hyphae involves molecular cross-talk, allowing the hyphae to penetrate root cells [30]. This process involves the exchange of signaling molecules and the activation of genes that facilitate the establishment of arbuscules, specialized structures within root cells where nutrient exchange occurs [31]. Nutrient exchange in AMF symbiosis is highly dependent on molecular processes. Within arbuscules, specific transporters and channels facilitate the movement of nutrients between the plant and the fungus ^[7][28][7,28]. The plant provides sugars obtained by photosynthesis and organic compounds from the plant’s own metabolic pathways through molecular transporters, while the fungus supplies the plant with essential nutrients, like phosphorus and nitrogen, which the fungi have solubilized from the soil ^[32][33][32,33]. AMF also influence plant defense and stress responses at the molecular level [9]. They can enhance the plant’s ability to withstand various environmental stresses, such as biotic and abiotic ^[2][10][2,10]. The molecular mechanisms involved in these responses include the activation of defense-related genes and the modulation of plant hormone signaling pathways ^[9][34][9,34]. The molecular interactions between plants and AMF are highly intricate and involve the exchange of signaling molecules, gene regulation and the coordination of various molecular processes to establish and maintain this symbiotic relationship ^[34][35][34,35]. The synergy between the two organisms at the molecular level results in improved nutrient uptake and enhanced plant resilience.

2.2. Mycorrhizal Symbiosis Genes

Mycorrhizal symbiosis involves the interaction between plants and mycorrhizal fungi, and several genes in both partners are crucial for the establishment and maintenance of this symbiotic relationship (Figure 1). Some of the key genes and molecular components involved in mycorrhizal symbiosis are established below.

2.2.1. In Plants

Sym genes (symbiosis genes): these are plant genes that are specifically involved in the establishment and regulation of mycorrhizal symbiosis [36]. They encode various proteins and transcription factors necessary for the recognition of fungal partners, the development of symbiotic structures and the regulation of nutrient exchange [37]. Receptor kinases: plant receptor kinases, such as the LysM receptor kinases, play a role in recognizing fungal signals and initiating the signaling cascade leading to mycorrhizal symbiosis ^[38][39][38,39]. These receptors are essential for the early recognition of mycorrhizal fungi [38]. Phosphate (Pi) transporters: plants upregulate genes encoding phosphate transporters in response to mycorrhizal colonization [40]. These transporters facilitate the uptake of phosphorus from the fungal partner [41]. Mycorrhiza-induced small RNAs: plants produce small RNAs in response to mycorrhizal colonization, which may play a role in regulating the symbiotic interaction [42]. Plant hormones: in general, plant hormones include activities directed towards the development and branching of plants. For example, strigolactones play an important role in stimulating branching activity in plants, contributing to the development of an extensive root system [43]. The gibberellin hormone promotes root elongation and influences plant growth and development [44]. Cytokinins are also involved in the regulation of plant branching. Cytokinins influence the overall architecture of the plant by promoting the growth of lateral buds [45]. Furthermore, various plant hormones, such as strigolactones, gibberellin and cytokinins, are involved in signaling and regulating mycorrhizal symbiosis ^[7][46][47][7,46,47]. Strigolactones, for example, are known to promote hyphal growth and root colonization by mycorrhizal fungi [6].

2.2.2. In Mycorrhizal Fungi

Mycorrhiza-specific genes: fungi possess genes that are specifically expressed during mycorrhizal symbiosis [48]. These genes are involved in hyphal growth, the establishment of symbiotic structures, like arbuscules, and nutrient exchange ^[48][49][48,49]. For example, Colard et al. [48] reported that AMF-specific AM1, AM2, AM3 and AM11 genes were activated at the pre-symbiotic stage. Transporter genes: mycorrhizal fungi have transporter genes that code for proteins responsible for the uptake and transfer of nutrients to the plant host [50] (Table 1). For example, phosphate transporters are crucial for delivering phosphorus to the plant [51]. Maldonado-Mendoza et al. [52] revealed that this was thanks to the GiPT gene for Glomus intraradices. Secreted proteins: fungi produce secreted proteins, some of which may be involved in facilitating the interaction with plant roots or modulating plant immune responses ^[53][54][53,54]. For example, Kamel et al.’s [55] study on Rhizophagus irregularis revealed that this species had a large number of putative secreted proteins (RiSPs), which could be of great importance in establishing symbiosis. Effector genes: some mycorrhizal fungi may produce effector proteins that can manipulate plant host defenses or signaling pathways to promote symbiosis ^[56][57][56,57]. Regulatory genes: fungi have genes involved in the regulation of their responses to the plant host and environmental cues ^[58][59][58,59]. These genes help the fungi adapt to different plant partners and environmental conditions [60]. For example, Huang et al. [59] reported that using mitogen-activated protein kinase (MAPK) signaling for the interactions between AMF and apple plant hosts was shown to increase apple drought tolerance. miRNA: there have also been some miRNAs identified as participants in this regulation, such as miR167, miR394 and miR156 [60]. Understanding the genetic and molecular basis of mycorrhizal symbiosis is an active area of research. These genes and molecular components play a critical role in the formation and maintenance of this beneficial mutualistic relationship, contributing to plant nutrient acquisition, stress tolerance and overall ecosystem health (Table 1).