Introduction: Mycorrhizal symbiosis, particularly arbuscular mycorrhiza, is recognized as one of the most widespread and beneficial interactions between plants and soil microorganisms and has been extensively studied. This symbiosis plays a significant role in improving plant nutritional status, enhancing resistance to both biotic and abiotic stresses, reducing the impact of certain pathogens, and promoting synergistic interactions with other beneficial microbial communities. Ultimately, these effects contribute to improved crop health and productivity. One of the most important benefits of this association is the enhancement of plant nutrition. Mycorrhizal plants acquire nutrients either directly through their roots or indirectly via the hyphal network of symbiotic fungi. The predominance of either pathway depends on a complex set of biochemical and molecular mechanisms within the common symbiosis pathway, as well as phosphorus availability, plant phenotypic traits (especially root characteristics), and environmental conditions. Despite its importance, the degree of mycorrhizal dependency varies among plant species and cultivars. Mycorrhizal dependency is defined as the extent to which a plant relies on mycorrhizal fungi to achieve optimal growth or yield. It is determined by the interaction between plant genotype and phenotype in combination with environmental factors.

Mycorrhizal symbiosis is the result of a complex molecular dialogue between plant and fungus, involving the exchange of signals at cellular, molecular, and genetic levels. In the early stages of symbiosis, plants secrete compounds such as strigolactones from their roots, which stimulate spore germination and hyphal branching. This is followed by fungal signalling molecules, including chito-lipo-oligosaccharides, which act as key signals in the initiation and regulation of arbuscular mycorrhizal symbiosis. Disruption in this shared signalling pathway can lead to failure of symbiosis establishment or reduced mycorrhizal dependency.

Under high nutrient conditions, particularly high phosphorus availability, plant secretion of strigolactones is typically reduced. This illustrates how excess soil phosphorus can interfere with the common symbiosis signalling pathway and thereby decrease mycorrhizal dependency. The synthesis and secretion of strigolactones are regulated by the upregulation of genes such as D27, CCD7, CCD8, and MAX1.

Recent advances in plant molecular biology and genetics have identified a set of genes involved in the common symbiosis pathway that play essential roles in fungal signal recognition, activation of cellular responses, and the formation of fungal structures such as arbuscules. These genes, including SYMRK, CCaMK, and CYCLOPS, will be discussed in detail later. Depending on the upregulation or downregulation of these genes in the plant–fungus symbiotic pathway, a higher or lower degree of mycorrhizal dependency can be expected, respectively.





Materials and Methods: This analytical review integrates findings from researchers in this field by utilizing up-to-date studies on the intensity of plant–fungus symbiosis and the degree of mycorrhizal dependency from morphological, physiological, biochemical, and especially molecular dimensions. These studies are accessible through major international databases, including Web of Science and Scopus. In addition, Google Scholar provides an open-access platform for identifying recent publications, while specialized resources such as ScienceDirect, SpringerLink, and Taylor & Francis offer valuable journal articles, book chapters, and authoritative reference works related to mycorrhizal dependency and the underlying mechanisms involved, particularly from a molecular perspective.

Results: The results of numerous studies indicate that the establishment of symbiosis between plants and arbuscular mycorrhizal fungi requires a series of complex physiological, biochemical, and molecular interactions. This process is initiated by the secretion of Myc-LCO signalling molecules by the fungus, which are subsequently recognized by specific receptors in plant roots, including SYMRK (DMI2). Following recognition, the common symbiosis signalling pathway is activated, and the release of calcium ions (Ca²⁺) through DMI1 generates rhythmic calcium oscillations within plant cells. These oscillations function as informational codes that are decoded by the calcium and calmodulin-dependent protein kinase (CCaMK). The calcium signals are then translated into molecular responses, leading to the activation of the transcription factor CYCLOPS and the regulation of symbiosis-related gene expression, ultimately resulting in arbuscule formation. Any disruption or interruption in this signalling pathway may prevent arbuscule development, particularly in non-host plants. The degree of plant dependence on mycorrhizal fungi is influenced by a range of factors, including plant, fungal, and environmental characteristics. Among environmental factors, abiotic stresses such as drought play a critical role. Under drought conditions, the influx of Ca²⁺ into the plant cell cytosol decreases, leading to disturbances in cellular ion homeostasis. However, the presence of mycorrhizal fungi enhances the expression of calcium-sensing and signalling genes, including CBLs, CDPKs, and MAPKs, thereby activating Ca²⁺-dependent signalling pathways. This process regulates and reinforces the increase in cytosolic Ca²⁺ concentration, which acts as an important secondary messenger during the establishment and maintenance of mycorrhizal symbiosis. Furthermore, numerous studies have demonstrated a significant relationship between mycorrhizal dependency and root morphological characteristics. In general, plant species possessing a magnolioid root architecture exhibit a greater degree of dependence on arbuscular mycorrhizal fungi than species with non-magnolioid root systems.

Conclusions: Although many plant species can establish symbiosis with mycorrhizal fungi, the degree of dependency and effectiveness of this association varies considerably. These differences are largely determined by the complexity of signalling pathways and the regulation of symbiosis-related genes. A deeper understanding of the underlying molecular and cellular mechanisms is essential for improving mycorrhizal symbiosis, although significant challenges remain in its practical application.