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

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.