Arbuscular mycorrhizal fungi for salinity stress: Anti-stress role and mechanisms

Salinity stress is considered one of the most harmful environmental plant stresses, as it reduces irrigated land crop production by over 20% worldwide. Hence, it is imperative to develop salt-tolerant crops in addition to understanding various mechanisms enabling plant growth under saline stress conditions. Recently, a novel biological approach that aims to address salinity stress has gained momentum, which involves the use of arbuscular mycorrhizal (AM) fungi in plant-microbe interactions. It has been observed that most terrestrial plant root systems are colonized by AM fungi, which modulate plant growth in multiple ways. In such interactions, AM fungi obtain organic compounds from the host plant while providing mineral nutrients, including nitrogen, phosphorus, potassium, calcium, and sulfur, to the host plant. Over recent decades, our understanding of the multifunctional roles played by AM fungi has been broadened and advanced, particularly regarding the mediation of mineral nutrients and the alleviation of stress (especially salt stress) in most crop plants. Increased uptake of phosphorus and augmented tolerance to salinity result in enhanced plant growth and yield. The evident anti-stress role of AM fungi and related mechanisms have been described separately, though they need to be analyzed and discussed together. Therefore, the present review addresses the major role of AM fungi in mitigating salt stress and their beneficial effects on plant growth and productivity. The mechanisms employed by AM fungi to amplify the salt tolerance of host plants by increased nutrient accession (e.g., phosphorus, nitrogen, and calcium), physiological changes (e.g., photosynthetic efficiency, cell membrane permeability, water status, and nitrogen fixation), and biochemical changes (e.g., the accumulation of different osmolytes such as proline and soluble sugars) are also discussed. Furthermore, this review highlights the role of AM fungi in the Na⁺/H⁺ antiporters. In plants, AM fungi inoculation increases the activities of multiple antioxidant enzymes, including superoxide dismutase, catalase, and peroxidase, which scavenge reactive oxygen species and relieve salt stress. In addition, AM fungi regulate the Na⁺/K⁺ ratio to maintain osmotic balance under salt stress. Further research is needed to gather in-depth knowledge about AM fungi-associated mechanisms to pave a way for the large-scale application of these fungal associations under saline stress conditions, with the main aim of building healthy, eco-friendly, cost-effective, and sustainable agricultural systems.