Arbuscular Mycorrhizal Fungi—alleviator for salinity stress: A review

Salinity stress is considered one of the most harmful environmental stresses as it decreases over 20% irrigated land crop production globally. Hence, it is imperative to develop salt-tolerant crops in addition to understanding various mechanisms enabling the plant growth under saline stress conditions. Recently, a novel biological approach, “plant-microbe interaction” (PMI) such as arbuscular mycorrhizal (AM) fungi, gained momentum as it aimed at addressing the salinity stress. Arbuscular mycorrhizal fungi colonize the plant root systems and modulate the plant growth in multiple ways. It has been observed that most land plant roots are colonized by AM fungi. During this process AM fungi obtain their main nutrients from the involved plants while mineral nutrients including N, P, K, Ca, and S are provided to the host plant. During the past few decades, broader understanding has advanced towards multifunctional role of AM fungi, particularly, in the mediation of mineral nutrients and alleviation of stress, especially salt stress, in most crop plants. In soil-plant systems these can increase uptake of root phosphorus as well as confer augmented tolerance to salinity with a consequent enhanced growth and yield. Despite the evident anti-stress role and mechanisms described separately, need arises to discuss and analyze these 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. In addition, the mechanisms AM fungi employ to amplify the salt tolerance of host plants by increased nutrient accession (phosphorus, nitrogen, calcium) physiological changes (efficiency of photosynthesis, permeability of cell membranes, water status, and nitrogen fixation), and biochemical changes (inducing the accumulation of different osmolytes like proline, soluble sugars and antioxidants) will be discussed. Furthermore, the review highlights the role of AM fungi on main antiporter Na⁺/H⁺). The AM fungi inoculation improves osmotic adjustment by accumulation of compatible solutes such as proline, soluble sugars and antioxidants. In plants, this inoculation increases the activity of multiple antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase stress. In addition, the AM alleviates the salt-induced APX), and peroxidase (POD), thereby scavenging the reactive oxygen species (ROS) generated under salt deleterious effects by regulating the osmotic balance of the Na⁺/K⁺ ratio through maintaining the lesser Na⁺ and K⁺ under salt stress conditions. Further research is necessary to gather in-depth knowledge on arbuscular AM fungi-associated mechanisms to pave a way for large-scale application of these fungal associations under saline stress conditions, with the main aim of building a healthy, eco-friendly, cost-effective, and sustainable agricultural system.