Arbuscular mycorrhizal fungi in alleviation of salt stress: a review

Abstract

Background: Salt stress has become a major threat to plant growth and productivity. Arbuscular mycorrhizal fungi colonize plant root systems and modulate plant growth in various ways.

Scope: This review addresses the significance of arbuscular mycorrhiza in alleviation of salt stress and their beneficial effects on plant growth and productivity. It also focuses on recent progress in unravelling biochemical, physiological and molecular mechanisms in mycorrhizal plants to alleviate salt stress.

Conclusions: The role of arbuscular mycorrhizal fungi in alleviating salt stress is well documented. This paper reviews the mechanisms arbuscular mycorrhizal fungi employ to enhance the salt tolerance of host plants such as enhanced nutrient acquisition (P, N, Mg and Ca), maintenance of the K(+) : Na(+) ratio, biochemical changes (accumulation of proline, betaines, polyamines, carbohydrates and antioxidants), physiological changes (photosynthetic efficiency, relative permeability, water status, abscissic acid accumulation, nodulation and nitrogen fixation), molecular changes (the expression of genes: PIP, Na(+)/H(+) antiporters, Lsnced, Lslea and LsP5CS) and ultra-structural changes. Theis review identifies certain lesser explored areas such as molecular and ultra-structural changes where further research is needed for better understanding of symbiosis with reference to salt stress for optimum usage of this technology in the field on a large scale. This review paper gives useful benchmark information for the development and prioritization of future research programmes.

Fig. 1

Effect of salt stress on plants. Salt stress causes physiological drought to plants, imbalance in nutrient composition and excessive toxicity due to Na and Cl ions thereby leading to reduction in osmotic potential of plants, disruption of cell organelles and their metabolism. These ultimately affect plant growth and reduce the yield.

Fig. 2

The intricate functioning of arbuscular mycorrhizal (AM) fungi in ameliorating salt stress in plants. In AM symbiosis, the fungus forms an appressorium (ap) on the root surface and enters the root cortex by extending its hyphae (h). The hyphae form arbuscules (a) and vesicles (v) in the cortex. Salinity deprives plants of the basic requirements of water and nutrients, causing physiological drought and a decrease in osmotic potential accompanied by nutrient deficiency, rendering plants weak and unproductive. Arbuscular mycorrhiza help plants in salt stress by improving water and nutrient uptake: a decrease in osmotic potential is countered by increasing accumulation of osmolytes, and water-use efficiency, photosynthesis and antioxidant production (to scavenge ROS) is more efficient in salt-stressed plants in the presence of AMF (see text).