Effect of Salinity on Plants and the Role of Arbuscular Mycorrhizal Fungi and Plant Growth-Promoting Rhizobacteria in Alleviation of Salt Stress

Abstract

Of the world’s 1.5 billion hectares of cultivable land, 77 million hectares (5 %) do not favor good yields due to high salt content and 20 % of the irrigated agricultural land is adversely affected by salinity. Salt-affected soils are increasing steadily in all continents, in particular in arid and semiarid areas which cover more than 7 % of the total land surface on earth. Due to the severe impact of salinity to crop production, salt stress is considered a major limiting factor in crop production. Under saline conditions plant growth is severely reduced, mainly due to osmotic stress. Low water potential in the roots causes water deficit within the plant. Toxic concentrations of Na+ and Cl− ions in plants affect cell membrane functions. Moreover, high amounts of these ions also reduce metabolic activities in plants which lead to growth inhibition and injury of the foliage. Furthermore, low nutrient uptake potential of root due to salinity stress causes nutrient imbalance and oxidative stress in plants which impede proper growth and development. Due to the scope of salinity affected soils and the deleterious effects of salinity on crop production, methods to remediate and/or improve crop production in salinity impacted soils are necessary. Management of soil salinity through land reclamation or improved techniques of irrigation provides only short-term solution and often expensive. Other methods like plant breeding for salt tolerance have been difficult and slow. The nature of salt tolerance traits and problems in developing appropriate testing environment makes it difficult to obtain. Microorganisms present in the rhizosphere soil play a vital role in improving plant growth and soil fertility. Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs that form symbiotic or mutualistic associations with roots of about 80 % of plant species. AMF widely exist in salt-affected soils and they infect the plant root cortical cells. AMF form arbuscules inside the cortical cells and utilize them as nutrient exchange sites. These also form vesicles between the cortical cells where all nutrients are stored. Under soil saline conditions, AMF extend their extraradical hyphae into non-rhizosphere soil to uptake more nutrients. Symbioses of halotolerant bacteria with plant roots also help plants withstand salinity stress. Halotolerant bacteria possess plant growth-promoting characters which may improve plant growth and nutrient uptake. The utilization of microorganisms is an economical and environmental friendly approach to alleviate stress in plants cultivated in salt-impacted agricultural fields.