At present, drought and soil salinity are among the most severe environmental stresses that affect the growth of plants through marked reduction of water uptake which lowers water potential, leading to osmotic stress. In general, osmotic stress causes a series of morphological, physiological, biochemical, and molecular changes that affect plant performance. Several studies have found that diverse types of soil microorganisms improve plant growth, especially when plants are under stressful conditions. Most important are the arbuscular mycorrhizal fungi (AMF) which form arbuscular mycorrhizas (AM) with approximately 80% of plant species and are present in almost all terrestrial ecosystems. Beyond the well-known role of AM in improving plant nutrient uptake, the contributions of AM to plants coping with osmotic stress merit analysis. With this review, we describe the principal direct and indirect mechanisms by which AM modify plant responses to osmotic stress, highlighting the role of AM in photosynthetic activity, water use efficiency, osmoprotectant production, antioxidant activities, and gene expression. We also discuss the potential for using AMF to improve plant performance under osmotic stress conditions and the lines of research needed to optimize AM use in plant production.

BACKGROUND: Arbuscular mycorrhizal (AM) fungi establish symbioses with most agricultural plants and improves growthunder soil stress conditions. The present study aimed to evaluate the functional contribution of 2 AM fungal inocula (a native consortium isolated from saline soils of the Atacama Desert, 'HMC', and a reference inoculum Claroideoglomus claroideum, 'Cc') on the growth and antioxidant compounds of two cultivars of lettuce (Lactuca sativa cvs. 'Grand Rapids' and 'Lollo Bionda') at increasing salt stress conditions (0, 40, and 80 mmol L -1 NaCl). At 60 days of plant growth, the symbiotic development, biomass production, lipid peroxidation, proline content, antioxidant enzymes, phenolic compound profiles and antioxidant activity were evaluated. RESULTS: The 2 AM inocula differentially colonized the roots of Grand Rapids and Lollo Bionda lettuce plants. The AM symbioses increased proline synthesis and superoxide dismutase, catalase and ascorbate peroxidase activities and diminished phenolic compound synthesis and oxidative damage in lettuce, which was related positively to a higher growth of inoculated plants under salt exposure. The higher concentration of phenolic compounds induced by salinity in non-inoculated plants was associated with high oxidative stress and low fresh biomass production.CONCLUSION: Modulation of salinity stress in lettuce by AM root colonization is a result of changes of antioxidant enzymatic systems that reduce oxidative damage and sustain growth. The application of AM fungi to improve crop production by means of directed inoculation with efficient AM fungal strains may enhance lettuce production on soils plagued with salinity worldwide.

Soil salinity is the biggest problem which hinders the productivity of agricultural crops, causing adverse effects on plant growth and development. In this regard, it has been shown that the arbuscular mycorrhizal fungi (AMF) can establish a symbiosis with most agricultural plants improving water and nutrient absorption under salinity stress conditions. The functional contribution of AMF strains (Claroideoglomus claroideum (Cc) and a native consortium of AMF (HM) isolated from saline soils) on the growth and nutrition of lettuce plants (Lactuca sativa var. longifolia) was evaluated under increasing salt stress conditions (0, 40, and 80 mM NaCl). At 60 days of growth, biomass production, nutrient content (N, P), ions (Ca 2+ , Mg 2+ , Na + , K +), chlorophyll, proline content, and AMF propagules were evaluated. The highest growth was observed in plants inoculated with Cc, which produced a higher percentage of root colonization and hyphal length at all levels of salinity, compared to plants inoculated with HM or non-inoculated plants. These results were directly related to higher biomass production, increased synthesis of proline, increased N uptake, and noticeable changes in ionic relations, based in a diminishing Na + , compared to non-mycorrhizal plants. Our results suggest that this improved ionic balance is due to a filtering effect of AMF structures both in the soil and in the root that prevents the entry of toxic Na + ions, which is important due to the level of lettuce production on saline soils improving the crop by means of directed inoculation with efficient AMF strains.