Localized and systemic effects of arbuscular mycorrhizal fungi on growth, antioxidant defense system, and nutrient uptake of alfalfa under uniform and non-uniform salt stress

Abstract Background and Aims: Salinity causes significant environmental stress restricting plant growth and development, and arbuscular mycorrhizal fungi (AMF) can protect plants against salt stress through different mechanisms. The aim of this study was to explore the local and systemic effects of AMF in alfalfa plants under non-uniform salinity and the mechanisms underlying these effects. Methods: Alfalfa plants were subjected to uniform and non-uniform salt stress using a split-root system, with one or both root compartments inoculated or not inoculated with AMF. Results: We observed that AMF inoculation ameliorated the negative effects of salt stress by enhancing the dry weight, plant growth rate, photosynthesis, and the activities of antioxidant enzymes including CAT, POD, and SOD. These effects contributed to maintaining ionic and nutritive balance and resulted in lower levels of lipid peroxidation and H2O2 contents under both uniform and non-uniform salinity treatments, particularly when the whole root system was inoculated with AMF. Under non-uniform salinity, AMF alleviated oxidative stress in the high-saline root side where it was inoculated, thus playing a key role in alleviating salt-induced damage. Conversely, when AMF was inoculated on the non-saline root side, both root compartments exhibited systemic oxidative defense mechanisms, highlighting the significance of functional equilibrium within the root system in enhancing salt tolerance. Conclusion: The findings indicate that AMF ameliorates the effects of salt stress through distinct antioxidant defense and ion regulation mechanisms under non-uniform salinity. P uptake and ion regulation were more effective in the AMF-inoculated root side, both under uniform and non-uniform salinity conditions.