Localized and systemic abilities of arbuscular mycorrhizal fungi to control growth, antioxidant defenses, and the nutrient uptake of alfalfa under uniform and non-uniform salt stress

Salinity causes significant environmental stress that restricts plant growth and development, and arbuscular mycorrhizal fungi (AMF) can use different mechanisms to protect plants against salt stress. The aim of this study was to explore the local and systemic effects of AMF in alfalfa plants under uniform and non-uniform salinity and the abilities of the AMF that underlie these effects.

Alfalfa (Medicago sativa L.) plants were subjected to uniform (200/200 mM NaCl) and non-uniform (0/200 mM NaCl) salt stress using a split-root system, with one or both root compartments inoculated or not inoculated with the AMF Rhizophagus irregularis.

We observed that the inoculation with AMF ameliorated the negative effects of salt stress by enhancing the dry weight, plant growth rate, photosynthesis, and the activities of antioxidant enzymes, including catalase, peroxidase, and superoxide dismutase. These effects contributed to maintaining an ionic and nutritive balance and resulted in lower levels of lipid peroxidation and contents of H₂O₂ under both uniform and non-uniform salinity treatments, particularly when the whole root system was inoculated with AMF. Under non-uniform salinity, when the high-saline root side was inoculated with AMF, the oxidative defense was restricted to that compartment, and the AMF played a key role in alleviating the damage caused by salt stress. Conversely, when AMF was inoculated on the non-saline root side, both root compartments exhibited systemic oxidative defense mechanisms, which highlighted the significance of functional equilibrium within the root system at enhancing salt tolerance.

The findings indicate that AMF ameliorates the effects of salt stress through distinct antioxidant defenses and ion regulatory mechanisms under non-uniform salinity. Phosphorus uptake and ion regulation were more effective in the AMF-inoculated root side under both the uniform and non-uniform salinity conditions.