Arbuscular mycorrhizal symbiosis enhances the cadmium stress tolerance of Medicago sativa in association with regulation of the aminolevulinic acid pathway.

Arbuscular mycorrhizal (AM) fungi can effectively enhance the photosynthesis and antioxidant capacity of plants under cadmium stress; however, the underlying regulatory mechanisms remain poorly understood. This study focuses on the 5-aminolevulinic acid (ALA) pathway, which regulates chlorophyll and heme synthesis in plants, to investigate its regulation by AM fungi under cadmium stress, using Rhizophagus irregularis and alfalfa (Medicago sativa) as the research materials. Results showed that AM symbiosis enhanced the osmotic regulation ability of alfalfa under Cd stress by increasing proline and soluble sugar content, improving root activity and morphological parameters to enhance water status of alfalfa. Additionally, AM symbiosis reduced Cd migration from roots to shoots in alfalfa, alleviating oxidative damage by reducing H2O2, O2·−, and MDA content. AM symbiosis also bolstered alfalfa’s photosynthesis by enhancing gas exchange parameters and chlorophyll fluorescence parameters, as well as enhancing antioxidant capacity via increased SOD, CAT, and POD activity. Furthermore, in the ALA pathway, AM symbiosis promoted Proto IX chelation in alfalfa under Cd stress, increasing ALA, PBG, Uro III, Mg-Proto IX, Pchl, chlorophyll, and heme concentrations. It also upregulated transcription levels of MsHO1, MsHEMA1, MsMCHI, MsPOR and MsFECH in ALA pathway. The findings suggest that AM fungi regulate the ALA pathway in M. sativa under cadmium stress, thereby promoting chlorophyll and heme synthesis, which may represent a potential regulatory mechanism for enhancing photosynthesis and antioxidant capacity. This provides new insights into understanding how AM fungi improve plant cadmium tolerance.