Metabolomics Reveals Concentration‐Specific Adaptive Mechanisms of Arbuscular Mycorrhizal Fungi in Cadmium Translocation and Detoxification in Arundinoideae ( <i>Phragmites australis</i> )

The symbiosis of arbuscular mycorrhizal fungi (AMF) and Phragmites australis (Pa) is an effective biological strategy for cadmium (Cd) remediation, however, the bioaccumulation and translocation mechanisms underlying this symbiosis remain underexplored. In this study, Cd and nutrient element concentrations in four Pa tissues were analyzed, along with ultrastructure observations and root metabolomics profiling, under different Cd concentrations (1 mg/L, 5 mg/L) and exposure durations (7 days, 30 days). The root metabolomics analysis, in combination with Cd accumulation patterns and ultrastructural observations, provided crucial insights into the biochemical pathways and molecular mechanisms involved in Cd detoxification, nutrient redistribution, and subcellular structural changes in the AMF-Pa symbiotic system. AMF reduced Cd accumulation in all Pa tissues under 1 mg/L Cd for 7 days and in roots under 5 mg/L Cd for 30 days. Conversely, with AMF, Pa accumulated more Cd in the other exposure groups. Under 5 mg/L Cd for 30 days, AMF facilitated Cd translocation from roots to aboveground parts. AMF altered Cu, Zn and P bioaccumulation in old roots and significantly influenced Fe accumulation in roots across all treatments. While 5 mg/L Cd disrupted cellular ultrastructure, AMF inoculation protected intracellular organ integrity and promoted cell wall thickening. This study reveals the dynamic mechanisms by which AMF regulate Cd translocation and accumulation under varying Cd concentrations. Under high Cd concentrations, AMF enhance energy metabolism and chelation, promoting Cd translocation from roots to aerial parts while mitigating Cd toxicity in the endodermis. In contrast, under low Cd concentrations, AMF suppress Cd uptake and promote its immobilization within root tissues by activating amino acid and nucleotide metabolism, reducing Cd translocation to aboveground parts. Additionally, AMF strengthen cell walls through phenylpropanoid biosynthesis, offering protection against Cd toxicity. These findings provide crucial theoretical insights for the application of AMF in phytoremediation of Cd-contaminated soils.