Root and Leaf-Specific Metabolic Responses of Ryegrass to Arbuscular Mycorrhizal Fungi Under Cadmium Stress

Cadmium (Cd) drastically inhibits plant growth and metabolism, whereas arbuscular mycorrhizal (AM) fungi can enhance plant Cd tolerance through metabolic regulation. To clarify tissue-specific responses, we conducted a pot experiment combined with GC-MS to examine how AM fungi influence root and leaf metabolism of ryegrass (Lolium perenne L.) under different Cd levels. Root and leaf metabolomes diverged substantially in composition and function. In total, 83 metabolites were identified in roots, mainly phenolics, amines, and sugars associated with carbon–nitrogen metabolism and stress-defense pathways, whereas 75 metabolites were identified in leaves, largely related to photosynthetic metabolism. Roots were more sensitive to Cd, showing significant metabolic alterations at Cd ≥ 5 mg·kg−1, including disruption of galactose metabolism, while leaves exhibited notable changes only at Cd ≥ 100 mg·kg−1, with suppression of citrate, L-aspartate, and starch and sucrose metabolism. AM fungi modulated plant metabolism more strongly under Cd stress. Specifically, AM fungi restored Cd-suppressed galactose and glyoxylate/dicarboxylate metabolism in roots, enhanced starch and sucrose metabolism and amino acid pathways in leaves, and increased stress-related amino acids and organic acids in both tissues. Overall, AM fungi substantially alleviated Cd-induced metabolic inhibition, particularly at Cd ≥ 50 mg·kg−1, providing mechanistic insight into AM-enhanced Cd tolerance and supporting the application of AM symbiosis in remediation of Cd-contaminated soils.