Arbuscular mycorrhizal fungi enhance Cd phytoremediation in C4 hyperaccumulator Amaranthus hybridus by modulating photosynthetic and metabolic pathways

Phytoremediation efficiency in cadmium (Cd)-contaminated soils is often constrained by a trade-off between biomass production and stress tolerance. Amaranthus hybridus , a fast-growing C 4 hyperaccumulator and industrial crop, holds promise for coupling metal extraction with biomass valorization. However, the physiological and metabolic mechanisms by which arbuscular mycorrhizal fungi (AMF) balance Cd accumulation and detoxification remain poorly understood, particularly for dual inoculation strategies. Here, we investigated the effects of single and dual inoculation with Funneliformis mosseae (FM) and Rhizophagus intraradices (RI) on photosynthesis and Cd detoxification in A. hybridus under Cd stress (30 mg kg −1 ), integrating physiological, biochemical, and metabolomic analyses. RI enhanced transpiration rate, light saturation point, Rubisco activity, abscisic acid levels, and superoxide dismutase activity, thereby improving overall stress tolerance. In contrast, FM preferentially elevated phosphoenolpyruvate carboxylase activity and chlorophyll content, while reducing Cd toxicity through growth dilution and enhanced root Cd retention, coupled with suppression of lipid peroxidation via the ascorbic acid-glutathione cycle. Notably, the dual inoculation (FR) exhibits environment‑dependent functional complementation, wherein FM dominates the metabolic response under Cd stress while RI contributes more under non‑stress conditions. Metabolomics revealed that under Cd stress, FR adopted an FM-dominant strategy, redirecting nitrogen metabolism from arginine biosynthesis toward glutathione synthesis and upregulating L-β-lysine to reinforce antioxidants. Collectively, FR coordinately regulated a "C 4 light capture-hyperaccumulation detoxification" module, enhancing photosynthetic efficiency and Cd tolerance. This study provides the first mechanistic insight into AMF-mediated co-adaptation of high photosynthetic efficiency and metal tolerance in a C 4 hyperaccumulator, offering a dual potential for Cd phytoremediation coupled with bioenergy crops valorization. • Amaranthus hybridus as a C4 hyperaccumulator with high Cd accumulation and biomass production. • Funneliformis mosseae and Rhizophagus intraradices synergistically exhibit environment-dependent functional complementation under Cd stress. • AMF inoculation promotes C 4 carbon metabolism and biomass dilution effect under Cd stress. • AMF modulate antioxidant defense, phytohormone (ABA and SL) balance, and global metabolic reprogramming. • AMF redirect nitrogen flux from arginine toward glutathione synthesis, coordinating detoxification and growth.