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We investigated how arbuscular mycorrhizal symbiosis can alter trace element uptake, distribution and toxicity in plants by examining some of the molecular mechanisms behind Populus trichocarpa tolerance to Cd and Zn, and the effects of AMF in metal homeostasis. Plants were grown under Cd and Zn contamination, with and without Rhizophagus irregularis inoculation. We determined organ metal concentrations, the expression of genes involved in trace element homeostasis, and the function of metallothionein PtMT2b by heterologous expression in yeast. P. trichocarpa was highly tolerant to both elements, with AMF increasing Zn accumulation. AMF altered the partitioning of Cd, but maintained the same patterns for Zn, indicating that despite being geochemically similar and carried mostly by the same transporters, the nutrient metal (Zn) is handled differently from the non-essential metal (Cd). High Zn and Cd …