Plant tolerance to stress can be improved by the presence of mycorrhiza. Cicatelli et al. study the response of poplar, Populus alba, with or without pre-inoculation with arbuscular mycorrhizal fungi (AMF) to soil contaminated with copper and Zn and find that on polluted soli the AMF restore plant biomass despite higher Cu and Zn accumulation in plant organs, especially the roots. The enhanced stress tolerance appears to derive from the transcriptional upregulation of several stress-related genes, and the protective role of polyamines.