Heavy metal stress in alders: Tolerance and vulnerability of the actinorhizal symbiosis

•Tolerance of Frankia alni ACN14a to 10 metals was investigated.•The actinorhizal symbiosis could establish under metal stress (Cu, Ni, Zn, Pb, Cd).•Biomass of alders increased with symbiosis, even when exposed to heavy metals.•Allocation of Ni, Mo, Cd, Pb in plant tissues was influenced by symbiotic status.•The actinorhizal symbiosis shows a significant potential for land reclamation. Alders have already demonstrated their potential for the revegetation of both mining and industrial sites. These actinorhizal trees and shrubs and the actinobacteria Frankia associate in a nitrogen-fixing symbiosis which could however be negatively affected by the presence of heavy metals, and accumulate them. In our hydroponic assay with black alders, quantification of the roots and shoots metal concentrations showed that, in the absence of stress, symbiosis increases Mo and Ni root content and simultaneously decreases Mo shoot content. Interestingly, the Mo shoot content also decreases in the presence of Ni, Cu, Pb, Zn and Cd for symbiotic alders. In symbiotic alders, Pb shoot translocation was promoted in presence of Pb. On the other hand, Cd exclusion in symbiotic root tissues was observed with Pb and Cd. In the presence of symbiosis, only Cd and Pb showed translocation into aerial tissues when present in the nutrient solution. Moreover, the translocation of Ni to shoot was prevented by symbiosis in the presence of Cd, Ni and Pb. The hydroponic experiment demonstrated that alders benefit from the symbiosis, producing more biomass (total, root and shoot) than non nodulated alders in control condition, and in the presence of metals (Cu, Ni, Zn, Pb and Cd). Heavy metals did not reduce the nodule numbers (SNN), but the presence of Zn or Cd did reduce nodule allocation. Our study suggests that the Frankia-alder symbiosis is a promising (and a compatible) plant–microorganism association for the revegetation of contaminated sites, with minimal risk of metal dispersion.