Culturable bacteria from Zn- and Cd-accumulating Salix caprea with differential effects on plant growth and heavy metal availability.

Aims: To characterize bacteria associated with Zn ⁄ Cd-accumulating Salix caprea regarding their potential to support heavy metal phytoextraction. Methods and Results: Three different media allowed the isolation of 44 rhizosphere strains and 44 endophytes, resistant to Zn ⁄Cd and mostly affiliated with Proteobacteria, Actinobacteria and Bacteroidetes ⁄ Chlorobi. 1-Aminocyclopropane- 1-carboxylic acid deaminase (ACCD), indole acetic acid and siderophore production were detected in 41, 23 and 50% of the rhizosphere isolates and in 9, 55 and 2% of the endophytes, respectively. Fifteen rhizosphere bacteria and five endophytes were further tested for the production of metal-mobilizing metabolites by extracting contaminated soil with filtrates from liquid cultures. Four Actinobacteria mobilized Zn and ⁄ or Cd. The other strains immobilized Cd or both metals. An ACCD- and siderophore-producing, Zn ⁄ Cd-immobilizing rhizosphere isolate (Burkholderia sp.) and a Zn ⁄ Cd-mobilizing Actinobacterium endophyte were inoculated onto S. caprea. The rhizosphere isolate reduced metal uptake in roots, whereas the endophyte enhanced metal accumulation in leaves. Plant growth was not promoted. Conclusions: Metal mobilization experiments predicted bacterial effects on S. caprea more reliably than standard tests for plant growth-promoting activities. Significance and Impact of the Study: Bacteria, particularly Actinobacteria, associated with heavy metal-accumulating Salix have the potential to increase metal uptake, which can be predicted by mobilization experiments and may be applicable in phytoremediation.