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.