title: Involvement of siderophores in the reduction of metal-induced inhibition of auxin synthesis in Streptomyces spp.
abstract: Unlike synthetic metal chelators, microbe-assisted phytoremediation provides plants with natural metal-solubilizing chelators which do not constitute a potential source of environmental pollution. Concurrently with microbial chelators, plant growth promotion can be enhanced through bacterially-produced phytohormones. In this work, the simultaneous production of siderophores and auxins by Streptomyces was studied to gain insight for future application in plant growth and phytoremediation in a metal-contaminated soil. Standard auxin and siderophore detection assays indicated that all of the investigated Streptomyces strains can produce these metabolites simultaneously. However, Al(3+), Cd(2+), Cu(2+), Fe(3+) and Ni(2+), or a combination of Fe(3+) and Cd(2+), and Fe(3+) and Ni(2+) affected auxin production negatively, as revealed by spectrophotometry and gas chromatography-mass spectrometry. This effect was more dramatic in a siderophore-deficient mutant. In contrast, except for Fe, all the metals stimulated siderophore production. Mass spectrometry showed that siderophore and auxin-containing supernatants from a representative Streptomyces species contain three different hydroxamate siderophores, revealing the individual binding responses of these siderophores to Cd(2+) and Ni(2+), and thus, showing their auxin-stimulating effects. We conclude that siderophores promote auxin synthesis in the presence of Al(3+), Cd(2+), Cu(2+) and Ni(2+) by chelating these metals. Chelation makes the metals less able to inhibit the synthesis of auxins, and potentially increases the plant growth-promoting effects of auxins, which in turn enhances the phytoremediation potential of plants.
authors.affiliation: Microbial Phytopathology, Institute of Microbiology, Friedrich Schiller University, Neugasse 25, Jena, Germany. C.Dimkpa@uni-jena.de
authors.forename: Christian O
authors.initials: CO
authors.lastname: Dimkpa
authors.forename: Ales
authors.initials: A
authors.lastname: Svatos
authors.forename: Paulina
authors.initials: P
authors.lastname: Dabrowska
authors.forename: Andre
authors.initials: A
authors.lastname: Schmidt
authors.forename: Wilhelm
authors.initials: W
authors.lastname: Boland
authors.forename: Erika
authors.initials: E
authors.lastname: Kothe
mesh_terms: D000537:Aluminum Oxide / Q000633:toxicity; D002104:Cadmium / Q000633:toxicity; D007210:Indoleacetic Acids / Q000378:metabolism*; D007501:Iron / Q000633:toxicity; D008670:Metals / Q000633:toxicity*; D009532:Nickel / Q000633:toxicity; D017262:Siderophores / Q000378:metabolism*; D013302:Streptomyces / Q000187:drug effects* / Q000378:metabolism
chemical_list: D007210:Indoleacetic Acids; D008670:Metals; D017262:Siderophores; D002104:Cadmium; D009532:Nickel; D007501:Iron; D000537:Aluminum Oxide