Hydroxamate siderophores produced by Streptomyces acidiscabies E13 bind nickel and promote growth in cowpea (Vigna unguiculata L.) under nickel stress Christian Dimkpa, Ales ˇ Svatos ˇ , Dirk Merten, Georg Bu ¨ chel, and Erika Kothe Abstract: The siderophore-producing ability of nickel-resistant Streptomyces acidiscabies E13 and the role of the elicited siderophores in promoting plant growth under iron and nickel stress are described. Siderophore assays indicated that S. acidiscabies E13 can produce siderophores. Electrospray ionization mass spectrometry (ESI-MS) revealed that the bacte- rium simultaneously produces 3 different hydroxamate siderophores. ESI-MS showed that in addition to iron, all 3 sidero- phores can bind nickel. In vitro plant growth tests were conducted with cowpea (Vigna unguiculata) in the presence and absence of the elicited siderophores. Culture filtrates containing hydroxamate siderophores significantly increased cowpea height and biomass, irrespective of the iron status of the plants, under nickel stress. The presence of reduced iron was found to be high in siderophore-containing treatments in the presence of nickel. Measurements of iron and nickel contents of cowpea roots and shoots indicated that the siderophore-mediated plant growth promotion reported here involves the si- multaneous inhibition of nickel uptake and solubilization and supply of iron to plants. We conclude that hydroxamate side- rophores contained in culture filtrates of S. acidiscabies E13 promoted cowpea growth under nickel contamination by binding iron and nickel, thus playing a dual role of sourcing iron for plant use and protecting against nickel toxicity. Key words: Streptomyces acidiscabies E13, siderophores, cowpea, iron, nickel. Re ´sume ´: La capacite ´ de production de side ´rophores de Streptomyces acidiscabies E13 re ´sistante au nickel, et le ro ˆle joue ´ par les side ´rophores produits dans la promotion de la croissance des plantes sous un stress en fer ou en nickel sont de ´crits. Un essai de de ´tection de side ´rophores a indique ´ que S. acidiscabies E13 peut produire des side ´rophores. La spectrome ´trie de masse par ionisation en mode electrospray (ESI-MS) a re ´ve ´le ´ que la bacte ´rie produit trois hydroxamates side ´rophores simultane ´ment. L’ESI-MS a montre ´ qu’en plus du fer, les trois side ´rophores peuvent lier le nickel. Des tests de croissance de plantes in vitro ont e ´te ´ re ´alise ´s avec la dolique a ` œil noir (Vigna unguiculata) en pre ´sence ou non des side ´rophores pro- duits. Des filtrats de culture contenant les hydroxamates side ´rophores ont augmente ´ significativement la hauteur et la bio- masse des plants de dolique a ` œil noir, sans e ´gard au statut en fer de la plante, sous un stress au nickel. La quantite ´ de fer re ´duit e ´tait e ´leve ´e lors des traitements en pre ´sence de side ´rophores et de nickel. Des mesures des contenus en fer et en nic- kel des racines et des pousses de doliques a ` œil noir ont indique ´ que la promotion de la croissance des plants assure ´e par les side ´rophores de ´crite ici implique l’inhibition simultane ´e de la captation de nickel et la solubilization et l’alimentation en fer des plants. Nous concluons que les hydroxamates side ´rophores contenus dans les filtrats de culture de S. acidiscabies E13 font la promotion de la croissance de la dolique a ` œil noir lors d’une contamination en nickel en liant le fer et le nickel, jouant ainsi un double ro ˆle en permettant l’utilisation de fer par la plante et en la prote ´geant de la toxi- cite ´ du nickel. Mots-cle ´s : Streptomyces acidiscabies E13, side ´rophores, dolique a ` œil noir, fer, nickel. [Traduit par la Re ´daction] Introduction Heavy metals exert different, mostly deleterious, effects on plants growing in contaminated environments. Apart from pH-induced insolubility and thus lack of iron (Fe) in calcareous soils, Fe deficiency symptoms in plants have also been linked to the presence of heavy metals. In this re- gard, nickel (Ni), for example, could affect Fe uptake in di- cotyledonous plants by either inhibiting Fe(III) reduction or by competing with Fe(II) at root uptake sites (Alca ´ntara et al. 1994). Moreover, it was hypothesized that Fe can form nonusable polymers with specific contaminating heavy met- als (Davis et al. 1971). To circumvent Fe deficiency, dicoty- ledonous plants release phenolic compounds and extrude Received 24 September 2007. Revision received 21 November 2007. Accepted 23 November 2007. Published on the NRC Research Press Web site at cjm.nrc.ca on 15 February 2008. C. Dimkpa 1 and E. Kothe. Microbial Phytopathology Group, Institute of Microbiology, Friedrich Schiller University, Neugasse 25, 07743 Jena, Germany. A. Svatos ˇ. Mass Spectrometry Research Group, Max Planck Institute for Chemical Ecology, Hans-Knoell-Str. 8, 07745 Jena, Germany. D. Merten and G. Bu ¨ chel. Applied Geology, Institute of Earth Sciences, Friedrich Schiller University, 07749 Jena, Germany. 1 Corresponding author (e-mail: C.Dimkpa@uni-jena.de). 163 Can. J. Microbiol. 54: 163–172 (2008) doi:10.1139/W07-130 # 2008 NRC Canada