Hydroxamate siderophores of root nodule bacteria

Sixty strains of root nodule bacteria were screened for siderophore production in low-iron broth, among them 40 strains from the Australian Inoculants Research and Control Service (AIRCS) which are the current commercial inoculants used in the pulse and legume pasture industries in Australia. Eleven new siderophore-producing strains were recognised including Sinorhizobium meliloti (WSM826, WSM352, SU47), Rhizobium leguminosarum biovar viciae (WU163, MNF3841, SU391), Rhizobium leguminosarum biovar trifolii (CB782, CC2483g, CC283b) and Rhizobium tropici (WSM1385, CB3060). Siderophores were identified by chemical characterisation for catecholate or hydroxamate, spectral studies, isoelectrofocusing and siderophore-mediated iron-uptake studies. The S. meliloti strains all produced dihydroxamate siderophores. Other siderophore-producing rhizobia, with the exception of R. tropici CB306c, excreted trihydroxamate-type siderophores. No bradyrhizobia were Chromazurol S-positive. 59Fe uptake studies revealed that all strains transported iron complexed to citrate. The sinorhizobia took up 5–10-fold more iron from dihydroxamate than trihydroxamate siderophores. Conversely, other rhizobia and the slow-growing bradyrhizobia transported iron complexed to trihydroxamates at rates 2–5 fold those of dihydroxamate siderophores. Rhizobactin 1021 was excreted by S. meliloti strains 1021, Rm2011 and SU47 and vicibactin by seven strains of R. leguminosarum (bv. viciae and bv. trifolii).