Siderophore activity in Rhizobium species isolated from different legumes

Rhizobium strains isolated from nodules of the different legumes including wild-growing plants were examined for their siderophore activity. Fifteen of the 84 screened rhizobial strains were able to grow under conditions of limited iron supply. Nine of them gave orange halos in the assay with Chrom azurol S. Among these strains were Rhizobium sp. (Ononis) and Rhizobium (Genista), producing hydroxamates and phenolates. These compounds could promote the growth of siderophore-negative bacteria on iron-deficient media. The results imply that the hydroxamates from G1 and O1 strains may belong to the monohydroxamate class of siderophores.