COMPARATIVE-STUDY OF SIDEROPHORE-LIKE ACTIVITY OF RHIZOBIUM-PHASEOLI AND PSEUDOMONAS-FLUORESCENS

Two strains of Rhizobium phaseoli (CP Mex. 46 and CP Mex. 3) and five of Pseudomonas fluorescens (A3, A5, A9, A10, and A12) were compared for their ability to inhibit the growth of six phytopathogenic bacterial strains on iron-rich and iron-deficient media. The strains CP Mex. 46 and CP Mex. 3 inhibited five and two of the phytopathogenic strains respectively, but only on the iron-deficient medium. Each displayed their highest inhibitory activity against Xanthomonas campestris CBP 123. Similarly, Agrobacterium tumefaciens was inhibited by the strains A3, A5, A9, and X. campestris by A 10 and A12. In all these cases it was inferred that the bacterial interference was mediated by competition for iron through excretion of siderophores. In addition Corynebactererium fscians was inhibited by A3; X. campestris pv phaseoli CBP 123 by A5 and A10; CBP 140 by A5, A9, and A10; and CBP 147 by A3 and A10; however, in these particular cases other mechanisms other than competition for iron may have been involved, since the inhibition was also clearly detected on the iron-rich medium as well. Addition of spent media from cultures of P. fluorescens to low-iron cultures of Phaseolus vulgaris plantlets regenerated in vitro induced the on set of chlorosis 10 days earlier than exhibited by control plants.