UTILIZATION OF HETEROLOGOUS SIDEROPHORES AND RHIZOSPHERE COMPETENCE OF FLUORESCENT PSEUDOMONAS SPP

In this study, the potential of different Pseudomonas strains to utili ze heterologous siderophores was compared with their competitiveness i n the rhizosphere of radish. This issue was investigated in interactio ns between Pseudomonas putida WCS358 and Pseudomonas fluorescens WCS37 4 and in interactions between strain WCS358 and eight indigenous Pseud omonas strains capable of utilizing pseudobactin 358. During four succ essive plant growth cycles of radish, strain WCS358 significantly redu ced rhizosphere population densities of the wild-type strain WCS374 by up to 30 times, whereas derivative strain WCS374(pMR), harboring the siderophore receptor PupA for ferric pseudobactin 358, maintained its population density. Studies involving interactions between strain WCS3 58 and eight different indigenous Pseudomonas strains demonstrated tha t despite the ability of these indigenous isolates to utilize pseudoba ctin 358, their rhizosphere population densities were significantly re duced by strain WCS358 by up to 20 times. Moreover, rhizosphere coloni zation by WCS358 was not affected by any of these indigenous strains, even though siderophore-mediated growth inhibition of WCS358 by a majo rity of these strains was demonstrated in a plate bioassay. In conclus ion, it can be stated that siderophore-mediated competition for iron i s a major determinant in interactions between WCS358 and WCS374 in the rhizosphere. Moreover, our findings support the common assumption tha t cloning of siderophore receptor genes from one Pseudomonas strain in to another can confer a competitive advantage in interactions in the r hizosphere. Interactions between WCS358 and the selected indigenous rh izosphere isolates, however, indicate that other traits also contribut e to the rhizosphere competence of fluorescent Pseudomonas spp.