Effects of bacterial colonization of tomato roots on subsequent colonization by Pseudomonas fluorescens MelRC2Rif

We investigated the effects of root colonization by soil microbes on the subsequent colonization by a bacterial inoculant, Pseudomonas fluorescens MelRC2Rif. When a soil bacterial community was introduced onto tomato roots, subsequent colonization by MelRC2Rif was significantly lowered. However, soil fungi had no suppressive effects, suggesting that bacteria may be involved in the suppression. When individual bacterial strains were examined, most strains except for P. fluorescens showed no significant suppressive effect on subsequent colonization by MelRC2Rif. In contrast, four out of six P. fluorescens strains significantly suppressed subsequent colonization by MelRC2Rif. Gram-negative bacteria, which had little suppressive effect on colonization by MelRC2Rif with individual inoculation, elicited suppression in some combination treatments. Simultaneous inoculation of individual bacterial strains with MelRC2Rif indicated that suppressive effect by these strains occurred only when MelRC2Rif was inoculated later to tomato roots. These results suggest that a less competitive bacterial strain may interfere with the colonization of a competitive strain on tomato rhizospheres. Key words: ecological niche, bacterial colonization, rhizosphere, competition.