Microbiological factors affecting the colonization of tomato roots byRalstonia solanacearumYU1Rif43lux

We investigated the effect of prior colonization of sterile soils by a range of soil microbes on the growth of Ralstonia solanacearum YUIRif43 in an earlier report (Soil Sci. Plant Nutr., 46, 449–459, 2000). Here, we report the effect of prior colonization of tomato roots, the host plant of the pathogen, by soil microbes on succeeding colonization by the pathogen. When the soil microbial community was introduced onto tomato roots, subsequent colonization by YUIRif431ux, a lux-marked mutant of YUIRif43, was remarkably suppressed compared with that in the absence of microorganisms which had previously colonized roots (hereafter referred to as priorcolonists). Soil bacteria, rather than fungi, were mainly responsible for the suppression. Next, the effect of individual microbial strains on the colonization by YUIRif43lux was examined. Some strains did not show any significant suppressive effect on subsequent colonization by YUIRif43Iux, unlike others. All the R. solanacearum strains used in this study, especially R. solanacearum NPSI (a weakly virulent mutant of YUl), were effective prior colonists against colonization by YUIRif431ux, that is, they markedly suppressed subsequent colonization by YUIRif431ux. Most of the strains that were effective prior colonists in soil in the earlier report were also effective at suppressing the colonization of tomato roots by YUIRif43Iux. YUIRif43lux competed on tomato roots with the strains that did not show previously an effect against colonization by YUIRif431ux, while root colonization by YUlRif43lux was reduced by co-inoculation with the strains that previously showed a colonization effect. These results suggested that soil bacteria were likely to be responsible for the suppression of colonization by YUIRif43lux of tomato roots, and that strains belonging to R. solanacearum among individual microbial strains might be the most effective antagonists against colonization by YUIRif431ux. Using Tn5-induced derivatives of a strain, Pseudomonas fluorescens MelRC2, one of the most effective priorcolonists, the contribution of motility, antibiosis, rhizosphere competence to the suppressiveness of P. fluorescens MelRC2 against colonization by YUIRif43lux were examined.