Microbiological factors affecting the colonization of tomato roots by Ralstonia solanacearum YU1Rif43lux

We investigated the effect of prior colonization of sterile soils by a rang e of soil microbes on the growth of Ralstonia solanacearum YU1Rif43 in an e arlier report (Soil Sci. Plant Nutr., 46, 449-459, 2000). Here, we report t he effect of prior colonization of tomato roots, the host plant of the path ogen, by soil microbes on succeeding colonization by the pathogen. When the soil microbial community was introduced onto tomato roots, subsequent colo nization by YU1Rif43lux, a lux-marked mutant of YU1Rif43, was remarkably su ppressed compared with that in the absence of microorganisms which had prev iously colonized roots (hereafter referred to as prior colonists). Soil bac teria, rather than fungi, were mainly responsible for the suppression. Next , the effect of individual microbial strains on the colonization by YU1Rif4 3lux was examined. Some strains did not show any significant suppressive ef fect on subsequent colonization by YU1Rif43lux, unlike others. All the R. s olanacearum strains used in this study, especially R. solanacearum NPS1 (a weakly virulent mutant of YU1), were effective priorcolonists against colon ization by YU1Rif43lux, that is, they markedly suppressed subsequent coloni zation by YU1Rif43lux. Most of the strains that mere effective priorcolonis ts in soil in the earlier report were also effective at suppressing the col onization of tomato roots by YU1Rif43lus. YU1Rif43lux competed on tomato ro ots with the strains that did not show previously an effect against coloniz ation by YU1Rif43lux, while root colonization by YU1Rif43lux was reduced by co-inoculation with the strains that previously showed a colonization effe ct. These results suggested that soil bacteria were likely to be responsibl e for the suppression of colonization by YU1Rif43lus of tomato roots, and t hat strains belonging to R. solanacearum among individual microbial strains might be the most effective antagonists against colonization by YU1Rif43lu s. Using Tn5-induced derivatives of a strain, Pseudomonas fluorescens MelRC 2, one of the most effective priorcolonists, the contribution of motility, antibiosis, rhizosphere competence to the suppressiveness of P. fluorescens MelRC2 against colonization by YU1Rif43lux were examined.