Suppression of colonization of the pathogenic strain and induction of resistance in tomato are involved in the biocontrol of bacterial wilt by using a phenotypic conversion mutant of <i>Ralstonia pseudosolanacearum</i>

<i>Ralstonia pseudosolanacearum</i> causes bacterial wilt in plants and undergoes spontaneous phenotypic conversion (PC) from a wild-type pathogenic to a non-pathogenic form. Pre-inoculation of plants with PC mutants suppressed bacterial wilt after challenge with pathogenic strains. In this study, we investigated the colonization of pathogenic strains and PC mutants and induction of disease defense response in tomato plants as the mechanisms of biological control against bacterial wilt in plants by the PC mutants. We created a PC mutant labelled with a red fluorescent protein gene (<i>DsRed2</i>) and a pathogenic strain labelled with a green fluorescent protein gene (<i>gfp</i>) and observed the colonization of both strains in roots under a fluorescence microscope. In plants with disease suppressed, the PC mutants colonized the roots at a high density, and colonization of pathogenic strains was suppressed. Conversely, in plants in which the disease was not suppressed, the colonization of PC mutants in the roots was low, and the colonization of pathogenic strains could not be suppressed. In tomato plants inoculated with the PC mutant, the expression levels of pathogenesis-related (PR) protein genes <i>PR-2b</i> and <i>PR-3a</i> were significantly increased in both the roots and leaves compared to those in the control plants. However, several gene expressions including <i>PR-1</i>, <i>PR-2a</i>, <i>PR-4, PR-5</i>, and defense-related enzymes induced by inoculation with PC mutant indicated contrasting patterns in leaves and roots. These results suggested that the biocontrol of bacterial wilt by PC mutant is related to the suppression of colonization of pathogenic strains and induction of resistance in plants.