Biological Role of Motility in the Plant Pathogenic Bacterium Ralstonia Solanacearum

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Most plant-associated bacteria can move, even though motility uses much energy and may elicit host defense responses. The ability to move is known to help bacteria pathogenic to animals, but the role of bacterial motility in plant pathogenesis is largely unexplored. Two well-characterized nonmotile mutants of the plant pathogen, Ralstonia solanacearum, caused significantly less disease on tomato in a realistic soil invasion virulence assay, showing that this trait does contribute to virulence. The goal of this project is to combine genetic, histopathological, and ecological approaches to comprehensively describe the role of motility in the life history of this plant-associated bacterium. The first objective is to determine how motility contributes to organismal fitness. The investigators will microscopically examine tomato roots infected with fluorescent motile or nonmotile bacteria to compare the behavior of these strains during the early stages of pathogenesis. These experiments will test the hypothesis that motility plays a key role early in host invasion and colonization. To test the hypothesis that motility late in disease development is actively disadvantageous, the investigators will measure the virulence of a regulatory mutant that is always motile. Because motility is likely to affect bacterial fitness outside the plant as well, they will compare soil survival of motile and nonmotile strains to test the hypothesis that motility increases saprophytic survival in soil and decaying plant material. The second objective is to separate the effects of bacterial movement from those of the flagella themselves, because flagella may mediate attachment to plant surfaces and recognition by the host. To determine if flagella attach bacteria to plant surfaces, investigators will compare attachment of non-flagellated and flagellated but paralyzed mutants to tomato roots. Analysis of plant biochemical responses and defense gene expression will reveal whether host plants recognize purified R. solanacearum flagella or their component flagellins. The third objective is to explore how R. solanacearum regulates its motility. This trait is likely controlled by a complex hierarchy involving an internally responsive flagellar regulon, known virulence factor regulators, and host plant signals. Investigators will use reporter gene fusions and regulatory mutants to define motility gene regulation in this species. Because R. solanacearum motility in culture does not reflect the behavior of the pathogen in its natural habitat, gene expression studies will be conducted in planta. Collectively, the experiments proposed here will yield an integrated understanding of how the ability to move helps, and possibly hinders, this bacterium throughout its complex life cycle.