Archivio Journal of Plant Pathology

Abstract:

Ralstonia solanacearum is a soil-borne pathogen that causes bacterial wilt in numerous plant species worldwide. In soil and culture media, the pathogen can undergo spontaneous phenotypic conversion (PC) from a pathogenic to a non-pathogenic form. This PC occurs also in Solanum plants and their homogenates. Little is known about the PC mechanism so that, to elucidate it, the culture conditions that influence bacterial PC were investigated using water extract of Solanum toxicarium. The pathogen was incubated in static culture in the water extract at various initial bacterial concentrations and incubation temperatures. PC did not occur during the exponential phase, but the pathogen underwent PC at the 4-5th day after reaching the level of 108 CFU ml-1. However, PC occurred infrequently when the bacterium was grown under conditions that did not allow a high concentration. Thus, R. solanacearum undergoes PC when the bacteria are present in the water extract for a certain period (4-5 days) at a high concentration (108 CFU ml-1). It is thought that the bacteria consume the nutrients in the water extract at the initial stage of culture, and that a starvation state will be reached if the pathogen is present for a certain period at a high concentration. Thus, it is supposed that the pathogen undergoes PC in order to express a phenotype advantageous to growth under nutrient starvation stress. Bacterial PC was not influenced by low temperature stress in the water extract, though it was induced at high temperature.