Ralstonia solanacearum – a plant pathogen in touch with its host: Response

Dr Denny gives an excellent short overview of the current ideas on how Ralstonia solanacearum interacts with its host. However, he suggests that a diffusible molecule could be responsible for the strong induction of the hrpB regulatory gene, in contrast to our proposition that the inducer is associated with the plant cell wall. Denny points out that a few clumps of the fluorescent bacteria are also found unattached to plant cells, an observation that might arise if hrpB were induced by a diffusible signal released from plant cell walls by degradative enzymes secreted by the pathogen. Two arguments, however, led us to reject this hypothesis: first, the use of a medium conditioned by co-cultivation of plant cells and bacteria does not increase the level of hrpB gene transcription compared with a medium conditioned only by plant cells (D. Aldon, unpublished). Second, in the model proposed by Denny, after a long period of co-cultivation the action of pathogen-secreted polygalacturonases or endoglucanases should make this plant signal freely diffusible, thereby leading to the induction of hrpB in the majority of bacteria in the medium. However, even after 16 h of co-cultivation, we still observe two distinct bacterial populations: one attached to plant cell surfaces and displaying strong hrpB gene expression, and the other non-attached and remaining mostly uninduced.