A bacterial sensor of plant cell contact controls the transcriptional induction of Ralstonia solanacearum pathogenicity genes

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Abstract
The hrp genes of the plant pathogen Ralstonia solanacearum are key pathogenicity determinants; they encode a type III protein secretion machinery involved in the secretion of mediators of the bacterium–plant interaction. These hrp genes are under the genetic control of the hrpB regulatory gene, expression of which is induced when bacteria are co‐cultivated with plant cell suspensions. In this study, we used hrp–gfp transcriptional fusions to demonstrate that the expression of the hrpB and type III secretion genes is specifically induced in response to the bacterium–plant cell contact. This contact‐dependent induction of hrpB gene expression requires the outer membrane protein PrhA, but not a functional type III secretion apparatus. Genetic evidence indicates that PrhA constitutes the first example of a bacterial receptor for a non‐diffusible signal present in the plant cell wall and which triggers the transcriptional activation of bacterial virulence genes.