PRHA CONTROLS A NOVEL REGULATORY PATHWAY REQUIRED FOR THE SPECIFIC INDUCTION OF RALSTONIA-SOLANACEARUM HRP GENES IN THE PRESENCE OF PLANT-CELLS

The Ralstonia solanacearum hrp gene cluster is organized in five trans criptional units. Expression of transcriptional units 2, 3 and 4 is in duced in minimal medium and depends on the hrp regulatory gene hrpB, w hich belongs to unit 1. This regulatory gene also controls the express ion of genes, such as popA, located to the left of the hrp cluster. He re, we show that, upon co-culture with Arabidopsis thaliana and tomato cell suspensions, the expression of the hrp transcriptional units 1, 2, 3 and 4 is induced 10- to 20-fold more than in minimal medium. This induction is not triggered by diffusible signals but requires the pre sence of plant cells. Moreover, we show that this specific plant cell induction of hrp genes is controlled by a gene, called prhA (plant reg ulator of hrp genes), located next to popA. This gene codes for a puta tive protein of 770 amino acids, which shows similarities with TonB-de pendent outer membrane siderophore receptors. Expression of prhA and h rp genes is not regulated by iron status, and we postulate that iron i s not the signal sensed by PrhA. In prhA mutants, the induction of hrp B and other hrp genes is abolished in cc-culture with Arabidopsis cell s, partially reduced in co-culture with tomato cells and not modified in minimal medium. prhA mutants are hypoaggressive on Arabidopsis (acc essions Col-0 and Col-5) but remain fully pathogenic on tomato plants, suggesting that the co-culture assays mimic the in planta conditions. A model suggesting that PrhA is a receptor for plant specific signals at the top of a novel hrp regulatory pathway is discussed.