Role Of Proteins Encoded By The Pseudomonas Solanacearum Hrp Regulon In The Control Of Plant-Bacteria Interactions

Like most other Gram-negative phytopathogenic bacteria, P. solanacearum possesses a cluster of genes, called hrp genes, which are key determinants of the plant-bacteria interactions. In several bacteria, this hrp gene cluster has been shown to be involved in the secretion of proteins which are active on plants (Wei and Beer, 1993; He et al, 1993; Arlat et al, 1994). In P. solanacearum, the hrp gene cluster consists of seven transcription units, genes encoded by five of them being required for the secretion of the PopAl protein that is able to induce HR-like necrosis on plants resistant to P. solanacearum (Arlat et al, 1994). The complete sequence of these five transcription units revealed the presence of 20 putative proteins, several of them showing homologies with proteins involved in protein secretion pathways (for a review Van Gijsegem et al,1993) but also with proteins involved in flagellum biogenesis. Here we propose a model which incorporates information provided by those homologies concerning the role of the Hrp proteins. We also present recent data towards the identification of additional genes controlling plant bacteria interactions and progress made to investigate the possibility that PopAl might act as the product of an avirulence gene.