Altered expression of Erwinia amylovora hrp genes in tobacco pretreated with bacterial protein-lipopolysaccharides

Infiltration of protein-lipopolysaccharide complexes (pr LPS) (250 μg/ml) of Pseudomonas syringae pv. aptata into tobacco leaves protected against subsequent elicitation of the hypersensitive response (HR) by Erwinia amylovora. The effect of prLPS on the expression of E. amylovora hrp (hypersensitive response and pathogenicity) gencs was tested in protected tobacco leaves and in a defined medium in which hrp genes were highly expressed. Two,E. amylovora hrp loci transcriptionally fused with the Escherichia coli β-glucuronidase (Gus) coding sequence were used as chromosomal reporter genes. The prLPS treatment did not affect hrp gene expression of Hrp- mutants both in planta and in vitro, so the effect of prLPS treatment on hrp gene expression of two E. amylovora Hrp+ transformants was assayed during HR development in unprotected tissue and during the same time in prLPS protected tissue. A plasmid containing the same Gus fusions previously located in the chromosome was introduced into a wild-type strain of E. amylovora. Gus activity was significantly lower in protected tissue. We suggest that HR inhibition by prLPS treatment requires the entire hrp gene cluster in order for the bacteria to send a signal to the plant. which, in turn, inhibits the expression of hrp genes.