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Infiltration of protein-lipopolysaccharide complexes (prLPS) (250 Cg/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) genes 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

The hypothesis of superoxide anion generation as a direct bactericidal mechanism exercised by hypersensitive tobacco leaf tissue against an incompatible phytopathogenic pseudomonad was tested. The detection of O2− was made directly by reducing Cytochrome c and by producing nitrite from hydroxylamine chloride and indirectly by measuring the death of endophytic bacteria in the presence of superoxide dismutase. At 4 h and 6 h the low net specific reduction of Cytochrome c in the hypersensitive tissue was not confirmed by the specific production of nitrite. At the same time active superoxide dismutase did not give significant protection to the incompatible bacteria as compared to inactive superoxide dismutase. The protection provided by active superoxide dismutase as compared with the controls (bovine serum albumine treated tissue and non-treated tissue) at 4 h and at 6 h was not correlated to its superoxide anion scavenger activity. The results indicated that the death of the incompatible pseudomonad during the hypersensitive reaction was not directly linked to O2− generation.