ERWINIA-AMYLOVORA HRPN MUTANTS, BLOCKED IN HARPIN SYNTHESIS, EXPRESS A REDUCED VIRULENCE ON HOST PLANTS AND ELICIT VARIABLE HYPERSENSITIVE REACTIONS ON TOBACCO

Erwinia amylovora is the bacterium responsible for fire blight, a necrotic disease affecting many rosaceous plants and especially pear tree and apple tree. A protein named harpin, secreted through the Hrp secretion pathway and able to elicit an hypersensitive reaction (HR) on tobacco has recently been isolated. Mutants in hrpN, the gene encoding harpin were described as non pathogenic on immature pear fruit and unable to elicit an HR on tobacco [Wei et al., 1992; Wei and Beer, 1993]. In this paper, the phenotype on plant of hrpN mutants was carefully determined. hrpN mutants expressed a weak but significant virulence on host plants. Furthermore, when infiltrated into tobacco leaf mesophyll, the hrpN mutants elicited varied responses that fluctuated from null reaction to full necrosis of the infiltrated area. These results show that harpin is not absolutely required neither for pathogenicity on host plant nor for elicitation of an hypersensitive reaction on tobacco. Furthermore, in all the tests performed, mutant blocked in harpin secretion remained non pathogenic and unable to elicit an HR on tobacco. This suggests that factor(s), different from harpin, involved both in pathogenicity and HR eliciting ability is (are) secreted through the Hrp secretion pathway.