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 necr otic disease affecting many rosaceous plants and especially pear tree and apple tree. A protein named harpin, secreted through the Hrp secre tion pathway and able to elicit an hypersensitive reaction (HR) on tob acco has recently been isolated. Mutants in hrpN, the gene encoding ha rpin were described as non pathogenic on immature pear fruit and unabl e to elicit an HR on tobacco [Wei et al., 1992; Wei and Beer, 1993]. I n this paper, the phenotype on plant of hrpN mutants was carefully det ermined. hrpN mutants expressed a weak but significant virulence on ho st plants. Furthermore, when infiltrated into tobacco leaf mesophyll, the hrpN mutants elicited varied responses that fluctuated from null r eaction to full necrosis of the infiltrated area. These results show t hat harpin is not absolutely required neither for pathogenicity on hos t plant nor for elicitation of an hypersensitive reaction on tobacco. Furthermore, in all the tests performed, mutant blocked in harpin secr etion remained non pathogenic and unable to elicit an HR on tobacco. T his suggests that factor(s), different from harpin, involved both in p athogenicity and HR eliciting ability is (are) secreted through the Hr p secretion pathway.