EXPRESSION OF THE PSEUDOMONAS-SYRINGAE AVIRULENCE PROTEIN AVRB IN PLANT-CELLS ALLEVIATES ITS DEPENDENCE ON THE HYPERSENSITIVE RESPONSE AND PATHOGENICITY (HRP) SECRETION SYSTEM IN ELICITING GENOTYPE-SPECIFIC HYPERSENSITIVE CELL-DEATH

The nonpathogenic bacteria Pseudomonas fluorescens and Escherichia col i can elicit a genotype-specific hypersensitive response (HR) in plant s if they express both the HR and pathogenesis (Hrp) protein secretion system and the HrpZ harpin from Fl syringae pv syringae 61 and a Fl s yringae avirulence (avr) gene whose presence is recognized by a corres ponding disease resistance gene in the plant. We have found that the r ecognition event appears to require transfer of the Avr protein into t he plant cell, Elicitation of a genotype-specific HR was observed with avrB(+) Fl fluorescens in soybean and Arabidopsis plants carrying res istance genes RPG1 and RPM1, respectively, and with avrPto(+) E. coli in tomato plants carrying resistance gene PTO, but only if the Hrp sec retion system, HrpZ, and the appropriate Avr proteins were produced in the same bacterial cell. The failure of avrB hyperexpression and exog enous AvrB or HrpZ to alleviate these requirements in soybean and Arab idopsis suggests that the site of AvrB action is not in the bacterial cell or plant apoplast, An Arabidopsis rps3 (rpm1) glabrous1 mutant wa s transformed with constructs expressing avrB and was crossed with an Arabidopsis ecotype Columbia (RPM1 GLABROUS1) plant. F-1 seedlings (id entified by their kanamycin-resistant, pubescent phenotype) exhibited extensive necrosis on cotyledon leaves 10 days postgermination. Ecotyp e Columbia and rps3-1 leaves biolistically cobombarded with plasmids e xpressing the beta-glucuronidase (GUS) gene and avrB failed to produce GUS activity (indicative of cell death) only when RPM1 and avrB were present in the leaf. Thus, both stable and transient expression of avr B in Arabidopsis resulted in RPM1-dependent necrosis, and the only dem onstrable site of action for AvrB was inside plant cells.