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Pseudomonas syringae pv. phaseolicola, a phytopathogenic bacterium, seemed very sensitive in planta to the adverse action of reactive oxygen species (ROS) produced by two chemical systems. The disease symptoms in host plants were also suppressed by ROS. Several other plant pathogenic bacteria (P. syringae pv. pisi, Erwinia amylovora. Xanthomonas campestris pv. pelargonii) as well as P. fluorescens were also sensitive in vitro to the inhibiting or killing action of ROS. It was shown that O2" and H2O2 were produced in our two chemical systems and were involved in the killing action. OH' however was not involved in the adverse action on bacteria of the ROS. Superoxide dismutase and catalase were able to reverse the killing action of ROS. When the EPS slime around bacteria was removed by washing and centrifuging the cells, bacteria were more sensitive to ROS. However, when the cells of EPS" mutants were washed and centrifuged, their sensitivity to the killing action of ROS did not change because the lack of slime around the mutant cells.The EPS" Tn5 mutants of P. syringae pv. phaseolicola and the natural EPS" mutant of E. amylovora were more sensitive to ROS than the wild type strains. These results support the idea that the EPS slime protects bacteria from ROS (O^ and H2O2). Zusammenfassung Die Bedeutung des aus extrazdlularen Polysacchariden (EPS) bestehenden ScUeims phytopathogener Bakterien fiir den Schutz der Zeilen vor reaktiven SauerstofTspeziesDas phytopathogene Bakterium Pseudomonas syringae pv. phaseolicola wurde in planta sehr leicht von reaktiven Sauerstoffspezies (ROS) geschadigt, welche durch zwei chemische Systeme gebildet wurden. Die ROS unterdruckten aucb die Krankheitssymptome von Wirtspflanzen. Verschiedene andere pbytopathogene Bakterien (P. syringae pv. pisi, Erwinia amylovora, Xanthomonas campestris pv. pelargonii) und P. fluorescens wurden auch in vitro von ROS gehemmt oder getotet. Es wurde gezeigt, daB in unseren beiden chemischen Systemen Oi" und H2O2 entstanden und ftir die abtotende Wirkung bedeutsam waren. OH' Radikale waren an der bakterienschadigenden Wirkung der ROS dagegen nicht beteiligt. Superoxid-Dismutase und Katalase konnten die abtotende Wirkung der ROS aufheben, Wurde der die Bakterien umgebende EPS-Schleim durch Waschen und Zentrifugieren der Zellen entfernt., so reagierten die Bakterien sensitiver auf die ROS. Wurden jedoch die Zellen von EPS~-Mutanten gewaschen und zentrifugiert, anderte sich ihre Sensitivitat fur die abtotende Wirkung der ROS aufgrund ihrer fehlenden Schleimhtllle nicbt.Die EPS~-Tn5-Mutanten von P. syringae pv. phaseolicola und die naturlich vorkommende EPS"-Mutante von E. amylovora reagierten im Vergleich zu den WildtypStammen sensitiver auf die ROS. Diese Ergebnisse untermauern die Ansicht, daB der EPS-Schleim die Bakterien vor ROS (O\~ und H2O2) schutzt.

This paper is an overview of a non-specific local early induced resistance (EIR) mechanism, distinct from the incompatible-specific hypersensitive reaction (HR). We have shown that the local induced resistance (LIR) described earlier is not a single and uniform response to pathogen infection, because an early (EIR) and a late form can be distinguished. EIR operates from 3-6 h post-inoculation (hpi) until about 20 hpi, and is inhibited by a short heat-shock or the eukaryotic protein synthesis inhibitor, cycloheximide. In contrast, LIR, which corresponds to the induced resistance forms discovered earlier, requires more time (about 24 h) and intensive illumination to develop, and is effective for a longer period. EIR develops parallel with HR and is sometimes able to prevent it when the induction time of HR is longer than the time required for the development of EIR. It seems that EIR inhibits the metabolism of bacteria and the activity of hrp genes which otherwise are required for the induction of HR. In a compatible host-pathogen relationship the effect of EIR fails to take place. The rapid development of EIR is greatly influenced by temperature and the physiological state of the plant. EIR activates the accumulation of hydrogen peroxide at the bacterial attachment, expressing new peroxidase isoenzymes in the initiated plant tissue. It seems that this is a native general local defence mechanism which can localise foreign organisms even at the penetration site.

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Following the in_ltration of heterologous pathovars of Pseudomonas syrin`ae two localized responses develop in a parallel manner in tobacco leaves] an early form of induced resistance "EIR# and the hypersensitive reaction "HR#[ The EIR inhibits the metabolic activity of in_l! trated bacteria and the leaf tissue remains symptomless whereas HR also inhibits bacteria\ but the leaf tissue shows con~uent necrosis or necrosis of individual plant cells\ depending on the inoculum concentration[ After in_ltration of a heterologous strain the rapid devel! opment of HR usually masks the e}ects of EIR which is developing at the same time[ The aim of this study was to characterize the conditions in which EIR occurs in the absence of HR following the in_ltration of heterologous pathovars of Pseudomonas syrin`ae pathovars[ A study was made of numerous experimental interactions of het! erologous bacteria:tobacco leaves in which the HR induc! tion time of the bacteria and:or the EIR response of the plant were modi_ed[ When "a# the HR induction time was extended by chloramphenicol treatment of bacterial cells or by using transposon mutants "Tn4#\ or "b# the time needed for the development of EIR was shortened "at 29>C or in young leaves# the heterologous strains were unable to induce visible HR[ But when the EIR was suppressed in tobacco leaves by pretreatment with cycloheximide or thermal shock "49>C for 02 s# the HR appeared[ Comparison of results of numerous experi! ments revealed that solely EIR was present if its devel! opment time was shorter than the induction period of HR[ This interpretation was con_rmed by the in_ltration of partially puri_ed harpin Pss into tobacco leaves\ where it caused plant cell death in the presence of EIR[ Zusammenfassung Die symptomlose resistente anstelle der hypersensitiven Reaktion in Tabakbla Ãttern nach In_ltration heterologer Pathovarieta Ãten von Pseudomonas syringae Nach der In_ltation heterologer Pathovarieta Ãten von U[ S[ Copyright Clearance Center Code Statement] 9820Ð0674:88:3697Ð9356 , 03[99:9 Pseudomonas syrin`ae werden in Tabakbla Ãttern lokal zwei Reaktionen parallel induziert\ die fru à he induzierte Resistenz "EIR# und die hypersensitive Reaktion "Hr#[ Die EIR hemmt den Sto}wechsel der in_ltrierten Bakte! rien\ so dass das Blattgewebe symptomfrei bleibt[ Auch die HR hemmt die Bakterien\ aber im Blatt enwickeln sich Ð in Abha Ãngigkeit von der Inokulumskonzentra! tion Ð gro)~a Ãchige Nekrosen\ oder einzelne Blattzellen nekrotisieren[ Nach der In_ltration eines heterologen Stammes maskiert die schnelle Entwicklung der HR meis! tens die Wirkungen der EIR\ die sich gleichzeitig ent! wickelt[ Es war das Ziel dieser Untersuchungen\ die Bedingungen zu charakterisieren\ unter denen sich die EIR bei Nichterscheinen der HR als Folge einer In_ltra! tion heterologer P[ syrin`ae!Pathovarieta Ãten entwickelt[ Deswegen wurden verschiedene Experimente mit hetero! logen Bakterien in Tabakbla Ãttern durchgefu à hrt\ in denen die HR!Induktionsperiode der Bakterien und:oder die Entwicklungszeit fu...