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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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Summary Cell wall peroxidases and plasma membrane‐localized NADPH oxidases are considered to be the main sources of the apoplastic oxidative burst in plants attacked by microbial pathogens. In spite of this established doctrine, approaches attempting a comparative, side‐by‐side analysis of the functions of extracellular reactive oxygen species (ROS) generated by the two enzymatic sources are scarce. Previously, we have reported the role of Arabidopsis NADPH oxidase RBOHD (respiratory burst oxidase homologue D) in plants challenged with the necrotrophic fungus Alternaria brassicicola . Here, we present results on the activity of apoplastic class III peroxidases PRX33 ( At3g49110 ) and PRX34 ( At3g49120 ) investigated in the same Arabidopsis – Alternaria pathosystem. ROS generated by Arabidopsis peroxidases PRX33 and PRX34 increase the necrotic symptoms and colonization success of A. brassicicola . In addition, the knockdown of PRX33 and PRX34 transcript levels leads to a reduced number of host cells showing an extracellular burst of ROS after inoculation with A. brassicicola . Our results also reveal an age‐dependent transcript distribution of ROS‐producing peroxidase and NADPH oxidase enzymes, and some potential new components of the RBOHD, PRX33 and PRX34 signalling networks.

Increasing evidence indicates that plants, like animals, use basal resistance (BR), a component of the innate immune system, to defend themselves against foreign organisms. Contrary to the hypersensitive reaction (HR)-type cell death, recognition in the case of BR is unspecific, as intruders are recognised based on their common molecular patterns. Induction of BR is not associated with visible symptoms, in contrast to the HR-type cell death. To analyse the early events of BR in tobacco plants we have carried out a subtractive hybridisation between leaves treated with the HR-negative mutant strain Pseudomonas syringae pv. syringae 61 hrcC and non-treated control leaves. Random sequencing from the 304 EBR clones yielded 20 unique EST-s. Real-time PCR has proved that 8 out of 10 clones are activated during BR. Six of these EST-s were further analyzed. Gene expression patterns in a time course showed early peaks of most selected genes at 3-12 h after inoculation (hpi), which coincided with the development-time of BR. Upon treatments with different types of bacteria we found that incompatible pathogens, their hrp mutants, as well as non-pathogens induce high levels of expression while virulent pathogens induce only a limited gene-expression. Plant signal molecules like salicylic acid, methyl jasmonate, ethylene and spermine, known to be involved in plant defense were not able to induce the investigated genes, therefore, an unknown signalling mechanism is expected to operate in BR. In summary, we have identified representative genes associated with BR and have established important features of BR by analysing gene-expression patterns.

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...

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The present study demonstrate that in tobacco leaves the diaminobenzidine (DAB) and 2¢,7¢-dichlorofluorescein diacetate (DCFH-DA) staining is a useful indicator of the basal (also known as general or innate) defence-associated reactions, especially of the early developing form of basal resistance (EBR). DAB and DCFH-DA, in the presence of H 2 O 2 and peroxidase converts to a brown polymer and fluorescent DCF respectively. In the present study, the hypersensitive response (HR)-inducing avirulent Pseudomonas syringae pv. syringae 61, its HR-negative hrp/hrc mutants and even non-pathogenic bacteria such as P. fluorescens and Escherichia coli caused DAB and DCFH-DA staining, if the dyes were injected 3-4 h after bacterial inoculation into tobacco leaves. The conditions that enable the staining of plant leaves infiltrated with HR-negative bacteria were persisted for 1 to several days depending on the physiological state of the plant, and plant activity was required to the development of the staining. The live virulent P. syringae pv. tabaci was able to suppress the development of the staining reaction. Bacteria that induced more intensive staining reaction triggered stronger local resistance response, which was verified by its ability to inhibit the HR by challenging avirulent bacteria and by expression analysis of genes that are activated during the basal defence response. The peroxidase enzyme activity increased in bacterially treated tobacco tissue, and inhibition of peroxidase activity blocked the development of the staining. The results showed that in tobacco leaves the staining reactions were associated with the general recognition and basal defence reaction of tobacco plant and can be used as markers in tobacco leaves for testing the occurrence of this type of defence.www.blackwell-synergy.com