Symptomless resistant response instead of the hypersensitive reaction in tobacco leaves after infiltration of heterologous pathovars of Pseudomonas syringae

Following the infiltration of heterologous pathovars of Pseudomonas syringa e two localized responses develop in a parallel manner in tobacco leaves: a n early form of induced resistance (EIR) and the hypersensitive reaction (H R). The EIR inhibits the metabolic activity of infiltrated bacteria and the leaf tissue remains symptomless whereas HR also inhibits bacteria, but the leaf tissue shows confluent necrosis or necrosis of individual plant cells , depending on the inoculum concentration. After infiltration of a heterolo gous strain the rapid development of HR usually masks the effects of EIR wh ich is developing at the same time. The aim of this study was to characteri ze the conditions in which EIR occurs in the absence of HR following the in filtration of heterologous pathovars of Pseudomonas syringae pathovars. A s tudy was made of numerous experimental interactions of heterologous bacteri a/tobacco leaves in which the HR induction time of the bacteria and/or the EIR response of the plant were modified. When (a) the HR induction time was extended by chloramphenicol treatment of bacterial cells or by using trans poson mutants (Tn5), or (b) the time needed for the development of EIR was shortened (at 30 degrees C or in young leaves) the heterologous strains wer e unable to induce visible HR. But when the EIR was suppressed in tobacco l eaves by pretreatment with cycloheximide or thermal shock (50 degrees C for 13 s) the HR appeared. Comparison of results of numerous experiments revea led that solely EIR was present if its development time was shorter than th e induction period of HR. This interpretation was confirmed by the infiltra tion of partially purified harpin(Pss) into tobacco leaves, where it caused plant cell death in the presence of EIR.