The Mechanism of Symptomless Reaction of Plants Induced by Pathogenic Pseudomonads

Abstract

Leaf spot-inducing pseudomonads usually cause a hypersensitive reaction (HR) in non-host plants. In certain cases, however, the necrotic response does not occur. The mechanism of this symptomless “immune” response was not known before. We have shown, that a single bacterial inoculation induces two iocai host responses at an early stage of pathogenesis. One of these is the HR and the other is the early induced resistance (ElR) of plants ². It was found that the symptomless response is a consequence of the development of the EIR prior to the induction of HR. This situation exists either a) when the EIR develops very quickly (e.g. at high temperatures) or b) when the HR induction time of pathogens is delayed (e.g. in some pathovars or mutants). We have results supporting both cases: a) the EIR developed in tobacco leaves at 20 °C by 3-6 h, but at 30 °C by 1-2 h. Therefore, Pseudomonas syringae pv. phaseolicola, the HR induction time of which is rather long (2.5-3 h) at both temperatures, causes HR at 20 °C but not at 30 °C; b) the induction time of some Tn5-mutants of pv. phaseolicola is delayed. During this longer induction time of HR (3-6 h) the EIR can develop also at normal temperature (20 °C). Therefore, the hypersensitive necrosis does not appear. These findings were verified in experiments in which the length of the HR induction time was extended by chloramphenicol treatment of wild strains of pv, phaseolicola. When the development of EIR was inhibited in tobacco leaves by inhibition of plant protein synthesis (cycioheximide or heat shock at 50 °C for 13 sec), the HR appeared.