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Systemic Resistance in Arabidopsis Induced by Rhizobacteria Requires Ethylene-Dependent Signaling at the Site of Application

August 1999 , Volume 12 , Number  8
Pages  720 - 727

Marga Knoester , 1 Corné M. J. Pieterse , 1 John F. Bol , 2 and Leendert C. Van Loon 1

1Institute of Biology, Section Phytopathology, Utrecht University, P.O. Box 80084, 3508 TB Utrecht, The Netherlands; 2Institute of Molecular Plant Sciences, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands

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Accepted 23 April 1999.

Root colonization of Arabidopsis thaliana by the nonpathogenic, rhizosphere-colonizing, biocontrol bacterium Pseudomonas fluorescens WCS417r has been shown to elicit induced systemic resistance (ISR) against Pseudomonas syringae pv. tomato (Pst). The ISR response differs from the pathogen-inducible systemic acquired resistance (SAR) response in that ISR is independent of salicylic acid and not associated with pathogenesis-related proteins. Several ethylene-response mutants were tested and showed essentially normal symptoms of Pst infection. ISR was abolished in the ethylene-insensitive mutant etr1-1, whereas SAR was unaffected. Similar results were obtained with the ethylene-insensitive mutants ein2 through ein7, indicating that the expression of ISR requires the complete signal-transduction pathway of ethylene known so far. The induction of ISR by WCS417r was not accompanied by increased ethylene production in roots or leaves, nor by increases in the expression of the genes encoding the ethylene biosynthetic enzymes 1-aminocyclopropane-1-carboxylic (ACC) synthase and ACC oxidase. The eir1 mutant, displaying ethylene insensitivity in the roots only, did not express ISR upon application of WCS417r to the roots, but did exhibit ISR when the inducing bacteria were infiltrated into the leaves. These results demonstrate that, for the induction of ISR, ethylene responsiveness is required at the site of application of inducing rhizobacteria.

Additional keyword: bacterial speck disease, ethylene response mutants.

The American Phytopathological Society, 1999