Plant Physiology Preview
Published on June 15, 2006; 10.1104/pp.106.081257
OPEN ACCESS ARTICLE
Received April 2, 2006
Returned for revision April 26, 2006
Accepted June 1, 2006
The Arabidopsis flavin-dependent monooxygenase FMO1 is an essential component of biologically induced systemic acquired resistance
Tatiana E. Mishina and Jürgen Zeier *
Julius-von-Sachs-Institute of Biological Sciences, University of Würzburg, Julius-von-Sachs-Platz 3, D-97082 Würzburg, Germany
* Corresponding author; email: zeier{at}botanik.uni-wuerzburg.de.
Upon localized attack by necrotizing pathogens, plants gradually develop increased resistance against subsequent infections at the whole plant level, a phenomenon known as systemic acquired resistance (SAR). In order to identify genes involved in the establishment of SAR, we pursued a strategy that combined gene expression information from microarray data with pathological characterization of selected Arabidopsis thaliana T-DNA insertion lines. A gene that is up-regulated in Arabidopsis leaves inoculated with avirulent or virulent strains of the bacterial pathogen Pseudomonas syringae pv. maculicola (Psm) showed homology to flavin-dependent monooxygenases (FMO) and was designated as FMO1. An Arabidopsis knockout line of FMO1 proved to be fully impaired in the establishment of SAR triggered by avirulent (Psm avrRpm1) or virulent bacteria (Psm). Loss of SAR in the fmo1 mutants was accompanied by the inability to initiate systemic accumulation of salicylic acid (SA) and systemic expression of diverse defense-related genes. In contrast, responses at the site of pathogen attack, including increases in the levels of the defense signals salicylic acid and jasmonic acid, camalexin accumulation, and expression of various defense genes, were induced in a similar manner in both fmo1 mutant and wild-type plants. Consistently, the fmo1 mutation did not significantly affect local disease resistance towards virulent or avirulent bacteria in naïve plants. Induction of FMO1 expression at the site of pathogen inoculation is independent of SA signalling, but attenuated in the Arabidopsis eds1 and pad4 defense mutants. Importantly, FMO1 expression is also systemically induced upon localized P. syringae infection. This systemic up-regulation is missing in the SAR-defective SA pathway mutants sid2 and npr1, as well as in the defense mutant ndr1, indicating a close correlation between systemic FMO1 expression and SAR establishment. Our findings suggest that the presence of the FMO1 gene product in systemic tissue is critical for the development of SAR, possibly by synthesis of a metabolite required for the transduction or amplification of a signal during the early phases of SAR establishment in systemic leaves.
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