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Published on May 1, 2008; 10.1104/pp.108.119420

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Received March 19, 2008
Accepted April 22, 2008

Characterization and biological function of the ISOCHORISMATE SYNTHASE 2 gene of Arabidopsis thaliana

Christophe Garcion , Antje Lohmann , Elisabeth Lamodiere , Jeremy Catinot , Antony Buchala , Peter Doermann , and Jean-Pierre Metraux *

Department of Biology, University of Fribourg, Route A. Gockel 3, 1700 Fribourg, Switzerland; Institute of Molecular Physiology and Biotechnology of Plants (IMBIO), University of Bonn, Karlrobert-Kreiten-Strasse 13, 53115 Bonn, Germany; UMR 1090 INRA-Universite Bordeaux 2, 71 Avenue Edouard Bourlaux, 33140 Villenave d'Ornon

* Corresponding author; email: jean-pierre.metraux{at}unifr.ch.

Salicylic acid (SA) is an important mediator of plant defense response. In Arabidopsis thaliana, this compound was proposed to derive mainly from isochorismate, itself produced from chorismate through the activity of ICS1 (Isochorismate Synthase1). Null ics1 mutants still accumulate some SA, suggesting the existence of an enzymatic activity redundant with ICS1 or of an alternative ICS-independent SA biosynthetic route. Here we studied the role of ICS2, second ICS gene of the Arabidopsis genome, in the production of SA. We have shown that ICS2 encodes a functional ICS enzyme and that, similarly to ICS1, ICS2 is targeted to the plastids. Comparison of SA accumulation in the ics1, ics2 and ics1 ics2 mutants indicates that ICS2 participates in the synthesis of SA but in limited amounts, that become clearly detectable only when ICS1 is lacking. This unequal redundancy relationship was also observed for phylloquinone, another isochorismate-derived end-product. Furthermore, detection of SA in the double ics1 ics2 double mutant that is completely devoid of phylloquinone provides genetic evidence of the existence of an ICS-independent SA biosynthetic pathway in Arabidopsis.






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