Characterization and Biological Function of the ISOCHORISMATE SYNTHASE2 Gene of Arabidopsis

Christophe Garcion,Elisabeth Lamodière,Jean-Pierre Métraux,Jérémy Catinot,Peter Doermann,Antje Lohmann,Antony Buchala

doi:10.1104/pp.108.119420

Christophe Garcion, Elisabeth Lamodière + Show 5 more

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https://doi.org/10.1104/pp.108.119420

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Journal: Plant Physiology	Publication Date: May 1, 2008
Citations: 346	License type: cc-by

Affiliation: University of Bonn, University of Fribourg

Abstract

Salicylic acid (SA) is an important mediator of plant defense response. In Arabidopsis (Arabidopsis thaliana), this compound was proposed to derive mainly from isochorismate, itself produced from chorismate through the activity of ISOCHORISMATE SYNTHASE1 (ICS1). 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, a second ICS gene of the Arabidopsis genome, in the production of SA. We have shown that ICS2 encodes a functional ICS enzyme and that, similar 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.

Highlights

Salicylic acid (SA) has been linked in various species with diverse physiological aspects, like thermogenesis, stomatal closure, senescence, leaf abscision or resistance to abiotic stresses (Raskin, 1992; Morris et al, 2000, Martinez et al, 2004)
No full-length ICS2 cDNA sequence was available from public databases and the current conceptual translation of the ICS2 coding sequence, relying on ESTs, predicted an ICS2 protein sequence lacking a N-terminal extension compared to ICS1
This concerns the implication of the two Arabidopsis isochorismate synthase (ICS) genes in the biosynthesis of SA

Summary

Introduction

SA has been linked in various species with diverse physiological aspects, like thermogenesis, stomatal closure, senescence, leaf abscision or resistance to abiotic stresses (Raskin, 1992; Morris et al, 2000, Martinez et al, 2004). In A. thaliana, a second pathway was proposed that is based on isochorismate, similar to the pathway described in some Pseudomonas species (Wildermuth et al, 2001) In this pathway, chorismate is converted into isochorismate through the action of an isochorismate synthase (ICS), and SA is generated from isochorismate by an isochorismate pyruvate-lyase. Chorismate is converted into isochorismate through the action of an isochorismate synthase (ICS), and SA is generated from isochorismate by an isochorismate pyruvate-lyase This scheme gained a strong support from studies with ics mutants that accumulate only low levels of SA, the conversion from isochorismate to SA has not yet been demonstrated in Arabidopsis (Wildermuth et al, 2001). These molecules are taxon-specific and so far have not been investigated in A. thaliana

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