Abstract
This communication describes the interactions of salicylic acid (SA) with plant ascorbate peroxidase (APX). Contrary to a recent report (Durner, J., and Klessig, D. F. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 11312-11316) we show conclusively that ascorbate oxidation by APX is not inhibited by SA (10 mM), but that SA is a slow reducing substrate of this enzyme. The suggestion that SA-dependent inhibition of APX in planta may result in the elevation of H2O2 levels, which in turn acts as a second messenger in systemic acquired resistance signaling, is therefore not tenable. We conclude that APX remains a key antioxidant during systemic acquired resistance following pathogenic infection of plants. The transient products of SA oxidation by APX appear to be SA free radicals that undergo subsequent chemistry. APX-dependent oxidation of SA could be essential for diminishing the detrimental effects of this phenolic acid on plant cells.
Highlights
Salicylic acid (2-hydroxybenzoic acid) is believed to play an important role in plant defense responses against pathogen attack
When the activities of all of the APX isoenzymes were assayed at 265 nm in a 1-mm cell containing 750 M ascorbic acid (AsA), the rates of ascorbate oxidation in the presence and absence of 1 mM salicylic acid (SA) were not significantly different (1500 Ϯ 50 and 100 Ϯ 5 mol minϪ1 mgϪ1 for chloroplastic and cytosolic ascorbate peroxidases respectively)
We have studied the effect of SA on cytosolic, chloroplastic thylakoid-bound, and chloroplastic stromal APX isoenzymes purified from tea plants as well as on an APX extract obtained from tobacco plants
Summary
Salicylic acid (2-hydroxybenzoic acid) is believed to play an important role in plant defense responses against pathogen attack (see Refs. 1 and 2 and references therein). APX with ascorbic acid (AsA) as a reducing substrate is believed to scavenge excess H2O2 formed in plants under both stress and normal conditions [18, 19]. We clearly demonstrate that SA is not an effective inhibitor of ascorbate oxidation but is a slow alternative reducing substrate of APX.
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