Salicylic acid is a reducing substrate and not an effective inhibitor of ascorbate peroxidase

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 may APX is not inhibited by SA (10 mM), hut that SA is a slow reducing substrate of this enzyme, The suggestion that SA-dependent inhibition of ABX 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 he essential for diminishing the detrimental effects of this phenolic acid on plant cells.