SALICYLIC-ACID IS A REDUCING SUBSTRATE AND NOT AN EFFECTIVE INHIBITOROF ASCORBATE PEROXIDASE

This communication describes the interactions of salicylic acid (SA) w ith plant ascorbate peroxidase (APX). Contrary to a recent report (Dur ner, 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 t herefore not tenable. We conclude that APX remains a key antioxidant d uring systemic acquired resistance following pathogenic infection of p lants, The transient products of SA oxidation by APX appear to be SA f ree radicals that undergo subsequent chemistry, APX-dependent oxidatio n of SA could he essential for diminishing the detrimental effects of this phenolic acid on plant cells.