HYDROGEN-PEROXIDE STIMULATES SALICYLIC-ACID BIOSYNTHESIS IN TOBACCO

Hydrogen peroxide induced the accumulation of free benzoic acid (EA) a nd salicylic acid (SA) in tobacco (Nicotiana tabacum L. cv Xanthi-nc) leaves. Six hours after infiltration with 300 mM H2O2, the levels of B A and SA in leaves increased 5-fold over the levels detected in contro l leaves. The accumulation of BA and SA was preceded by the rapid acti vation of benzoic acid 2-hydroxylase (BA2H) in the H2O2-infiltrated ti ssues. This enzyme catalyzes the formation of SA from BA. Enzyme activ ation could be reproduced in vitro by addition of H2O2 or cumene hydro peroxide to the assay mixture. H2O2 was most effective in vitro when a pplied at 6 mM. In vitro activation of BAZH by peroxides was inhibited by the catalase inhibitor 3-amino-1,2,4-triazole. We suggest that H2O 2 activates SA biosynthesis via two mechanisms. First, H2O2 stimulates BA2H activity directly or via the formation of its substrate, molecul ar oxygen, in a catalase-mediated reaction. Second, higher BA levels i nduce the accumulation of BA2H protein in the cells and provide more s ubstrate for this enzyme.