Accumulation of H2O2 in xylem fluids of cucumber stems during ASM-induced systemic acquired resistance (SAR) involves increased LOX activity and transient accumulation of shikimic acid

Systemic acquired resistance (SAR) to Colletotrichum orbiculare was induced in young cucumber (Cucumis sativus) plants within 3 h of ASM (acibenzolar-S-methyl) application onto the first leaves. A potent signal associated with significant accumulation of hydrogen peroxide in xylem fluids from severed stems appeared to be rapidly translocated from elicited lower leaves within 3 h and 6 h after treatment. Some metabolites of the shikimate, phenylpropanoid and lignin biosynthetic pathways were quantified and significant increases in the levels of shikimic acid were observed in ASM-treated plants challenge-inoculated with the anthracnose fungus. Furthermore, the expression of the 5-enolpyruvylshikimate-3-phosphate synthase gene (EPSPS) was 1.5 times higher within 12 h after ASM treatment in challenge-inoculated plants than in the untreated control. The involvement of lipoxygenase activity, shikimic acid and others such as caffeic acid in the induction of SAR is discussed.