Hydrogen Peroxide Acts as a Second Messenger for the Induction of Defense Genes in Tomato Plants in Response to Wounding, Systemin, and Methyl Jasmonate

The systemic accumulation of both hydrogen peroxide (H 2 O 2 ) and proteinase inhibitor proteins in tomato leaves in re- sponse to wounding was inhibited by the NADPH oxidase inhibitors diphenylene iodonium (DPI), imidazole, and pyri- dine. The expression of several defense genes in response to wounding, systemin, oligosaccharides, and methyl jasmonate also was inhibited by DPI. These genes, including those of four proteinase inhibitors and polyphenol oxi- dase, are expressed within 4 to 12 hr after wounding. However, DPI did not inhibit the wound-inducible expression of genes encoding prosystemin, lipoxygenase, and allene oxide synthase, which are associated with the octadecanoid signaling pathway and are expressed 0.5 to 2 hr after wounding. Accordingly, treatment of plants with the H 2 O 2 -gener- ating enzyme glucose oxidase plus glucose resulted in the induction of only the later-expressed defensive genes and not the early-expressed signaling-related genes. H 2 O 2 was cytochemically detected in the cell walls of vascular paren- chyma cells and spongy mesophyll cells within 4 hr after wounding of wild-type tomato leaves, but not earlier. The cu- mulative results suggest that active oxygen species are generated near cell walls of vascular bundle cells by oligogalacturonide fragments produced by wound-inducible polygalacturonase and that the resulting H 2 O 2 acts as a second messenger for the activation of defense genes in mesophyll cells. These data provide a rationale for the se- quential, coordinated, and functional roles of systemin, jasmonic acid, oligogalacturonides, and H 2 O 2 signals for sys- temic signaling in tomato plants in response to wounding.