INTRACELLULAR LEVELS OF FREE LINOLENIC AND LINOLEIC ACIDS INCREASE INTOMATO LEAVES IN RESPONSE TO WOUNDING

An intracellular signaling pathway for activating plant defense genes against attacking herbivores and pathogens is mediated by a lipid-base d signal transduction cascade. In this pathway, linolenic acid (18:3) is proposed to be liberated from cell membranes and is converted to cy clopentanones that are involved in transcriptional regulation of defen se genes, analogously to prostaglandin synthesis and function in anima ls. Levels of 18:3 and linoleic acid in tomato (Lycopersicon esculentu m) leaves increased within 1 h when the leaves were wounded with a hem ostat across the main vein to simulate herbivore attacks. The increase correlated with the time course of accumulation of jasmonic acid, a c yclopentanone product of 18:3, that had previously been shown to incre ase in leaves in response both to wounding and to elicitors of plant d efense genes. One hour after wounding, at least a 15-fold excess of 18 :3 was found over that required to account for the levels of newly syn thesized jasmonic acid. The free fatty acids in both control and wound ed leaves accounted for less than 0.25% of the total fatty acids. Howe ver, the total lipid contents of the leaves remained relatively unchan ged up to 8 h after wounding, indicating that extensive loss of lipids did not occur, although a gradual decrease in polar lipids was observ ed, mainly in monogalactosyl diacylglycerol of chloroplast lipids. The data support a role for lipid release as a key step in the signaling events that activate defense genes in tomato leaves in response to wou nding by attacking herbivores.