Lipoxygenases (LOXs) catalyze the dioxygenation of polyunsaturated fatty acids and consequently the synthesis of hydroperoxy derivatives, which are converted by a multitude of enzymes into signalmolecules and antimicrobial compounds, among others. These derivatives are involved in the plant pathogen defense. In potato cells, the 9-LOX pathway is preferentially stimulated after treament with a preparation of crude elicitor from Phytophthora infestans. The main metabolites are the divinylether colneleic acid, which exhibits antimicrobial activity, the trihydroxy derivatives 9,10,11-TriHOE and 9,10,11-TriHOD as well as the hydroxy derivative 9-HOD. 9-LOX reactions are also stimulated in potato leaves, in the compatible interaction with P. infestans and in the non-host pathogen interaction with Pseudomonas syringae pv. maculicola. The trihydroxy derivatives 9,10,11-TriHOD and 9,12,13-TriHOD are the most prominent oxylipins. An antifungal activity is suggested for these compounds. In parallel, the hydroxy derivative 9-HOT and the divinylethers colneleic acid (CA) and colnelenic acid (CnA) also accumulate in both systems but to lower amounts. The stimulation of the 9-LOX pathway occurs in parallel to browning of the cells or the development of necroses on the pathogen-treated leaves. After treatment with the photodynamic substance eosin, the 9-LOX metabolites 9,10,11-TriHOD and 9,12,13-TriHOD as well as 9-HOT accumulate in temporal correlation with the development of necroses. "Gain-of-function" experiments, in which the exogenouse application of solutions of LOX metabolites were investigated in a concentration dependent manner, demonstrated the correlation of stimulation of the LOX activity and the activation of defense reactions. All this leads to the conclusion that these oxylipins may be directly involved in the induction of defense reactions. To analyze the pathogen-induced expression of 9-LOXs, posttranscriptional gene silencing via interference of double-stranded RNA (RNAi) was used. Expression of all known 9-LOXs from potato is inhibited in transgenic plants. Apart from an amplification of autoxidative processes a pathogen-induced accumulation of free fatty acids and divinylethers occurred in some RNAi plants but could not be correlated to increased resistance.