A glycopeptide elicitor prepared from germ tubes of the rust fungus Puccinia graminis Pers. f. sp. tritici Erikss. & Henn (Pgt), as well as chitin oligosaccharides, chitosan, and methyl jasmonate (MJ) stimulated lipoxygenase (LOX) activity (E.C. 1.1 3.1 1.1 2) in wheat (Triticum aestivum) leaves. lmmunoblot analysis using anti-LOX antibodies revealed the induction of 92-and 103-kD LOX species after Pgt elicitor treatment. In contrast, MJ treatment led to a significant increase of a 100-kD LOX species, which was also detected at lower levels in control plants. The effects of chitin oligomers and chitosan resembled those caused by MJ. In conjunction with other observations the results suggest that separate reaction cascades exist, and that jasmonates may not be involved in Pgt elicitor action. LOX-92 appears to be mainly responsible for the increase in LOX activity after Pgt elicitor treatment because its appearance on western blots coincided with high LOX activity in distinct anion-exchange chromatography fractions. It is most active at pH 5.5 to 6.0, and product formation from linoleic and a-linolenic acid is clearly in favor of the 9-LOOHs. It is interesting that a 92-kD LOX species, which seems to correspond to the Pgt elicitor-induced LOX species, was also detected in rust-inoculated leaves.The ubiquitous LOXs (E.C. 1.13.11.12) identified in animais and plants are non-heme-iron-containing enzymes, which catalyze the oxidation of polyunsaturated fatty acids containing a cis,cis-1,4-pentadiene site. Linoleic (182) and linolenic (18:3) acid, components of plant lipids, represent potential substrates for these enzymes, which in plants are predominantly present as soluble proteins in the cytosol but are also found in membranes and in different organelles (for review, see Siedow [1991]). Generally, free fatty acids have been regarded as the natural LOX substrate; however, in vitro reactions of plant lipoxygenases with membrane components have been shown (Maccarrone et al., 1994;Regdel et al., 1994). A precise physiological role for LOX in plants has not been defined so far, but the diversity of isozymes and the subcellular distribution suggest multiple functions (Siedow, 1991;Rosahl, 1996). LOX is required for the syn-