Publikationen - Molekulare Signalverarbeitung

Stress-induced gene expression in barley (Hordeum vulgare cv Salome) leaves has been correlated with temporally changing levels of octadecanoids and jasmonates, quantified by means of gas chromatography/mass spectrometry-single ion monitoring. Application of sorbitol-induced stress led to a low and transient rise of jasmonic acid (JA), its precursor 12-oxophytodienoic acid (OPDA), and the methyl esters JAME and OPDAME, respectively, followed by a large increase in their levels. JA and JAME peaked between 12 and 16 h, about 4 h before OPDA and OPDAME. However, OPDA accumulated up to a 2.5-fold higher level than the other compounds. Dihomo-JA and 9,13-didehydro-OPDA were identified as minor components. Kinetic analyses revealed that a transient threshold of jasmonates or octadecanoids is necessary and sufficient to initiate JA-responsive gene expression. Although OPDA and OPDAME applied exogenously were metabolized to JA in considerable amounts, both of them can induce gene expression, as evidenced by those genes that did not respond to endogenously formed JA. Also, coronatine induces JA-responsive genes independently from endogenous JA. Application of deuterated JA showed that endogenous synthesis of JA is not induced by JA treatment. The data are discussed in terms of distinct signaling pathways.

Allene oxide cyclase (AOC; EC 5.3.99.6) catalyzes the cyclization of 12,13(S)-epoxy-9(Z),11,15(Z)-octadecatrienoic acid to 12-oxo- 10,15(Z)-phytodienoic acid, the precursor of jasmonic acid (JA). This soluble enzyme was purified 2000-fold from dry corn (Zea mays L.) kernels to apparent homogeneity. The dimeric protein has a molecular mass of 47 kD. Allene oxide cyclase activity was not affected by divalent ions and was not feedback-regulated by its product, 12-oxo-l0,15(Z)-phytodienoic acid, or by JA. ([plus or minus])-cis- 12,13-Epoxy-9(Z)-octadecenoic acid, a substrate analog, strongly inhibited the enzyme, with 50% inhibition at 20 [mu]M. Modification of the inhibitor, such as methylation of the carboxyl group or a shift in the position of the epoxy group, abolished the inhibitory effect, indicating that both structural elements and their position are essential for binding to AOC. Nonsteroidal anti-inflammatory drugs, which are often used to interfere with JA biosynthesis, did not influence AOC activity. The purified enzyme catalyzed the cyclization of 12,13(S)-epoxy-9(Z),11,15(Z)-octadecatrienoic acid derived from linolenic acid, but not that of 12,13(S)-epoxy-9(Z),11- octadecadienoic acid derived from linoleic acid.