Octadecanoid-derived alteration of gene expression and the "Oxylipin signature" in stressed barley leaves. Implications for different signaling pathways

Stress-induced gene expression in barley (Hordeum vulgare cv Salome) leaves has been correlated with temporally changing levels of octadecanoids and j asmonates, quantified by means of gas chromatography/mass spectrometry-sing le 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, ab out 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 we re identified as minor components. Kinetic analyses revealed that a transie nt threshold of jasmonates or octadecanoids is necessary and sufficient to initiate JA-responsive gene expression. Although OPDA and OPDAME applied ex ogenously 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 indep endently from endogenous JA. Application of deuterated JA showed that endog enous synthesis of JA is not induced by JA treatment. The data are discusse d in terms of distinct signaling pathways.