Formation of lipoxygenase-pathway-derived aldehydes in barley leaves upon methyl jasmonate treatment

In barley leaves, the application of jasmonates leads to dramatic alteratio ns of gene expression. Among the up-regulated gene products lipoxygenases o ccur abundantly. Here, at least four of them were identified as 13-lipoxyge nases exhibiting acidic pH optima between pH 5.0 and 6.5. (13S,9Z,11E,15Z)- 13-hydroxy-9,11,15-octadecatrienoic acid was found to be the main endogenou s lipoxygenase-derived polyenoic fatty acid derivative indicating 13-lipoxy genase activity in vivo. Moreover, upon methyl jasmonate treatment > 78% of the fatty acid hydroperoxides are metabolized by hydroperoxide lyase activ ity resulting in the endogenous occurrence of volatile aldehydes. (2E)-4-Hy droxy-2-hexenal, hexanal and (3Z)- plus (2E)-hexenal were identified as 2,4 -dinitrophenylhydrazones using HPLC and identification was confirmed by GC/ MS analysis. This is the first proof that (2E)-4-hydroxy-2-hexenal is forme d in plants under physiological conditions. Quantification of (2E)-4-hydrox y-2-hexenal, hexanal and hexenals upon methyl jasmonate treatment of barley leaf segments revealed that hexenals were the major aldehydes peaking at 2 4 h after methyl jasmonate treatment. Their endogenous content increased fr om 1.6 nmol.g(-1) fresh weight to 35 nmol.g(-1) fresh weight in methyl-jasm onate-treated leaf segments, whereas (2E)-4-hydroxy-2-hexenal, peaking at 4 8 h of methyl jasmonate treatment increased from 9 to 15 nmol.g(-1) fresh w eight. Similar to the hexenals, hexanal reached its maximal amount 24 h aft er methyl jasmonate treatment, but increased from 0.6 to 3.0 nmol.g(-1) fre sh weight. In addition to the classical leaf aldehydes, (2E)-4-hydroxy-2-he xenal was detected, thereby raising the question of whether it functions in the degradation of chloroplast membrane constituents, which takes place af ter methyl jasmonate treatment.