[Involvement of polar lipids in polyenoic fatty acid formation in greening wheat seedlings]

After the dark incorporation of [1-14C]acetate into etiolated wheat seedlings the bulk of the labelled fatty acids was found in the neutral lipids of mitochondria and microsomes; the incorporation into etioplast fatty acids was considerably smaller. The saturated fatty acids, namely palmitic and stearic ones, were predominantly synthesized in the dark; the incorporation of the label into the polyenoic fatty acids was not detected. Upon continuous illumination the radioactivity of fatty acids was increased, firstly in the phospholipids and then in the galactolipids. The illumination caused desaturation of oleic acid to linoleic and linolenic acids. The formation of linoleate was associated with microsomal phosphatidylcholine, and that of linolenate--with plastidal galactosyldiacylglycerol. The kinetics of the label incorporation into the unsaturated fatty acids is consistent with the concept that microsomal phosphatidylcholine can act as a linoleate donor for its further desaturation within the composition of chloroplast galactolipids. Galabiosyldiacylglycerol differs from galactosyldiacylglycerol by a lower content of [14C]linolenic acid and unsaturated 14C--fatty acids of the hexadecanoate type. The specific localization of [14C] linolenate in galactosyldiacylglycerol suggests that either lynoleoyl-CoA (with the further acyl transfer on galactosylmonoacylglycerol) or linoleate-containing molecular species of galactosyldiacylglycerol can be true substrates fo linoleate delta 15-desaturase. A scheme for the proposed subcellular localization of dark- and light-induced paths of free and complex lipid-bound fatty acids synthesis is presented.