Metabolism of lipids during chloroplast differentiation in barley (Hordeum vulgare).

Plants grown in the dark contain plastids that are quite different in structure to normal chloroplasts. The latter possess sheets of lamellar membranes and granal stacks consisting of closely associated thylakoid discs. In contrast, plastids from darkgrown plants are termed etioplasts and contain a threedimensional lattice of membrane tubes called the prolamellar body. They do not contain chlorophyll, although the organelle does contain carotenoids. On exposure to light, the etioplast will differentiate into a chloroplast. This process involves a great deal of membrane reassembly as the prolamellar body gradually breaks down and stromal lamellar membranes form. After 48 h, granal stacks are present. It was hoped that an examination of lipid composition and metabolism in this system would help in understanding the role of lipids in the specialized chloroplast membranes. Experiments with greening barley (Hordeurn vulgare) have included lipid analysis of the various membrane fractions separated from chloroplasts or etioplasts in order to show the differences before and after greening. Results of these analyses showed that trans-hexadec-3enoic acid was absent in the etioplast, was synthesized during greening, was always esterified to phosphatidylglycerol and was localized in the granal and stromal lamellae. There was also a large increase (4472%) in the relative amounts of linolenic acid in the galactosylacylglycerols, diacylgalactosylglycerol (monogalactosyldiacylglycerol) and diacylgalabiosylglycerol (digalactosyldiacylglycerol). The galactosylacylglycerols were the major acyl lipid components of the etioplasts (50% of the total) and the chloroplasts (75% of the total). Phosphatidylglycerol (lo%), phosphatidylcholine (2%) and diacylsulphoquinovosylglycerol (1 1%) accounted for the majority of the other components. These results agreed with previously published data for wheat (Triticum aestivum) (Bahl et al., 1976), broad bean (Viciu faba) (Mackender & Leech, 1974) and barley (Tevini, 1977). Etiolated plants were illuminated with white light, and then leaves removed at times of up to 72h and incubated with ["Clacetate. The acyl lipids were extracted, separated by t.1.c. (Khan & Williams, 1977) and analysed (Wharfe & Harwood, 1978). Incorporation of radioactivity was greatest into linoleic acid. Palmitic, oleic and linolenic acids were labelled at similar rates, which were about half that for linoleic acid. There was little label found in stearic or trans-hexadec-3-enoic acids. trans-Hexadec-3-enoic acid has been suggested to have a role in photosynthesis, though its exact function is unclear at present (Percival et al., 1979). The relatively small amount of labelling in this acid was probably due to the desaturation of an existing pool of unlabelled palmitoylphosphatidylglycerol. After 2 h the rate of incorporation of radioactivity into all fatty acids except linoleic acid decreased. Then between 4 and 48h the relative labelling of all fatty acids remained constant (linoleic acid, 47%; oleic acid, 15%; linolenic acid, 17%; palmitic acid, 15%; trans-hexadec-3-enoic acid, 3%; stearic acid, 3%). The complex lipids with the highest amounts of incorporation were diacylgalactosylglycerol and phosphatidylcholine. Label was found in high amounts in phosphatidylcholine first, then as the amount of incorporation into this phospholipid levelled off, the rate of labelling of diacylgalactosylglycerol rose. From 24 h onwards the amounts of radioactivity in diacylgalactosylglycerol remained approximately constant, whereas the labelling of phosphatidylcholine increased. Incorporation into the fatty acids of other lipids was low, with no major changes occurring in their relative rates of labelling (Table 1). Sandoz herbicides (San 9785 and San 6706) inhibit the desaturation of linoleate and of both linoleate and palmitate (to trans-hexadec-3-enoate) respectively. Both these herbicides were found to only affect fatty acid labelling patterns and not the distribution of radioactivity between acyl lipids. This result, with greening barley, agreed with observations for other plant tissues (Murphy et al., 1980).