The kinetics of [1-14C]acetate incorporation in Arabidopsis thaliana L. (Heyn) showed almost equal labelling of phosphatidylcholine (PC) and diacylgalactosylglycerol (DGG) at early times and the transfer of radioactivity from PC to DGG and diacyldigalactosylglycerol (DDG) at longer times. These kinetics demonstrated the parallel operation of the prokaryotic and eukaryotic pathways of lipid synthesis [Roughan & Slack (1982) Annu. Rev. Plant Physiol. 33, 97-132] in this tissue. At 2 h after the application of [1-14C]acetate, more than 85% of the radioactivity at the sn-2 position of each chloroplast lipid was in 16-carbon fatty acids. However, after 60 h, molecular species containing labelled C18 fatty acids at position sn-2 and presumably derived from microsomal PC made a large contribution (20-70%) to each chloroplast lipid except phosphatidylglycerol. These findings are consistent with the contention that the chain length of the fatty acid at the sn-2 position of glycerol is an accurate predictor of whether a particular lipid molecule has been synthesized by the prokaryotic or eukaryotic pathway. At 30 min after the start of [1-14C]acetate labelling, only 12.3% of the radioactivity in PC was in saturated fatty acids, but the proportion increased steadily to 24.3% after 142 h. It is suggested that steps involved in the conversion of PC to chloroplast lipids on the eukaryotic pathway discriminate against palmitate-containing species. The step involved does not appear to be transfer of PC to the chloroplast because extrachloroplastic and chloroplast membranes purified from Arabidopsis mesophyll protoplasts each contained PC with a fatty acid composition similar to that of the same lipid from leaves. Positional analysis of unlabelled lipids, together with the information summarized above, is used to construct a quantitative scheme of the fluxes through the prokaryotic and eukaryotic pathways during lipid synthesis in Arabidopsis. This scheme shows that 38% of the fatty acids synthesized de novo in the chloroplast enter the prokaryotic pathway in the chloroplast envelope. Of the 62% which are exported as acyl-CoA species to enter the eukaryotic pathway, 56% (34% of the total) are returned to complete synthesis of the chloroplast's complement of glycerolipids.

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