Import of lyso-phosphatidylcholine into chloroplasts likely at the origin of eukaryotic plastidial lipids

Plastids rely on the import of extraplastidial precursor for the synthesis of their own lipids. This key phenomenon in the formation of plastidial pho sphatidylcholine (PC) and of the most abundant lipids on earth, namely gala ctolipids, is poorly understood. Various suggestions have been made on the nature of the precursor molecule(s) transferred to plastids, but despite ge neral agreement that PC or a close metabolite plays a central role, there i s no clear-cut answer to this question because of a lack of conclusive expe rimental data. We therefore designed experiments to discriminate between a transfer of PC, 1-acylglycero phosphorylcholine (lyso-PC), or glycerophosph orylcholine. After pulse-chase experiments with glycerol and acetate, plast ids of leek (Allium porrum L.) seedlings were purified. The labels of the g lycerol moiety and the sn-1- and sn-2-bound fatty acids of plastidial lipid s were determined and compared with those associated with the extraplastidi al PC. After import, plastid lipids contained the glycerol moiety and the f atty acids esterified to the sn-1 position originating from the extraplasti dial PC; no import of sn-2-bound fatty acid was detected. These results rul e out a transfer of PC or glycerophosphorylcholine, and are totally explain ed by an import of lyso-PC molecules used subsequently as precursor for the synthesis of eukaryotic plastid lipids.