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Plant and Cell Physiology, 1982, Vol. 23, No. 5 767-773
© 1982

Article

Glycerolipid synthesis in Avena Leaves during Greening of Etiolated Seedlings III. Synthesis of {alpha}-Linolenoyl-monogalactosyl Diacylglycerol from Liposomal Linoleoyl-phosphatidylcholine by Avena Plastids in the Presence of Phosphatidylcholine-exchange Protein

Jun-ichi Ohnishi1 and Mitsuhiro Yamada2,2

1Department of Biochemistry, Faculty of Science, Saitama University Urawa 338, Japan
2Department of Biology, University of Tokyo Komaba, Meguro, Tokyo 153, Japan

2To whom correspondence should be addressed

The synthesis of {alpha}-linolenoyl-monogalactosyl diacylglycerol (MGDG) by purified Avena plastids from linoleoyl-phosphatidylcholine (PC) was investigated. When Avena plastids were incubated with liposomal [14C]acyl-[3H]glycerol-PC, the label was transferred from the liposomes to the plastids with no change in the 3H/14C ratio. More than 99% of the label appeared as PC and the remainder as diacylglycerol (DG), MGDG and free fatty acids. The transfer was enhanced by a phosphatidylcholine-exchange protein (PCEP) from germinated castor bean endosperms. Addition of UDP-galactose resulted in the increased synthesis of MGDG.

The distribution of 14C among the molecular species of liposomal PC was very low in the hexaene and pentaene species and high in the tetraene and triene species. The labeling pattern of the molecular species was similar for liposomal PC and plastid DG, whereas the 14C in plastid MGDG shifted from the tetraene and triene species toward the hexaene and pentaene species. These results suggest the conversion of liposomal PC to MGDG through DG in Avena plastids and the desaturation of linoleate to {alpha}-linolenate on MGDG molecules.

In contrast, the major labeled fatty acids were oleic and palmitic acids, when plastids were incubated with [1-14C]acetate, and no further desaturation occurred in each lipid class. This supports our hypothesis that {alpha}-linolenoyl-MGDG is synthesized from the linoleoyl-PC transferred to the plastid from the endoplasmic reticulum by a PCEP, not from oleoyl-MGDG [Ohnishi and Yamada, Plant & Cell Physiol. 21: 1607 (1980)].

(Received March 6, 1982; Accepted April 30, 1982)
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