Glycerolipid synthesis in Avena leaves during greening of etiolated seedlings II. α-Linolenic acid synthesis.

To determine the synthesis of α-linolenic acid esterified to galactolipids, etiolated leaves from Avena sativa L. were fed with [1-(14)C]acetate for the first 3 hr of greening, and the redistribution of (14)C incorporated into the fatty acid moieties of lipid classes was examined during a 21-hr chase. Phosphatidylcholine (PC) was most heavily labeled, but lost a large portion of its (14)C during the chase. Galactolipids, such as monogalactosyl-diglyceride (MGDG) and digalactosyldiglyceride (DGDG), were only slightly labeled at the start, but gradually gained (14)C during the chase. When greening Avena leaves were pulse-labeled with [2-(3)H]glycerol and chased in the same manner, a similar incorporation and redistribution of (3)H were observed in the glycerol moieties of lipid classes. A [1-(14)C]oleic acid-feeding experiment also showed the same redistribution of (14)C from PC to galactolipids and little change of the (14)C incorporated into phosphatidylglycerol and phosphatidylethanolamine. These results are evidence that galactolipids were synthesized from PC in greening Avena leaves. Time courses for the (14)C in the fatty acid moieties of lipid classes in both [1-(14)C]-acetate- and [1-(14)C] oleic acid-feeding experiments showed dominant labeling of oleoyl-PG during the early hours and the subsequent transfer of the label from oleoyl-PG to linoleoyl-PC, linoleoyl-MGDG and finally to α-linolenoyl-MGDG. From these results, the major pathway of α-linolenic acid synthesis in greening Avena leaves is proposed: oleate synthesized de novo is first acylated to PG then desaturated to linoleoyl-PC, which is in turn converted to MGDG and desaturated to α-linolenoyl-MGDG. Because of the slow incorporation of (14)C into α-linolenoyl-DGDG, in contrast to the rapid incorporation of (14)C into a-linolenoyl-MGDG, in the [1-(14)C] acetate- and [1-(14)C]linoleic acid-feeding experiments, the former lipid class seems to be formed by the galactosylation of the latter rather than by the desaturation of linoleoyl-DGDG. The involvement of PC-exchange protein in the transfer of linoleoyl-PC from ER to the plastid is discussed.