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- ¹⁴C]acetate for the first 3 hr of greening, and the redistribution of ¹⁴C 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 ¹⁴C during the chase. Galactolipids, such as monogalactosyl-diglyceride (MGDG) and digalactosyldiglyceride (DGDG), were only slightly labeled at the start, but gradually gained ¹⁴C during the chase. When greening Avena leaves were pulse-labeled with [2- ³H]glycerol and chased in the same manner, a similar incorporation and redistribution of ³H were observed in the glycerol moieties of lipid classes. A [1- ¹⁴C]oleic acid-feeding experiment also showed the same redistribution of ¹⁴C from PC to galactolipids and little change of the ¹⁴C incorporated into phosphatidylglycerol and phosphatidylethanolamine. These results are evidence that galactolipids were synthesized from PC in greening Avena leaves. Time courses for the ¹⁴C in the fatty acid moieties of lipid classes in both [1- ¹⁴C]-acetate- and [1- ¹⁴C] 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 ¹⁴C into α-linolenoyl-DGDG, in contrast to the rapid incorporation of ¹⁴C into a-linolenoyl-MGDG, in the [1- ¹⁴C] acetate- and [1- ¹⁴C]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.