A New Arabidopsis Mutant with Reduced 16:3 Levels

Abstract

Higher plants possess two distinct pathways for the synthesis of chloroplast glycerolipids in leaf cells [1]. The chloroplast is the sole site of de novo fatty acid synthesis. The final products of fatty acid synthesis and of the soluble 18:0-ACP desaturase are 16:0-ACP and 18:1-ACP. These either enter the prokaryotic pathway of the chloroplast inner envelope to produce chloroplastic lipids or they are exported as CoA thioesters to the ER to enter the eukaryotic pathway. Both pathways are initiated by the synthesis of phosphatidic acid (PA). Because of the specificities of the plastid acyltransferases, the PA made by the prokaryotic pathway has 16:0 at the sn-2 position and, in most cases, 18:1 at the sn-1 position. This PA is used for the synthesis of PG or is converted to DAG by a phosphatidic acid phosphatase. This DAG pool is the precursor for the synthesis of MGDG, DGDG, and SL, the major plastid membrane lipids. The PA synthesized in the ER by a different set of acyltransferases than the plastid isozymes is characteristically enriched in 18-carbon fatty acids at the sn-2 position, 16:0 when present, is confined to the sn-1. This PA is used to produce phospholipids characteristic of the various extrachloroplast membranes of the cell. In Arabidopsis, a portion of PC produced by the eukaryotic pathway is returned to the chloroplast and used in the production of chloroplast lipids. Consequently, both pathways contribute about equally to the synthesis of MGDG, DGDG, and SL, and the leaf lipids characteristically contain substantial amounts of 16:3 which is found only in MGDG and DGDG molecules produced by the prokaryotic pathway. Mutants of Arabidopsis with altered fatty acid composition have been isolated [2]. One of them, actl, is deficient in chloroplast acyl-ACP:sn--glycerol-3-phosphate acyltransferase activity, and its fatty acid composition is characterized by greatly reduced levels of 16:3 [3]. Here we show that another mutant with also reduced 16:3 levels is not allelic to actl suggesting that other mutations can (partly) block the prokaryotic pathway.