Fig. 10. Simplified model for galactolipid biosynthesis in Arabidopsis wild type (A) and the tgd1 mutants (B). The endoplasmic reticulum (ER), the outer (oE) and inner (iE) chloroplast envelope and the thylakoids (Thy) are shown (Cyt, cytosol; CS, chloroplast stroma). Lipid species derived from the plastid and ER pathways are indicated by subscript P or E, respectively. In addition, ER-derived species are emphasized in bold. Molecular species from the two pathways can be distinguished based on their fatty acid composition. Greek letters indicate the anomeric proton configuration of the second galactose in TGDG or DGDG. The configuration is diagnostic for the respective enzyme. For simplicity, the export of fatty acids from the plastid is not shown. Four key proteins and their experimentally determined location are shown: MGD1, the predominant MGDG synthase; DGD1, the predominant DGDG synthase; PGT, a processive galactosyltransferase highly active in tgd1; TGD1, a putative membrane transporter mutated in tgd1. Paralogs of MGD1 (Awai et al., 2001) and DGD1 (Kelly et al., 2002) present in Arabidopsis are not considered here, because they may only be active under certain conditions or in specific tissues. Because the exact nature of the lipid returning from the ER is not known, the generic designation DAGE precursor is chosen. ACP; acyl carrier protein; DAG, diacylglycerol; DGDG, digalactosyldiacylglycerol; MGDG, monogalactosyldiacylglycerol; TGDG, trigalactosyldiacylglycerol; UDP-Gal, UDP-galactose.