Phosphatidylglycerol is a ubiquitous phospholipid that is also present in the photosynthetic membranes of plants. Multiple independent lines of evidence suggest that this lipid plays a critical role for the proper function of photosynthetic membranes and cold acclimation. In eukaryotes, different subcellular compartments are competent for the biosynthesis of phosphatidylglycerol. Details on the plant-specific pathways in different organelles are scarce. Here, we describe a phosphatidylglycerol biosynthesis-deficient mutant of Arabidopsis, pgp1. The overall content of phosphatidylglycerol is reduced by 30%. This mutant carries a point mutation in the CDP-alcohol phosphotransferase motif of the phosphatidylglycerolphosphate synthase (EC 2.7.8.5) isoform encoded by a gene on chromosome 2. The mutant shows an 80% reduction in plastidic phosphatidylglycerolphosphate synthase activity consistent with the plastidic location of this particular isoform. Mutant plants are pale green, and their photosynthesis is impaired. This mutant provides a promising new tool to elucidate the biosynthesis and function of plastidic phosphatidylglycerol in seed plants.Phosphatidylglycerol (PG) is one of the most common phosphoglycerolipids found in nature. It is the only major phospholipid present in the thylakoid membranes of plant chloroplasts (Marechal et al., 1997) and the only phospholipid in cyanobacteria, which strikingly resemble plant chloroplasts in their lipid composition (Murata and Nishida, 1987). A large body of correlative and direct evidence suggests that PG is critical for the structural and functional integrity of the thylakoid membrane. Thus, the presence of specific molecular species of PG in photosynthetic membranes correlates well with lowtemperature-induced photoinhibition and chilling sensitivity of plants and cyanobacteria (Murata et al., 1992;Somerville, 1995). Light-harvesting pigmentprotein complexes of PSII are specifically enriched in PG (Murata et al., 1990;Tremolieres et al., 1994). Moreover, PG is crucial for the in vitro trimerization of the major peripheral light-harvesting pigmentprotein complexes (Nussberger et al., 1993; Hobe et al., 1994;Kü hlbrandt, 1994) and the dimerization of the reaction/center core pigment-protein complexes of PSII (Kruse et al., 2000). It is also an integral component of the PSI reaction center (Jordan et al., 2001) and is required for the in vitro reconstitution of the light-harvesting pigment-protein complexes of PSI (Schmid et al., 1997). Thylakoid membranes treated with phospholipase A 2 are PG depleted and are inhibited in their photosynthetic electron transport activities (Jordan et al., 1983;Siegenthaler et al., 1987). Furthermore, the anionic lipid PG interacts with the transit peptide of chloroplast precursor proteins during protein import into chloroplasts (van't Hof et al., 1993).PG-deficient auxotrophic mutants of the cyanobacterium Synechocystis sp. PCC6803 that are severely impaired in photosynthesis have recently been isolated (Hagio et al., 2000;Sato et al., ...