;Phosphatidylglycerol is a ubiquitous phospholipid in the biological membranes of many organisms. In plants, phosphatidylglycerol is mainly present in thylakoid membranes and has been suggested to play specific roles in photosynthesis. Here, we have isolated two T-DNA tagged lines of Arabidopsis thaliana that have a T-DNA insertion in the PGP1 gene encoding a phosphatidylglycerolphosphate synthase involved in the biosynthesis of phosphatidylglycerol. In homozygous plants of the T-DNA tagged lines, the PGP1 gene was completely disrupted. The growth of these knockout mutants was dependent on the presence of sucrose in the growth medium, and these plants had pale yellow-green leaves. The leaves of the mutants had remarkably large intercellular spaces due to the reduction in the number of mesophyll cells. The development of chloroplasts in the leaf cells was severely arrested in the mutants. Mesophyll cells with chloroplast particles are only found around vascular structures, whereas epidermal cells are enlarged but largely conserved. The content of phosphatidylglycerol in the mutants was reduced to 12% of that of the wild type. These results demonstrate that PGP1 plays a major role in the biosynthesis of phosphatidylglycerol in chloroplasts, and that phosphatidylglycerol is essential for the development of thylakoid membranes in A. thaliana.

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., ...

To investigate the role of phosphatidylglycerol (PG) in photosynthesis, we constructed a mutant defective in the CDP-diacylglycerol synthase gene from a cyanobacterium, Synechocystis sp. PCC6803. The mutant, designated as SNC1, required PG supplementation for growth. Growth was repressed in PG-free medium concomitantly with the decrease in cellular content of PG. These results indicate that PG is essential, and that SNC1 is defective in PG synthesis. Decrease in PG content was accompanied by a reduction in the cellular content of chlorophyll, but with little effect on the contents of phycobilisome pigments, which showed that levels of chlorophyll-protein complexes decreased without alteration of those of phycobilisomes. Regardless of the decrease in the PG content, CO2-dependent photosynthesis by SNC1 was similar to that by the wild type on a chlorophyll basis, but consequently became lower on a cell basis. Simultaneously, the ratio of oxygen evolution of photosystem II (PSII) measured with p-benzoquinone to that of CO2-dependent photosynthesis, which ranged between 1.3 and 1.7 in the wild type. However, it was decreased in SNC1 from 1.3 to 0.4 during the early growth phase where chlorophyll content and CO2-dependent photosynthesis were little affected, and then finally to 0.1, suggesting that PSII first lost its ability to reduce p-benzoquinone and then decreased in its level and actual activity. These results indicate that PG contributes to the accumulation of chlorophyll-protein complexes in thylakoid membranes, and also to normal functioning of PSII. Biological membranes with special functions show specific protein and lipid compositions. The membrane lipids form a barrier between the inside and the outside of the membranes, supply the membrane proteins with sites for their action, and regulate the functions of the membrane proteins. So far, research on the functioning of membrane systems has focused mainly on the proteins with actual catalytic activities themselves and to a much smaller extent on the membrane lipids that surround or are integrated into the membrane proteins. Studies on the latter kind include (i) examination of the effects of the removal or addition of specific lipids on some purified membrane protein complexes, (ii) analysis of lipids bound to the complexes, and (iii) characterization of the membrane functions of mutants defective in some particular lipid. Although the former two strategies have led to the accumulation of a substantial amount of information on the possible roles of lipids, a final conclusion as to their roles in vivo cannot be drawn owing to potential artifacts produced by, for example, the use of detergents with in vitro experimental systems. In contrast, the last strategy has been successfully used to obtain in vivo evidence regarding the roles of lipids.Thylakoid membranes, sites for photosynthetic electron transport and photophosphorylation, contain monogalactosyl diacylglycerol (MGDG), digalactosyl diacylglycerol (DGDG), sulfoquinovosyl diacylglycerol (SQDG), and phos...