The galactolipids, mono-and digalactosyldiacylglycerol (DGDG), are the most common nonphosphorous lipids in the biosphere and account for 80% of the membrane lipids found in green plant tissues. These lipids are major constituents of photosynthetic membranes (thylakoids), and a large body of evidence suggests that galactolipids are associated primarily with plastid membranes in seed plants. A null-mutant of Arabidopsis (dgd1), which lacks the DGDG synthase (DGD1) resulting in a 90% reduction in the amount of DGDG under normal growth conditions, accumulated DGDG after phosphate deprivation up to 60% of the amount present in the wild type. This observation suggests the existence of a DGD1-independent pathway of galactolipid biosynthesis. The fatty acid composition of the newly formed DGDG was distinct, showing an enrichment of 16-carbon fatty acids in the C-1 position of the glycerol backbone of DGDG. Roots with their rudimentary plastids accumulated large amounts of DGDG after phosphate deprivation, suggesting that this galactolipid may be located in extraplastidic membranes. Corroborating evidence for this hypothesis was obtained directly by fractionation of subcellular membranes from leaf tissue and indirectly by lipid analysis of the phosphate-deprived fad3 mutant primarily deficient in extraplastidic fatty acid desaturation. The discovery of extraplastidic DGDG biosynthesis induced by phosphate deprivation has revealed a biochemical mechanism for plants to conserve phosphate. Apparently, plants replace phospholipids with nonphosphorous galactolipids if environmental conditions such as phosphate deprivation require this for survival.O ne of the most powerful environmental stimuli affecting the overall glycerolipid composition of bacterial membranes is phosphate deprivation. The replacement of phospholipids by nonphosphorous lipids was discovered first in nonphotosynthetic bacteria (1) and later in those capable of photosynthesis (2, 3). Mutants of different photosynthetic bacteria lacking the acidic nonphosphorus sulfolipid sulfoquinovosyldiacylglycerol showed impaired growth only under phosphate-limited growth conditions (2, 4). This led to the hypothesis that sulfolipid can replace the acidic thylakoid phospholipid phosphatidylglycerol under these growth conditions (5). An inverse relationship between sulfolipid and phosphatidylglycerol as a function of phosphate availability was also observed for Arabidopsis (6). Experiments with the phosphate-deficient pho1 mutant of Arabidopsis (7) suggested that not only sulfolipid but also the amounts of digalactosyldiacylglycerol are increased after phosphate deprivation (8). Thus, it seemed reasonable to ask whether the substitution of phospholipids by nonphosphorous lipids is a more general phenomenon in plants.The nonphosphorous galactolipids mono-and digalactosyldiacylgycerol (MGDG and DGDG) constitute the bulk of membrane lipids in green tissues of seed plants where they are known to be located in the photosynthetic membranes (thylakoids) of the chloroplasts (9, 1...

The sulfolipid 6-sulfo-alpha-D-quinovosyldiacylglycerol is associated with the thylakoid membranes of many photosynthetic organisms. Previously, genes involved in sulfolipid biosynthesis have been characterized only in the purple bacterium Rhodobacter sphaeroides. Unlike plants and cyanobacteria, photosynthesis in this bacterium is anoxygenic due to the lack of a water splitting photosystem II. To test the function of sulfolipid in an organism with oxygenic photosynthesis, we isolated and inactivated a sulfolipid gene of the cyanobacterium Synechococcus sp. PCC7942. Extensive analysis of the sulfolipid-deficient null mutant revealed subtle changes in photosynthesis related biochemistry of O2. In addition, a slight increase in the variable room temperature chlorophyll fluorescence yield was observed. Regardless of these changes, it seems unlikely that sulfolipid is an essential constituent of a functional competent water oxidase or the core antenna complex of photosystem II. However, reduced growth of the mutant under phosphate-limiting conditions supports the hypothesis that sulfolipid acts as a surrogate for anionic phospholipids under phosphate-limiting growth conditions.

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., ...

Xanthophyll-cycle kinetics as well as the relationship between the xanthophyll de-epoxidation state and Stern-Volmer type nonphotochemical chlorophyll (Chl) fluorescence quenching (qN) were investigated i n barley (Hordeom vulgare L.) leaves comprising a stepwise reduced antenna system. For this purpose plants of the wild type (WT) and the Chl b-less mutant chlorina 3613 were cultivated under either continuous (CL) or intermittent light (IML). Violaxanthin (V) availability varied from about 70% in the WT up to 97 to 98% in the mutant and IML-grown plants. I n CL-grown mutant leaves, de-epoxidation rates were strongly accelerated compared to the WT. This is ascribed to a different accessibility of V to the de-epoxidase due to the existence of two V pools: one bound to light-harvesting Chl a/b-binding complexes (LHC) and the other one not bound. Epoxidation rates ( k ) were decreased with reduction in LHC protein contents: kWT > k,,,,,,,, >> k,(IML This supports the idea that the epoxidase activity resides on certain LHC proteins. lrrespective of huge zeaxanthin and antheraxanthin accumulation, the capacity to develop qN was reduced stepwise with antenna sim. l h e qN leve1 obtained in dithiothreitol-treated CL-and IML-grown plants was almost identical with that in untreated IML-grown plants. l h e findings provide evidence that structural changes within the LHC proteins, mediated by xanthophyll-cycle operation, render the basis for the development of a maior proportion of qN.Higher plants are able to nonradiatively dissipate energy absorbed in excess to what can be utilized in photosynthesis and thus protect themselves from photodamage. This is reflected as qN. The buildup of qN was found to be influenced in a complex manner. The major (rapidly reversible) proportion of qN is associated with acidification of the thylakoid lumen in response to light exposure (for a review, see Horton and Ruban, 1992). There is mounting evidence that this proyortion of qN arises from the LHCs (for a recent review, see Horton et al., 1994). Studies of plants with altered antenna composition have served to substantiate this idea (Lokstein et al., 1993(Lokstein et al., , 1994 Briantais, * This work was supported in part by the Deutsche Forschungsgemeinschaft (grants Nos. Ho 1757/1 and Gr 936/4) and by a doctoral fellowship of the "Studienstiftung des deutschen Volkes"

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