Galactolipids in Plant Membranes

Abstract

Photosynthetic membranes of plants contain high amounts of galactolipids (monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG)) that are indispensable for the efficiency of photosynthetic light reactions. Galactolipids make up the bulk of the thylakoid membranes, and they are also found as integral constituents of the photosystems I and II. The presence of galactolipids is conserved from cyanobacteria and green algae to plants, in accordance with the endosymbiont theory. However, cyanobacteria and plants use different pathways for the synthesis of galactolipids. The ratio of MGDG to DGDG in the thylakoids is crucial for the stabilisation of the membrane bilayer. Under freezing and drought stress, a certain proportion of MGDG is converted into DGDG and oligogalactolipids to prevent fissions and fusions of chloroplast membranes. When plants are grown under phosphate‐limiting conditions, phospholipids are partially replaced by galactolipids in plastidial and extraplastidial membranes, thereby releasing phosphate for other essential cellular processes.

Key Concepts:

  • Galactolipids are phosphorous‐free glycoglycerolipids in plants.

  • Galactolipids make up the bulk of photosynthetic membranes.

  • Oxygenic photosynthesis in cyanobacteria and plants depends on the presence of galactolipids.

  • The ratio of the two galactolipids carrying one or two galactoses in the head group is crucial for the maintenance of membrane integrity during stress.

  • Phospholipids in plants are replaced by galactolipids during phosphate deprivation.

Keywords: lipid; galactose; chloroplast; photosynthesis; phosphate

Figure 1.

Plants contain two major galactolipids, β‐MGDG and α,β‐DGDG. Note that the linkage between glycerol and the first galactose in MGDG and DGDG is β‐glycosidic, whereas the terminal galactose of DGDG is bound in α‐configuration. β,β‐DGDG and oligogalactolipids (e.g. β,β,β‐TGDG) with all‐β‐anomeric configuration are produced in chloroplasts during freezing and drought conditions.

Figure 2.

The enzymes of plant galactolipid synthesis localise to the envelope membranes of chloroplasts. The MGDG synthase MGD1 of Arabidopsis is localised to the inner envelope where it produces MGDG from diacylglycerol and UDP‐galactose. MGDG is subsequently transported to the thylakoid membranes. MGD2 and MGD3 are found in the outer envelope. α,β‐DGDG is produced by the DGDG synthases DGD1 and DGD2 in the outer envelope. SFR2, a GGGT, converts two molecules of MGDG into β,β‐DGDG. Further galactosylation results in the production of oligogalactolipids such as β,β,β‐TGDG. This pathway is activated during freezing or drought stress. The galactose is represented by a black hexagon, and the two fatty acids of the diacylglycerol moiety by waved lines.

Figure 3.

Galactolipids are structural components of photosynthetic complexes in thylakoid membranes. Galactolipids not only make up the bulk lipid phase of the thylakoids, but also in addition, they are found in the crystal structures of light‐harvesting complex II (LHCII), the cytochrome b6f complex (Cytb6f), photosystem I (PSI) and photosystem II (PSII). Electron flow is indicated by red arrows. Yellow hexagons in the head groups of lipids indicate galactose of MGDG or DGDG. Green hexagons depict the sulfoquinovosyl group of sulfolipid SQDG. The letter ‘P’ in the head group of lipids indicates phospholipids (phosphatidylglycerol). WOC, water‐oxidising complex.

Figure 4.

During growth on phosphate‐limiting soils, large amounts of phospholipids are substituted by galactolipids. The phosphate released from phospholipids (indicated by the letter ‘P’ in the head group) is made available for nucleic acid and sugar‐phosphate synthesis. The diacylglycerol moiety is converted into MGDG and DGDG by MGDG and DGDG synthases. Under phosphate‐limiting conditions, DGDG replaces phospholipids in plastidial and extraplastidial membranes.

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Further Reading

Benning C and Ohta H (2005) Three enzyme systems for galactoglycerolipid biosynthesis are coordinately regulated in plants. Journal of Biological Chemistry 280: 2397–2400.

Fyfe PK, Hughes AV, Heathcote P and Jones MR (2005) Proteins, chlorophylls and lipids: x‐ray analysis of a three‐way relationship. Trends in Plant Sciences 10: 275–282.

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Moellering ER and Benning C (2010) Galactoglycerolipid metabolism under stress: a time for remodeling. Trends in Plant Sciences 16: 98–107.

Shimojima M and Ohta H (2011) Critical regulation of galactolipid synthesis controls membrane differentiation and remodeling in distinct plant organs and following environmental changes. Progress in Lipid Research 50: 258–266.

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Dörmann, Peter(Jan 2013) Galactolipids in Plant Membranes. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020100.pub2]