The Role of Glycolipids in Photosynthesis

Abstract

SummaryA variety of sugars with different epimeric and anomeric configurations and linkages are incorporated into head groups of glycolipids. The occurrence of glycoglycerolipids, glycolipids containing sugar residues directly linked to diacylglycerol, is restricted to photosynthetic organisms and some bacteria. Thylakoid membranes of chloroplasts and cyanobacteria are characterized by a unique set of three glycoglycerolipids, that is, monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldia-cylglycerol (SQDG), and one phosphoglycerolipid, phosphatidylglycerol (PG). While glycoglycerolipids are the predominant lipids in thylakoids, DGDG and SQDG accumulate to even higher levels during phosphate deprivation. The strict correlation of galactolipids with oxygenic photosynthesis was previously taken as evidence for a specific role in photosynthetic light reactions. This was later confirmed by numerous studies on cyanobacteria, Chlamydomonas and plants. Mutants with decreased content of galactolipids are characterized by growth retardation, decreased chlorophyll content and reduced photosynthetic activity. DGDG is crucial for the structural integrity of the photosystem II (PS II) donor site, assembly of light harvesting complex II (LHCII) trimers and stability of PS I. MGDG plays an important role in the xanthophyll cycle activity, and the two galactolipids are integral constituents of different photosynthetic pigment—protein complexes. The analysis of transgenic plants accumulating alternative glycoglycerolipids showed that galactose is the preferred sugar in thylakoid lipids for photosynthesis. Analysis of mutants disrupted in SQDG biosynthesis revealed that SQDG is dispensable for photosynthesis in anoxygenic bacteria and in the cyanobacterium Synechococ-cus. In contrast, photoautotrophic growth of Synechocystis, another cyanobacterium, and of Chlamydomonas, depends on SQDG. SQDG is not essential for photosynthesis in Arabidopsisunder optimal conditions. Under phosphate limitation, SQDG is important for photosynthetic activity because SQDG as an anionic lipid partially replaces PG. Anoxygenic photosynthetic bacteria contain a more diverse set of phospho- and glycoglyc-erolipids, but little information is available on the role of the glycoglycerolipids in photosynthesis. MGDG is an important constituent of the chlorosome monolayer, and a glucosylgalactosyldiacylglycerol is associ- ated with the Rhodobacter reaction center. Taken together, glycoglycerolipids in plants and bacteria not only establish the lipid matrix of thylakoids, but they also play an important role for the activity of photosynthetic pigment—protein complexes and replace phospholipids during phosphate deprivation.