Do Galactolipid Synthases Play a Key Role in the Biogenesis of Chloroplast Membranes of Higher Plants?

Abstract

A unique feature of chloroplasts is their high content of the galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), which constitute up to 80% of their lipids. These galactolipids are synthesized in the chloroplast envelope membrane through the concerted action of galactosyltransferases, the so-called ‘MGDG synthases (MGDs)’ and ‘DGDG synthases (DGDs),’ which use uridine diphosphate (UDP)-galactose as donor. In Arabidopsis leaves, under standard conditions, the enzymes MGD1 and DGD1 provide the bulk of galactolipids, necessary for the massive expansion of thylakoid membranes. Under phosphate limited conditions, plants activate another pathway involving MGD2/MGD3 and DGD2 to provide additional DGDG that is exported to extraplastidial membranes where they partly replace phospholipids, a phosphate-saving mechanism in plants. A third enzyme system, which relies on the UDP-Gal-independent GGGT (also called SFR2 for SENSITIVE TO FREEZING 2), can be activated in response to a freezing stress. The biosynthesis of galactolipids by these multiple enzyme sets must be tightly regulated to meet the cellular demand in response to changing environmental conditions. The cooperation between MGD and DGD enzymes with a possible substrate channeling from diacylglycerol to MGDG and DGDG is supported by biochemical and biophysical studies and mutant analyses reviewed herein. The fine-tuning of MGDG to DGDG ratio, which allows the reversible transition from the hexagonal II to lamellar α phase of the lipid bilayer, could be a key factor in thylakoid biogenesis.

Effects of salicylic acid on photosynthetic activity and chloroplast morphology under light and prolonged darkness

“…Moreover, photosynthetic membranes are rich in the non-phosphorous and uncharged glyceroglycolipids. The two main types, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) play also a role in the stabilization of photosystem protein complexes in chloroplasts (Rocha et al 2018). It has been observed recently that 1 mM SA changed the MGDG/DGDG ratio and decreased the MGDG content in flax leaves (Linum usitatissimum L.), which can contribute to the changes in biogenesis and stabilization of chloroplast membranes (Belkadhi et al 2015).…”

Characterisation of genes involved in galactolipids and sulfolipids metabolism in maize and Arabidopsis and their differential responses to phosphate deficiency

“…The photosynthetic membranes surrounding the chloroplast and thylakoid of higher plants are lipid bilayers with embedded proteins, which provide a platform for photosynthesis (Kobayashi et al 2007;Rocha et al 2018). The four main types of glycerolipids that make up the photosynthetic membrane are monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG), and phosphatidylglycerol (PG), and the outer envelope of chloroplast membrane includes a higher scale of typical eukaryotic lipids for instance the phospholipid phosphatidylcholine (PC) (Denev and Minkov 2001;Narayanan et al 2018).…”

“…The galactolipids MGDG and DGDG are synthesised in the chloroplast membrane with synthesis catalysed by two galactosyltransferases -MGDG synthase (MGD) and DGDG synthase (DGD) respectively (Denev and Minkov 2001;Rocha et al 2018). MGD catalyses the galactose conversion from UDPgalactose to the diacylglycerol (DAG) backbone to form MGDG, and then the second galactose is transferred from UDP-galactose to MGDG by DGD for the final formation of DGDG (Li et al 2016a).…”

“…A phosphate-saving mechanism involving the enhanced accumulation of DGDGs has been discovered in plants under phosphate deficient conditions. This mechanism could produce extra-plastidial membranes where they substitute phospholipids (Kobayashi et al 2006;Rocha et al 2018). The proportion of non-phospholipids to phospholipids in Arabidopsis increases sharply under phosphate stress (Härtel et al 2000).…”

An efficient and comprehensive plant glycerolipids analysis approach based on high‐performance liquid chromatography–quadrupole time‐of‐flight mass spectrometer

“…Triacylglycerol supplies energy and essential fatty acids for human health (Calder, 2015;Song et al, 2018;Spector & Kim, 2015), and it also supplies energy for plant seed germination and maintains cellular energy balance for plant growth (Tsai et al, 2015;Xu & Shanklin, 2016;Yang & Benning, 2018). MGDG and DGDG are the major components of the membrane system in chloroplast (Rocha et al, 2018), while DAG plays key roles in cell signaling as a second messenger and a precursor of TAG (Vermeer et al, 2017). Thus, quantitative determination of glycerolipids is becoming important for understanding lipids metabolism and their functions in plants (Peng & Ahrends, 2016).…”