Encyclopedia of Lipidomics

Living Edition
| Editors: Markus R. Wenk

Monogalactosyldiacylglycerol (MGDG) in Plants: Functional Diversity of

  • Krzysztof ZienkiewiczEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-94-007-7864-1_141-1

Keywords

Phosphatidic Acid Phosphatidic Acid Galactose Residue Octadecatrienoic Acid Neutral Glycolipid 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Synonyms

Definitions

Glycolipid – organic molecule composed of lipid moiety and sugar residue attached with its anomeric carbon to the lipid hydroxyl group by a glycosidic bond. The saccharide part may vary from small sugar units to large polysaccharide chains. Among plant glycolipids, the most common are ethers of glucose, galactose, and sulfoquinovose, serving as headgroups in sphingolipids and glycerolipids.

Structure and Occurrence

Monogalactosyldiacylglycerol (MGDG) is a neutral glycolipid where one galactose residue is bound to the glycerol at sn-3 position to form a hydrophilic headgroup whereas sn-1 and sn-2 positions of the glycerol backbone are esterified with acyl chains (Fig. 1). MGDG is a predominant galactoglycerolipid of photosynthetic membranes. The complete structure of MGDG is 1,2-diacyl-3-O-(β-D-galactopyranosyl)-sn-glycerol and was discovered by Carter et al. (1956). Together with another galactoglycerolipid – digalactosyldiacylglycerol (DGDG) – MGDG contributes to almost 80% (w/w) of all galactolipids being unique for plastid membranes (Table 1), whereas it is underrepresented in non-photosynthetic membranes (Joyard et al. 1998).
Fig. 1

Structure of the monogalactosyldiacylglycerol (MGDG). R1 and R2 indicate acyl chains

Table 1

Relative MGDG content in diverse plant tissues. The lipid content is expressed as % of total lipid (TL)

Specie/tissue/cell compartment

% TL

Reference

Maize (Zea mays) leaf

42

Boudière et al. (2014)

Spinach (Spinacia oleracea) thylakoids

52

Li-Beisson et al. (2016)

Chlamydomonas reinhardtii thylakoids

55

Li-Beisson et al. (2016)

Potato tuber

6

Boudière et al. (2014)

Apple fruit

1

Boudière et al. (2014)

A high content of polyunsaturated fatty acids, like cis-7,10,13-hexadecatrienoic acid (16:3n-3) and cis-9,12,15-octadecatrienoic acid (18:3n-3), is characteristic for MGDG in higher plants. Additionally in algae, MGDG often contains fatty acids of more than 20 carbon atoms and with more than three double bonds (Li-Beisson et al. 2016).

Biosynthesis

MGDG is synthesized in the plastid envelope from diacylglycerol (DAG) by the action of MGDG synthase (MGD), formally known as uridine 5-diphosphate (UDP)-galactose:1,2-diacylglycerol 3-β-D-galactosyltransferase. This enzyme catalyzes the formation of MGDG by transferring a β-galactose residue from a UDP-galactose to the sn-3 position of DAG (Fig. 2). The DAG used for MGDG synthesis can originate either from plastids by dephosphorylation of phosphatidic acid (PA) (prokaryotic pathway) or from the complementary pathway in the endoplasmic reticulum (ER) (eukaryotic pathway) (Botté et al. 2011). Three isoforms of MGD were found in Arabidopsis thaliana and classified into two types: type A (MGD1) and type B (MGD2 and MGD3) (Awai et al. 2001). The knockout of the MGD1 encoding gene results in death soon after embryogenesis revealing the crucial role of MGD1 in proper development of chloroplasts and photosynthetic machinery, whereas MDG2 and MDG3 are thought to be involved in membrane remodeling under phosphate limitation (Kobayashi et al. 2004, 2007, 2013).
Fig. 2

The overview of MGDG biosynthesis and its role in lipid metabolism of plant cells. DAG Diacylglycerol, DGDG Digalactosyldiacylglycerol, JA Jasmonic acid, puMGDG Polyunsaturated monogalactosyldiacylglycerol, TAG Triacylglycerol, UDP Uridine diphosphate, UDP-Gal Uridine diphosphate galactose (Adapted and modified from (Botté et al. (2011))

Function

A conserved presence of MGDG in photosynthetic membranes of evolutionary diverse photosynthetic organisms suggests its prominent role in the structural and functional organization of the photosynthetic apparatus. Complete loss of MGDG synthesis leads to lethal phenotypes, likely because of its role as precursor of the main lipid class of thylakoids. Reduction of MGDG production in higher plants can be however achieved by mutations in the MGD1-encoding gene (Jarvis et al. 2000) or by applying a specific inhibitor of MGDG synthesis – galvestine-1 (Botté et al. 2011). The most common phenotypes resulting from reduction of MGDG synthesis are (1) deficiency (Jarvis et al. 2000) or complete lack (Kobayashi et al. 2007) of chlorophyll content, (2) impaired thylakoid and chloroplast development (Botté et al. 2011; Jarvis et al. 2000), and (3) significant growth defects (Botté et al. 2011). At structural level, MGDG is a non-bilayer-forming lipid that can form reverse micelles. It was suggested that it enables formation of highly curved membrane domains and/or proper structural organization of the membranes at the sites occupied by large protein complexes and within the thylakoid stacks (Bruce 1998). Together with other major glycolipids, MGDG was shown to directly associate with photosystems I and II (Mizusawa and Wada 2012, Loll et al. 2007) as well as with proteins involved in plastid import (Schleiff et al. 2003).

MGDG is a direct substrate for synthesis of the other membrane galactolipid – digalactosyldiacylglycerol (DGDG) (Fig. 2). This reaction is catalyzed by DGDG synthases (DGS) and occurs via MGDG galactosylation (Benning 2009). This leads to the formation of a glycosidic bond in α-configuration between the galactose residues. MGDG as well as DGDG seem to play an essential role in maintaining the physicochemical properties of the thylakoid membranes. The sugar moiety of MGDG can be alternatively transferred to another galactolipid molecule resulting in formation of DAG and galactolipids with a digalactose polar headgroup. This reaction is catalyzed by an enzyme encoded by the Sensitive to Freezing 2 (SFR2) gene in response to cold stress and induces changes in physical properties of chloroplast membranes in the chloroplast leading to their stabilization during freezing (Moellering et al. 2010). However, this reaction leads to the formation of a glycosidic bond in β-configuration between the galactose residues. Additionally, desaturation of fatty acids esterified to MGDG by desaturases (FADs) leads to formation of MGDG reach in polyunsaturated fatty acids (PUFAs). These PUFAs can be in turn converted into oxylipins, like jasmonic acid (JA) (Andreou et al. 2009), or may serve as substrates for TAG synthesis (Padham et al. 2007) (Fig. 2).

Cross-References

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Copyright information

© Springer Science+Business Media B.V. 2017

Authors and Affiliations

  1. 1.Department of Plant Biochemistry, Albrecht-von-Haller-Institute for Plant SciencesGeorg-August-University GöttingenGöttingenGermany