Galactolipid deficiency and abnormal chloroplast development in the Arabidopsis MGD synthase 1 mutant

Abstract

The lipid monogalactosyl diacylglycerol (MGD) is a major structural component of photosynthetic membranes in chloroplasts. Its formation is catalyzed by the enzyme MGD synthase. In many plants, MGD derives from two different biosynthetic pathways: the prokaryotic pathway, which operates entirely within the plastid, and the eukaryotic pathway, which involves steps in the endoplasmic reticulum. Here, we describe the identification and characterization of an Arabidopsis mutant with a defective MGD synthase gene (MGD1). The mutant was identified in a screen of T-DNA lines for individuals with defects in chloroplast biogenesis. It has a yellowgreen phenotype that correlates with a Ϸ50% deficiency in total chlorophyll per plant. A single T-DNA insertion is located adjacent to the transcription initiation site of the MGD1 gene, and the abundance of MGD1 mRNA is reduced by 75% compared with wild type. Correlation between steady-state MGD1 transcript levels and MGD synthase activity (also reduced by 75% in mgd1) suggests that MGD1 is the most important MGD synthase in green tissues. The amount of MGD in mutant leaves is reduced by 42% compared with wild type. MGD from the mutant contains 23% less 16:3 fatty acid and 10% more 18:3 fatty acid. Because 16:3 is a characteristic feature of MGD from the prokaryotic pathway, it is possible that MGD1 operates with some preference in the prokaryotic pathway. Finally, the MGD-deficiency of mgd1 is correlated with striking defects in chloroplast ultrastructure, strongly suggesting a unique role for MGD in the structural organization of plastidic membranes.G alactose-containing lipids are the predominant nonproteinaceous components of photosynthetic membranes in plants, algae, and a variety of bacteria. The two most common galactolipids are monogalactosyl diacylglycerol (MGD) and digalactosyl diacylglycerol (DGD). In plants, MGD and DGD occur exclusively in plastidic membranes, where they account for about 50 and 20 mol% of the lipid matrix, respectively (1). Up to 80% of all lipids in plants are associated with photosynthetic membranes, and MGD is widely considered to be the most abundant membrane lipid on earth. Most vegetables and fruits in human and animal diets are rich in galactolipids, and their breakdown products represent an important dietary source of galactose and polyunsaturated fatty acids (2, 3).Galactolipids play an important role in the organization of photosynthetic membranes. The abundance and physical properties of MGD make it particularly important in this respect. Its small galactose head group and large unsaturated fatty acid chains give it a cone-like molecular shape and a consequent predisposition to form nonlamellar, hexagonal-phase aggregates (4). The molecular shape of MGD may be important for the structural organization of thylakoid membranes. Because much of the photosynthetic apparatus is embedded within thylakoids, the lipids that make up these membranes are of profound importance. Evidence also suggests that MGD is more directly involved in certain ph...

Knocking Down of Isoprene Emission Modifies the Lipid Matrix of Thylakoid Membranes and Influences the Chloroplast Ultrastructure in Poplar

“…These results are consistent with previous studies that indicated a role for MGDGs and DGDGs in the structure of thylakoid membranes (Dörmann et al, 1995;Jarvis et al, 2000). In this study, we observed a significant reduction of grana stacks per chloroplast in NE compared with IE poplar lines (Fig.…”

DGD1-independent biosynthesis of extraplastidic galactolipids after phosphate deprivation in Arabidopsis

“…1 I) is impaired and 16-carbon fatty acid-containing lipids are decreased (13). The mgd1 mutant of Arabidopsis deficient in MGDG synthase activity revealed that MGD1 is responsible for the bulk of MGDG biosynthesis (14), whereas the dgd1 mutant phenotype suggested that the DGD1 gene product (Fig. 1) is responsible for over 90% of DGDG biosynthesis (15).…”

The Regulation of Photosynthetic Structure and Function during Nitrogen Deprivation in Chlamydomonas reinhardtii

“…Thylakoid membrane lipids are central to photosynthetic function, including the stabilization of photosynthetic complexes and oxygen evolution (Jarvis et al, 2000;Jones, 2007;Leng et al, 2008;Mizusawa and Wada, 2012;Boudière et al, 2013). The most abundant classes are the neutral lipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) and the anionic lipids sulfoquinovosyldiacylglycerol (SQGD) and phosphatidylglycerol (PG; Frentzen, 2004;Shimojima et al, 2010;Mizusawa and Wada, 2012).…”