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

Abstract

The galactolipids, mono- and digalactosyldiacylglycerol (DGDG), are the most common nonphosphorous lipids in the biosphere and account for 80% of the membrane lipids found in green plant tissues. These lipids are major constituents of photosynthetic membranes (thylakoids), and a large body of evidence suggests that galactolipids are associated primarily with plastid membranes in seed plants. A null-mutant of Arabidopsis (dgd1), which lacks the DGDG synthase (DGD1) resulting in a 90% reduction in the amount of DGDG under normal growth conditions, accumulated DGDG after phosphate deprivation up to 60% of the amount present in the wild type. This observation suggests the existence of a DGD1-independent pathway of galactolipid biosynthesis. The fatty acid composition of the newly formed DGDG was distinct, showing an enrichment of 16-carbon fatty acids in the C-1 position of the glycerol backbone of DGDG. Roots with their rudimentary plastids accumulated large amounts of DGDG after phosphate deprivation, suggesting that this galactolipid may be located in extraplastidic membranes. Corroborating evidence for this hypothesis was obtained directly by fractionation of subcellular membranes from leaf tissue and indirectly by lipid analysis of the phosphate-deprived fad3 mutant primarily deficient in extraplastidic fatty acid desaturation. The discovery of extraplastidic DGDG biosynthesis induced by phosphate deprivation has revealed a biochemical mechanism for plants to conserve phosphate. Apparently, plants replace phospholipids with nonphosphorous galactolipids if environmental conditions such as phosphate deprivation require this for survival.

Figure 1

Schematic representation of the three pathways of galactolipid biosynthesis discussed in the text. The glycerol backbones with the typical carbon length of fatty acids in the C-1 and C-2 positions are indicated to illustrate the main molecular species derived from each pathway. The head groups are always at the glycerol C-3 position. Three pools of galactolipids are indicated, representing the plastid-derived pool (I) in the plastid, the ER-derived pool in the plastid (II), and the ER-derived pool located in extraplastidic membranes (III). Pool III accumulates after phosphate deprivation. Reactions that are blocked in the act1 and dgd1 mutants are indicated. Broken lines separate plastid, cytosol, and ER. DAG, diacylglycerol; FAS, fatty acid synthase; Gal, galactose; PC, phosphatidylcholine; PCho, phosphocholine; PA, phosphatidic acid.

Figure 2

Appearance (Top) and lipid composition (Bottom) of Arabidopsis homozygous wild type and pho1, dgd1, and dgd1/pho1 mutant lines grown for 6 weeks on soil. Lipids were separated on activated ammonium sulfate-impregnated silica gel TLC plates and visualized with iodine vapor. The lipids were identified by cochromatography with lipid extracts of known composition. In addition, glycolipids were verified by specific staining with α-naphthol reagent. The numbers at the bands indicate the relative proportion (mol%) of the respective lipids. Abbreviations not mentioned in the legend to Fig. 1. PE, phosphatidylethanolamine; PG, phosphatidylglycerol; PI, phosphatidylinositol; SQDG, sulfoquinovosyldiacylglycerol.

Figure 3

Fatty acid composition of MGDG and DGDG in the wild-type and pho1, dgd1, and dgd1/pho1 mutant lines. (Upper) Ratio of total 16- to 18-carbon fatty acids in each lipid. (Lower) Relative abundance (mol%) of 16- (open bars) and 18-carbon fatty acids (closed bars) at the glycerol C-2 position of MGDG and DGDG. The proportion of fatty acids (FA) in the C-2 position of the glycerol backbone was determined from the lyso derivatives after digestion of the lipids with Rhizopus sp. lipase. Values represent the means of at least three measurements. The standard error is indicated.

Figure 4

Changes in the total amounts of inorganic phosphate (A and B) and relative amounts of DGDG (C and D) in leaves (A and C) and roots (B and D) of wild-type and the dgd1 mutant of Arabidopsis with decreasing phosphate concentration in the nutrient medium. Open symbols denote wild-type and closed symbols dgd1 mutant values. The means of at least four measurements are shown.