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.O ne of the most powerful environmental stimuli affecting the overall glycerolipid composition of bacterial membranes is phosphate deprivation. The replacement of phospholipids by nonphosphorous lipids was discovered first in nonphotosynthetic bacteria (1) and later in those capable of photosynthesis (2, 3). Mutants of different photosynthetic bacteria lacking the acidic nonphosphorus sulfolipid sulfoquinovosyldiacylglycerol showed impaired growth only under phosphate-limited growth conditions (2, 4). This led to the hypothesis that sulfolipid can replace the acidic thylakoid phospholipid phosphatidylglycerol under these growth conditions (5). An inverse relationship between sulfolipid and phosphatidylglycerol as a function of phosphate availability was also observed for Arabidopsis (6). Experiments with the phosphate-deficient pho1 mutant of Arabidopsis (7) suggested that not only sulfolipid but also the amounts of digalactosyldiacylglycerol are increased after phosphate deprivation (8). Thus, it seemed reasonable to ask whether the substitution of phospholipids by nonphosphorous lipids is a more general phenomenon in plants.The nonphosphorous galactolipids mono-and digalactosyldiacylgycerol (MGDG and DGDG) constitute the bulk of membrane lipids in green tissues of seed plants where they are known to be located in the photosynthetic membranes (thylakoids) of the chloroplasts (9, 1...

Glycerolipid transfer for the building of membranes in plant cells

“…In plants, phosphate deprivation has been reported to decrease the phospholipid content, consistent with a mobilization of the phosphate reserve, and conversely to increase nonphosphorous membrane lipids such as DGDG and SQDG [3,4]. Moreover a form of DGDG with specific fatty acids: 16:0 at glycerol sn1 position and 18:2 at sn2 position is especially enhanced [5,6].…”

A Shoot-Specific Hypoxic Response of Arabidopsis Sheds Light on the Role of the Phosphate-Responsive Transcription Factor PHOSPHATE STARVATION RESPONSE1

“…Thus, the induction of MGD2 and MGD3 expression contributes to a balanced galactolipid biosynthetic pathway to meet the altered fluxes of lipid precursors through the prokaryotic or eukaryotic lipidsynthesizing pathways. In contrast to this scenario, upon phosphate deprivation, MGD2 and MGD3 together with DGD1 and DGD2 are strongly expressed to produce large amounts of DGDG for plastidial and extraplastidial membranes (Härtel et al, 2000;Kelly et al, 2003;Li et al, 2006). During phosphate deprivation, expression of two sulfolipid synthesis genes, SQD1 and SQD2, is induced, resulting in the strong increase in sulfolipid content (Essigmann et al, 1998;Yu et al, 2002), but these alterations were not observed under hypoxia (Supplemental Fig.…”

“…Germination and seedling growth was carried out in growth chambers at 23°C with a 16-h photoperiod and about 100 mmol photons m -2 s -1 . For phosphate starvation experiments, seedlings grown under control conditions on solidified MS medium were transferred to solidified control (2.5 mM Pi) or Pi-deficient (0 mM Pi) medium (Härtel et al, 2000) at the age of 7 d and grown for another 2 to 10 d. For light/dark treatments on Pi-differing medium, seedlings were transferred to the stress conditions 2 h after the onset of the photoperiod, and for dark incubation, plates were wrapped in aluminum foil immediately after transfer of the seedlings. Growth was continued under long-day conditions for indicated time periods.…”

“…Galactolipids are essential for plastid biogenesis, consequently photosynthesis, and, under normal growth conditions, are produced by Monogalactosyldiacylglycerol synthase1 (MGD1) and Digalactosyldiacylglycerol synthase1 (DGD1) in Arabidopsis (Dörmann et al, 1999;Awai et al, 2001;Kobayashi et al, 2007). During phosphate deficiency, the expression of MGD2 and MGD3 as well as DGD1 and DGD2, encoding MGDG synthases and DGDG synthases, respectively, is induced, and DGDG is produced to replace phospholipids in plant membranes to save phosphate (Härtel et al, 2000;Kobayashi et al, 2009;Shimojima and Ohta, 2011).…”

Double Knockouts of Phospholipases Dζ1 and Dζ2 in Arabidopsis Affect Root Elongation during Phosphate-Limited Growth But Do Not Affect Root Hair Patterning

“…Upon phosphate starvation, plants increase levels of digalactosyldiacylglycerol in roots; this lipid class is believed to functionally replace acidic phospholipids (Härtel et al, 2000). A nonspecific phospholipase C, which hydrolyzes common membrane phospholipids, such as phosphatidylcholine to diacylglycerol, has been shown to increase in expression and activity during phosphate deprivation (Nakamura et al, 2005).…”