Disruption of the Two Digalactosyldiacylglycerol Synthase Genes DGD1 and DGD2 in Arabidopsis Reveals the Existence of an Additional Enzyme of Galactolipid Synthesis

Abstract

Two genes ( DGD1 and DGD2 ) are involved in the synthesis of the chloroplast lipid digalactosyldiacylglycerol (DGDG). The role of DGD2 for galactolipid synthesis was studied by isolating Arabidopsis T-DNA insertional mutant alleles ( dgd2-1 and dgd2-2 ) and generating the double mutant line dgd1 dgd2 . Whereas the growth and lipid composition of dgd2 were not affected, only trace amounts of DGDG were found in dgd1 dgd2 . The growth and photosynthesis of dgd1 dgd2 were affected more severely compared with those of dgd1 , indicating that the residual amount of DGDG in dgd1 is crucial for normal plant development. DGDG synthesis was increased after phosphate deprivation in the wild type, dgd1 , and dgd2 but not in dgd1 dgd2 . Therefore, DGD1 and DGD2 are involved in DGDG synthesis during phosphate deprivation. DGD2 was localized to the outer side of chloroplast envelope membranes. Like DGD2, heterologously expressed DGD1 uses UDP-galactose for galactosylation. Galactolipid synthesis activity for monogalactosyldiacylglycerol (MGDG), DGDG, and the unusual oligogalactolipids tri-and tetragalactosyldiacylglycerol was detected in isolated chloroplasts of all mutant lines, including dgd1 dgd2 . Because dgd1 and dgd2 carry null mutations, an additional, processive galactolipid synthesis activity independent from DGD1 and DGD2 exists in Arabidopsis. This third activity, which is related to the Arabidopsis galactolipid:galactolipid galactosyltransferase, is localized to chloroplast envelope membranes and is capable of synthesizing DGDG from MGDG in the absence of UDP-galactose in vitro, but it does not contribute to net galactolipid synthesis in planta.

Quantitative Profiling of Arabidopsis Polar Glycerolipids in Response to Phosphorus Starvation. Roles of Phospholipases Dζ1 and Dζ2 in Phosphatidylcholine Hydrolysis and Digalactosyldiacylglycerol Accumulation in Phosphorus-Starved Plants

“…During phosphorus starvation, expression of MGD2/3 and DGD1/2 are induced, activating the galactolipid biosynthetic pathway leading to DGDG accumulation (Kelly and Dö rmann, 2002). This pathway is crucial for DGDG accumulation because the dgd1dgd2 double knockout mutants show undetectable accumulation of DGDG even under phosphorus starvation conditions (Kelly et al, 2003). The induction of PLDzs during phosphorus starvation synchronized well with that of purple acid phosphatases, MGD2/3 and DGD1/2, suggesting that these enzymes may function coordinately in the alteration of membrane lipid composition during phosphorus starvation conditions.…”

Comparative Genomic Analysis Revealed a Gene for Monoglucosyldiacylglycerol Synthase, an Enzyme for Photosynthetic Membrane Lipid Synthesis in Cyanobacteria

“…In plants, the gene for bulk synthesis of DGDG and its paralog have been identified (Dörmann et al, 1999). These enzymes are reported to utilize UDP-Gal as the substrate (Kelly and Dörmann, 2002;Kelly et al, 2003). Cyanobacteria DGDG synthase is also presumed to utilize a nucleotide sugar (probably UDP-Gal; Sato and Murata, 1982a); however, no ortholog of plant DGDG synthase has been found in any cyanobacteria genome.…”

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.…”

“…Previous experiments on phosphate deficiency demonstrated induction of galactolipid and sulfolipid synthesis genes within 24 to 48 h (Essigmann et al, 1998;Awai et al, 2001;Yu et al, 2002;Kelly et al, 2003;Misson et al, 2005;Kobayashi et al, 2009;Bustos et al, 2010;Woo et al, 2012), while changes in galactolipid and sulfolipid contents were generally monitored only after several days of phosphate deficiency. Because hypoxic incubation in our system caused death of the seedlings after more than 24 h, prolonged stress treatments were only performed under submergence conditions.…”