The Role of Diglycosyl Lipids in Photosynthesis and Membrane Lipid Homeostasis in Arabidopsis

Abstract

The galactolipid digalactosyldiacylglycerol (DGD) is an abundant thylakoid lipid in chloroplasts. The introduction of the bacterial lipid glucosylgalactosyldiacylglycerol (GGD) from Chloroflexus aurantiacus into the DGD-deficient Arabidopsis (Arabidopsis thaliana) dgd1 mutant was previously shown to result in complementation of growth, but photosynthetic efficiency was only partially restored. Here, we demonstrate that GGD accumulation in the double mutant dgd1dgd2, which is totally devoid of DGD, also complements growth at normal and high-light conditions, but photosynthetic efficiency in the GGD-containing dgd1dgd2 line remains decreased. This is attributable to an increased susceptibility of photosystem II to photodamage, resulting in reduced photosystem II accumulation already at normal light intensities. The chloroplasts of dgd1 and dgd1dgd2 show alterations in thylakoid ultrastructure, a phenotype that is restored in the GGD-containing lines. These data suggest that the strong growth retardation of the DGD-deficient lines dgd1 and dgd1dgd2 can be primarily attributed to a decreased capacity for chloroplast membrane assembly and proliferation and, to a smaller extent, to photosynthetic deficiency. During phosphate limitation, GGD increases in plastidial and extraplastidial membranes of the transgenic lines to an extent similar to that of DGD in the wild type, indicating that synthesis and transport of the bacterial lipid (GGD) and of the authentic plant lipid (DGD) are subject to the same mechanisms of regulation.Higher plants contain two galactolipids, monogalactosyldiacylglycerol (MGD) and digalactosyldiacylglycerol (DGD), that constitute about 75 mol % of the thylakoid lipids in chloroplasts (Benson, 1971;Joyard et al., 1998; Dörmann and Benning, 2002). MGD and DGD are synthesized from UDP-Gal and diacylglycerol by MGD and DGD synthases in the chloroplast envelope membranes. While MGD is mostly produced from diacylglycerol originating from the plastid (''prokaryotic lipid''), DGD is largely derived from endoplasmic reticulum (ER) lipid precursors (''eukaryotic lipid'';Heinz and Roughan, 1983;Browse et al., 1986b). The lipid transport from the ER to the plastid is not well understood. However, recent results suggest that an ATP-binding cassette-type transport complex is involved in the transfer of eukaryotic lipids from the ER to the chloroplast (Xu et al., 2003). Galactolipids do not only establish the lipid bilayer into which the photosynthetic complexes are embedded. Structural analysis of crystallized protein complexes revealed that galactolipids are also found within the structures of PSI and PSII, light-harvesting complex II (LHCII), and cytochrome b 6 /f (Jordan et al., 2001;Stroebel et al., 2003;Liu et al., 2004;Loll et al., 2005;Jones, 2007). MGD and DGD play crucial roles in photosynthetic light reactions, and this is in agreement with results obtained from the analysis of galactolipid-deficient Arabidopsis (Arabidopsis thaliana) mutants (Härtel et al., 1997;Reifarth et al., 1997;Guo et al., 2...

Reduced Biosynthesis of Digalactosyldiacylglycerol, a Major Chloroplast Membrane Lipid, Leads to Oxylipin Overproduction and Phloem Cap Lignification in Arabidopsis

“…It has been shown that replacing DGDG with GGDG in the dgd1 mutant can restore plant growth and chloroplast shape but not photosynthesis (Hölzl et al, 2006(Hölzl et al, , 2009. Together with our data, this shows that the dgd1 phenotypes are caused by three factors.…”

“…When observed by electron microscopy, instead of being spindle-shaped, like wild-type chloroplasts, dgd1-1 chloroplast envelopes are usually more rounded, but the thylakoids remain elongated, resulting in bent thylakoids and large thylakoid-free stromal areas (Dörmann et al, 1995;Hölzl et al, 2009). As shown in Figure 7, the dgd1-2 allele showed the same phenotype as the dgd1-1 allele, whereas chloroplasts in the aos and coi1-30 mutants had the same morphology as the wild-type chloroplasts, and all double mutants exhibited rounded chloroplasts with bent thylakoids, as in the dgd1 mutants.…”

“…It also has an altered chloroplast morphology, with a rounded envelope enclosing bent thylakoid membranes and large thylakoid-free stromal areas; interestingly, the most pronounced phenotype is stunted growth, in particular, extremely short inflorescence stems, short petioles, and ruffled leaves (Dörmann et al, 1995). Complementation of the dgd1 single mutant or the dgd1 dgd2 double mutant with a bacterial glucosyl transferase, thereby replacing DGDG with glucosylgalactosyl diacylglycerol (GGDG), does not fully rescue the photosynthesis defect, showing that the galactose moiety in DGDG has specific functions in photosynthesis and cannot be replaced by glucose, but the bacterial glucosyl transferase does restore growth and chloroplast morphology (Hölzl et al, 2006(Hölzl et al, , 2009. Therefore, it was suggested that the altered chloroplast morphology is the primary cause of growth retardation.…”

Configuration of the sugar head of glycolipids in thylakoid membranes

“…In Synechococcus, accumulation of ββGlcGDG by introducing β-GlcT could not complement the knockout of dgdA (Maida and Awai, 2016). Considering that ββGlcGDG can partially complement photosynthetic efficiency with increased susceptibility of photosystem II to photodamage (Hölzl et al, 2009) and that dgdA mutants of Synechocystis have a similar phenotype (Mizusawa et al, 2009a(Mizusawa et al, , 2009b, this result implies that αβDGDG has specific roles in the recovery from photodamage.…”

“…This led to various defects in the mutant, especially the dwarf phenotype, mainly because of overproduction of oxylipin and lignification of the phloem cap (Lin et al, 2016). These phenomena were partially complemented by introducing Chloroflexus aurantiacus glycosyltransferase (β-GlcT) (Hölzl et al, 2006(Hölzl et al, , 2009, which synthesizes…”