SUGAR-DEPENDENT6 Encodes a Mitochondrial Flavin Adenine Dinucleotide-Dependent Glycerol-3-P Dehydrogenase, Which Is Required for Glycerol Catabolism and Postgerminative Seedling Growth in Arabidopsis

Abstract

The aim of this study was to clone and characterize the SUGAR-DEPENDENT6 (SDP6) gene, which is essential for postgerminative growth in Arabidopsis (Arabidopsis thaliana). Mutant alleles of sdp6 were able to break down triacylglycerol following seed germination but failed to accumulate soluble sugars, suggesting that they had a defect in gluconeogenesis. Mapbased cloning of SDP6 revealed that it encodes a mitochondrial flavin adenine dinucleotide (FAD)-dependent glycerol-3-P (G3P) dehydrogenase:ubiquinone oxidoreductase called FAD-GPDH. This gene has previously been proposed to play a role both in the break down of glycerol (derived from triacylglycerol) and in NAD 1 /NADH homeostasis. Germinated seeds of sdp6 were severely impaired in the metabolism of [U-14 C]glycerol to CO 2 and accumulated high levels of G3P. These data suggest that SDP6 is essential for glycerol catabolism. The activity of the glycolytic enzyme phosphoglucose isomerase is competitively inhibited by G3P in vitro. We show that phosphoglucose isomerase is likely to be inhibited in vivo because there is a 6-fold reduction in the transfer of 14 C-label into the opposing hexosyl moiety of sucrose when [U- 14C]glucose or [U-14 C]fructose is fed to sdp6 seedlings. A block in gluconeogenesis, at the level of hexose phosphate isomerization, would account for the arrested seedling growth phenotype of sdp6 and explain its rescue by sucrose and glucose but not by fructose. Measurements of NAD 1 and NADH levels in sdp6 seedlings also suggest that NAD 1 /NADH homeostasis is altered, and this observation is consistent with the hypothesis that SDP6 participates in a mitochondrial G3P shuttle by cooperating with the cytosolic NAD-dependent GPDH protein GPDHC1.

Central Metabolic Responses to Ozone and Herbivory Affect Photosynthesis and Stomatal Closure

“…The suppression of photosynthesis and the regulation of glycerol and mitochondrial metabolism during O 3 stress may dissipate excess energy to avoid the formation of oxidative radicals (Hoefnagel et al, 1998), combining fast NAD + recycling with the maintenance of central metabolism and growth (Dizengremel et al, , 2012. This hypothesis is supported by previous studies on the physiological functions of glycerol metabolism in NADH/NAD + homeostasis (Shen et al, 2006;Quettier et al, 2008), osmotic stress responses (Biela et al, 1999;Eastmond, 2004;Chen et al, 2011;Geijer et al, 2012), and plant development (Hu et al, 2014). These pathways were redirected in the sequential treatment, during which glycerolipid resources may have been reallocated toward JA signaling and defense against herbivores (Turner et al, 2002;Kachroo et al, 2004;Havko et al, 2016).…”

“…Pathway visualization in KEGG/KaPPAView4 ( Fig. 6F) showed that GPDHc1 and SDP6 (located in the cytosol and on the mitochondrial membrane, respectively) constitute the G3P shuttle that is responsible for transporting reducing equivalents to the mitochondrial ETC via dihydroxyacetone phosphate recycling (Shen et al, 2003(Shen et al, , 2006Quettier et al, 2008).…”

Seed lipases: sources, applications and properties - a review

“…Figure 3 shows the hydrolysis of seed triacylglycerol to free fatty acids and glycerol by the action of one or more lipases. The glycerol formed is phosphorylated and subjected to glycogenesis after its conversion to dihydroxyacetone phosphate (DHAP) (Quettier et al 2008). The free fatty acids are transported into the peroxisome, where they are activated into acytil-COA and initiate β-oxidation.…”

Storage oil hydrolysis during early seedling growth

“…Briefly, TAGs are hydrolysed to free fatty acids and glycerol by TAG lipases. The glycerol is phosphorylated and enters gluconeogenesis after its conversion to dihydroxyacetone phosphate [4]. The free fatty acids are transported to the glyoxysome where they are activated to acyl-CoAs and enter the ß-oxidation spiral.…”