Storage oil hydrolysis during early seedling growth

Storage oil breakdown plays an important role in the life cycle of many plants by providing the carbon skeletons that support seedling growth immediately following germination. This metabolic process is initiated by lipases (EC: 3.1.1.3), which catalyze the hydrolysis of triacylglycerols (TAGs) to release free fatty acids and glycerol. A number of lipases have been purified to near homogeneity from seed tissues and analysed for their in vitro activities. Furthermore, several genes encoding lipases have been cloned and characterised from plants. However, only recently has data been presented to establish the molecular identity of a lipase that has been shown to be required for TAG breakdown in seeds. In this review we briefly outline the processes of TAG synthesis and breakdown. We then discuss some of the biochemical literature on seed lipases and describe the cloning and characterisation of a lipase called SUGAR-DEPENDENT1, which is required for TAG breakdown in Arabidopsis thaliana seeds

Implication des protéines tyrosine phosphatases dans la signalisation de l' acide abscissique chez Arabidopsis thaliana

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

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

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. Map-based 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 breakdown of glycerol (derived from triacylglycerol) and in NAD(+)/NADH homeostasis. Germinated seeds of sdp6 were severely impaired in the metabolism of [U-C-14] glycerol to CO2 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 C-14-label into the opposing hexosyl moiety of sucrose when [U-C-14] glucose or [U-C-14] 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(+) and NADH levels in sdp6 seedlings also suggest that NAD(+)/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

Numerical simulation of effectively nonergodic systems

Triacylglycerol (TAG) is a major storage reserve in many plant seeds. We previously identified a TAG lipase mutant called sugar-dependent1 (sdp1) that is impaired in TAG hydrolysis following Arabidopsis (Arabidopsis thaliana) seed germination (Eastmond, 2006). The aim of this study was to identify additional lipases that account for the residual TAG hydrolysis observed in sdp1. Mutants were isolated in three candidate genes (SDP1-LIKE [SDP1L], ADIPOSE TRIGLYCERIDE LIPASELIKE, and COMPARATIVE GENE IDENTIFIER-58-LIKE). Analysis of double, triple, and quadruple mutants showed that SDP1L is responsible for virtually all of the residual TAG hydrolysis present in sdp1 seedlings. Oil body membranes purified from sdp1 sdp1L seedlings were deficient in TAG lipase activity but could still hydrolyze di- and monoacylglycerol. SDP1L is expressed less strongly than SDP1 in seedlings. However, SDP1L could partially rescue TAG breakdown in sdp1 seedlings when expressed under the control of the SDP1 or 35S promoters and in vitro assays showed that both SDP1 and SDP1L can hydrolyze TAG, in preference to diacylglycerol or monoacylglycerol. Seed germination was slowed in sdp1 sdp1L and postgerminative seedling growth was severely retarded. The frequency of seedling establishment was also reduced, but sdp1 sdp1L was not seedling lethal under normal laboratory growth conditions. Our data show that together SDP1 and SDP1L account for at least 95% of the rate of TAG hydrolysis in Arabidopsis seeds, and that this hydrolysis is important but not essential for seed germination or seedling establishment

PHOSPHATIDIC ACID PHOSPHOHYDROLASE1 and 2 Regulate Phospholipid Synthesis at the Endoplasmic Reticulum in Arabidopsis

Phospholipid biosynthesis is essential for the construction of most eukaryotic cell membranes, but how this process is regulated in plants remains poorly understood. Here, we show that in Arabidopsis thaliana, two Mg2+-dependent phosphatidic acid phosphohydrolases called PAH1 and PAH2 act redundantly to repress phospholipid biosynthesis at the endoplasmic reticulum (ER). Leaves from pah1 pah2 double mutants contain similar to 1.8-fold more phospholipid than the wild type and exhibit gross changes in ER morphology, which are consistent with massive membrane overexpansion. The net rate of incorporation of [methyl-C-14]choline into phosphatidylcholine (PC) is similar to 1.8-fold greater in the double mutant, and the transcript abundance of several key genes that encode enzymes involved in phospholipid synthesis is increased. In particular, we show that PHOSPHORYLETHANOLAMINE N-METHYLTRANSFERASE1 (PEAMT1) is upregulated at the level of transcription in pah1 pah2 leaves. PEAMT catalyzes the first committed step of choline synthesis in Arabidopsis and defines a variant pathway for PC synthesis not found in yeasts or mammals. Our data suggest that PAH1/2 play a regulatory role in phospholipid synthesis that is analogous to that described in Saccharomyces cerevisiae. However, the target enzymes differ, and key components of the signal transduction pathway do not appear to be conserved

A phosphatidate phosphatase double mutant provides a new insight into plant membrane lipid homeostasis

Phospholipids make up the bulk of most eukaryotic cell membranes, but how their synthesis is regulated remains relatively poorly understood in plants. In our article1 we provide evidence that two Mg ( 2+) -dependent phosphatidic acid phosphatase enzymes, called PAH1 and PAH2, are capable of repressing phospholipid biosynthesis at the endoplasmic reticulum in Arabidopsis thaliana. The precise mechanism of repression remains unclear and it does appear to vary in several respects from that already described in Saccharomyces cerevisiae. ( 2,3)