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Genes involved in galactolipid formation and degradation in chloroplasts of Arabidopsis

Peter Dörmann

iIMBIO Institute, University of Bonn, Germanyj

14:25

Roles of phosphatidate phosphatases in glycerolipid biosynthesis

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Acyl transferases involved in plant oil biosynthesis

Sten Stymne

iSwedish University of Agricultural Sciences, Swedenj

16:20

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Peter Dörmann

Genes involved in galactolipid formation and degradation in chloroplasts of Arabidopsis

 

 

Plants contain two galactolipids, mono- and digalactosyldiacylglycerol (MGDG, DGDG), which are synthesized in the chloroplasts by sequential galactosyl transfer from UDP-galactose onto diacylglycerol and MGDG, respectively. MGDG and DGDG are the two most abundant lipids in chloroplasts. During phosphate deprivation, the amount of DGDG increases in the chloroplast and in extraplastidial membranes to replace phospholipids. UDP-galactose is produced from UDP-glucose in the cytosol of the plant cell. Arabidopsis contains five UDP-glucose epimerase genes, (UGE1...5). The isolation of single and multiple uge mutants has previously revealed that these genes are involved in producing UDP-galactose for cell wall synthesis in different plant organs. Lipid measurements in the different uge mutants grown on soil, or on media containing or lacking phosphate revealed overlapping activities for the UGE isoforms with regard to galactolipid synthesis. Quantification of different carbohydrates (glucose, sucrose, raffinose) in the uge mutants after exposure to cold temperature showed that UGE isoforms are involved in generating UDP-galactose for the synthesis of raffinose-family oligosaccharides. Furthermore, the UGE-dependent conversion of UDP-galactose into UDP-glucose is critical for mobilizing carbohydrates during carbon starvation (extended night).

While the biosynthesis of galactolipids has been well studied, much less is known about their degradation during senescence and abiotic stress. We previously showed that fatty acid phytyl esters (FAPEs) accumulate in chloroplasts during senescence. Furthermore, FAPEs were shown to increase in the chs1 (chilling sensitive 1) mutant of Arabidopsis after exposure to low temperature. Hexadecatrienoic acid, 16:3, is the most abundant fatty acid in FAPEs from Arabidopsis, indicating that MGDG-derived fatty acids accumulate in this ester class. FAPEs presumably serve as transient sink for lipid degradation products (phytol and fatty acids). Two genes ("phytyl ester synthases", PES1, PES2) were identified in Arabidopsis by sequence homology searches. T-DNA insertion mutants were obtained, and a double homozygous mutant, pes1 pes2, was generated. Lipid measurements showed that the pes1 pes2 double mutant is free of 16:3-FAPE and other FAPEs, indicating that these genes represent the major FAPE synthesizing activities in Arabidopsis.

 

 

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Roles of phosphatidate phosphatases in glycerolipid biosynthesis

 

 

Diacylglycerol (DAG) supply is a crucial step for glycerolipid biosynthesis in plants. For both membrane and storage lipid biosyntheses, the DAG is provided by phosphatidate phosphatases (PAPs) that hydrolyze phosphatidic acid and produce the DAG and an inorganic phosphate. This DAG is further utilized as a substrate to synthesize various glycerolipids such as galactolipids, phospholipids and triacylglycerol.

We recently reported that Arabidopsis lipin homologs are essential for membrane lipid remodeling under phosphate starvation (Nakamura et al PNAS 2009). Two Arabidopsis lipins, AtPAH1 and AtPAH2, encode PAPs involved in galactolipid biosynthesis. Double mutant pah1pah2 plants had decreased phosphatidic acid hydrolysis, thus affecting the eukaryotic pathway of galactolipid synthesis. Upon phosphate starvation, pah1pah2 plants were severely impaired in growth and membrane lipid remodeling. These results indicate that PAH1 and PAH2 are the PAP responsible for the eukaryotic pathway of galactolipid synthesis and the membrane lipid remodeling mediated by these two enzymes is an essential adaptation mechanism to cope with phosphate starvation. Another role of the PAH1/2 in triacylglycerol synthesis will be discussed in this talk.

 

 

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Sten Stymne

Acyl transferases involved in plant oil biosynthesis

 

 

Triacylglycerols (oils) are formed by acyl transferases acylating each of the three sn positions of the glycerol backbone. Apart from the three acyl-CoA dependent acyl transferases in the straight glycerol 3-phosphate pathway, also the PDAT enzyme, which transfer acyl groups from phospholipids, plays a major role in oil biosynthesis. Further, in order for the oil to contain polyunsaturated fatty acids and unusual fatty acids produced by desaturase-like enzymes, the acyl groups have to do a detour into phospholipids for modification before ending up in the triacylglycerols, a pathway that also requires acyl transferases. I will in this presentation review what is known about the nature of the different acyl transferases leading to triacylglycerol accumulation. I will show that introduction of acyl transferases with certain acyl specificities are pivotal to achieve high amount of species-foreign fatty acids, such as medium chain, very long chain monounsaturated and very long chain polyunsaturated fatty acids, in transgenic crops. I will also point out the knowledge gaps we still have regarding seed acyl transferases.