Article

Analysis of Acyl Fluxes through multiple pathways of Triacylglycerol synthesis in developing Soybean embryos

Departments of Biochemistry and Molecular Biology , Michigan State University, East Lansing, Michigan 48824-1312, USA.
Plant physiology (Impact Factor: 7.39). 04/2009; 150(1):55-72. DOI: 10.1104/pp.109.137737
Source: PubMed

ABSTRACT The reactions leading to triacylglycerol (TAG) synthesis in oilseeds have been well characterized. However, quantitative analyses of acyl group and glycerol backbone fluxes that comprise extraplastidic phospholipid and TAG synthesis, including acyl editing and phosphatidylcholine-diacylglycerol interconversion, are lacking. To investigate these fluxes, we rapidly labeled developing soybean (Glycine max) embryos with [(14)C]acetate and [(14)C]glycerol. Cultured intact embryos that mimic in planta growth were used. The initial kinetics of newly synthesized acyl chain and glycerol backbone incorporation into phosphatidylcholine (PC), 1,2-sn-diacylglycerol (DAG), and TAG were analyzed along with their initial labeled molecular species and positional distributions. Almost 60% of the newly synthesized fatty acids first enter glycerolipids through PC acyl editing, largely at the sn-2 position. This flux, mostly of oleate, was over three times the flux of nascent [(14)C]fatty acids incorporated into the sn-1 and sn-2 positions of DAG through glycerol-3-phosphate acylation. Furthermore, the total flux for PC acyl editing, which includes both nascent and preexisting fatty acids, was estimated to be 1.5 to 5 times the flux of fatty acid synthesis. Thus, recycled acyl groups (16:0, 18:1, 18:2, and 18:3) in the acyl-coenzyme A pool provide most of the acyl chains for de novo glycerol-3-phosphate acylation. Our results also show kinetically distinct DAG pools. DAG used for TAG synthesis is mostly derived from PC, whereas de novo synthesized DAG is mostly used for PC synthesis. In addition, two kinetically distinct sn-3 acylations of DAG were observed, providing TAG molecular species enriched in saturated or polyunsaturated fatty acids.

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    • "DAG can also be converted to phosphatidylcholine (PtdC) via the action of phosphatidylcholine:diacylglycerol cholinephosphotransferase (PDCT) or sn-1,2-diacylglycerol:cholinephosphotransferase (CPT). It has been reported in soybean seeds that about 60% of newly synthesized acyl chains directly incorporate into the sn-2 position of PC through an acyl-editing mechanism rather than a pathway for sequential acylation of G3P [89]. PDCT has a clear seed-specific expression, but DAG-CPT does not have a clear tissue-expression–this does not immediately suggest a major role for DAG-CPT in tri-ricinolein synthesis as reported in castor [83]. "
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    • "ies ( 34 : 1 , 34 : 2 , 36 : 2 , and 36 : 3 ) increased significantly ( by 1. 5 - to 2 . 5 - fold at 24 h ) under N - conditions ( Figure 3 ; Supplemental Data Set 3 ) , indicating the possible presence of PC - based acyl editing in membrane lipid turnover and TAG synthesis . PC acyl editing involves rapid deacylation of PC to release acyl - CoA ( Bates et al . , 2007 , 2009 ) , which is then incorporated into TAG . However , N deprivation did not cause any significant changes in most of the PE species , suggesting that these " housekeeping " lipid species may be involved in maintaining the integrity of cellular membrane structure and function ."
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    • "The PA is converted to 1,2-sn-diacylglycerol (DAG) [64] by the action of PA phosphatase and then DAG can be acylated to produce triacylglycerol (TAG). In developing soybean seeds, the major pathway for TAG formation is through conversion of PC to DAG and acylation of DAG to produce TAG [65]. Recently, Lee et al. [51,52] reported that the lipid profile was changed by suppression of PLDα in the soybean seeds and the total lipids and TAG signals tended to decrease in fresh seeds of PLDα-knockdown soybean. "
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