title: Tracking synthesis and turnover of triacylglycerol in leaves.
abstract: Triacylglycerol (TAG), typically represents <1% of leaf glycerolipids but can accumulate under stress and other conditions or if leaves are supplied with fatty acids, or in plants transformed with regulators or enzymes of lipid metabolism. To better understand the metabolism of TAG in leaves, pulse-chase radiolabelling experiments were designed to probe its synthesis and turnover. When Arabidopsis leaves were incubated with [(14)C]lauric acid (12:0), a major initial product was [(14)C]TAG. Thus, despite low steady-state levels, leaves possess substantial TAG biosynthetic capacity. The contributions of diacylglycerol acyltransferase1 and phospholipid:diacylglycerol acyltransferase1 to leaf TAG synthesis were examined by labelling of dgat1 and pdat1 mutants. The dgat1 mutant displayed a major (76%) reduction in [(14)C]TAG accumulation whereas pdat1 TAG labelling was only slightly reduced. Thus, DGAT1 has a principal role in TAG biosynthesis in young leaves. During a 4h chase period, radioactivity in TAG declined 70%, whereas the turnover of [(14)C]acyl chains of phosphatidylcholine (PC) and other polar lipids was much lower. Sixty percent of [(14)C]12:0 was directly incorporated into glycerolipids without modification, whereas 40% was elongated and desaturated to 16:0 and 18:1 by plastids. The unmodified [(14)C]12:0 and the plastid products of [(14)C]12:0 metabolism entered different pathways. Although plastid-modified (14)C-labelled products accumulated in monogalactosyldiacylglycerol, PC, phosphatidylethanolamine, and diacylglcerol (DAG), there was almost no accumulation of [(14)C]16:0 and [(14)C]18:1 in TAG. Because DAG and acyl-CoA are direct precursors of TAG, the differential labelling of polar glycerolipids and TAG by [(14)C]12:0 and its plastid-modified products provides evidence for multiple subcellular pools of both acyl-CoA and DAG.
authors.affiliation: Department of Plant Biology and Department of Energy, Great Lakes Bioenergy Research Center Michigan State University, East Lansing, MI 48824, USA henrik@msu.edu.
authors.forename: Henrik
authors.initials: H
authors.lastname: Tjellström
authors.affiliation: Department of Plant Biology and Department of Energy, Great Lakes Bioenergy Research Center Michigan State University, East Lansing, MI 48824, USA.
authors.forename: Merissa
authors.initials: M
authors.lastname: Strawsine
authors.affiliation: Department of Plant Biology and Department of Energy, Great Lakes Bioenergy Research Center Michigan State University, East Lansing, MI 48824, USA.
authors.forename: John B
authors.initials: JB
authors.lastname: Ohlrogge
mesh_terms: D017360:Arabidopsis / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D002247:Carbon Isotopes / Q000032:analysis / Q000378:metabolism; D018515:Plant Leaves / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D013194:Staining and Labeling; D014280:Triglycerides / Q000096:biosynthesis* / Q000737:chemistry / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D002247:Carbon Isotopes; D014280:Triglycerides
keywords: Acyl-CoA; DGAT; diacylglycerol acyltransferase; leaf TAG; lipids; triacylglycerol.