title: A mutant of the Arabidopsis thaliana TOC159 gene accumulates reduced levels of linolenic acid and monogalactosyldiacylglycerol.
abstract: Previous studies have shown that a mutant of Arabidopsis that lacks the Toc159 receptor is impaired in chloroplast biogenesis. The mutant is referred as plastid protein import 2 or ppi2 and has an albino phenotype due to its inability to import the photosynthetic proteins. In this study, we measured fatty acid composition and transcript levels of plastid-localized fatty acid desaturases in the wild type and ppi2 mutant. The objective was to evaluate whether the Toc159 receptor was critical in the import of lipid-synthesizing enzymes. The ppi2 mutant accumulated decreased levels of oleic acid (18:1) and α-linolenic acid (18:3). The mutant accumulated drastically reduced amounts of the chloroplast lipid monogalactosyldiacylglycerol (MGDG), which contains more than 80% of 18:3. The expression of genes that encode stearoyl-ACP desaturase and MGD1 synthase were down-regulated in the ppi2 mutant, and this corresponded to decreased levels of 18:1 and MGDG, respectively. We conclude that in the ppi2 mutant the impaired synthesis of MGDG resulted in decreased amounts of 18:3. The mutant however, had a 30-fold increase in fad5 transcript levels; this increase was mirrored by a 16- to 50-fold accumulation of hexadecatrienoic acid (16:3), a fatty acid found exclusively in MGDG. Taken together, these data suggest that the Toc159 receptor is required in the import of stearoyl-ACP desaturase and MGD1 synthase into the chloroplasts. Since the expression of fad5 gene was up-regulated in the ppi2 mutant, we propose that fad5 desaturase is imported into plastids through the atToc132/atToc120 protein import pathway.
authors.affiliation: Department of Biological Sciences, Western Illinois University, Waggoner Hall 311, Macomb, IL 61455, USA. Electronic address: m-afitlhile@wiu.edu.
authors.forename: Meshack
authors.initials: M
authors.lastname: Afitlhile
authors.forename: Samantha
authors.initials: S
authors.lastname: Workman
authors.forename: Kayla
authors.initials: K
authors.lastname: Duffield
authors.forename: Danielle
authors.initials: D
authors.lastname: Sprout
authors.forename: Mark
authors.initials: M
authors.lastname: Berhow
mesh_terms: D017360:Arabidopsis / Q000201:enzymology / Q000235:genetics* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism; D002736:Chloroplasts / Q000378:metabolism*; D015536:Down-Regulation; D044943:Fatty Acid Desaturases / Q000235:genetics* / Q000378:metabolism; D020558:GTP Phosphohydrolases / Q000235:genetics* / Q000378:metabolism; D038983:Galactolipids / Q000235:genetics* / Q000378:metabolism; D005700:Galactosyltransferases / Q000235:genetics / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D008565:Membrane Proteins / Q000235:genetics* / Q000378:metabolism; D009154:Mutation; D019301:Oleic Acid / Q000235:genetics / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism; D017962:alpha-Linolenic Acid / Q000235:genetics* / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D038983:Galactolipids; D008565:Membrane Proteins; D011956:Receptors, Cell Surface; C403444:TOC159 protein, Arabidopsis; C495595:Toc120 protein, Arabidopsis; C009909:monogalactosyldiacylglycerol; D017962:alpha-Linolenic Acid; D019301:Oleic Acid; D044943:Fatty Acid Desaturases; D005700:Galactosyltransferases; C109990:1,2-diacylglycerol 3-beta-galactosyltransferase; D020558:GTP Phosphohydrolases
keywords: 16:3; 18:3; Arabidopsis thaliana; Fad; Fad5; MGD1 synthase; MGDG; TOC; fatty acid desaturases; hexadecatrienoic acid; monogalactosyldiacylglycerol; plastid protein import 2; ppi2; translocon at the outer envelope of chloroplasts; α-linolenic acid