Location: Functional Foods ResearchTitle: A mutant of the Arabidopsis thaliana TOC159 gene accumulates reduced levels of linolenic acid and monogalactosyldiacylglycerol Author
Submitted to: Plant Physiology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2013
Publication Date: 10/21/2013
Citation: Afitlhile, M., Workman, S., Duffield, K., Sprout, D., Berhow, M. 2013. A mutant of the Arabidopsis thaliana TOC159 gene accumulates reduced levels of linolenic acid and monogalactosyldiacylglycerol. Plant Physiology and Biochemistry. 73:344-350. Interpretive Summary: This work details more information on what genetic controls are in play on the synthesis and accumulation of lipids in the model plant system Arabidopsis. The objective was to evaluate whether Toc159 receptor was critical in the import of lipid synthesizing enzymes. A mutant line, which is defective in plastic protein import, was examined for lipid composition as compared to wild type. The data showed that mutant accumulates the normal level of fatty acids and transcripts level of fatty acid desaturase enzymes were up-regulated at low temperatures. The mutant exhibited 30-fold increase in fad5 transcript levels and this increase was mirrored by 50-fold accumulation of a fatty acid found exclusively in the chloroplast lipid. This information will be used by researchers to develop a further understanding of how enzymes encoded by genes affected by this mutation are imported into plastids in a pathway that is independent of the normal Toc159 receptor, and are most likely imported by alternate Toc132/Toc120 import pathway.
Technical 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 a-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.