|AFITLHILE, MESHACK - Western Illinois University|
|WORKMAN, SAMANTHA - Western Illinois University|
|DUFFIELD, KAYLA - Western Illinois University|
|SPOUT, DANIELLE - Western Illinois University|
Submitted to: Plant Physiology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/14/2013
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 was used to propose that the enzymes encoded by the genes affected by the 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. This research will be of value to researchers through greater understanding of how plants control the synthesis of oils.
Technical Abstract: Previous studies have shown that a null mutant of Arabidopsis that lacks Toc159 receptor is impaired in chloroplast biogenesis and incapable of importing photosynthetic proteins. The mutant is referred to as plastid protein import 2 or ppi2, and has an albino phenotype. In this study, we measured fatty acid composition and the transcript levels of plastid-localized fatty acid desaturases in wild type and ppi2. The objective was to evaluate whether Toc159 receptor was critical in the import of lipid synthesizing enzymes. The data showed that ppi2 accumulates the normal level of fatty acids and transcripts level of fatty acid desaturases 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 hexadecatrienoic acid (16:3), a fatty acid found exclusively in the chloroplast lipid monogalactosyldiacylglycerol (MGDG). Although the expression level of MGD1 synthase was comparable between wild type and ppi2, the mutant had reduced levels of MGDG. This indicates that expression of MGD1 might be regulated mainly at posttranscriptional level and MGD1 activity could be tightly linked to the developmental status of the plastids. Nevertheless, in cold-acclimated plants the expression of MGD1 synthase was repressed in both wild type and ppi2. Interestingly, in wild type the repressed MGD1 expression was mirrored by 15 mol% decrease in levels of MGDG. In non-acclimated and cold-acclimated ppi2, transcript levels of MGD1 synthase were similar and levels of MGDG were the same in both treatments. This data shows tight correlation between the expression of MGD1 synthase and levels of MGDG in ppi2 and wild type. Since ppi2 accumulated high levels of 16:3, we conclude that in the mutant MGD1 synthase was efficient at utilizing prokaryotic diacylglycerol and fad5 desaturase was catalytically more active. Furthermore, since ppi2 accumulated normal level of fatty acids and the expression of stearoyl-ACP desaturase, fad5 and MGD1 genes were comparable or higher than in wild type, we propose that the enzymes encoded by these genes are imported into plastids in a pathway that is independent of Toc159 receptor, most likely atToc132/Toc120 import pathway.