The toc132toc120 heterozygote mutant of Arabidopsis thaliana accumulates reduced levels of hexadecatrienoic acid

A null and heterozygous mutant for the Arabidopsis thaliana TOC132 and TOC120 genes accumulates increased levels of 16:0 and decreased 16:3, suggesting altered homeostasis in fatty acid synthesis. The FAD5 gene encodes a plastid desaturase that catalyzes the first step in the synthesis of 16:3 in monogalactosyldiacylglycerol (MGDG). In non-acclimated toc132. toc120+/- mutant plants, the FAD5 gene was repressed and this correlated with decreased levels of 16:3. In cold-acclimated mutant however, the FAD5 gene was upregulated and there was a small increase in 16:3 levels relative to the non-acclimated mutant plants. The MGD1 gene was expressed at control levels and the mutant accumulated levels of MGDG that were similar to the wild type. In the mutant however, MGDG had decreased 16:3 levels, suggesting that the activity of FAD5 desaturase was compromised. In the mutant, the FAD2 and FAD3 genes were downregulated but levels of 18:3-PC were increased, suggesting posttranscriptional regulation for the ER-localized fatty acid desaturases. The Toc120 or Toc159 receptor is likely to compensate for a defective Toc132 receptor. In the cold-acclimated mutant, the TOC159 gene was repressed ca. 300-fold, whereas the TOC120 gene was repressed 7-fold relative to the non-acclimated wild type. Thus, the TOC159 gene is more sensitive to cold-stress and might not compensate for defect in the TOC132 gene under these conditions. Overall, these data show that a mutation in the TOC132 gene results in decreased 16:3 levels, indicating the need for an intact Toc132/Toc120 receptor, presumably to facilitate the import of the FAD5 preprotein into chloroplasts.