| Summary: | Sunflower (<i>Helianthus annuus</i> L.) is an important oil crop with rich nutrients, and genetically engineered breeding has become an important way to improve its quality. In this study, five varieties of oilseed sunflower were analyzed for fatty acid (FA) content. The seed embryos of one of the high oleic acid (OA) varieties were transcriptome sequenced at different stages. The results showed that OA synthesis dominated the unsaturated fatty acid (UFA) synthesis pathways in seed embryos. Substantially differentially expressed genes were detected at various post-flowering stages. Specifically, the up-regulated gene numbers were highest at 10 d after flowering, while most genes were down-regulated at 20 d after flowering. The enriched genes were rather consistent with almost all experimental groups exhibiting enrichment to the <i>FAD2</i> gene. The expression of <i>FAD2</i> was highly negatively correlated with the expressions of <i>FAD6, FAD3,</i> and <i>FAD7</i>. During seed embryo development, the expression level of <i>FAD2</i> was highly negatively correlated with the final OA content and was highly positively correlated with the final linoleic acid (LA) content. This suggests that the <i>FAD2</i> is a key enzyme catalyzing the OA to LA conversion.
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