De novo transcriptome sequencing of Torreya grandis reveals gene regulation in sciadonic acid biosynthesis pathway

Torreya grandis (T. grandis), a conifer of the Cephalotaxaceae family, yields rare and unique dried fruits of great nutritional value that contains a high level of novel UFA, sciadonic acid. The composition of sciadonic acid in different tissues and cultivars, and the molecular mechanisms underlying the biosynthesis of sciadonic acid in T. grandis, both of which remain unknown however, were investigated. Two cultivars, T. grandis cv. Yuanfei and Xifei were used to reveal in comparison the fatty acid composition of roots, stems, leaves, arils and kernels. It was found that the Yuanfei kernels had the highest levels of sciadonic acid with 17% in oil. In transcriptome analysis, a total of 80,235 and 82,454 unigenes were generated from five libraries of Yuanfei and Xifei cultivars, respectively. KEGG orthology enrichment of differentially expressed genes showed that lipid metabolism pathways were the most highly represented. In particular, six candidate unigenes (Delta(6)-elongase and Delta(5)-desaturase) encoding sciadonic acid elongase and desaturases with high expression levels in Yuanfei and Xifei kernels were identified. This study provides the first comprehensive genomic analysis of sciadonic acid biosynthesis and contributes to the understanding of the molecular mechanisms responsible for de novo fatty acid biosynthesis and high sciadonic acid accumulation in gymnosperm species.