Comparative transcriptome analysis reveals key genes in the regulation of squalene and beta-sitosterol biosynthesis in Torreya grandis

Torreya grandis (T. grandis, Taxaceae) is both economically and medicinally valuable species being rich in various bioactive compounds (e.g., squalene and beta-sitosterol). However, the contents of these compounds are various and cultivar specific, and the complicated regulatory mechanisms of their biosynthesis in T. grandis are still unknown. To uncover the underlying molecular mechanisms that control the differences in the accumulation of squalene and beta-sitosterol, a comprehensive transcriptome was constructed from nine different T. grandis cultivars. A total of more than 60,372 unigenes were obtained, of which over 60% were successfully annotated. Identification and expression analysis of the differentially-expressed genes (DEGs) showed that 39 candidate genes were involved in squalene and beta-sitosterol biosynthesis in T. grandis seeds. In particular, the expression patterns of genes related to the mevalonate (MVA) and methylerythritol phosphate (MEP) pathways indicates that both pathways promote the upstream biosynthesis of isopentenyl diphosphate (IPP) and dimethylallyl di phosphate (DMAPP) in different T. grandis cultivars. Moreover, several key regulatory steps controlling the differential accumulation of squalene and beta-sitosterol between T. grandis cultivars were also discussed.