Steroidal saponin profiles and their key genes for synthesis and regulation in Asparagus officinalis L. by joint analysis of metabolomics and transcriptomics

Asparagus officinalis L. is a worldwide cultivated vegetable enrichened in both nutrient and steroidal saponins with multiple pharmacological activities. The upstream biosynthetic pathway of steroidal saponins (USSP) for cholesterol (CHOL) synthesis has been studied, while the downstream pathway of steroidal saponins (DSSP) starting from cholesterol and its regulation in asparagus remains unknown. Metabolomics, Illumina RNAseq, and PacBio IsoSeq strategies were applied to different organs of both cultivated green and purple asparagus to detect the steroidal metabolite profiles contents and to screen their key genes for biosynthesis and regulation. The results showed that there is a total of 427 compounds, among which 18 steroids were detected with fluctuated concentrations in roots, spears and flowering twigs of two garden asparagus cultivars. The key genes of DSSP include; steroid-16-hydroxylase (S16H), steroid-22-hydroxylase (S22H) and steroid-22-oxidase-16-hydroxylase (S22O-16H), steroid-26-hydroxylase (S26H), steroid-3-β-glycosyltransferase (S3βGT) and furostanol glycoside 26-O-beta-glucosidases (F26GHs) which were correlated with the contents of major steroidal saponins were screened, and the transcriptional factors (TFs) co-expressing with the resulted from synthetic key genes, including zinc fingers (ZFs), MYBs and WRKYs family genes were also screened. Based on the detected steroidal chemical structures, profiles and contents which correlated to the expressions of screened synthetic and TFs genes, the full steroidal saponin synthetic pathway (SSP) of asparagus, including its key regulation networks was proposed for the first time.