Transcriptomic investigation of the biochemical function of 7-dehydrocholesterol reductase 1 from the traditional Chinese medicinal plant Anemarrhena asphodeloides Bunge

Anemarrhena asphodeloides Bunge (Liliaceae) is an important Traditional Chinese Medicine herb, which contains up to 6 % total steroidal saponins (timosaponins) and has multiple pharmacological properties. However, the timosaponin biosynthetic pathway has not been extensively investigated. Here we conducted de novo transcriptome sequencing and analysis of A. asphodeloides Bunge and screened for candidate genes involved in the timosaponin biosynthetic pathway. Targeted metabolite analysis showed that timosaponins primarily accumulated in rhizomes, while phytosterols (including cholesterol) were distributed throughout various organs. Most of the identified candidate genes of the timosaponin biosynthetic pathway were also highly expressed in the rhizome, consistent with the results of metabolic analysis. Based on the transcriptome results, two candidate 7-dehydrocholesterol reductase genes were cloned and heterologously expressed in the yeast Saccharomyces cerevisiae. The purified and identified products supported that Aa7DR1 possessed Delta(7)-reduction activity in yeast and therefore may be involved in the timosaponins biosynthetic pathway in A. asphodeloides Bunge. Phylogenetic analysis showed Aa7DR1 belongs to monocotyledonous Delta(7) reductase of phytosterol biosynthesis. These data expand our understanding of timosaponin biosynthesis.

Automated summary

Anemarrhena asphodeloides Bunge, an important Traditional Chinese Medicine herb containing high levels of total steroidal saponins, primarily accumulates timosaponins in the rhizomes. Analysis of candidate genes involved in the timosaponin biosynthetic pathway revealed their high expression in the rhizomes, consistent with metabolic analysis results. Cloning and expression of two 7-dehydrocholesterol reductase genes in yeast supported their involvement in timosaponin biosynthesis. Phylogenetic analysis identified Aa7DR1 as a monocotyledonous Delta(7) reductase in phytosterol biosynthesis. This study expands our understanding of timosaponin biosynthesis.