Discovery of genes involved in onjisaponin biosynthesis from Polygala tenuifolia

Radix Polygalae, which is the root of Polygala tenuifolia has been used as a traditional medicine for tonic, sedative, and expectorant, and it has been recently reported to be effective for cognitive disorders, e.g. Alzheimer's disease. Onjisaponins possess complicated chemical structures and they are known to be major active constituents that exhibit various pharmacological action in Radix Polygalae. Biosynthetic route of onjisaponins is speculated as follows: (1) cyclization of β-amyrin from 2,3-oxidosqualene, (2) hydroxylation steps from β-amyrin to tenuigenin, and (3) glycosylation and esterification of the sapogenin. However, their biosynthetic enzymes/genes have not been fully elucidated. Thus, we have attempted to discover the genes involved in onjisaponin biosynthesis in P. tenuifolia. For the purpose, we have performed a high-throughput RNA-seq analysis using MiSeq. Approximate 20 million clean reads were generated and assembled de novo into 70,989 unique transcripts and 55,133 unigenes. After annotation of gene description, gene ontology, and conserved protein domain search, candidate genes involved in saponin biosynthesis were selected on the basis of gene annotation with combination of the expression data in aerial part and roots. As results, a putative β-amyrin synthase and a putative CYP450 could be predicted to be involved in biosynthesis of oleanolic acid, which is an important intermediate of onjisaponin biosynthesis. In addition, other CYP450 gene candidates (43 unigenes), those predominantly expressed in roots and 26 putative UDP glycosyl transferase unigenes were also identified. These data might help to understand saponin biosynthetic machinery in P. tenuifolia.