Transcriptomic Analysis, Cloning, Characterization, and Expression Analysis of Triterpene Biosynthetic Genes and Triterpene Accumulation in the Hairy Roots of Platycodon grandiflorum Exposed to Methyl Jasmonate

Platycodon grandiflorum is a perennial plant that has been used for medicinal purposes. Specifically, it is widely used in Northern China and Korea for the treatment of various diseases. Terpenoids belong to a group called secondary metabolites and have attracted a wide range of interest. Here, we determined the expressed sequence tag (EST) library of the methyl jasmonate (MeJA)-treated hairy root of P. grandiflorum. In total, 5760 ESTs were obtained, but after deleting the vector sequences and removing poor-quality sequences, a total of 2536 ESTs were attained. Of these, 811 contigs and 1725 singletons were annotated. The data were further analyzed with a focus on the gene families of the terpenoid biosynthetic pathway (TBP). We identified and characterized four TBP genes; among these were three full-length cDNAs encoding PgHMGS, PgMK, and PgMVD, whereas PgHMGR had a partial sequence based on the EST library database. Phylogenetic analysis and a pairwise identity matrix showed that these identified genes were closely related to those of other higher plants. Moreover, the tertiary structure and multiple alignment analysis showed that several distinct conserved motifs were present. Quantitative reverse transcription-polymerase chain reaction results revealed that TBP genes were constitutively expressed in all organs of P. grandiflorum, while the expression of transcript levels of these genes varied distinctly among the organs. Additionally, the total amount of platycosides was highly detected in the root, accumulating in concentrations 7.8 and 2.6 times higher than in the hairy root and stem, respectively, and 1.4 times higher than in the leaf and flower. The highest amount of total phytosterols was found to accumulate in the leaves at 9.3, 9.1, 1.8, and 1.6 times higher than that of the stem, root, hairy root, and flower, respectively. After the hairy root was exposed to the MeJA treatment, the transcript levels of PgHMGS, PgHMGR, PgMK, and PgMVD had significantly increased. The highest level of transcript accumulation occurred at 3 h after initial exposure for most of the genes. Meanwhile, triterpene saponin accumulation increased with the increase in the time of exposure, and at 48 h after initial exposure, the total saponin content was the highest recorded.