Molecular cloning, functional characterization and expression of the β-amyrin synthase gene involved in saikosaponin biosynthesis in Bupleurum chinense DC

The chromosome-level genome of Centella asiatica provides insights into triterpenoid biosynthesis

Centella asiatica is a well-known herbal plant that makes a significant contribution to the treatment of various chronic ailments. Triterpenoid saponins are the main active components extracted from C. asiatica, which have rich pharmacological activity. However, only a few studies have systematically elucidated the molecular mechanism underlying the biosynthesis of triterpenoid saponins in C. asiatica. Here, we report a chromosome-level reference genome of C. asiatica, by using Illumina, PacBio HiFi, and Hi-C technologies. The assembled genome exhibits high quality with a size of 455 Mb and a contig N50 of 36 Mb. A total of 26,479 protein-coding genes were predicted. Comparative genomic analysis revealed that the gene families involved in triterpenoid saponin biosynthesis, including squalene synthase (SS) and farnesyl diphosphate synthase (FPS), rapidly expanded in the C. asiatica genome. In particular, we have discovered two whole-genome duplication events in C. asiatica genomes. A further comprehensive analysis of the metabolome and transcriptome was performed using different tissues of C. asiatica in order to identify the key genes associated with triterpenoid saponin biosynthesis. Consequently, seven enzyme genes were considered to play important roles in triterpenoid biosynthesis. Subsequent functional characterization of CaOSC4 demonstrated that it is responsible for the biosynthesis of three ursane-type triterpenoids in C. asiatica. Our research establishes a genomic data platform that can be employed in the excavation of genes and precision breeding in C. asiatica. Additionally, the results offer new insights into the biosynthesis of triterpenoid saponins.

Adjusting Catalytic Activity of β-Amyrin Synthase GgBAS by Utilizing the Plasticity Residues of an Active Site

β-Amyrin synthase (bAS) is a representative plant oxidosqualene cyclase (OSC), and previous studies have identified many functional residues and mutants that can alter its catalytic activity. However, the regulatory mechanism of the active site architecture for adjusting the catalytic activity remains unclear. In this study, we investigate the function of key residues and their regulatory effects on the catalytic activity of Glycyrrhiza glabra β-amyrin synthase (GgbAS) through molecular dynamics simulations and site-directed mutagenesis experiments. We identified the plasticity residues located in two active site regions and explored the interactions between these residues and tetracyclic/pentacyclic intermediates. Based on computational and experimental results, we further categorize these plasticity residues into three types: effector, adjuster, and supporter residues, according to their functions in the catalytic process. This study provides valuable insights into the catalytic mechanism and active site plasticity of GgbAS, offering important references for the rational enzyme engineering of other OSC enzyme.

Integrated metabolome and transcriptome analysis identifies candidate genes involved in triterpenoid saponin biosynthesis in leaves of Centella asiatica (L.) Urban

Centella asiatica (L.) Urban is a well-known medicinal plant which has multiple pharmacological properties. Notably, the leaves of C. asiatica contain large amounts of triterpenoid saponins. However, there have only been a few studies systematically elucidating the metabolic dynamics and transcriptional differences regarding triterpenoid saponin biosynthesis during the leaf development stages of C. asiatica. Here, we performed a comprehensive analysis of the metabolome and transcriptome to reveal the dynamic patterns of triterpenoid saponin accumulation and identified the key candidate genes associated with their biosynthesis in C. asiatica leaves. In this study, we found that the key precursors in the synthesis of terpenoids, including DMAPP, IPP and β-amyrin, as well as 22 triterpenes and eight triterpenoid saponins were considered as differentially accumulated metabolites. The concentrations of DMAPP, IPP and β-amyrin showed significant increases during the entire stage of leaf development. The levels of 12 triterpenes decreased only during the later stages of leaf development, but five triterpenoid saponins rapidly accumulated at the early stages, and later decreased to a constant level. Furthermore, 48 genes involved in the MVA, MEP and 2, 3-oxidosqualene biosynthetic pathways were selected following gene annotation. Then, 17 CYP450s and 26 UGTs, which are respectively responsible for backbone modifications, were used for phylogenetic-tree construction and time-specific expression analysis. From these data, by integrating metabolomics and transcriptomics analyses, we identified CaHDR1 and CaIDI2 as the candidate genes associated with DMAPP and IPP synthesis, respectively, and CaβAS1 as the one regulating β-amyrin synthesis. Two genes from the CYP716 family were confirmed as CaCYP716A83 and CaCYP716C11. We also selected two UGT73 families as candidate genes, associated with glycosylation of the terpenoid backbone at C-3 in C. asiatica. These findings will pave the way for further research on the molecular mechanisms associated with triterpenoid saponin biosynthesis in C. asiatica.