Genome-Wide Analysis of Oxidosqualene Cyclase Genes in Artemisia annua: Evolution, Expression, and Potential Roles in Triterpenoid Biosynthesis

Abstract

Plant triterpenoids are structurally diverse specialized metabolites with significant ecological, medicinal, and agricultural importance. Oxidosqualene cyclases (OSCs) catalyze the crucial cyclization step in triterpenoid biosynthesis, generating the fundamental carbon skeletons that determine their structural diversity and biological functions. Genome-wide identification of OSC genes was performed using bioinformatics tools, including HMMER and BLASTP, followed by phylogenetic analysis, gene structure analysis, conserved domain and motifs identification, cis-regulatory element prediction, protein-protein interaction analysis, and expression profiling using publicly available transcriptome data from UV-B treated A. annua six-week-old seedlings. We identified 24 AaOSC genes, classified into CAS, LAS, LUS, and unknown subfamilies. Phylogenetic analysis revealed evolutionary relationships with OSCs from other plant species. Gene structure analysis showed variations in exon-intron organization. Promoter analysis identified cis-regulatory elements related to light responsiveness, plant growth and development, hormone signaling, and stress response. Expression profiling revealed differential expression patterns of AaOSC genes under UV-B irradiation. This genome-wide characterization provides insights into the evolution and functional diversification of the OSC gene family in A. annua. The identified AaOSC genes and their regulatory elements lay the foundation for future studies aimed at manipulating triterpenoid biosynthesis for medicinal and biotechnological applications, particularly focusing on enhancing stress tolerance and artemisinin production.

Keywords: Artemisia annua; gene family; genome-wide analysis; oxidosqualene cyclase (OSC); triterpenoids.

Figure 6

Predicted protein–protein interaction (PPI) network of AaOSC genes. Nodes represent proteins, and edges represent predicted interactions. Node size indicates ‘popularity’ (number of interactions). Node color indicates the protein domain. Edge color indicates the interaction weight based on a gradient from light orange (lower weight) to dark orange (higher weight).

Figure 1

Comparative analysis of physicochemical properties of oxidosqualene cyclase (OSC) genes across five plant species. Gene length distribution (amino acid count). Molecular weight (MW, kDa) of OSC proteins. Hydrophobicity (GRAVY index). Isoelectric point (pI). Species: Arabidopsis thaliana, Artemisia annua, Helianthus annuus, Oryza sativa Japonica, and Vitis vinifera.

Figure 2

Phylogenetic relationships, domain architecture, conserved motifs, and gene structure of AaOSC genes.

Figure 3

Cis-regulatory element analysis of AaOSC gene promoters. The phylogenetic tree on the left indicates the relationships between the 24 AaOSC genes analyzed. The stacked bar chart displays the relative proportion of CRE categories (light responsiveness, plant growth and development, plant hormone-related, and stress-related). The heatmap represents the number of each CRE type found in the promoter region of each gene. The color intensity in the heatmap corresponds to the number of elements as indicated in the legend.

Figure 4

Phylogenetic relationships of AaOSC genes. The tree was constructed using neighbor-joining method based on the amino acid sequences of 12 AaOSC genes and other OSCs from various plant species. AaOSC genes are marked with asterisks. Numbers at the nodes indicate bootstrap support values (%).

Figure 5

Gene Ontology (GO) enrichment analysis of AaOSC genes. GO terms are categorized into Biological Process (BP), Cellular Component (CC), and Molecular Function (MF). The size of each circle represents the number of AaOSC genes associated with the corresponding GO term, and the x-axis indicates the number of genes associated with each GO term.

Figure 7

Heatmap representation of the expression profiles of AaOSC genes under UV irradiation. Columns correspond to different UV treatment conditions (0 h, 2 h, 4 h, and 6 h of UV exposure), with numbers indicating replicates. Bootstrap values for the phylogenetic tree are indicated by node shading.