Integration of gene expression profiling and triterpenic acids content reveals optimal harvesting seasons in Salvia rosmarinus

Rosemary ( Salvia rosmarinus Spenn.) is one of the most important medicinal and aromatic plants of the Lamiaceae family, widely used in food, pharmaceutical, cosmetic, and hygienic industries. The plant is rich in triterpenic acids including betulinic, oleanolic, and ursolic acids, which possess diverse biological activities. Understanding the biosynthetic pathways and the expression patterns of genes involved in triterpenoid production is essential for optimizing the commercial exploitation of this plant. In addition, seasonal variation in metabolite accumulation highlights the importance of harvesting at an appropriate developmental stage. In the present study, the content of triterpenic acids and the expression of five biosynthetic genes, farnesyl diphosphate synthase ( FDS ), squalene synthase ( SQS ), squalene epoxidase ( SQE ), β-amyrin synthase ( BAS ), and lupeol synthase ( LUS ), were analyzed in monthly harvested leaves of rosemary over one year. Real-time quantitative PCR revealed that the highest relative expression of LUS (7.47 ± 0.10) occurred in summer (September). In contrast, SQS exhibited its maximum expression (8.03 ± 0.13) in June (early summer), whereas the highest expression levels of SQE (5.19 ± 0.15), FDS (4.86 ± 0.06), and BAS (4.84 ± 0.08) were observed during colder months (February, November, and January, respectively). Consistently, the highest contents of betulinic (7.46 ± 0.11 mg/g DW), oleanolic (7.67 ± 0.13 mg/g DW), and ursolic acids (11.21 ± 0.15 mg/g DW) were detected in autumn and winter. Overall, the coordinated increase in gene expression and triterpenic acid accumulation during the cold months suggests that the optimal harvesting period for maximizing the yield of bioactive triterpenoids in rosemary is from October to January.