Integrative analysis of transcriptomic and volatile compound profiles sheds new insights into the terpenoid biosynthesis in tree peony

Tree peony (Paeonia suffruticosa Andr.), a traditional Chinese ornamental plant, is famous for its large, colorful, and fragrant flowers. Studies on tree peony have mainly focused on the improvement of flower color and the regulation of the flowering stage, but the molecular mechanisms controlling the formation of floral fragrance has yet to be understood. In this study, the aromatic tree peony variety ‘High Noon’ was used to determine whether volatile compounds (volatiles) in flowers were synthesized at the balloon, initial opening, half opening, or full opening stages. Transcriptomic sequencing was conducted on petals and stamens collected at the balloon and full opening stages. The relative content of linalool peaked during the full opening stage of flowering, and linalool was the main component of flower fragrance. As flowering progressed, the relative content of monoterpenes gradually increased and the relative content of sesquiterpenes gradually decreased. Transcriptome data revealed 23695 differentially expressed genes (DEGs), 142 of which were related to terpene accumulation. Transcriptome analysis and correlation analysis between floral volatiles and gene expressions showed that, 1-deoxy-d-xylulose 5- phosphate synthase (DXS), 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR), acetyl-CoA acetyltransferase (AACT), geranyl pyrophosphate synthase (GPPS), and terpene synthase (TPS) may be major regulators controlling floral terpene biosynthesis in ‘High Noon’. And PsGPPS may regulate the expression of GPPS, and then participate in the synthesis of terpenes. This study provides a possible molecular mechanism for terpene formation in tree peony ‘High Noon’ flowers as well as a theoretical basis upon which to study tree peony flowers.