Syringa oblata is a high ornamental value tree owing to its elegant colors, unique aromas and wide adaptability, however, studies on the molecular mechanism underlying the formation of its ornamental traits are still lacking. Here, we presented a chromosome-scale genome assembly of S. oblata and the final genome size was 1.11 Gb with a contig N50 of 4.75 Mb, anchored on 23 chromosomes and was a better reference for S. oblata transcriptome assembly. Further by integrating transcriptomic and metabolic data, it was concluded that F3H, F3’H, 4CL and PAL, especially the F3’H, were important candidates involved in the formation of floral color differences among S. oblata individuals. Genome-wide identification and analysis revealed that the TPS-b subfamily was the most abundant subfamily of TPS family in S. oblata, which together with the CYP76 family genes determined the formation of the major floral volatiles of S. oblata. Overall, our results provide an important reference for mechanistic studies on the main ornamental traits and molecular breeding in S. oblata.

Geranyl diphosphate synthase (GPS) and farnesyl diphosphate synthase (FPS) catalyzes the biosynthesis of monoterpenoids and sesquiterpenoids, two key precursors for terpenoids biosynthesis. Here, the open reading frame (ORF) sequences of GPS (named CiGPS) and FPS (named CiFPS) with lengths of 1278 bp and 1035 bp were cloned from Chrysanthemum indicum var. aromaticum (C. indicum var. aromaticum). Phylogenetic analysis showed that CiGPS and CiFPS genes were closely related to Artemisia annua and Chrysanthemum × morifolium, respectively. qRT-PCR results showed that CiGPS and CiFPS gene had the highest expression level in leaves, while the expression of CiFPS was induced to methyl jasmonate. Overexpression of CiGPS and CiFPS in tobacco resulted in shorter plant height, darker leaf color, higher chlorophyll, and carotenoid content; the number of long handle trichomes increased significantly, while short handle trichomes only increased significantly on the upper epidermis of leaves of CiGPS transgenic tobacco lines. The relative content of terpenes of CiGPS and CiFPS transgenic tobacco lines increased significantly and monoterpenoids and sesquiterpenoids were detected in transgenic tobacco without wild type (WT) and empty vector (EV) lines. The activity of GPS enzyme was increased in CiGPS transgenic tobacco lines, but FPS enzyme activity in CiFPS transgenic tobacco lines was non-significant. Transcriptional analyses revealed that the expression levels of terpenoid biosynthesis pathway genes were higher in transgenic tobacco lines than in WT and EV lines. All the results demonstrated that GPS and FPS cDNA were effective to improve growth traits and alter terpenoid metabolism of C. indicum var. aromaticum.