Integrative metabolome and transcriptome analyses reveal the molecular mechanism underlying variation in floral scent during flower development of Chrysanthemum indicum var. aromaticum

Identification and functional analysis of terpene synthases revealing the secrets of aroma formation in Chrysanthemum aromaticum

C. aromaticum is widely cultivated for its aromatic, medicinal, and tea-applicable properties, earning the nickname 'lavender in composite'. Terpenoids are the major compounds of C. aromaticum fragrance. To reveal the molecular mechanisms of terpenoid biosynthesis in C. aromaticum, NGS and SMRT sequencing were employed to identify the key terpene synthase genes. A total of 59,903 non-redundant transcripts were obtained by the transcriptome analysis. Twenty-nine terpene synthase genes (TPSs) were identified, and phylogenetic analysis showed that they belong to four subfamilies of terpene synthases. Five CaTPSs were successfully cloned. Subcellular localization showed they were present in the nucleus and cytosol. Structure models of five terpene synthases were predicted, and molecular docking results showed good binding affinities with FPP/GPP. In vitro enzymatic tests showed that CaTPS7, CaTPS8, CaTPS10 and CaTPS20 could catalyze substrates to produce terpenoids. CaTPS7 and CaTPS20 were both able to effectively convert the precursor FPP into caryophyllene. CaTPS8 could convert FPP to trans-nerolidol and nerolidyl acetate, while CaTPS10 could convert FPP to elemene and aristolochene. This study lays the groundwork for further research to depict the metabolism network of terpenoid in C. aromaticum. These identical terpene synthase genes could be introduced into the cultivated chrysanthemums to enhance their fragrance.

Delineating Molecular Regulatory of Flavonoids Indicated by Transcriptomic and Metabolomics Analysis during Flower Development in Chrysanthemum morifolium 'Boju'

Flavonoids are pharmacologically active compounds in flowers of Chrysanthemum morifolium 'Boju' (C. morifolium); however, the molecular regulatory network governing flower development remains largely elusive. Flower samples were collected at four stages, namely budding (BD), bud breaking (BB), early blooming (EB), and full blooming (FB), for omics analysis. We revealed distinct transcriptional regulation patterns at these four stages of the flower from the perspective of differentially expressed unigenes (DEGs). There are 152 DEGs shared among the three comparative groups (BD vs. BB, BB vs EB, EB vs FB), wherein the expression of 44 DEGs (including AtADT6, MDL3, and ROMT) continues to be upregulated, and 85 DEGs (including CYP81E, TPS-Cin-1, and TPS-Cin-2) showed persistent downregulation with flower development. Flavonoid-targeted metabolomics identified 118 differentially abundant metabolites (DAMs) in the FB group compared to the BD stage; the top three upregulated and downregulated metabolites are Cyanidin-3-O-(6″-O-malonyl)glucoside-5-O-glucoside, Luteolin-7-O-(6″-caffeoyl)rhamnoside, Kaempferol-3-O-(6″-p-coumaroyl)glucoside and Chrysoeriol-6,8-di-C-glucoside-7-O-glucoside, Kaempferol, Kaempferol-3,7-O-dirhamnoside, respectively. These DAMs were predominantly enriched in "flavonoid biosynthesis", "isoflavonoid biosynthesis", and "flavone and flavonol biosynthesis" pathways. AtADT6, MDL3, ROMT, CYP81E, TPS-Cin-1, and TPS-Cin-2 were correlated with kaempferol. Our findings provide a new idea for interfering with flavonoid production, especially kaempferol, in flowers.

Identification of Floral Volatile Components and Expression Analysis of Controlling Gene in Paeonia ostii 'Fengdan' under Different Cultivation Conditions

In order to explore the release rule of floral volatile substances and the diurnal variation of different flower development stages of Paeonia ostii 'Fengdan' in potted and ground-planted conditions, dynamic headspace adsorption combined with gas chromatography-mass spectrometry(GC-MS) was used to analyze the dynamic changes in floral volatile components and contents. Quantitative real-time PCR (qRT-PCR) was used to analyze changes in flower fragrance-regulating genes PsPAL, PsTPSs, and PsbHLH at different flower development stages and a daily change process at the full-blooming stage. The results show that there were differences in aroma components and contents of Paeonia ostii 'Fengdan' at different flower development stages and different time quantum of every day. There were 25 and 28 aroma components identified in 7 flower development stages of tree peonies planted in pots and in the field, respectively, and 23 and 22 aroma components identified at different time quantum of the day, of which the largest and highest content was alkanes. The main characteristic aroma substances were (E)-β-ocimene, 1,3,5-trimethoxybenzene, 2,4-di-tert-butylphenol, methyl jasmonate, nerol, and cinnamyl alcohol; released amounts of the abovementioned substances varied depending on the development stage and the time of the day. The expression of flower fragrance-controlling genes (PsPAL, PsTPSs, and PsbHLH) in tree peonies varied greatly in different conditions. The results of this study provide a valuable resource to investigate floral fragrance formation in tree peonies.

Integrated metabolome and transcriptome analysis provides insights on the floral scent formation in Hydrangea arborescens

Hydrangea (Hydrangea arborescens var. 'Annabelle') flowers are composed of sweet aroma sepals rather than true petals and can change color. Floral volatiles play important roles in plants, such as attracting pollinators, defending against herbivores, and signaling. However, the biosynthesis and regulatory mechanisms underlying fragrance formation in H. arborescens during flower development remain unknown. In this study, a combination of metabolite profiling and RNA sequencing (RNA-seq) was employed to identify genes associated with floral scent biosynthesis mechanisms in 'Annabelle' flowers at three developmental stages (F1, F2, and F3). The floral volatile data revealed that the 'Annabelle' volatile profile includes a total of 33 volatile organic compounds (VOCs), and VOCs were abundant during the F2 stage of flower development, followed by the F1 and F3 stages, respectively. Terpenoids and benzenoids/phenylpropanoids were abundant during the F2 and F1 stages, with the latter being the most abundant, whereas fatty acid derivatives and other compounds were found in large amount during the F3 stage. According to ultra performance liquid chromatography - tandem mass spectrometer (UPLC-MS/MS) analysis, benzene and substituted derivatives, carboxylic acids and derivatives, and fatty acyls play a significant role in the floral metabolite profile. The transcriptome data revealed a total of 17,461 differentially expressed genes (DEGs), with 7,585, 12,795, and 9,044 DEGs discovered between the F2 and F1, F3 and F1, and F2 and F3 stages, respectively. Several terpenoids and benzenoids/phenylpropanoids biosynthesis-related DEGs were identified, and GRAS/bHLH/MYB/AP2/WRKY were more abundant among transcription factors (TFs). Finally, DEGs interlinked with VOCs compounds were determined using cytoscape and k-means analysis. Our results paves the way for the discovery of new genes, critical data for future genetic studies, and a platform for the metabolic engineering of genes involved in the production of Hydrangea's signature floral fragrance.