Elucidating Scent and Color Variation in White and Pink-Flowered Hydrangea arborescens ‘Annabelle’ Through Multi-Omics Profiling

The color and scent of flowers are vital ornamental attributes influenced by a complex interaction of metabolic and transcriptional mechanisms. Comparative analyses were performed to determine the molecular rationale for these features in <i>Hydrangea arborescens</i>, between the white-flowered variety 'Annabelle' (An) and its pink-flowered variant 'Pink Annabelle' (PA). Gas chromatography-mass spectrometry (GC-MS) detected 25 volatile organic compounds (VOCs) in 'An' and 21 in 'PA', with 18 chemicals common to both types. 'An' exhibited somewhat higher VOC diversity, whereas 'PA' emitted much bigger quantities of benzenoid and phenylpropanoid volatiles, including benzaldehyde, benzyl alcohol, and phenylethyl alcohol, resulting in a more pronounced floral scent. UPLC-MS/MS metabolomic analysis demonstrated obvious clustering of the two varieties and underscored the enrichment of phenylpropanoid biosynthesis pathways in 'PA'. Transcriptomic analysis revealed 11,653 differentially expressed genes (DEGs), of which 7633 were elevated and linked to secondary metabolism. Key biosynthetic genes, including <i>PAL</i>, <i>4CL</i>, <i>CHS</i>, <i>DFR</i>, and <i>ANS</i>, alongside transcription factors such as <i>MYB</i>-specifically <i>TRINITY_DN5277_c0_g1</i>, which is downregulated in 'PA' (homologous to <i>AtMYB4</i>, a negative regulator of flavonoid biosynthesis)-and <i>TRINITY_DN23167_c0_g1</i>, which is significantly upregulated in 'PA' (homologous to <i>AtMYB90</i>, a positive regulator of anthocyanin synthesis), as well as <i>bHLH</i>, <i>ERF</i>, and <i>WRKY</i> (notably <i>TRINITY_DN25903_c0_g1</i>, highly upregulated in 'PA' and homologous to <i>AtWRKY75</i>, associated with jasmonate pathway), demonstrating a coordinated activation of color and fragrance pathways. The integration of metabolomic and transcriptome data indicates that the pink-flowered 'PA' variety attains its superior coloring and aroma via the synchronized transcriptional regulation of the phenylpropanoid and flavonoid pathways. These findings offer novel molecular insights into the genetic and metabolic interplay of floral characteristics in Hydrangea.