Hydrangea arborescens 'Annabelle' Flower Formation and Flowering in the Current Year

The perennial woody plant Hydrangea arborescens 'Annabelle' is of great research value due to its unique mechanism of flower development that occurs in the current year, resulting in decorative flowers that can be enjoyed for a relatively long period of time. However, the mechanisms underlying the regulation of current-year flower development in H. arborescens 'Annabelle' are still not fully understood. In this study, we conducted an associated analysis to explore the core regulating network in H. arborescens 'Annabelle' by combining phenological observations, physiological assays, and transcriptome comparisons across seven flower developmental stages. Through this analysis, we constructed a gene co-expression network (GCN) based on the highest reciprocal rank (HRR), using 509 differentially expressed genes (DEGs) identified from seven flowering-related pathways, as well as the biosynthesis of eight flowering-related phytohormones and signal transduction in the transcriptomic analysis. According to the analysis of the GCN, we identified 14 key genes with the highest functional connectivity that played critical roles in specific development stages. We confirmed that 135 transcription factors (AP2/ERF, bHLH, CO-like, GRAS, MIKC, SBP, WRKY) were highly co-expressed with the 14 key genes, indicating their close associations with the development of current-year flowers. We further proposed a hypothetical model of a gene regulatory network for the development of the whole flower. This model suggested that the photoperiod, aging, and gibberellin pathways, along with the phytohormones abscisic acid (ABA), gibberellin (GA), brassinosteroid (BR), and jasmonic acid (JA), work synergistically to promote the floral transition. Additionally, auxin, GA, JA, ABA, and salicylic acid (SA) regulated the blooming process by involving the circadian clock. Cytokinin (CTK), ethylene (ETH), and SA were key regulators that affected flower senescence. Additionally, several floral integrators (HaLFY, HaSOC1-2, HaAP1, HaFULL, HaAGL24, HaFLC, etc.) were dominant contributors to the development of H. arborescens flowers. Overall, this research provides a comprehensive understanding of the dynamic mechanism underlying the entire process of current-year flower development, thereby offering valuable insights for further studies on the flower development of H. arborescens 'Annabelle'.