Functional Characterization of Floral Gene Network Reveals a Critical FT1–AP1 Interaction in Flowering Regulation in Longan

Longan (<i>Dimocarpus longan</i> Lour.) is a commercially valuable tropical fruit crop that contains two antagonistic <i>FLOWERING LOCUS T</i> (<i>FT</i>) homologs involved in regulating flowering time. However, how these <i>FT</i> genes interact with flowering regulators <i>FLOWERING LOCUS D</i> (<i>FD</i>) and <i>APETALA1</i> (<i>AP1</i>) remains unknown. Four flowering-related genes in longan, <i>DlFT1</i>, <i>DlFT2</i>, <i>DlAP1</i> and <i>DlFD</i>, were successfully isolated. Expression profiling revealed that all four genes were expressed in leaves and buds across different stages of natural and KClO₃-induced floral bud differentiation. Functional characterization through heterologous overexpression in <i>Arabidopsis thaliana</i> showed that <i>DlAP1</i> significantly promotes early flowering under long-day conditions and induced morphological changes in floral organs and leaves. In contrast, <i>DlFD</i> overexpression had no effect on flowering time. Subcellular localization assays revealed that DlFT1 and DlFT2 localized to both the nucleus and the plasma membrane, while DlAP1 and DlFD localized exclusively to the nucleus. Yeast two-hybrid and bimolecular fluorescence complementation (BiFC) analyses revealed a novel regulatory node: DlFT1 directly interacts with DlAP1, a finding that expands the classical FT-FD-AP1 flowering model. Additionally, DlFD interacts more strongly with DlFT1 than with DlFT2, whereas DlFT1 only interacts with DlAP1, but not DlFT2. These results demonstrate that DlFT1 promotes flowering not only via the conserved FD-dependent pathway but also through direct association with AP1. These findings advance our understanding of the regulatory mechanisms of flowering in longan and provide valuable insights into flowering pathways of perennial woody species.