Low ambient temperature induces flowering of Osmanthus fragrans via SOC1A/SOC1B protein complex-mediated activation of FT

Temperature critically influences flowering time, yet the molecular mechanisms governing this process in perennial woody species remain largely elusive. Based on flowering characteristics, Osmanthus fragrans can be divided into two major groups: once-flowering types and continuous-flowering types (the Asiaticus group). Among these, the floral transition of continuous-flowering types is precisely regulated by ambient temperature. Here, using O. fragrans 'Sijigui' (a cultivar of the Asiaticus group) as experimental material, we combined phenotypic observation with paraffin sectioning analysis under different ambient temperature treatments. The results showed that relatively low ambient temperature induced floral transition, whereas relatively high ambient temperature suppressed this process. By conducting transcriptome sequencing and weighted gene co-expression network analysis, we found that SOC1A and SOC1B, members of the MADS-box family, play key roles in ambient temperature induced flowering of O. fragrans. SOC1A/1B are localized in the nucleus and cytoplasm, and their expression and promoter activity are significantly induced by low ambient temperature. Overexpression of SOC1A/1B significantly activates the expression of flowering-related genes and promotes flowering in transgenic plants. Yeast one-hybrid (Y1H), electrophoretic mobility shift assay (EMSA) and dual-luciferase reporter assays verified that both SOC1A and SOC1B directly target and activate the downstream flowering gene FT. Intriguingly, GST pull-down, split-luciferase complementation (Split-LUC), and bimolecular fluorescence complementation (BiFC) assays confirmed that SOC1A physically interacts with SOC1B both in vitro and in vivo to form a protein complex. Furthermore, their protein interaction enhances the transcriptional activation of FT, thereby synergistically modulating low ambient temperature induced flowering in O. fragrans. In conclusion, our study initially identified the key genes involved in low ambient temperature-induced flowering of O. fragrans, laying a foundation for revealing the molecular mechanism of ambient temperature regulating flowering in perennial woody plants.