Figure 1.

Major flowering genes in the photoperiodic and temperature network in Arabidopsis, rice, wheat and orchids. (A) Flowering of Arabidopsis is promoted by long days (LDs). The photoperiod signaling cascade involves GI and CO with modulation by EARLY FLOWERING4 (ELF4). ELF4 negatively regulates CO expression by sequestering GI from the CO promoter. CO positively regulates expression of floral integrator FT that mediates SOC1activation. The vernalization response in Arabidopsis through VERNALIZATION INSENSITIVE3 (VIN3) family proteins represses FLC gene family members, which represses FT and SOC1. In addition, AGL24 participates in the FLC–independent vernalization response to regulate LFY expression. The ambient temperature affects flowering time in Arabidopsis through SVP-FLM-β repressor complexes that delay flowering by repressing FT and SOC1 transcription at low ambient temperature. (B) In rice, a short day plant, the OsGI -Hd1 (CO in Arabidopsis)-Hd3a (FT in Arabidopsis) cascade is well conserved. Hd1 and Ehd1 positively regulate Hd3a and promote flowering under inductive short days (SDs). However, under LDs, Ghd7 is a major flowering suppressor that represses Ehd1 and Hd1 also represses Hd3a expression. Instead, RFT1 is activated through OsMADS50 and Ehd1 for eventual flowering under LDs. OsMADS50 and OsMADS56 function antagonistically to affect flowering under LDs by controlling expression of Ehd1. Rice AP1 homologs such as OsMADS14 and OsMADS15 are activated by florigens (e.g., Hd3a and RFT1) in the SAM. (C) In wheat, the TaFT1/VRN3 gene integrates photoperiod (via PPD1–WCO) and vernalization (via VRN1–VRN2). The photoperiodic signals involving PPD1 are transmitted to negative regulator WCO1 and positive regulator TaHd1 to control TaFT1/VRN3 expression under SD. VRN2 prevents flowering before vernalization but later, vernalization induces VRN1, which is followed by the downregulation of VRN2, thereby releasing TaFT1/VRN3 which leads to flowering. ODDSOC2 (TmOS2/TaAGL33) functions as a vernalization-regulated flowering repressor. It is downregulated by cold independently of VRN1. (D) In orchids, Phalaenopsis PhalCOL and PaFT1 were regulated by photoperiod and low ambient temperature, respectively. Doritaenopsis DhEFL2, 3 and 4, as floral repressors, coordinate floral transition in the photoperiodic flowering pathway. DnVRN1 and DnFT from Dendrobium nobile are linked to low temperature (10°C)-induced floral transition. Dendrobium Chao Praya Smile DOSOC1 is expressed specifically in floral meristemsduring the transition from the vegetative to the reproductive phase and coupled with the upregulation of its LFY. DnAGL19 expression of D. nobile repressed by polycomb-group complexes is activated after vernalization reminiscent of Arabidopsis AGL19 pathway to activate LFY and AP1 for flowering. Arrows and T-shaped bars are regulatory links for the promotion and repression of gene transcription, respectively. The numbers next to the arrows and T-shaped bars are references to related studies and the dashed lines indicate predictions of the flowering regulation networks. Prediction of orchids flowering networks should be confirmed by more extensive studies.