Previous studies suggested thatHd1promoted heading under short-day conditions (SD) and delayed heading under long-day conditions (LD). However in this study,Hd1was demonstrated to consistently promote heading date in Zhenshan 97 (ZS97) background by upregulatingEhd1,Hd3aandRFT1expression under both SD and LD. While the high photoperiod sensitivity ofHd1was observed in Minghui 63 (MH63) background, with heading being suppressed in LD but promoted in SD. Comparative analysis of two sets of near isogenic lines ofHd1in MH63 and ZS97 backgrounds indicated that the alternative functions ofHd1in promoting or suppressing heading under LD are dependent on the previously cloned flowering repressor geneGhd7. The interaction between proteins Ghd7 and Hd1 occurred through binding of the CCT domain of Ghd7 to the transcription-activating domain of Hd1, resulting in suppression ofEhd1and florigen gene expression. The involvement of the transcription-activating domain of Hd1 in this protein-protein interaction probably blocked or weakened its transcriptional activity. These findings suggest thatHd1alone essentially acts as a promoter of heading date, and the protein interaction between Ghd7 and Hd1 determines photoperiod sensitivity and integratedHd1-mediated andEhd1-mediated flowering pathways in rice.