Provided is a crystal of a florigen activation complex, including a florigen, a 14-3-3 protein, and a bZIP transcription factor bound to each other. In addition, flowering of a plant is regulated by controlling mechanisms of interactions among those proteins utilizing the crystal. These are achieved as follows. With attention focused on the fact that Hd3a is bound to FD1 via GF14c to form a florigen activation complex, a crystal of the florigen activation complex is produced, conformational information is obtained through the use of the crystal of the florigen activation complex, and the flowering of a plant is regulated by controlling mechanisms of interactions among the florigen and the like utilizing such conformational information.