Involved in regulation of circadian rhythm, and in the control of the photoperiodic flowering. Acts as suppressor of flowering under short-day (SD) and long-day (LD) conditions. Activates Hd1/CONSTANS gene. Expressed with a circadian rhythm with the highest level 8 hours into the light and the lowest level at dawn. The peak of expression in long days is slightly lower, shifted later and the decrease is slower. Belongs to the GIGANTEA family.