Probable transcription factor involved in the regulation of flower development. Required for the promotion of flowering under short day (SD) conditions and the suppression of flowering under long day (LD) conditions. Regulates positively the floral activator HEADING DATE 3a (HD3A) under SD and negatively under LD conditions. Interacts with HAL3 in the dark. Expressed with a circadian rhythm showing a peak at night and then decreasing to reach the lowest levels around the middle of the day in LD conditions. The levels of expression in SD conditions are slightly lower. Phosphorylated by OSK4 in the presence of HDR1. The cultivar Taichung 65 exhibits a long basic vegetative growth and reduced response to photoperiod due to loss-of-function alleles of HD1 and EHD1. This confers almost constant and sufficient vegetative growth periods even in low latitudes where short photoperiod conditions continue almost throughout the year. The GIGANTEA-CONSTANS-FLOWER LOCUS T (GI-CO-FT) pathway to control photoperiodic flowering under LD is conserved between Arabidopsis and rice, but the regulation of the downstream gene by the upstream regulatory gene is reversed in the two species. In Arabidopsis, GI acts as an activator of CO, which in turn activates the floral activator FT under LD conditions. In rice, GI activates HD1/CO in a similar manner to that in Arabidopsis. However, under LD conditions, HD1 suppresses HD3A/FT expression, causing the suppression of flowering. Belongs to the CONSTANS family.