Transcription factor that acts in the long day flowering pathway and may mediate between the circadian clock and the control of flowering. Plays a role in the regulation of flowering time by acting on 'SUPPRESSOR OF OVEREXPRESSION OF CO1', 'TERMINAL FLOWER 1' and 'FLOWERING LOCUS T'. Also regulates P5CS2 and ACS10 (involved in proline and ethylene biosynthesis, respectively). Regulates the expression of NAKR1 by binding to the 5'-TGTG(N2-3)ATG-3' motif (PubMed:27255839). Interacts with ADO3, SPA1, SPA2, SPA3 and SPA4 (PubMed:16854975, PubMed:22628657). Interacts with MRG1 and MRG2 (via MRG domain) (PubMed:25211338). Interacts (via B-box) with MIP1A (PubMed:27015278). Interacts with AS1 to form a functional complex regulating FT expression (PubMed:21950734). Interacts with NFYC9 (PubMed:25105952). Component of a red light-dependent nuclear complex made of PHL, PHYB and CO. Interacts directly with PHL in the presence of PHYB (PubMed:24127609). Expressed in leaves, shoots and shoot apical meristem. Detected in the vascular tissue of the hypocotyl, the cotyledons and the leaves. Restricted to the protoxylem and phloem in young inflorescence stems and to the phloem only in older inflorescences. Also detected in the vascular tissue of the root. Expressed with a circadian rhythm showing a broad peak between 12 hours and dawn. Higher expression under long days. The CCT domain is essential for the interaction with SPA1. 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.