Hd1,a CONSTANS ortholog in rice, functions as an Ehd1 repressor through interaction with monocot‐specific CCT‐domain protein Ghd7

Flowering time is an important agronomic trait that affects crop yields. In cereals, several CCT-domain proteins unique to monocots, such as Grain number, plant height, and heading date 7 (Ghd7) gene, have been identified as key floral repressors, although the corresponding molecular mechanisms have been unknown until now. In rice, a short-day plant, Heading date 1 (Hd1) gene, a rice ortholog of Arabidopsis floral activator CONSTANS (CO), represses flowering under non-inductive long-day (LD) conditions and induces it under inductive short-day (SD) conditions. Here, we report biological interactions between Ghd7 and Hd1, which together repress Early heading date 1 (Ehd1), a key floral inducer under non-inductive LD conditions. In addition to this genetic interaction between them, Co-IP experiments further demonstrated that a Ghd7-Hd1 protein formed a complex in vivo and ChIP and luciferase reporter analyses suggested that this complex specifically binds to a cis-regulatory region in Ehd1 and represses its expression. These findings imply that Hd1, an evolutionally conserved transcriptional activator, can function as a strong transcriptional repressor within a monocot-specific flowering-time pathway through with Ghd7.