Flowering plants sense various environmental and endogenous signals to trigger the floral transition and start the reproductive growth cycle. CONSTANS (CO) is a master transcription factor in the photoperiod floral pathway that integrates upstream signals and activates the florigen gene FLOWERING LOCUS T (FT). Here, we performed comprehensive structural and biochemical analyses to study the molecular mechanism underlying the regulation of FT by CO in Arabidopsis thaliana. We show that the four previously characterized cis-elements in the FT promoter proximal region, CORE1, CORE2, P1, and P2, are all direct CO binding sites. Structural analysis of CO in complex with NUCLEAR FACTOR-YB/YC (NF–YB/YC) and the CORE2 or CORE1 elements revealed the molecular basis for the specific recognition of the shared TGTG motifs. Biochemical analysis suggested that CO might form a homomultimeric assembly via its N-terminal B-Box domain and simultaneously occupy multiple cis-elements within the FT promoter. We suggest that this multivalent binding gives the CO–NF–Y complex high affinity and specificity for FT promoter binding. Overall, our data provide a detailed molecular model for the regulation of FT by the master transcription factor complex CO–NF–Y during the floral transition.

Dual recognition of H3K4me3 and DNA by the CRA-type ISWI complex subunit ARID5 is required for gene expression, development, and the floral transition.

Significance: TEM1 is a flowering repressor that suppresses the expression of FT florigen. However, the specific FT targeting mechanism of TEM1 and the molecular basis for the repression function of TEM1 remain unclear. In this research, we performed structural, biochemical, and genetic studies on TEM1. We revealed the molecular mechanism underlying the combinatorial binding of the FT gene 5′-untranslated region (UTR) by TEM1 AP2 and B3 domains. We found that TEM1 interacts with the PRC2 subunit CLF and is responsible for deposition of the repressive histone mark H3K27me3 at the FT 5′-UTR region. Altogether, we revealed a molecular mechanism underlying the flowering repression function of TEM1.