Molecular condensation of the CO/NF-YB/NF-YC/FT complex gates floral transition in Arabidopsis

The plant master photoperiodic regulator CONSTANS (CO) interacts with Nuclear Factor-Y subunits B2 (NF-YB2) and C9 (NF-YC9) and transcriptionally activates the florigen gene FLOWERING LOCUS T ( FT ), regulating floral transition. However, the molecular mechanism of the functional four-component complex assembly in the nucleus remains elusive. We report that co-phase separation of CO with NF-YB2/NF-YC9/ FT precisely controls heterogeneous CO assembly and FT transcriptional activation. In response to light signals, CO proteins form functional percolation clusters from a diffuse distribution in a B-box-motif-dependent manner. Multivalent coassembly with NF-YC9 and NF-YB2 prevents inhibitory condensate formation and is necessary to maintain proper CO assembly and material properties. The intrinsically disordered region (IDR) of NF-YC9, containing a polyglutamine motif, fine-tunes the functional properties of CO/NF-YB/NF-YC condensates. Specific FT promoter recognition with polyelectrolyte partitioning also enables the fluidic functional properties of CO/NF-YB/NF-YC/ FT condensates. Our findings offer novel insights into the tunable macromolecular condensation of the CO/NF-YB/NF-YC/ FT complex in controlling flowering in the photoperiod control. Synopsis Phase separation of transcription factors plays a vital role in regulating gene expression. This study identifies a multicomponent condensation of the transcription factors CO, NF-YB, and NF-YC which can form functional CO assemblies that enable transcriptional activation of FLOWERING LOCUS T ( FT ). CO proteins accumulate from a diffuse distribution to functional percolation clusters. Coassembly with NF-YC9 and NF-YB2 is necessary to maintain proper CO assembly status and material properties to activate FT expression. The polyglutamine (polyQ) motif within IDR of NF-YC9 modulates the functional material properties of CO/NF-YB/NF-YC condensates. Specific FT promoter recognition enables the fluidic properties of CO/NF-YB/NF-YC/ FT condensates. Control of the material properties of CONSTANS (CO) condensates at FLOWERING LOCUS T by NF-YC9 and NF-YB2 regulates transcriptional activation.