NF-YC3, NF-YC4 and NF-YC9 are required for CONSTANS-mediated, photoperiod-dependent flowering in Arabidopsis thaliana

P>NF-Y transcription factors represent a complex of three proteins called NF-YA, NF-YB and NF-YC. Each protein is highly conserved in eukaryotes, and in the plant lineage has undergone numerous rounds of duplication. Individual NF-Y are emerging as important regulators of several essential plant processes, including embryogenesis, drought resistance, maintenance of meristems in nitrogen-fixing nodules and photoperiod-dependent flowering time. Building on the recent finding that NF-YB2 and NF-YB3 have overlapping functionality in Arabidopsis photoperiod-dependent flowering (Kumimoto et al., 2008), we have identified three NF-YC (NF-YC3, NF-YC4, and NF-YC9) that are also required for flowering, and physically interact in vivo with both NF-YB2 and NF-YB3. Furthermore, NF-YC3, NF-YC4 and NF-YC9 can physically interact with full-length CONSTANS (CO), and are genetically required for CO-mediated floral promotion. Collectively, the present data greatly strengthens and extends the argument that CO utilizes NF-Y transcription factor complexes for the activation of FLOWERING LOCUS T (FT) during photoperiod-dependent floral initiation.