IMPORTIN β4 Facilitates CONSTANS Nuclear Accumulation and FT Activation in Arabidopsis.

The timing of the vegetative-to-reproductive transition is critical for plant reproductive success. In Arabidopsis, the B-box transcription factor CONSTANS (CO) is a central regulator of flowering that activates FLOWERING LOCUS T (FT). Although the transcriptional regulation and post-translational stability of CO have been extensively characterised, the mechanisms governing its nuclear accumulation remain poorly defined. Here, we identify IMPORTIN β4 (IMB4) as a positive regulator of flowering time. Loss of IMB4 delayed flowering and reduced FT expression without affecting CO transcript abundance. IMB4 interacts with the C-terminal CCT domain of CO, promoted its nuclear accumulation, and thereby enables efficient FT activation. IMB4 and CO show coincident diurnal expression peaks at dusk, suggesting temporal coordination of this transport step. Together, these findings identified IMB4 as a regulator of the CO-FT module and support a role for nucleocytoplasmic partitioning in flowering-time regulation.