Alternative splicing provides a proactive mechanism for the diurnal CONSTANS dynamics in Arabidopsis photoperiodic flowering.

The circadian clock control of CONSTANS (CO) transcription and the light-mediated stabilization of its encoded protein coordinately adjust photoperiodic flowering by triggering rhythmic expression of the floral integrator flowering locus T (FT). Diurnal accumulation of CO is modulated sequentially by distinct E3 ubiquitin ligases, allowing peak CO to occur in the late afternoon under long days. Here we show that CO abundance is not simply targeted by E3 enzymes but is also actively self-adjusted through dynamic interactions between two CO isoforms. Alternative splicing of CO produces two protein variants, the full-size COα and the truncated COβ lacking DNA-binding affinity. Notably, COβ, which is resistant to E3 enzymes, induces the interaction of COα with CO-destabilizing E3 enzymes but inhibits the association of COα with CO-stabilizing E3 ligase. These observations demonstrate that CO plays an active role in sustaining its diurnal accumulation dynamics during Arabidopsis photoperiodic flowering.